Chemical structures of cellulosic and synthetic polymeric membranes for blood purification.
\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.
When functions of the living kidney decrease down to under survival level, patients are required to be treated with a system that supports kidney functions. There are several such treatment modalities available, including peritoneal dialysis (PD), hemodialysis (HD), hemofiltration (HF), hemodiafiltration (HDF), hemoadsorption (HA), and their advanced derivatives, among which the most popular treatment system is HD. The artificial kidney device used in HD is called “hemodialyzer” or more simply “dialyzer” that includes membrane to separate the waste products and excess water from blood.
\nArtificial kidney is also expected to correct pH that is usually acidic before treatment by balancing electrolytes in addition to removing waste products and excess water. All these functions are dependent upon the permeability of the membrane used in a dialyzer and since the quality of treatment is strongly dependent on the performance of the dialyzer, many materials have been proposed as a candidate of the membrane. We have currently several commercial materials available, including natural polymers and synthetic polymeric ones.
\nIn this chapter, dialysis membrane and its materials are extensively discussed from the physicochemical points of view, including microscopic views taken by scanning electron microscope (SEM), mathematical expressions of membrane transport, fundamental in vitro experiments as well as in vivo trials or clinical experiences.
\nDialysis is a phenomenon at which two different fluids (usually liquids) are separating flowing on either side of the membrane (usually counter-currently) and the solute of interest in higher concentration transports across the membrane due to concentration gradient in accordance with the Fick’s 1-st law of diffusion, i.e.,
\nwhere x is the co-ordinate in the diffusion direction [m], J\n Ax\n is the mass flux of solute A in x direction [kg/(s m2)], D\n Ax\n is the diffusion coefficient of A in x direction [m2/s], and C\n A is the concentration of A [kg/m3]. Dialysis, therefore, is one of separation techniques of the solute of interest by using the membrane and is applied elsewhere in many industrial as well as laboratory situations. Letting C\n A0 and C\n\n AL to be the concentrations of A at x=0 and x=L, respectively (Figure 1), Eq.(1) is integrated in a straight-forward manner to get,
\nwhere k\n M is the membrane permeability [m/s] defined by (D\n Ax\n /L). From Eq.(2), one would alternately mention that the rate of diffusion is proportional to the concentration difference between either side of the membrane. The value of k\n M is discussed in section 4.
\nDiffusion across a piece of membrane assuming no existence of boundary film adjacent to either side of the membrane.
Application of dialysis to blood purification, hemodialysis (HD), was first performed for canines reported by Abel et al. in 1914 [1]. A chemical substance (sodium salicylate) was added to the subject as a marker prior to the experiment, mimicking the clinical situation of kidney failure in which waste products accumulate in the human body. Then the marker substance was removed by the dialyzer that included the membrane made of collodion. The dialyzer included 16 collodion tubes whose length was 40 cm that is 1.5 times longer than a currently available normal commercial model and the diameter was about 8 mm that is approximately 40 times larger than a popular hollow fiber membrane currently utilized worldwide. Since the collodion was too fragile to perform dialysis experiments, many other membranes cast from natural materials were examined whether or not they were suited as a separation membrane. Finally collodion was replaced by cellophane, and the first clinical trial was performed by Kolff et al. in 1943 with a rotating drum dialyzer, designed and assembled by themselves [2]. Separation performance of these dialysis membranes, however, was not discussed extensively at that time because mechanical strength of the materials was more important for performing experiments or treatments than the permeability of the membrane.
