Threshold Limit Value
\r\n\tA number of advanced combustion technologies have been introduced to improve performance, fuel economy and emissions levels. Research in combustion technology has highlighted the importance of new fuels in reducing the petroleum dependence and achieving high efficiency with low pollutant formation.
\r\n\tThe purpose of this book is to collect interesting and original studies on combustion methods, advanced combustion strategies and new fuels able to achieve efficiency improvements and environment compliance.
\r\n\tContributions in which experimental, theoretical and computation approaches are applied to explore how fuel properties and composition affect advanced combustion systems and how advanced combustion technology can maximize engine efficiency and be environment-friendly are invited and appreciated.
Robots are necessary for search and rescue purposes, to access concealed places and environments that fire fighters and rescue personnel cannot gain entry to. Three hundred and forty-three firefighters died at the World Trade Center during the September 11 attacks in 2001 (Wiens, 2006). Often these rescuers unnecessarily entered an environment that had unstable structures as there were no live victims to rescue. Sixty-five of these rescuers died due to searching confined spaces that had flooded (Kleiner, 2006). Rescue workers have about 48 hours to retrieve victims (Gloster, 2007). Several hours are lost when rescuers are unsure of buildings stability. After a disaster the structures are often unstable and rescuers need to evacuate until the rubble has stabilized. Frequently the rescuers have to evacuate even though a body part of a possible survivor is seen, due to unstable surroundings (Roos, 2005). Robots can stay in the unstable area and continue searching for survivors. In the future, robots could possibly also be used to access mines after an accident prior to rescuers workers (Trivedi, 2001).
\n\t\t\tUrban Search And Rescue (USAR) Robots were first extensively tested at the collapsing of the World Trade Center site in 2001 (Greer et al., 2002). The University of South Florida were involved in these rescue attempts. The robots that they used are shown in figure 1. The advantage of these robots above rescue members is that the disaster areas can be entered immediately after a disaster.
\n\t\t\tProblems identified at the World Trade Center as well as at the testing grounds of the National Institution of Standards and Technology (NIST) are that the robot\'s traction system malfunctioned. (Greer et al., 2002). More research is needed for the robots to withstand the harsh conditions of a fire (Wiens, 2006). Other problems observed were unstable control system, chassis designed for narrow range of environmental conditions and limited wireless communication range in urban environment as well as unreliable wireless video feedback (Calson et al., 2004). Some robots were either too large or not easily maneuverable (Gloster, 2007).
\n\t\t\tFurther problems experienced were that the setup time of the robots was too extensive and the human to robot ratio for transport and controlling were not ideally 1:1 (Greer et al., 2002). Problems were identified regarding the communication with the robots (Remley et al., 2007).
\n\t\t\tThe Inuktun MicroVGTV and I-Robot Packbot was used in the rescue attempts at the World Trade Center in September 2001
Communication is critical as the rescuers need to send instructions to the robots, but at the same time receive vital information about the environment. This could save lives as it could indicate poor structural areas, dangerous gases and extreme temperatures. Research has been performed to determine improvements and possible solutions to these problems experienced. These solutions include a combination of communication reliability in these environments, and a sensory system to allow the robots to maneuver across the terrain successfully.
\n\t\t\tThe communication and sensory system is discussed as it was implemented on the CAESAR (Contractible Arms Elevating Search And Rescue) robot. These developments include communication protocols, hardware interface and artificial intelligence to indicate the safety and danger levels for both humans and the robot.
\n\t\tThe interferences that were experienced before are mainly due to the robots using Industrial, Scientific and Medical (ISM) bands. Many electronic communication units use the ISM bands which are unlicensed frequencies that have certain constraints. As USAR robots are used to save lives, it is suggested that licensed frequencies are utilized. This will significantly prevent interferences. The output power between the control unit and the robot can be constrained to prevent a signal from one unit overwhelming the signals from other units.
\n\t\t\tAnother reason for failed robot communication is the loss of signals between the robot and its control unit. This is mainly caused by the frequency used. As wavelength is inversely proportional to the frequency and the antenna size is proportional to the wavelength therefore the higher the frequency, the smaller the antenna will be. Transmission efficiency decreases as higher frequencies are used. The signal penetration into buildings is also effected by the frequency used. Higher frequencies are capable of penetrating more dense materials that lower frequencies. The disadvantage of higher frequencies is that small items, such as dust particles, resonate at the high frequency therefore causing it to absorb the power of the signal. Therefore it is best to use a frequency in the center of the two extremes that will allow optimization for radio communication. The comparison of the different factors that are considered are shown in figure 2. Subsequently the decision is, to use UHF frequencies as these are able to penetrate with a relatively low power output and have a relatively good signal penetration property.
\n\t\t\tComparison of factors considered as frequency increases
Different modules and units are needed for the successful communication of video, audio and data. The modules that were used for the CAESAR robot, are discussed and explained.
\n\t\t\t\tThe Radiometrix narrow band FM multi-channel UHF TR2M-433-5 transceiver modules is used for the data communication. A photograph of one of these modules is shown in figure 3 (Radiometrix, 2004).
\n\t\t\t\t\tRadiometrix TR2M radio module
The features of the TR2M modules are:
\n\t\t\t\t\tCan be programmed to operate on any 5 MHz band from 420 MHz to 480 MHz.
Fully screened
1200 baud dumb modem
Pertaining to the above features, these data modules will be valuable for the USAR robot. It enables the programming of the modules to operate on the frequencies supplied by the fire department. The power consumption is low which is vital for power saving. With this large range of operating temperatures the heat from the outside could be insulated and limited to the module.
\n\t\t\t\t\tThe only problem that occurs regarding these modules is their inability to transmit more than 10 mW. An output power of 5 W is required for efficient communication with the restrictions of buildings and other power absorbers. A RF amplifier is needed to solve this problem.
\n\t\t\t\t\tRF amplifiers that amplify 10 mw to at least 5 W are either not readily available or they are expensive. In order to solve this problem, the final stages of Motorola MCX100 radios were used. The need arose for two of the three RF amplification stages as the amount of power that these final stages produce is sufficient, whereas the three final stages produce more than 5 W output power. Refer to figure 4 for the interconnection of these stages. The disassembly and reconstruction of these stages require the addition of discrete components. Not all the modules in the radio were used. These impedances of the missing modules are to be replaced. The circuit of the RF amplifier is traced with a probe to determine the amplification of each stage. There are two positive power supply points. Tracing the power point that was not powering the circuit of the first stage of the RF amplifier, it was found that there was a discontinuation for a closed loop circuit. This closed loop circuit was terminated to another module not used. By modifying the impedance on this point, a different output power was produced from the RF amplifier. It was discovered that a resistance of 300Ω made the RF amplifier produce 5W output.
\n\t\t\t\t\tTransmission process block diagram
A problem occurred in the reception, as the signal was not able to reach the TR2M module from the antenna due to the RF amplifier not being bi-directional. This could possibly be solved by connecting the antenna directly to the TR2M module and then reception would be possible, but the high output power from the RF amplifiers would terminate the operation of the TR2M module, as there is high power penetrating the sensitive module.
\n\t\t\t\t\tThis problem was solved by the implementation of a switching circuit on the output to the antenna. Figure 5 illustrates the concept of this circuitry. While the two relays are in position 1, the TR2M module can receive data. Should the TR2M module need to transmit, then the relays are switched over to position 2, which will connect the TR2M module to the RF amplifier and in turn with the antenna. This prevents the need for two antennas and allows for only one radio module for data communication at each station.
\n\t\t\t\tTR2M and RF Amplifier with the appropriate switching
The use of protocols is important for data to be successfully transmitted. Using available protocols is an option, but the performance and efficiency must be considered. Most existing protocols have been developed over many years and by various people. These protocols are optimized for best performance for a specific task.
\n\t\t\t\t\tThe IEEE 802.11 protocol could be used for communication between the robots, but there is not always an Access Point available for the wireless communication. The communication between the robots will be an Ad-Hoc style. Since UHF frequencies are being used, the data rate will be less in comparison to that used by wireless communication, as they use frequencies in the 2.4 GHz band and the quality factor bandwidth decreases as frequency decreases. Due to the bandwidth being decreased, additional collisions might occur and therefore smaller packet sizes are needed. More data transmission from other stations is able to occur when the packet sizes are smaller.
\n\t\t\t\tThe Robot Communication Protocol (RCP) uses different aspects from the wired and wireless LAN protocols. The problem when using wireless communication technology is that it uses the 2.4 GHz band which causes the small particles of buildings to resonate at this frequency and to absorb energy which can prevent penetration through buildings. A further problem with the use of the IEEE 802.11 protocol is that its packets contain header details that will not be utilized for the USAR robots. This is therefore unnecessary data that will be transmitted and will occupy the use of the medium. In view of the fact that the baud rate of the data communication modules can be low, unnecessary data must be prevented as this can saturate the medium.
\n\t\t\t\t\tAnother problem pertaining to the existing protocols is that they may possibly contain non-printable characters that cannot be processed by certain computers and microcontrollers. The printable characters are those that have an ASCII value between 31 and 127.
\n\t\t\t\t\tA new wireless communication protocol is required for USAR robots to utilize. A decision was made to use callsigns to identify the robots and control units to prevent communication interference. A six character callsign that consist of letters of the alphabet and numbers is assigned to each robot and control unit. This gives a combination of 366 = 2.17 x 109 different callsigns available.
\n\t\t\t\t\tThere are two types of protocols that need to be transmitted namely: a “one way packet” that is sent from one station to the other and that needs no confirmation (referred to from now on as a Robotic One-way (RO) packet) and a packet which is sent from one station to the other and which replies with an acknowledgment of reception packet (referred to from now on as a Robotic Confirmation (RC) packet).
\n\t\t\t\t\tThere are four packets for the robotic network namely, Request-To-Send (RTS), Clear-To-Send (CTS), Acknowledgment (ACK) and Data packet. The different packets with their fields are explained below.
\n\t\t\t\t\tRTS / CTS / ACK Packet\n\t\t\t\t\tThe packet format for the RTS, CTS and ACK packets are shown in figure 6.
\n\t\t\t\t\tRTS / CTS / ACK Packet.