\nCuprophan® is a registered name of the membrane made of cuprammonium rayon made from cellulose dissolved in cuprammonium solution, produced by Enka Co. in West Germany, later Membrana in Polypore Co., Germany. Another cuprammonium rayon membrane with nearly the same chemical and physical structures was developed by Asahi-Kasei Co. (Tokyo, Japan) termed Bemberg®, followed by Terumo (Tokyo, Japan). These membranes were also called regenerated cellulosic (RC) membrane since they were cast from cellulose or cotton fibers. Chemical modifications were made for RC membranes mostly because of improving their biocompatibility by replacing their hydroxyl group(s) with acetate group(s). They are called cellulose acetate (CA), cellulose diacetate (CDA), and cellulose triacetate (CTA) in accordance with the number of introduction of acetate groups to the cellulose backbone. Although RC membranes are no longer commercially available, CA, CDA, and CTA membranes still have fairly good market share since they have much higher solute and hydraulic permeabilities as well as better biocompatibility than original RC membranes.
\nThe first synthetic polymeric membrane was developed in 1969 by Rhône-Poulenc (France) and was named AN-69®, since the main material of the membrane was acrylonitrile (AN). The brand name of the dialyzer assembled with a flat sheet AN-69® membrane was RP-6® and it was also the first dialyzer sterilized by the gamma-ray irradiation. Although the production company of AN-69® membrane has been changed over time from Rhône-Poulenc to Hospal, Gambro, and Baxter, dialyzers with AN-69® membrane are still available worldwide, especially in the field of acute kidney injury (AKI) therapy since it has strong adsorption characteristic to specific substances such as inflammatory cytokines.
\nThe first dialyzer with a cellulosic hollow fiber membrane was developed by chemical engineers, Stuart and Lipps in 1967 [3] in Massachusetts Institute of Technology (Boston, MA, U.S.A.), and the commercial product was available in 1972 from Cordis-Dow Co. (Miami, FL, U.S.A.). The basic structure of the hollow-fiber dialyzer is the same as the one of multi-tube heat exchanger that is compact and has large surface area. Because of these advantages, dialyzers with hollow fiber membrane have been become widely used. The first dialyzer with a synthetic polymeric hollow fiber membrane sterilized by gamma-ray was introduced by Toray Co. (Tokyo, Japan), in which polymethylmethacrylate (PMMA) was used as a main material of the membrane [4].
\nIn order to improve solute and hydraulic permeabilities as well as biocompatibilities, many synthetic polymeric membranes have been introduced to the market since early 1980’s, and currently these membranes are the main stream. Among them, polysulfone (PSf) and the like (including polyethersulfone (PES), polyarylethersulfone (PAES), etc.) have the highest market share over the world. Since these membranes are made from petroleum, they are hydrophobic in nature. Then most of these membranes include so-called hydrophilic agent that also plays a role of pore-forming agent when cast. The role of the hydrophilic agent is discussed later from the chemical (section 3), mass transport (section 4), and biological (section 5) points of view.
\nChemical structure of the dialysis membrane usually refers to the chemical structural formula of the main material(s) of the dialysis membrane. Most chemical structural formulae of the main material of the membrane are tabulated in Table 1, including both natural and synthetic polymers. Among them AN-69®, ethylenevinylalcohol (EVAL) co-polymer, polyester polymer alloy (PEPA, Nikkiso Co., Tokyo, Japan, Figure 2) include two main materials. Actually PMMA is also a stereo complex co-polymer of two kinds of PMMA, isotactic and syndiotactic. Isotactic PMMA has acetate groups on only one side of the main chain, resulting a curled structure, whereas syndiotactic PMMA has acetate groups alternately on either side of the main chain, resulting a fairly straight structure. Combining these two polymers, membranes with low to high hydraulic permeability has been brought to realization [4].
\nChemical structures of polyester polymer alloy (PEPA) composed of PES and PAR with polyvinylpyrroridone (PVP).
Chemical structures of cellulosic and synthetic polymeric membranes for blood purification.