Start: The start character is for stations to identify the commencement of the packet. This is indicated with the hash (#) character. Should a station only start receiving in the middle of a transmission it will then recognize this and discard the packet. The purpose for the necessity of a start byte is that the transmission is asynchronous on a single channel.
\n\t\t\t\t\tType: This field indicates the type of packet that is being sent. The indication for the RTS, CTS and ACK packets are the characters 0, 1 and 2 respectively.
\n\t\t\t\t\tDuration: The duration of the transmission is specified in this field. This provides the other stations with the time period to delay before attempting to transmit. The duration is specified by the number of characters. Time periods are calculated from the sum of the two bytes multiplied with x, where x is the time period for each character to transmit.
\n\t\t\t\t\tShould these values be a “#” or “!”, then the most significant byte must be incremented and the least significant byte must be decremented.
\n\t\t\t\t\tRA: This is the address of the receiving station. This field presents the opportunity for other stations to identify whether that the packet is for them or not. Should the packet not be intended for the station, the rest of the incoming packet can be disregarded and the station can start processing other incoming packets after the delay duration.
\n\t\t\t\t\tTA: This is the address of the transmitting station and is used by the receiving station to identify if the packet is from its approved station.
\n\t\t\t\t\tChecksum: This verifies the integrity of the packet. The field value consist of the sum of all ASCII values of all characters in packet modular 94 and the addition of 32. Should the receiving station receive a packet that is not approved then it is subsequently dropped. If the value of this field should be equal to “#” or “!” then the duration field is incremented and the checksum is recalculated. This field must be a printable character and not a control character (I.e. the character must have an ASCII value between 31 and 127)
\n\t\t\t\t\tEnd: This indicates the end of the packet with an exclamation mark (!) character.
\n\t\t\t\t\tData Packet\n\t\t\t\t\tThe format of the Data packet is shown in figure 7.
\n\t\t\t\t\tData Packet
Start: The start character is for stations to identify the beginning of the packet. This is indicated with the hash (#) character. In the event that a station only starts receiving in the middle of a transmission, this will be identified and the packet will be discarded. The motivation for a start byte is that the transmission is asynchronous on a single channel.
\n\t\t\t\t\tType: This field indicates the type of packet that is being sent. The identification of a RO Data packet is the character 3 while for a RC Data packet it is the character 4. The other possible values (except for the character values for # and !) for this field are reserved for future use.
\n\t\t\t\t\tDuration: The duration of the transmission is given here. This provides the other stations with the time period that they have to delay with before attempting to transmit. The duration is given by the number of characters. Time periods are calculated from the sum of the two bytes multiplied with x, where x is the time period for each character to transmit.
\n\t\t\t\t\tShould these values exist of a “#” or “!”, then the most significant byte must be incremented and the least significant byte must be decremented.
\n\t\t\t\t\tRA: This is the address of the receiving station. This gives the opportunity for other stations to identify whether the packet is meant for it or not. In the event that it is not, the station can ignore the rest of the incoming packet and start processing other incoming packets after the delay duration.
\n\t\t\t\t\tTA: This is the address of the transmitting station. This is used by the receiving station to identify that the packet is from its relative approved station.
\n\t\t\t\t\tData: The data for specific instruction or information between the stations is stored in this field. The only characters that are not allowed in this field are the hash (#) and the exclamation mark (!) seeing that these are the start and end characters respectively. Control characters are also not allowed in this field.
\n\t\t\t\t\tChecksum: This verifies the integrity of the packet. The field value consist of the sum of all ASCII values of all characters in packet modular 94 and the addition of 32. Should the receiving station receive a packet that is not approved it is subsequently dropped. If the value of this field is equal to “#” or “!”, the duration field is then incremented and the checksum is recalculated. Furthermore this field must be a printable character and not a control character (I.e. the character must have an ASCII value between 31 and 127)
\n\t\t\t\t\tEnd: This indicates the end of the packet with an exclamation mark (!) character.
\n\t\t\t\t\tThe description of the communication procedure is described by means of two stations; station A and station B. Should station A want to transmit, it would observe whether no transmissions are occurring. If none are detected, then station A starts transmitting a RTS packet. All the stations in the vicinity of station A will delay transmission for the period of the duration field in the RTS packet. The delay duration period consists of the sum of the following:
\n\t\t\t\t\t\tthe time period needed to transmit the RTS packet
the time period needed to transmit a CTS packet
the time period for the Data packet
the time period to transmit an ACK packet (if this is needed)
the sum of the processing time at each station
Station B receives the RTS packet and replies with a CTS packet which contains a delay duration period which is:
\n\t\t\t\t\t\tthe sum of the time period for the CTS packet
the time period to transmit the Data packet
the time period to transmit an ACK packet (if this is needed)
the sum of the processing time at each station.
Station A responds with the Data packet that contains a delay duration period which is the sum of the time period for:
\n\t\t\t\t\t\tthe time period to transmit the Data packet,
the time period to transmit an ACK packet if this is needed
the sum of the processing time at each station.
Station B will reply with an ACK packet should the last received packet have a type value of 100. This packet will contain a delay duration period which is the sum of the time period to transmit the ACK packet as well as the processing time at each station.
\n\t\t\t\t\t\tGiven that there is no Access point that is stationary, there is no station that controls communication within the network. In figure 8 four stations are shown with their respective radio coverage. C1 and R1 are control unit 1 and robot 1 respectively and C2 and R2 are control unit 2 and robot 2 respectively.
\n\t\t\t\t\t\tRadio Coverage of two control units and two robots
As noted in figure 8, C1 is in radio coverage with R1 and C2; R1 is in radio coverage with R2 and C2; R2 is in radio coverage with C2. Since C1 and R2 are not in radio coverage packets to request transmission will not be received between these two stations. This is not a great disadvantage as the different stations operate in an ad hoc system. Of importance is the aspect that each robot is able to communicate with its own control unit.
\n\t\t\t\t\t\tShould a RTS packet be transmitted by C1 then R1 will subsequently receive the request and reply with a CTS packet. This CTS packet, which will contain a duration field, will be received by R2 as well. In view of the fact that R2 has received this packet, it will delay with any transmission for this time period before trying to transmit again.
\n\t\t\t\t\t\tIn the instance that both C1 and C2 transmit a RTS at the same time, R1 will then receive data that will be a combination of data from the two control units. R1 will reject this data, as it will not recognize it or because it will not exist of an acceptable packet. After a time-out period C1 will realize that R1 has not responded and will transmit the RTS again if required.
\n\t\t\t\t\t\tAs the RTS packets are relatively small, the overhead of retransmission would be small if two stations should transmit the same time. The sum of data being sent in the Data packet is limited to 128 characters and it need not be necessarily sent in a specific format, providing the format is understandable between the respective control units and the robots.
\n\t\t\t\t\t\tThe advantage of the RCP is that a computer system could be connected to a modem that uses the same protocol and this modem could then transmit and receive instructions and data to a large network of robots. In this situation the computer will be the control unit and will not be dedicated to only a single robot. This network of robots could then be controlled to perform a task that could have a greater efficiency than a single robot.
\n\t\t\t\t\t\tThe RCP packets that are used to control a robot have smaller packets sizes of at least 38 % compared to those used by hard-wired computer network protocols and 33 % compared to that used by IEEE 802.11 protocol. Communication between the robots and their control units are more reliable when used in a network scenario. The use of a computer network protocol could be valuable when the robots have to transmit data and information that involves more than just the basic instructions.
\n\t\t\t\t\tA layered model similar to the OSI model is needed for data communication. Each layer has its unique task to optimize the communication. The advantage of having a layered model is that each layer can be modified and optimized without affecting the other layers. The layered model can be represented as indicated in figure 9.
\n\t\t\t\t\tA three layered model
This model has been divided into three layers as each layer will be controlled by a separate module or microcontroller. The Physical layer consists of the hardware that will be used. In the case of the USAR robot, this will be the radio modules that will act as the transceivers.
\n\t\t\t\t\tThe layer that is a combination of the Data link, Transport, Session and Presentation will be controlled by a single microcontroller. The Data link layer is in control of the packets that are being sent, while the Transport and Session layer is responsible for the packet’s control and transmission permission respectively. All the received data must be presented in a format for the computer to understand. This is achieved by the Presentation layer.
\n\t\t\t\t\tThe Application Layer is involved in the displaying of the information and with the interaction with the user. This layer is also involved in the output, being the movement of the motors and any other attachments of the robot. This layer will be controlled by a microcontroller which could be attached to other microcontrollers or modules, depending on the complex of the attached module.
\n\t\t\t\tVoice communication between the robot and the rescuers is essential for the rescuers to get information from survivors. Rescuers can also calm the victims when the robot approaches and notify victims that help is on the way and of possible ways to save themselves. The voice communication between the robot and the rescuers will be achieved through the video communication but communication between the rescuers and the robot is still required.
\n\t\t\t\tTwo radios are needed for this communication to occur. Since one of the assigned frequencies is used for the data communication, the other assigned frequency is to be used for the voice communication. It was thus decided to use Amateur radios for this communication as it was possible to purchase them due to a license obtained.
\n\t\t\t\tThe decision was to use the Yaesu VX-7R and VX-3E transceivers. These radios can be modified to operate on these emergency bands and have different useful features. Diagrams of the Yaesu VX-7R (Vertex, 2002) and VX-3E (Vertex, 2007) are shown in figure 10.
\n\t\t\t\tDiagram of the Yaesu VX-7R and VX-3E radios.
The Yaesu VX-7R have the feature to operate on UHF bands. The Yaesu VX-7R is used in the control unit. It has the useful characteristic of a keypad, allowing the rescuers to tune into frequencies other than those used for the robot, if so required. With this radio it is possible for the rescuers to tune into the audio frequency of the video transmission from the robot, should the sound from the television be unclear.
\n\t\t\t\tThe Yaesu VX-3E has feature that it can receive between 420 and 470 MHz. The Yaesu VX-3E is used mainly for reception of audio in the robot. The useful characteristics of the Yaesu VX-3E is that it is small in size, light weighted and can operate at temperatures that could possibly occur in the robot.