In general, cellulosic membranes are hydrophilic in nature, including original RC and its derivatives such as CA, CDA, and CTA in which hydroxyl group(s) are replaced by acetate group(s). On the contrary, since synthetic polymeric membranes are originated from petroleum, generally speaking they are hydrophobic in nature. Blood coagulation usually occurs soon after blood interacts with hydrophobic materials. Most synthetic polymeric membranes, therefore, include so-called hydrophilic agent such as polyvinylpyrrolidone (PVP) to make membrane hydrophilic. Figure 2 includes the chemical structure of PVP together with two other polymers (polyarrylate and polyethersulfone). PEPA is composed of these two polymers with or without PVP, the former shows little hydrophobicity, while the latter has strong adsorptive characteristic to various proteins due to its hydrophobicity (see section 4).
\nSince PVP is water-soluble, excess amount of PVP may be rinsed out from the membrane after cast that forms pores for solute and water transport. Therefore PVP is also known as a pore-forming agent. Namely, it should be understood that PVP residues in or on the membrane after rinse behave as a hydrophilic agent. Both an average and a distribution of molecular weight of PVP are important as well as the amount of PVP used in the membrane. Moreover, PVP may be cross-linked together and to the main material of the membrane by irradiating gamma-ray in the final sterilization process. With this procedure, PVP should be tightly attached together and/or on the membrane that does not allow PVP to behave as a “cushion” (cushion effect) to the blood corpuscles [5].
\nAcrylic acid is specifically chosen for polyacrylonitrile (PAN, Asahi Kasei, different from AN-69®) as a hydrophilic agent, whereas no hydrophilic agent is used in PMMA, EVAL and the original PEPA in which micro-layer separation technology plays a significant role in casting procedure.
\nPhysical structures can be demonstrated in the following two ways, i.e., microscopic view analysis and a theoretical analysis based on mathematical models. Microscopic views are usually taken by a scanning electron microscope (SEM). Recently the microscope technology has been advancing drastically and a field-emission SEM (FE-SEM) that has much higher resolutions can be utilized widely.
\nA cross-sectional view of EVAL hollow fiber membrane (Asahi-Kasei, Tokyo, Japan) taken by FE-SEM.
\n Figure 3 is a FE-SEM of intersection of EVAL membrane (Asahi-Kasei). It is entirely a dense membrane and the entire thickness contributes to the transport resistance for solutes and water. Membranes of this kind are usually called “homogeneous.” Besides EVAL, PMMA, and AN-69®, most cellulosic membranes are homogeneous. Figure 4 shows a cross-sectional view of PSf membrane (Toray). One should realize that a dense thin layer exists on the inner surface of the membrane, called “skin layer” from which the density is gradually decreasing in the radial direction. Since most part excluding the skin layer is known to have little resistance for solute and water transport, it is called the “support layer” (Figure 5). The support layer, however, has an important role for the membrane to have enough mechanical strength with little resistance for transport. Membranes of this kind are called “asymmetry.” Most synthetic polymeric membranes (except for PMMA, EVAL, and AN-69®) are asymmetry. In general, although the physical thickness of synthetic polymeric membranes is thicker (approximately 35 μm) than that of cellulosic membranes (approximately 15 μm), the thickness that contributes to the separation (Δx) of the former is approximately 0.5-2 μm that is much thinner than the latter. As mentioned before, synthetic polymeric membranes are main stream these days because much higher solute and hydraulic permeabilities are achieved with the thinner Δx.
\nA cross-sectional view of PSf hollow fiber membrane (Toray, Tokyo, Japan) taken by FE-SEM.
Cross-sectional views of dialysis membranes.
The pore theory is often used to analyze and to design physical structures of the membrane. The original pore theory was introduced by Pappenheimer et al. [6] to analyze the Glomerular filtration in the living kidney (Figure 6), and was later modified by Verniory et al. [7], introducing steric hindrance effect. Sakai [8] further modified the model by introducing the tortuosity for transporting across the membrane. Followings are the equations for modified pore theory.