\n\t\t\t\tThe audio input to the video transmitter, (discussed in Chapter 2.3 – Video Communication) needs to have an impedance of 600 Ω and a maximum voltage of 1VP-P or 0.775 VRMS. A 600 Ω dynamic microphone was initially connected to the input of the audio as there was no verification as to whether the transmitter had a build in preamplifier. This did not seem to work, so a mono microphone preamplifier is used to amplify the signal from the dynamic microphone. While the preamplifier is connected to the transmitter, the preamplifier output is tested on an oscilloscope and the gain is altered to get a maximum output of 1VP-P. The schematic of the microphone preamplifier is shown in figure 11 (Excellence, 1998).
\n\t\t\t\t\tSchematic of the microphone pre-amplifier.
The dynamic microphone used is manufactured from plastic which will result in a problem at high temperatures. Research has been performed to determine the availability of high temperature microphones but the research proved unsuccessful. It is therefore decided to continue using the plastic microphone to enable the testing of the principles being discussed.
\n\t\t\t\t\tAn ear piece with microphone is used in the Yaesu VX-7R radio, to allow the controller to communicate with any victims. The VOX-activation function could be set to allow transmission of spoken voice.
\n\t\t\t\t\tA 1.5mm earphone plug is used for the Yaesu VX-3E radio and connected to an 8Ω speaker. Research was performed to determine whether speakers were available that would be able to resist the high temperatures, but none were found. An ordinary speaker is used to prove the principle.
\n\t\t\t\tThe video is from the FLIR PathFindIR thermal camera shown in figure 12 (FLIR, 2006). It has the following specifications:
\n\t\t\t\tSize: (58mm x 57mm x 72mm)
Input Voltage range: 6V – 16V
Power dissipation: Less than 2W
Weight: less than 0.4 kg
The PathFindIR is ideal for this project, as it is small, does not weigh much, and is affordable compared to other available thermal cameras. It has a low power dissipation and can operate from -40 C to 80 C. Should the temperature decrease below -40 C, the heating element is switched on, therefore allowing images to be transmitted in cold environments.
\n\t\t\t\tThe video from the PathFindIR needs to be transmitted. ICASA (Independent Communication Association of South Africa) and Sentech have given permission to use channel 54 (735 MHz) for video transmission, on the condition that the output power is less than 1W, and the transmitter is calibrated by one of their approved dealers.
\n\t\t\t\tFLIR PathFindIR thermal camera.
The modulator and IF converter is used to generate the video on the required frequency. This signal is then amplified to 1W. This amplifier is shown in figure 13 (Jackel, 2008).
\n\t\t\t\tUHF amplifier.
These modules can operate between 470 – 862 MHz. It has been confirmed that all output power for communication must be at least 5W(Reynolds, 2008) for search and rescue reasons. As there is a restriction for the video output power, 1W is used to prove the concept for this robot. It is suggested that a video frequency is assigned for search and rescue purposes so that the output power can be increased to 5W.
\n\t\t\t\tA block diagram of the interconnection between the PathFindIR, converter/modulator, microphone, audio preamplifier, video amplifier and antenna is shown in figure 14.
\n\t\t\tPathFindIR connected to the modulator/converter, 1W UHF amplifier, audio preamplifier and antenna
Antennas are the source of transmission into the medium of air and the absorber of signals from the medium of air. Different antennas have different properties of radiation patterns and polarization. This is a topic in communication that is often neglected, but the antenna used has an effect on the performance of transmission and reception of signals. The antenna of a radio can influence many factors that can be the cause of many problems. Calibrating and selecting an antenna influences the efficiency of output power and signal strength that will be radiated from a radio.
\n\t\t\t\tThe antennas used were investigated. The orientation of the antenna effects the polarization of the transmitted waves. It would be ideal to have vertical and horizontal polarization. The best antenna for this purpose is the egg-beater type. It gives vertical and horizontal polarization, but it has the disadvantage of being relatively large, which is not ideal, as one of the objectives of a USAR robot is to design it as small as possible.
\n\t\t\t\tVertical antennas were investigated and a problem encountered is that the base plane shields the signals from being transmitted through it. Different fractions of the wavelength antennas have got different properties. A ½ wavelength antenna has radiation lobes that are perpendicular to the antenna, while the ¼ wavelength antenna has radiation lobes that are at an angle of about 45 degrees. The use of the property of the ¼ wavelength antenna will work well as it was found that it has a degree of output power directed towards the end point of the antenna. The only disadvantage of this type of antenna is that there is no radiation past the base plane.
\n\t\t\t\tThis problem is solved by removing the base plane and replacing it with a piece of coaxial cable that is longer than the ¼ wavelength. The reason for the need of the base plane or coaxial cable is that it produces the negative part of the modulated sine wave. With the removal of the base plane, the radiation from the antenna is relatively isotropic, with low radiation towards the end points of the antenna. The antenna then is seen as a ½ wavelength dipole antenna. This isotropic radiation pattern is caused by the minor lobes that are allowed to be radiated next to the main lobe. When the robot is a number of wavelengths above the ground, the radiation pattern will become more isotropic because of more lobes, and will lower the elevation angle of the lowest angle lobe (Roos, 2005). This antenna has the disadvantage in that it is not being vertically and horizontally polarized. This is solved by using an egg-beater type antenna that is scaled in size at the receiving unit. It will then be able to receive any polarized signal (discussed in section 2.4.2 Eggbeater Antenna Design).
\n\t\t\t\t½ wavelength radiation pattern
¼ wave radiation pattern
Radiation pattern of antenna that is used.
Communication is improved with the use of UHF frequencies because, the penetration of the signal is increased, the antenna is relatively small and the transmission efficiency is still acceptable. With the use of a dipole antenna that has coaxial cable for the ground plane, the radiation pattern is increase by 100 % in terms of direction compared to an antenna that has a base plane. The radiation distance decreases as the output energy remains the same and is spread over a larger angle. The polarization of the radiated waves are in the same orientation as the antenna\'s orientation and can be received with an egg-beater antenna that is capable of receiving any polarized signal.
\n\t\t\t\tThe length of the full wave antenna in free space is calculated from equation 3. This equation is valid for transmission in free space.
\n\t\t\t\t\twhere: λ= wavelength in meters
\n\t\t\t\t\tf = frequency in MHz
\n\t\t\t\t\tThe surrounding air has an effect on the antenna, so a factor η has to be multiplied with equation 4. The value of η is variable and depend on the antenna\'s surroundings. As the robot will be operated in conditions of smoke, heat and with various objects surrounding it, the value of η would vary.
\n\t\t\t\t\t\n\t\t\t\t\t\tEquation 4 is used to calculate the wavelength of the antenna. This length of antenna wire is then cut and connected to the radio with a Standing Wave Ratio (SWR) meter, which is connected in series with the feed line. Millimeters of the antenna is trimmed away until the SWR value is very close to a SWR ratio of 1:1.
\n\t\t\t\t\tSWR is the ratio of the forward and reflective power. Power is reflected back into the transmitter when the load does not have a matching impedance to that of the characteristic impedance. The SWR of a specific load can be calculated with equation 4 (Frenzel, 2001).
\n\t\t\t\t\tFrom equation 4, it is seen that as PR decreases, the SWR will tend to 1. To determine the SWR of a specific antenna, the meter is calibrated so that there is maximum deflection for the forward transmission of a signal, and then the reflective signal back into the system is read. This reading is performed every time an alteration of the antenna is made, until the SWR is close to 1:1. The ideal situation is to have a SWR of 1:1, but there are many factors that can influence this reading, such as surrounding objects.
\n\t\t\t\t\tAn antenna tuned for a frequency in the UHF band is compatible for most frequencies in the UHF band. This characteristic is used to tune the antenna for a frequency of 450 MHz. With the use of equation 3, the antenna wavelength is calculated as:
\n\t\t\t\t\tThe full wavelength is 667 mm, but since a quarter wavelength antenna is to be used, the antenna length required will be 166.75 mm. From this length, the antenna is lengthened or trimmed until a SWR of 1:1 is obtained. This is needed as the antenna is operated in an environment that is not free space. The final antenna length is 170 mm.
\n\t\t\t\tDifferent forms of the eggbeater antenna design were considered. The testing of the antennas were performed with the use of a RF generator and a SWR meter. A receiver with a horizontal antenna was set up. The strength of the signal received by the receiver is displayed on a signal analyzer.
\n\t\t\t\t\tA folded dipole antenna was initially considered. This is a dipole antenna that is bent into a loop, bringing the ground and live point to each other, but not touching each other. The same configuration is used for another folded dipole antenna that is placed 90 degrees to the first one. The two loop antennas are separated with a quarter wave stub, so that the transmission between the two loops are 90 degrees out of phase and therefore prevent cancellation. The quarter wavelength coaxial cable stub must be shortened depending on the velocity factor of the transmission line. This value typically varies between 0.6 and 0.7. The velocity factor of a RG-174 coaxial cable is 0.66. This quarter wave coaxial cable length can be calculated by equation 6 (Frenzel, 2001).
\n\t\t\t\t\twhere F is the velocity factor of the coaxial cable.
\n\t\t\t\t\tIt is very difficult to determine the exact length of the coaxial cable stub, as the theoretical value does not correlate to that of the practical assessment. Therefore a Dip Meter is used to cut the exact length of the coaxial stub. The Kenwood DM-81 Dip Meter was used for this. A photo of this Dip Meter is illustrated in figure 18.
\n\t\t\t\t\tThe Dip Meter has a connection for a coil for the required frequency. A coil for a harmonic of 450 MHz was used. As the dial of the Dip Meter is not very accurate, the frequency counter that is on the Yaesu VX-7R was used to get the resonating frequency close to 450 MHz.
\n\t\t\t\t\tThe Dip Meter is calibrated on the resonated frequency and a portion of coaxial cable is then placed next to the coil. A single loop then is made from a piece of wire and is soldered between the center conductor and the outside braid. Initially this loop is placed around the coil to get a broader band reading. The dial is cautiously turned until the Dip Meter is at full deflection. With this configuration, 2 mm pieces are cut from the coaxial cable, until it is detected that the Dip Meter is deflected towards zero. This is an indication that the coaxial cable being tested is absorbing most of the power at that frequency and that the coaxial cable is exactly a quarter wavelength which also incorporates the velocity factor.
\n\t\t\t\t\tThe tests proved that the antenna is relatively omni-directional, but there is a couple of cancellations of signals where two parallel or perpendicular antennas cancel each other.
\n\t\t\t\t\tThe eggbeater antenna was then considered. The eggbeater antenna consists of two loops that are perpendicular to each other. A quarter wavelength stub is placed between the two loops to cause the transmission between the two antennas to be 90 degrees out of phase. The tests confirmed that this type of antenna is more omni-directional and has relatively rare cases of signal cancellation. There is more dips in the signal strength than complete cancellation.
\n\t\t\t\t\tThe problem with this type of antenna is that the loops must have a full wavelength circumference, making the diameter of the loop relatively large. This causes a space problem in the robot casing for this type of antenna (at 450 Mhz). The loop can be made smaller, but then higher frequencies must be used. As we want to use UHF frequencies, it would not be ideal to use smaller loops.
\n\t\t\t\t\tResulting from this, the decision is to use the eggbeater antennas in the control unit where there is not as much constraints in size. Should the robot contain an antenna that is polarized in a single direction, then the eggbeater antenna (that has horizontal and vertical polarization) will be able to receive the transmitted signal. The tested eggbeater antenna is resonating between 440 MHz and 490 MHz, which is ideal for the available frequencies.
\n\t\t\t\t\tKenwood DM-81 Dip Meter
Eggbeater Antenna
The gases that are of main importance in a search and rescue event is carbon dioxide, carbon monoxide, hydrogen sulphide, methane and oxygen (Gloster, 2007). Most sensors give an output of gas concentration, which is measured in parts per million (ppm). This data may be meaningless to the controller as it might hold no threat. An example will be the detection of methane gas. Should the robot detect 1ppm of methane, this could possibility not be dangerous, as it could be either a natural gas in the environment or of such a small quantity that it won\'t cause an explosion.
\n\t\t\tFurther analysis of the gases and their respective properties need to be investigated. Different aspects of the gases were analyzed, to determine the concentrations that would be considered dangerous.
\n\t\t\t\tThe Immediately Dangerous to Life or Health (IDLH) levels are used for non-emergency and emergency scenarios (Aerotech, 2009). These IDLH concentrations are determined with the following considerations (NIOSH, 2004):
\n\t\t\t\tPeople must be able to escape the danger without the loss of life or irreversible health effects that could happen after exposure to that environment for a time period of 30 minutes.
Prevention of severe eye or respiratory irritation, which will prevent a person from escaping the dangerous environment.
The compiled properties of the gases are represented in table 1.
\n\t\t\t\tSubstance | \n\t\t\t\t\t\t\tIDLH (ppm) | \n\t\t\t\t\t\t\tTLV * (ppm) | \n\t\t\t\t\t\t\tSmell | \n\t\t\t\t\t\t\tFlammability percentage | \n\t\t\t\t\t\t\tNFPA ? Health / Flammability / Reactivity | \n\t\t\t\t\t\t
CO 2\n\t\t\t\t\t\t\t | \n\t\t\t\t\t\t\t40 000 | \n\t\t\t\t\t\t\t5 000 | \n\t\t\t\t\t\t\tOdorless | \n\t\t\t\t\t\t\tNon flammability | \n\t\t\t\t\t\t\t3 / 0 / 0 | \n\t\t\t\t\t\t
CO | \n\t\t\t\t\t\t\t1 000 | \n\t\t\t\t\t\t\t25 | \n\t\t\t\t\t\t\tOdorless | \n\t\t\t\t\t\t\t12% - 75% | \n\t\t\t\t\t\t\t3 / 4 / 0 | \n\t\t\t\t\t\t
H 2 S | \n\t\t\t\t\t\t\t100 | \n\t\t\t\t\t\t\t10 | \n\t\t\t\t\t\t\tRotten Egg | \n\t\t\t\t\t\t\t4.3% - 46% | \n\t\t\t\t\t\t\t4 / 4 / 0 | \n\t\t\t\t\t\t
Methane | \n\t\t\t\t\t\t\t5 000 *** | \n\t\t\t\t\t\t\t1 000 | \n\t\t\t\t\t\t\tOdorless | \n\t\t\t\t\t\t\t5% - 15% | \n\t\t\t\t\t\t\t1 /4 / 0 | \n\t\t\t\t\t\t
Oxygen | \n\t\t\t\t\t\t\t** | \n\t\t\t\t\t\t\t** | \n\t\t\t\t\t\t\tOdorless | \n\t\t\t\t\t\t\tNon flammability ** | \n\t\t\t\t\t\t\tN/A ** | \n\t\t\t\t\t\t
Threshold Limit Value
** Oxygen is not flammable, but assist with combustion Oxygen level that are required to function mentally is 19.5% (NIOSH, 2004). Higher concentrations of Oxygen does not have serious effect on a person, but could cause sever explosions.
\n\t\t\t\t*** As methane is an asphyxiant, there is no IDLH data available (Stanford, 2008). A value for IDLH of five times that of the TLV is used.
\n\t\t\t\t\n\t\t\t\t\tTable 1. Properties of gases of interest.
\n\t\t\t\tOxygen reacts with carbon to form carbon dioxide, therefore, as the carbon dioxide levels increase by x%, the oxygen levels will decrease by x% (U.S. OSHA et al., 2001). In the event that the oxygen levels decreases by x%, and the normal levels of oxygen in our atmosphere is considered to be 20.9%, then the oxygen level will decrease by (x / 20.9)%.
\n\t\t\t\tConcentrations of the gases up to the level of the Threshold Level Value (TLV) are considered to be safe. Any gas concentrations between the TLV and the IDLH are considered unsafe, while any concentration above the IDLH are dangerous. Table 2 shows a combination of all these properties in percentages.
\n\t\t\t\tSubstance | \n\t\t\t\t\t\t\tUnsafe human (%) | \n\t\t\t\t\t\t\tDangerous human (%) | \n\t\t\t\t\t\t\tFlammable (%) | \n\t\t\t\t\t\t
CO 2 | \n\t\t\t\t\t\t\t0.5 | \n\t\t\t\t\t\t\t4 | \n\t\t\t\t\t\t\tNon-flammable | \n\t\t\t\t\t\t
CO | \n\t\t\t\t\t\t\t0.0025 | \n\t\t\t\t\t\t\t0.1 | \n\t\t\t\t\t\t\t12% - 75% | \n\t\t\t\t\t\t
H 2 S | \n\t\t\t\t\t\t\t0.001 | \n\t\t\t\t\t\t\t0.01 | \n\t\t\t\t\t\t\t4.3% - 46% | \n\t\t\t\t\t\t
CH 4 | \n\t\t\t\t\t\t\t0.1 | \n\t\t\t\t\t\t\t0.5 | \n\t\t\t\t\t\t\t5% - 15% | \n\t\t\t\t\t\t
O 2 | \n\t\t\t\t\t\t\t< 19.5 | \n\t\t\t\t\t\t\t< 16.9 | \n\t\t\t\t\t\t\tNon-flammable | \n\t\t\t\t\t\t
Gas properties in percentages
Using the above data, it is possible to alert the rescuers in time of different possible conditions that could occur in the environment. These conditions could be either considered dangerous for humans or for the robot. With the information the rescuers could determine whether to risk their lives or the robot to enter a room with this environmental conditions.
\n\t\t\t\tFuzzy logic is the way to determine logical expressions that are neither true or false. This type of reasoning is used to determine the unsafe, danger and flammable possibilities. The standard rules for fuzzy truth (T) are the following (Russell & Norvig, 2003):
\n\t\t\t\t\tFor the above rules to be applied to the gas concentrations, it needs to be associated with a relationship referenced to the percentages in table 2, which are the boundaries. The value gn which is the specific gas concentration read from the sensors, is a value per million. This value has to be normalized with respect to 1 million to get a ratio. The unsafe value for humans (uh), dangerous value for humans (dh) and flammable value (f) are also normalized with respect to 1 million to get a ratio for the boundary values. Equations (10), (11) and (12) are used to determine A, B and C respectively.
\n\t\t\t\t\tIn the event that the values of A,B and C are negative, the environment is safe for humans. Should any of the values become positive, it indicates that the gas concentration is either unsafe, dangerous or flammable.
\n\t\t\t\t\tUsing equation (8) and equation (9), and only the positive numbers of A, B and C, (denoted by pos()), then
\n\t\t\t\t\tCombining equations (10), (11) and (12) with equation (13), results,
\n\t\t\t\t\tLet equation {14} relate to a function (K) that returns a solution to the logical expression. The relationship to determine if the gas concentration are unsafe, dangerous or flammable, the boundaries uh, dh and fmin are compared to K, which concludes the possible decision (D). With this model, it specifies P(Safety of the environment | specific gas concentration).
\n\t\t\t\t\tDifferent solutions are expected from each gas analysis. All the solutions from the different gases are required to determine the safety of the environment. As the order of safety (being unsafe, dangerous and flammable) decreases and the gas concentration increases, the final decision is considered in respect to the worst solution from the different gases. This is expressed as the worst safety which is max (Dn). Should one gas concentration be flammable but another only unsafe for humans, then the flammability of the gas takes priority.
\n\t\t\t\t\tFor the above equations to be valid, the gas concentration in table 2 needs to be converted to a ratio with respect to 1 million. As it becomes more dangerous for humans when the oxygen decreases, the values required are subtracted from 1 million. This allows for monitoring values that will be increasing throughout the table. The measurements for the oxygen concentration will also need to be deducted from 1 million to get an accurate decision. This is shown in table 3.
\n\t\t\t\t\tSubstance | \n\t\t\t\t\t\t\t\tUnsafe human (ppm) | \n\t\t\t\t\t\t\t\tDangerous human (ppm) | \n\t\t\t\t\t\t\t\tFlammable min (ppm) | \n\t\t\t\t\t\t\t
CO 2 | \n\t\t\t\t\t\t\t\t500 | \n\t\t\t\t\t\t\t\t4000 | \n\t\t\t\t\t\t\t\tNon-flammable | \n\t\t\t\t\t\t\t
CO | \n\t\t\t\t\t\t\t\t25 | \n\t\t\t\t\t\t\t\t1000 | \n\t\t\t\t\t\t\t\t120 000 | \n\t\t\t\t\t\t\t
H 2 S | \n\t\t\t\t\t\t\t\t10 | \n\t\t\t\t\t\t\t\t100 | \n\t\t\t\t\t\t\t\t43 000 | \n\t\t\t\t\t\t\t
CH 4 | \n\t\t\t\t\t\t\t\t1000 | \n\t\t\t\t\t\t\t\t5000 | \n\t\t\t\t\t\t\t\t50 000 | \n\t\t\t\t\t\t\t
O 2 | \n\t\t\t\t\t\t\t\t805 000 | \n\t\t\t\t\t\t\t\t831 000 | \n\t\t\t\t\t\t\t\tNon-flammable | \n\t\t\t\t\t\t\t
Gas properties in ratio with respect to 1 million
With the above information certain decisions can be made. Should the gas concentration be between Unsafehuman and Dangeroushuman then the robot could continue to search for victims. In the event that the gas concentrations is higher than the Dangeroushuman level, the possibility for humans to survive in these conditions are decreasing and the rescuers must decide about entering the environment depending on other safety issues. These safety issues could be falling debris or unstable surfaces. As the gas concentration for the flammablemin condition is much higher than that of the Dangeroushuman levels, it could imply that humans would not survive in these environments. The robot could make the decision to evacuate the environment and possibly save itself from an explosion, or searching for survivors in other areas of the disaster. These logical decisions will be determined from a weighting table shown in table 4.
\n\t\t\t\t\tThere are two types of warnings that have to be considered. The unsafe / danger factor for humans and the danger factor for the robot. A model is required to determine the danger for humans. This is achieved with equation (15).
\n\t\t\t\t\tSubstance | \n\t\t\t\t\t\t\t\tUnsafe human | \n\t\t\t\t\t\t\t\tDangerous human | \n\t\t\t\t\t\t\t\tDangerous robot | \n\t\t\t\t\t\t\t
CO 2 | \n\t\t\t\t\t\t\t\t1 | \n\t\t\t\t\t\t\t\t2 | \n\t\t\t\t\t\t\t\t0 | \n\t\t\t\t\t\t\t
CO | \n\t\t\t\t\t\t\t\t1 | \n\t\t\t\t\t\t\t\t2 | \n\t\t\t\t\t\t\t\t1 | \n\t\t\t\t\t\t\t
H 2 S | \n\t\t\t\t\t\t\t\t1 | \n\t\t\t\t\t\t\t\t2 | \n\t\t\t\t\t\t\t\t1 | \n\t\t\t\t\t\t\t
CH 4 | \n\t\t\t\t\t\t\t\t1 | \n\t\t\t\t\t\t\t\t2 | \n\t\t\t\t\t\t\t\t1 | \n\t\t\t\t\t\t\t
O 2 | \n\t\t\t\t\t\t\t\t1 | \n\t\t\t\t\t\t\t\t2 | \n\t\t\t\t\t\t\t\t0 | \n\t\t\t\t\t\t\t
Gas weighting factors
where n = number of gases being considered
\n\t\t\t\t\tp = number of gases that give an unsafe warning
\n\t\t\t\t\tq = number of gases that give a danger warning
\n\t\t\t\t\twu = unsafe human weighting factor
\n\t\t\t\t\twd = Dangerous human weighting factor
\n\t\t\t\t\tThe above model will give a percentage of danger for humans. As the number of unsafe and dangerous factors for humans increases, the model increases the percentage value.
\n\t\t\t\t\tA model is also required for the danger of the robot. As seen in table 4, carbon dioxide and oxygen does not have a weighting factor, as these gases are not flammable. This danger for the robot is expressed by the model shown in equation (16).
\n\t\t\t\t\twhere m = number of gases not giving Danger warnings and that wn ≠ 0
\n\t\t\t\t\tDn = danger that gas n has on robot (flammablemin)
\n\t\t\t\t\tShould any one gas have a concentration that is higher than flammablemin, the environment is considered to be 100% dangerous for the robot. The danger for the robot could increase as other gas concentrations increase, but it will never decrease below the highest danger percentage.
\n\t\t\t\t\tEquation {28} could be used to determine the danger or unsafe value for humans, but Dn will be the danger or unsafe value that gas n has on humans. This is a more accurate result compared to equation {27}, which only monitors the limits of the gas concentrations.
\n\t\t\t\t\tThe models shown give a probability that humans would be able to survive in the surrounding environments and the probability that the robot is in a dangerous environment. All the decisions described above are performed by the control station, as it randomly requests for environmental status. The CAESAR robot responds to the request and awaits for it\'s next instruction. The control station considers the procedures that the rescuers and the robot must follow from the information received.
\n\t\t\t\tThe Radiometrix TR2M modules with its related features, as well as the programming of the modules are discussed. Protocols and the basic procedure of the IEEE 802.11 protocol are explained, and a new robotic communication protocol, with its procedures of operation, are explained. The Robotics Communication Protocol (RCP) has a decreased size of 33 % to 38 % compared to IEEE 802.11 and hard-wired computer protocols respectively.
\n\t\t\tSpecific features of voice communication with the Yaesu VX-7R and VX-3E radios are explained. The modification of the Yaesu VX-7R are also discussed, enabling communication over the allocated frequencies.
\n\t\t\tFurther research should be performed in the development of microphones and speakers that are able to withstand high temperatures. An explanation is given for the video communication between the thermal camera and a television receiver for a successful observation of the robot\'s surrounding environment.
\n\t\t\tRadiation properties of feasible antennas are discussed, and the advantages and disadvantages of the vertical and eggbeater antenna are clarified. The testing procedures and verification of the different antennas and the radiation performance of the chosen antennas are also discussed.
\n\t\t\tDecisions are made from the analysis of gases and their concentrations in the environment. Models are developed that enable this analysis, determining if the environment is dangerous to humans or for the robot. This will assist rescuers to determine whether it is safe or worthwhile to risk their lives to enter the disaster.
\n\t\t\tWith the improvements made on the communication and AI for gas detection it thereby allows a reliable control and communication interaction between the control station and the USAR robot. This also supplies the rescuers with critical information about the environment, before they enter and risk their lives in the unstable conditions.
\n\t\tSeveral advantages on the design of proton exchange membrane (PEM) fuel cells are the cost-effective and innovative synthesis methods, which are necessary for new catalyst discovery and catalyst performance optimization. In addition, the carbon support functionality should be emphasized in terms of the active surface increase, the coordination effect of catalyst and support, and the distribution of active catalytic sites.
The main focus is the oxygen reduction reaction (ORR); this electrochemical reaction plays an important role in the operation of fuel cells. Nevertheless, due to its complexity, we are far to reach a full comprehension about the mechanisms involved in these systems. The development and study of novel materials that have useful electrocatalytic properties to carry out the reactions involved in these electrochemical devices is needed.
Platinum is considered in such a traditional catalyst for reactions involved in PEM fuel cells. However, their high costs keep us researching on new approaches to reduce the platinum load on the electrocatalytic material, and, therefore, Pt loading catalyst is still the main issue. Some methodologies for the preparation of disperse transition of metal nanoparticles and carbon nanostructures (CNS) have been developed and are described here.
Catalysis with transition metal sulfides (TMS) also play a crucial role in petroleum industry, owing to their exceptional resistance to poisons. TMS are unique catalysts for the removal of heteroatoms (S, N, O) in the presence of a large amount of hydrogen [1]. In particular, they are the optimal materials to carry out the numerous reactions [2, 3, 4, 5]. Through effective synthesis procedures, new non-noble catalysts have been discovered. TMS synthesized by carbonyl route using sulfides and selenides are promising. Besides platinum and noble metal nanoparticles and its alloys, other kinds of materials have shown important electrocatalytic activity in PEM fuel cells. Alonso-Vante and coworkers have proposed semiconducting TMS (sulfides and selenides) as efficient catalysts for cathode fuel cell reactions with significant oxygen reduction activity and high stability in acidic environment. A strategy to synthetize these materials in nanodivided way, is using carbonyl-based molecular clusters as precursors [6]; this route of synthesis offers the possibility to produce well-shaped nanoparticles with right stoichiometries. Ruthenium carbonyl (Ru3(CO)12) is extensively employed as feedstock to obtain diverse types of compounds and metallic clusters for new electrocatalysts; the main objective in the catalyst design is to replace and overcome the platinum properties [6, 7, 8, 9, 10].
However, platinum metal and its alloys with other transition metals are important catalysts for low-temperature fuel cells. The catalysts are typically developed in a form of nanoparticles for a better dispersion and/or minimum loading of platinum. Since they have the best activities and chemical stability, the problem is the high costs of Pt loadings in operating cathodes. ORR has been examined in the presence of Pt and Pt alloy nanoparticles on carbon-supported, CoN4 catalysts, Chevrel-type chalcogenide materials, and RuxSey clusters [7, 11]. The ability to fabricate new model systems in which one can control the number of particles, size, and shape would be of tremendous fundamental importance in catalysis and electrocatalysis, as well as in other technologically important areas that use nanoparticles.
On the other hand, chalcogenides are synthesized under mild conditions in the nano-length scale by simple and fast methods. In the final form of the catalyst, chalcogenide atoms interact with surface metal atoms in a chemical way to avoid poisoning. Evident effects were observed in the presence of organic molecules as CH3OH or HCOOH. Synthesized catalysts have been compared with commercial Pt/C [7, 11]. Further, the ORR kinetics was not perturbed, assessing this phenomenon wherein the sulfur atoms and organic molecules showed a little effect against the molecular oxygen adsorption. Some results demonstrated that the fuel crossover is no longer a major concern; however, the nature of the active sites on the chalcogenides and more investigations on dispersion and synthesis methods will follow for the development of very small and low-cost fuel cells, such as microsystems [12]. Therefore, results suggest the development of novel systems that is not size restricted, and its operation is mainly based on the selectivity and nature of its electrodes.
The challenges of scale-up and commercialization of fuel cells depend on the optimal choice of fuel as well as on the development of cost-effective catalysts. One approach for the ORR is the use of transition metal chalcogenides (TMCs) or dichalcogenides (TMDs), which also have the great advantage of being selective in the presence of methanol. However, the target is to develop materials based essentially on non-noble metals and reduction of the Pt loading [5, 13]. These results promise new opportunities to design cathodic catalysts.
On the other hand, W6S8(PEt3)6 was reported as the first soluble model clusters of the molybdenum Chevrel phases and their (unknown) tungsten analogs [14]. However, according to the literature reviewed, until 2003 tungsten, Chevrel phases had not been reported, despite many years of effort. As reported in many studies, chalcogenides are markedly less sensitive than platinum catalysts to methanol. In accordance with this idea, we endeavored to explore the nature of chalcogenides based on sulfur and thiosalts. These results described a significant tolerance toward some carbonaceous species like monoxide and methanol. Likewise, we called “the decorative nanoexfoliation of platinum model” to explain the effect of sulfur species on the surface of platinum, and further studies demonstrated how the WS2 planes are highly exfoliated around platinum nanoparticle to avoid the poisoning (see Figure 1).
(a) HRTEM image of the unsupported catalyst PtxWySz, (b) HRTEM image at high magnification of one platinum nanoparticle decorated by WS2 nanostructures, and (c) current-potential curves for oxygen reduction for PtxMoySz/C, PtxWySz/C, Pt/C commercial, and PtxSy/C. All samples were immobilized on a glassy carbon RDE, and the measurements were carried out in O2-saturated 0.5 M H2SO4 solution at 5 mV s−1 at 1600 rpm rotation speed and 25°C. The current densities were normalized to the geometric surface area.
This idea is to design selective catalysts with high activity for PEM fuel cells based on sulfur. We reported novel platinum chalcogenides as cathodic catalysts from platinum with tungsten and molybdenum thiosalts, as well as platinum and sulfide in acid media, and in other studies, we also analyzed the promising results for anodic electrode [15, 16]. In addition, we have studied the interaction with the supported TMS on Vulcan carbon. Figure 1(a) and (b) shows HRTEM images of the unsupported PtxWySz. In concordance, Figure 1(c) shows a significant effect of the chalcogenide on the platinum surface and the catalytic activity is better in comparison with the commercial platinum at 20 wt.% metal loading [16].
Carbon-supported PtW nanoparticles are usually prepared by impregnation or chemical co-reduction of chloroplatinic acid and ammonium tungstate. However, these methods are not suitable for preparing carbon-supported PtW nanoparticles with well-controlled particle size and homogeneous composition [17]. In Figure 1(c), we report the ORR polarization curves for three synthesized catalysts and compared it to commercial Pt/C Vulcan at 20 wt.% of metal load. As shown, in all samples, the current density values are higher than the Pt/C. Furthermore, it was noticeable that cathodic current due to the reduction of O2 commences at much more positive potential for PtWS/C catalyst than the synthesized samples and similar than commercial sample but increases upon further cathodic scan, and overall it shows a significant enhancement versus the Pt/C.
TMCs are a group of materials that show activity toward ORR. It is worthwhile to mention that TMS are the optimal catalysts to carry out the numerous reactions of hydrogenation and hydrogenolysis on different processes for the refining industry. We have reported catalytic materials sulfided by DMDS, and their activities are similar than H2S. It is an advantage, in order to determine the effect of sulfur on trimetallic catalysts and explore other sulfiding agents. This experimental procedure is also on research by our group [18].
Ruthenium (Ru)-based chalcogenide catalysts synthesized by Alonso-Vante et al. [8, 10, 11] have been among the most promising, due to their high activity and stability toward the ORR in acidic media [19]. Particularly, RuS2 also has been extensively employed as catalyst for hydrodesulfurization (HDS) reactions. It has been shown that semiconducting transition metal sulfides, such as PdS, PtS, Rh2S3, Ir2S3, and RuS2, have higher catalytic activity than the metallic sulfides [20]. However, the electronic environments of the surface of Ru atoms are also compared to the electronic environments and reactivities of metal centers found in d6 transition metal complexes that incorporate thiophenic ligands [20, 21].
Cluster compounds of the Chevrel type (MosXs) contain molybdenum octahedral and form metals with the Fermi level clearly below the energy gap. It clearly shows the molybdenum cluster octahedron (accommodating 20 electrons) surrounded by a cube of chalcogen atoms. It is also possible to distinguish the crystal channels between the clusters into which guest atoms can be inserted.
Alonso-Vante and Tributsch were the first that communicated that semiconducting ruthenium-molybdenum chalcogenides having the general formula MoxRuyXO2 (with X = chalcogen: essentially, one of the elements O, S, Se, and Te) and forming Chevrel phases exhibit good catalytic activity for ORR in acidic solutions and catalyze the four-electron reduction to H2O over the H2O2 route [22]. It was soon found that the catalytic activity is not restricted to Chevrel phases, but other varieties of such chalcogenides are active as well. Many other studies go on; using similar compounds are synthesized in different ways, and this is the purpose of this contribution, in order to enhance the catalytic activity, selectivity, and stability; thus, new modifications on active phases and carbon supports have been explored.
The morphology, structure, and composition of the support material significantly affect the catalytic activity of the fuel cell catalyst [23]. Carbon is most often used as catalyst support in cathodes because it is inexpensive; it can be prepared in a pure form as high-surface area powders, and it is electrically conductive. However, the atomic arrangement of carbon atoms on the network is the key to determine well-defined properties and therefore specific applications. In order to improve the electrocatalytic efficiency, various carbon support materials such as carbon nanotubes and graphene have been applied recently by our group. Some requirements for these supports are electrical conductivity, good metal-carbon interaction, high surface area, and high inertness in harsh chemical and electrochemical conditions.
Since Iijima’s landmark paper in 1991 [24], carbon nanotubes (CNTs) have been studied by many researchers all over the world. Their large length (up to several microns) and small diameter (a few nanometers) give them a large aspect ratio. CNTs are mainly produced by three techniques: arc discharge, laser ablation, and chemical vapor deposition. Research has been targeted toward finding more cost-efficient ways to produce these structures.
According to theoretical models, all of these structures may appear due to non-hexagonal carbon rings that are incorporated in the hexagonal network of the graphene sheet. In particular, coiled carbon nanotubes were first predicted to exist in the early 1990s by Ihara [25] and Dunlap [26], but they were experimentally observed until 1994 by Zhang [27]. On a microscale, periodic incorporation of pentagon and heptagon pairs into the predominantly hexagonal carbon framework in order to create positively and negatively curved surfaces, respectively, can generate a carbon nanotube with regular coiled structure [28].
A large variety of tubule morphologies as straight, coiled, waved, branched, beaded, and regularly bent have been synthesized and observed; however, there are no studies about the growth time which affects CNT morphology. Herein, the growth time promotes the arrangement by hexagonal lattices to produce different shapes [26]. Hence, to prepare high-quality metal catalyst supports, it is necessary to deposit dispersed metal particles onto nanotubes, ideally particles that have diameters within the nanometric range. It is worthwhile to mention that a combination of catalytic metals, chiefly transition metals such as iron, cobalt, or nickel, leads to the growth of extremely forms of CNTs such as helically wound graphite spirals. Under catalytic conditions, a wide variety of carbon nanotubes, which may not be linear but resemble spaghetti piles, are possible and may not be recognized as carbon.
Recently, aligned and coiled multiwalled carbon nanotubes were successfully obtained inside of quartz tubing by our group using the modified spray pyrolysis method. In Figure 2, two types of morphology of multiwalled carbon nanotubes (MWCNTs) are shown. In concordance to these results, variable control is essential to produce CNTs [25, 29].
(a) TEM image of straight MWCNT and (b) TEM image of coiled MWCNT synthesized by modified spray pyrolysis method.
On the other hand, preparative methods of synthesis of CNS such as graphene are also currently a heavily researched and important issue. The search for a methodology that can reproducibly generate high-quality monolayer graphene sheets with large surface areas and large production volumes is greatly sought after. A popular aqueous-based synthetic route for the production of graphene utilizes GO. It is produced via graphite oxide by various different routes. Hummer’s method, for example, involves soaking graphite in a solution of sulfuric acid and potassium permanganate to produce graphite oxide. In this method, we have done some modifications on the variables of synthesis. Our focus to take advantage of the TMD catalytic activity is on the development of different pathways of synthesis to accelerate the electron transport. Therefore, carbon support is another factor that affects the catalysis. Some studies have Wilkinson reported the effect of carbon support on catalytic activity and found the relation between the kinetic and the specific surface areas, pore size distribution, and the N or O content of the carbon support [7].
Here, it is worth to mention that various syntheses and preparations of catalyst routes have been reviewed, with emphasis on the problems and prospects associated with the different methods. However, we reported a simpler synthesis method to prepare Pt-WS2 nanoparticles supported on Vulcan carbon [30] and later on MWCNT synthesized by modified spray pyrolysis. These results were used to compare the catalytic electroactivity toward the ORR in acid media, in order to carry out studies about the influence of the exfoliated sulfides on Pt nanoparticles to modify its catalytic properties and to enhance the activity of pure Pt. In Figure 3, the result of chalcogenides versus Pt on carbon supports is shown. It is clear to observe the effect of the arrangement of carbon atoms on the kinetic response to increase the current density. The overview of several studies has also suggested that a strong coupling (synergistic effect) interaction between catalysts and substrates is a promising approach for promoting electrocatalytic performance [7, 11, 15, 30].
ORR polarization curves in oxygen-saturated 0.5 M H2SO4 as a function of potential for different platinum electrocatalysts. Pt/C commercial and electrocatalysts synthesized from sulfur (PtxSy/C), tungsten thiosalt, and Pt/MWCNT. All samples have 20 wt.% of active phase. Measurements were carried out in O2-saturated 0.5 M H2SO4 solution at 5 mV s−1 at 1600 rpm rotation speed and 25°C.
It should be noted that the constituent atoms of graphite, fullerenes, and graphene share the same basic structural arrangement in what structure begins with six carbon atoms which are tightly bound together (chemically, with a separation of approx. 0.142 nm) in the shape of a regular hexagonal lattice. Moreover, at the next level of organization, graphene is widely considered as the “mother of all graphitic forms.” In this sense, compared to black carbon, CNTs show much higher catalyst loading efficiency, electrical conductivity, better durability, and lower impurities. However, due to their high aspect ratio and strong π-π interactions, the dispersion and difficulty to achieve uniform deposition of metal nanoparticles are some challenges in this field. In contrast, the graphene displays better electrical, mechanical, and physical properties and much larger surface area than MWCNTs, which are highly desirable for the catalyst support [31].
In PEM fuel cells, platinum-based electrocatalysts are still widely utilized as anode and cathode electrocatalysis. However, carbon nanostructures (nanotubes and graphene), supported on Fe or Co nanoparticles, show promise for fuel cells, and these nanostructured metal chalcogenides (NMCs), CNS, or even NMC-CNS could also be applied for other energy devices. Some recent reports about utilized GNSs and nitrogen-doped GNS as catalyst supports for Pt nanoparticles toward the ORR, where the constructed fuel cells exhibited the power densities of 440 and 390 Mw cm−2 for nitrogen-doped GNS-Pt and GNS-Pt, respectively. It is clear that the nitrogen-doped device exhibited an enhanced performance, with improvements attributed to the process of nitrogen doping which created pyrrolic nitrogen defects that acted as anchoring sites for the deposition of Pt nanoparticles and is also likely due to increased electrical conductivity and/or improved carbon-catalyst binding. On the other hand, Pt nanoparticles deposited on graphene submicroparticles (GSP) in addition to carbon black and CNT via reduction method. Results demonstrated that the Pt/GSP was two to three times more durable than the CNT and carbon black alternatives [30].
The main issues about graphene-based materials are focused on structural characteristics, interaction between nanoparticles or functional groups, and their electrochemical performance as catalysts, and a wide variety of graphene-based hybrid nanocomposites are grouped into the next categories: doped/modified graphene, noble metal/graphene hybrids, and graphene/nonmetal composites.
Figure 4 shows catalyst prepared from nitrogen-doped graphene-carbon nanotube hybrids (NGSHs) and their electrochemical behavior toward ORR for graphene-SWCNT hybrids (GSHs), NGSHs, and Pt/C supported on GC electrodes [32]. Those edge planes of GNS also provide defects for the uniform dispersion of Pt nanoparticles, subsequently increasing catalytic activity by increasing the surface area of an electrode as well. However, nitrogen dopants increase the number of defects on the CNT surface, subsequently improving the distribution of a catalyst. Since nitrogen is introduced into the growth process of GNS-CNT hybrid nanostructure, these substituted nitrogen sites prevent the Pt nanoparticles from aggregation [33].
(a) Schematic illustration of the preparation of the nitrogen-doped graphene-carbon nanotube hybrids (NGSHs). (b) TEM image of the NGSHs. (c) ORR polarization for graphene-SWCNT hybrids (GSHs), NGSHs, and Pt/C supported on GC electrodes at a rotating rate of 1225 rpm.
The fast development of nanocarbon materials like graphene enables them to play an increasingly important role in the improvement of non-precious metal-based catalyst (NPMC) performance. ORR activity of Co9S8-N-C catalysts, for instance, was much higher than that of the state-of-the-art Pt/C 0.1 M NaOH solution. Dai et al. synthesized a CoxS-reduced graphene oxide (RGO) hybrid material by a mild solution-phase reaction followed by a solid-state annealing step. Strong electrochemical coupling of the RGO support with the CoxS nanoparticles and the desirable morphology, size, and phase of the CoxS nanoparticles mediated by the RGO template rendered the hybrid with a high ORR catalytic performance in acid media [5, 33]. Figure 5 shows an illustration of carbon nanostructures and nanoparticles, synthesis, and functionalization methods commonly used by our group.
Schematic illustration of carbon nanostructures and nanoparticles, synthesis, and functionalization methods reported by our group. Potential applications could be reached with these preparation routes in terms of catalytic activity, time, and cost-effectiveness.
Nowadays, the nanoscience has reached the status of a leading science with basics and applied implications in all physics, life, earth sciences, as well as in engineering and materials sciences. Figure 6 shows the schematic illustration of the focus on research from the synthesis methods of carbon support materials, such as carbon nanotubes and graphene, and metallic nanoparticles that also can be obtained by different methodologies, until the surface modification of these nanomaterials. It could be on TMS or non-noble metals as the active phase of the catalysts for PEM fuel cells.
ORR polarization curves in oxygen-saturated 0.5 M H2SO4 as a function of potential for different Pt catalysts at the rotation speed of 1600 rpm. (Reprinted with permission from Royal Society of Chemistry. Lic. No. 4171470897994).
In this regard, our strategy is to generate nanomaterials that could be fabricated by simple methods with the purpose of controlling and understanding at nanoscale the properties of the catalysts based on NMCS and CNS through the atomic behavior at specific conditions, in order to enhance the catalytic activity. This concept focuses on the design and the creation of novel morphology and structure to probe, tune, and optimize the properties to develop functional materials for multiple applications. Nevertheless, significant electrochemical effects have been observed in different samples of platinum. Morphology and structure dependence can be shown in Figure 6. It displays the ORR polarization curves in oxygen-saturated 0.5 M H2SO4 as a function of potential for different geometries of Pt at the rotation speed of 1600 rpm. The response of the kinetic behavior on the atomic structure is clear to observe [5].
On the other hand, it is worth to mention some synthesis methods that are well known and developed by our group. Table 1 shows some catalysts based on TMC and their method to obtain materials with high catalytic activity on specific reactions [34]. However, a recent development in the field of organometallic chemistry has been the use of organometallic complexes for the high-yield catalytic synthesis of CNT [35, 36, 37].
Catalyst | Synthesis method conditions | Reference |
---|---|---|
NEBH2S NEB DMDS NEBDMS | Two aqueous solutions were prepared (A and B). Solution A consisted of ammonium heptamolybdate and ammonium metatungstate dissolved in water at 363 K under stirring. The pH of this solution was maintained at about 9.8 by adding NH4OH. Solution B consisted of nickel nitrate dissolved in water at 363 K while stirring; solution B was slowly added to solution A at 363 K; a precipitate was formed; and then the solid was filtered, washed with hot water, and dried at 393 K. The molar ratio Mo:W:Ni of precipitate was 1:1:2 and was represented as NH4-Ni-Mo0.5 W0.5-O. Sulfidation was carried out in a tubular furnace at 673 K for 2 h using H2S, DMDS, or DMS (10 vol. % in hydrogen). | Gochi Y et al., 2005 [2] |
PtxSy/C | First, the synthesis of catalytic precursor is from molecular sulfur, and ammonium hexachloroplatinate ((NH4)2PtCl6, Alfa Aesar) was reacted under a constant agitation for 12 h at room temperature. The solution was mixed with carbon Vulcan (E-TEK) and stirred continuously for 24 h at room temperature. The precipitates were filtered, washed with distilled water, and dried for 12 h at room temperature on a drier. Finally, the precursor was treated thermally at 350°C under (75% v/v) N2/H2 atmosphere for 2 h. | Gochi-Ponce Y et al., 2006 [15] |
PtxMoySz/C, PtxWySz/C, or MWCNT | Tungsten or molybdenum thiosalts, as appropriate, and ammonium hexachloroplatinate were reacted under constant agitation for 12 h at room temperature. The solution was mixed with the carbon support and is stirred for 24 h at room temperature. The precipitates were filtered, washed with distilled water, and dried for 12 h at room temperature. The supported precursor was treated at 400°C under N2/H2 atmosphere for 2 h. | Gochi-Ponce Y et al. 2006 [16] |
Pt/MWCNT-Fe PtFe/MWCNT Pt/MWCNT | The coordination complex salt of Pt was synthesized by Burst-Schiffrin method. Ammonium hexachloroplatinate was dissolved into 10 ml triply distillated water. This solution was added to 15 ml of a TOAB in 2-propanol solution at room temperature (25°C). The Pt precursor was filtered under vacuum, washed with deionized water, and dried at 70°C for 8 h. MWCNTs (raw, treated, or cleaned and synthesized by spray pyrolysis) are added to 2-propanol and dispersed in an ultrasonic bath for 1 h. The Pt precursor dissolved in 5 ml 2-propanol solution was added to the MWCNT-Fe suspension and stirred for 1 hr. Finally, 10 mL aqueous solution of NaBH4 in excess, 1:10 was added by drip during 5 min to the suspension, which was stirred at room temperature for 12 h to reduce Pt4+ to Pt0. The obtained mixture was then filtered and washed with acetone and water, to be finally dried at 70°C for 4 h. | Rodriguez JR et al. 2014 [35] |
Pt-Ni/MWCNT | MWCNTs were synthesized in a spray pyrolysis. For the MWCNT-Ni, it was necessary to use a thin film (manganese oxide) as substrate previously deposited in the inner walls of the Vycor tubing. The temperatures of MWCNT synthesis were 900 and 800°C for ferrocene and nickelocene, respectively. After the process, once the substrate was completely cold, the MWCNTs were removed (scratched) from the Vycor tubing. | Valenzuela-Muñiz AM et al. 2013 [36] |
RuxSey | Carbon-supported RuxSey (20 wt.%) nanoclusters were prepared in aqueous media using RuCl3_xH2O and SeO2. Typically, 0.124 g carbon (Vulcan XC-72) was dispersed in 100 mL of water under nitrogen under vigorous stirring. The resulting suspension was heated to 80°C, mixed at this temperature for 30 min to remove oxygen in water, and then cooled down to room temperature. Subsequently, 4 mmol RuCl3_xH2O and 1 mmol SeO2 were added to the above suspension and then mixed for another 1 h. Thereafter, 100 mL of a mixture solution containing 0.1 M NaBH4 and 0.2 M NaOH was added dropwise (1.25 mL min−1) to the suspension to reduce the metal ions. The suspension was kept for further reaction for another 10 min and then heated to 80°C for 10 min. The final black powder was collected on the Millipore filter membrane washed with water and dried under vacuum at room temperature. | Saul Gago A et al., 2012 [12] |
An overview of synthesis reports using platinum, sulfur, or selenium.
Table 1 An overview of synthesis reports using platinum, sulfur, or selenium.
Some results reported about the ORR activity of the thiospinel compounds were directly related to the type of metal utilized, with an order of Co > Ni > Fe. Moreover, decreased performance was also observed when sulfur was partially replaced with O, Se, or Te. Table 1 shows an overview of catalyst synthesized for PEM fuel cells. The main methods that we have used to obtain catalysts are spray pyrolysis and Hummer’s method, electrochemical methods, ultrasonic techniques, and green synthesis.
First, the experimental procedure of modified spray pyrolysis is simple and is one of the most commonly used; this methodology represents advantages among others due to its characteristics of using non-sophisticated equipments as well as easiness of scalability. To start, an aqueous solution containing the metal precursor is nebulized into a carrier inert gas that is passed through a furnace. Second, the nebulized precursor solution deposits onto Vycor tube as a substrate, where it reacts and forms the final product. To form nanoparticles, the aerosol is pyrolyzed under inert atmosphere and a set temperature [17, 29].
Recently, we are also producing graphene for PEM fuel cells and other specific applications. In accordance with Hummer’s method, we modify some steps in the original method. However, it is worth to mention about a specific application, for instance, about the storage energy, the combination of carbon nanostructures as support, and the functionalization with a pseudocapacitive material which generates a synergistic effect in capacitance, thus, in the energy density with an excellent electrochemical performance throughout the system. The main determining factor on this material is the surface area of each electrode that makes up the supercapacitor. Through the synthesis methods of carbon nanostructured materials such as graphene and nanotubes, the size and morphology of the compounds are tunable. This approach favors some specific properties for applications on fuel cell systems such as high surface area, stability, electroconductivity and catalytic activity.
Some progress has been made in catalytic materials and supports preparation techniques, although none of these catalysts has reached the level of a Pt- or Ru-based catalyst in terms of catalytic activity, durability, and chemical/electrochemical stability. In order to make non-noble catalysts commercially feasible, cost-effective, and innovative, synthesis methods are needed for new catalyst discovery and catalyst performance optimization. The use of electrochemical methods, such as galvanic displacement and ultrasonic techniques, for instance, was chosen to describe here.
Figure 7 shows the preparation of core-shell nanoparticle catalysts. We also report here the electrochemical response obtained by PtPd/MWCNT. The parameters investigated were Pt concentration and sonication by a simple and fast galvanic displacement (GD) method, finding that both play a key role in the physicochemical features and, thereby, modifying the performance of the catalysts toward the oxygen reduction reaction (ORR) activity and according to results highly dispersed Pt10Pd90/MWCNT was produced [13, 36, 38].
Illustration of basic synthesis approaches for the preparation of core-shell nanoparticle catalysts. Electrochemical (acid) dealloying/leaching results in (a) dealloyed Pt bimetallic core-shell nanoparticles, and (b) Pt-skeleton core– shell nanoparticles, respectively. Reaction process routes generate segregated Pt skin core-shell nanoparticles induced by either (c) strong binding to adsorbates or (d) thermal annealing. The preparation of (e) heterogeneous colloidal core-shell nanoparticles and (f) Pt monolayer core-shell nanoparticles is via heterogeneous nucleation and UPD followed by galvanic displacement, respectively. (Reprinted with permission from Royal Society of Chemistry. Lic. No. 4171470897994).
In addition, it is of great significance to explore different methods to obtain efficient catalysts for the PEM fuel cells. Ultrasonic-assisted strategy is known as a unique synthesis method in materials chemistry. Sonochemical reaction techniques have been introduced in the 1980s by Suslick’s group. However, most of the literature works on electrocatalysis published until 2010 are cited by Eunjik Lee (2016) [39]. A number of alloy and core-shell NPs are well discussed. During the past years, a number of new alloy and core-shell NPs based on Pt and Pd have been synthesized by sonochemistry and studied for their electrocatalytic properties [40]. Therefore, in light of the importance of finding more dependable catalysts in the present status of FC researches. Some works cited here are the syntheses of Pt-Pd/MWCNT for enhanced ORR of Pt/MWCNT and PtNi/MWCNT catalysts with high electroactivity, and further ultrasound treatment is used because carbon nanotubes are uniform in size and well dispersed by this via [32]. We also reported about Pt/CNT/TiO2 catalyst, and here we note the effect of the amount of MWCNT with the current density. In addition, the CO tolerance performance increases in the next sequence of Pt/CNT < Pt/TiO2 < Pt/CNT/TiO2 [41].
According to the principle of green chemistry, the feed stock of any industrial process must be renewable rather than depleting a natural resource. Moreover, the process must be designed to achieve maximum incorporation of the constituent atoms (of the feed stock) in to the final product [39].
A great advantage is the use of aqueous solutions instead of any surfactants, additive reagent, or posttreatment in the nanoparticles and CNS synthesis. The preparation of sulfide chalcogenides as reference PtxSy, PtxWySz, and PtxMoySz catalysts were carried out only with water and at room temperature [19, 20] as well as other synthesis methods to produce CNS such as graphene or MWCNTs and nanoparticles, recently cupper nanoparticles, for instance [42].
Illustration of the chemistry of carbon nanotubes in biomedical applications. Reprinted with permission from (Royal Society of Chemistry. Lic. No. 4171820715591).
The functionalization of carbon materials is essential processes for the utilization of these materials. Functional groups or molecules can be directly attached on the periphery of the surfaces of the carbons through various treatments with acids, etc. A large number of oxygen functional groups are created during the activation process by saturation of dangling bonds with oxygen. This creates a rich surface chemistry which is used for selective adsorption. In addition, it determines the ion exchange properties that are relevant for catalyst loading with active components. In Figure 8, an illustration of multiple routes of the chemistry of carbon nanotubes in biomedical applications is shown [43, 44]. Although the applications of functionalized carbon nanotubes are numerous, the modification surface of the individual carbon nanotubes by decorating the surface with OH, COOH, NH2, F, or other groups promotes dispersion in a wide variety of solvents and polymers enabling the use of nanotubes in many more applications and different fields of studio. The image above details only one specific application enabled the functionalized carbon nanotubes.
Maximum power density achieved with (A) Pt-based and (B) CoSe2 cathodes of a H2/O2 PEM fuel cell, an LFFC, a Y-type MRFC, and a multichannel mMRFC (this work). The dashed bar in (B) corresponds to the use of 10 mgcm−2 Pd at the anode, 10 m HCOOH, and pure O2. Preparation of MEAs for the H2/O2 systems was done under the same conditions as those used for Pt and CoSe2 systems. (Reprinted with permission from John Wiley and Sons. Lic. No. 4166570806290).
Another example of the modification of carbon nanostructures for different applications is on the design of ultrasensitive biosensors with advantages in the detection of organic molecule. The preparation of the CNT-graphene hybrid, with regard to the complex molecules and nanoparticles that can be anchored to the surface of these nanostructured materials after the oxidation. These results are a significant contribution to the properties that have the nanomaterials mentioned here. Recently, carbon-supported highly dispersed RuxSey chalcogenide nanoparticles (1.7 nm) were synthesized; here, Ru and Se precursors in a simple microwave-assisted polyol process. In other studies, Ir85Se15/C was synthesized with an average particle size less than 2 nm by the same method [13].
Different routes of modification of CNS have been used by our group. Some synthesis and modification methods by microwave-assisted are used, the oxidizing agents are acids or even, hydrogen peroxide. On the other hand, the heat treatment is also a key factor of the nanostructures obtained [2, 15, 16, 44, 45, 46]. Traditionally, acids have been widely used for attaching to CNT. However, the microwave-assisted polyol is a versatile method for synthesis, dispersion, and surface modification of chalcogenides and CNS. Other important aspects of CNT and graphene are on chemistry, the level of purity and functionalization degree of the starting materials. Actually, our interests are on this direction, and the focus is the search of new catalysts for PEM fuel cell based on chalcogenides and CNS synthesized by rapid and efficient methods.
To date, microscale system research has focused mostly on miniaturization of functional components, for instance, specialized devices such as clinical and diagnostic test, microanalytical systems for field tests, and various portable devices. Thus, here we mention about chalcogenide such as RuxSey, CoSe2, PtxSey, and PtxSy that have showed a remarkable selectivity toward the oxygen reduction reaction (ORR) for membraneless microlaminar-flow fuel cell. Figure 9 shows a significant comparison between Pt, PtxSy, and CoSe2. The maximum power density for fuel cells are achieved with (A) Pt-based and (B) CoSe2 cathodes of a H2/O2 for the PEM fuel cell, an LFFC, a Y-type MRFC, and a multichannel mMRFC [12].
This work is inspired by the excellent electrocatalytic activity of chalcogenides and carbon nanostructures which open the door for the development of a novel type of micro- or even nano-fuel cell. Figure 10 displays a schematic illustration of an application for a PEM fuel cell. Some basic concepts about advantages and disadvantages of these devices were reported by Taner [47, 48]. It is a challenge to develop an active cathode catalyst for the ORR that is tolerant at the same time. One strategy proposed is the use of chalcogenides as anodic catalyst and CNS as cathodic catalyst. On the one hand, this type of chalcogenides can be used as anode, because are tolerant to CO molecules and by other sides of carbon nanostructures can be placed as cathode because of the atomic arrangement of the carbons can behaviors as metal and also can be modified on the surface, it means, doped or well-functionalized to support non-platinum metals, N2, B, P, S, etc. Either as cathode or anode, chalcogenides based on sulfur are promising. The target is to generate a maximum power density, and the key is on the methods of synthesis such as here we described. Moreover, many other studies about these materials are furthered from here. Nevertheless, in addition we report on micro-fabricated membraneless fuel cells with PtxSy- and CoSe2-tolerant cathodes and show how such materials can be used for developing smaller, simpler, and cheaper for PEM fuel cells.
Schematic illustration of a PEM fuel cell and the use of chalcogenides and carbon nanostructures as anodic and cathodic electrodes.
The authors are grateful to Dr. F. Paraguay Delgado for TEM analysis and to Marco Ovalle, student of Nanotechnology Engineering, for their technical support and design of figures and to the National Institute of Technology of México/Technological Institute of Tijuana and Technological Institute of Oaxaca, Mexico, for the collaboration.
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