\nPore theory (pore diffusion model). Assuming pores whose radius is uniformly r\n p [m] with a membrane thickness of Δx [m], through which a solute of interest whose radius is r\n s [m] is passing.
where k\n M is the membrane permeability [m/s] (see also section 1), D\n w is the diffusion coefficient for the solute of interest in pure water [m2/s], A\n k is the surface porosity of the membrane [-], Δx is the membrane thickness that contributes to the transport resistance [m], r\n s is the solute radius [m], r\n p is the pore radius of the membrane [m], L\n p is the hydraulic permeability of the membrane [m2 s/kg], σ is the Staverman’s reflection coefficient [-], τ is the tortuosity of the membrane [-], q is the ratio of r\n s to r\n p [-], S\n D, S\n F, f(q), and g(q) are the dimensionless stereo correction factors defined as functions of q. The pore theory can be applied to the situation in which q < 0.8 is satisfied.
\nFrom Eqs.(3) and (4), it is clear that A\n k/(τ Δx) is an important factor both for solute and water transport because both k\n M and L\n p include this value. Figure 7 shows two examples of L [m] x L [m] portions of the membrane, i.e., membrane (A) with four pores with the same radius of a [m], and membrane (B) with one pore with a radius of 2a. Then the surface porosity can be calculated, respectively for membranes (A) and (B) with subscripts (A) and (B), i.e.,
\n\n \n
Portions of two modeled membranes with the same surface porosity.
Then one would realize that membranes (A) and (B) have the same surface porosities, although the situations are quite different in terms of the pore diameter.
\nExample)
\nCompare the two membranes (A) and (B) that have the same surface porosity (Figure 7), tortuosity and the thickness in terms of
\nhydraulic permeability
solute permeability
under the following two conditions
\n r\n s is negligibly small compared with a\n
\n r\n s\n =a/3
Solution) As stated above, A\n \n k\n , τ, and Δx are the same in two membranes, A\n \n k\n /(τ Δx) is just a constant.
\nRecalling Eq. (4) to get,
\n \n
Therefore, the membrane (B) has four times higher hydraulic permeability than the membrane (A).
\nSince q=0 may reasonably be applied in this case, recalling Eqs.(7)-(10) to get,
\n S\n D=S\n F=f(q)=g(q)=1
\nin both membranes (A) and (B). Therefore Eq.(3) may be simplified as follows,
\nConsequently, there is no difference between membranes (A) and (B) in terms of transport of small solutes.
\nRecalling Eq.(5),
Then recalling Eqs.(7) and (9) with q values calculated above,
\n\n S\n D(A)=(1-q\n (A))2=0.8889
\n\n S\n \n D(B)=(1-q\n (B))2=0.9722
\n\n f(q\n (A))=0.3707
\n\n f(q\n (B))=0.6587
\nThen from Eq.(3),
\n\n \n
Finally one would conclude that the membrane (B) has almost two times higher solute permeability than the membrane (A) for those solutes whose r\n s\n =a/3.
\nChemical characteristic determines the hydrophilicity and hydrophobicity of the material, whereas physical structure determines the pore sizes as well as the thickness that contributes to the transport resistance. Therefore, both chemical and physical features are important for designing dialysis membrane.
\nIn this section, we discuss the performances under in vitro ultrafiltration experiments and those under on-line HDF in clinical situations because the former is suited for evaluation of maximal performance of the membrane and the latter takes a responsibility of the real performance under advanced clinical situations.
\nSix filters (dialyzers), one with PSf (PS-1.6UW, Fresenius-Kawasumi Co., Tokyo, Japan) and other five with PEPA (Nikkiso Co., Tokyo, Japan) were investigated (Table 2). Since both PSf and PEPA are hydrophobic in nature, these membranes include PVP for anti-thrombosis purpose, except for one dialyzer that includes PEPA membrane with no additives (FLX). Amount of PVP used in the membrane is semi-quantitatively shown as (+++), (++), (+), and (-), respectively for “most”, “much”, “small” and “none”.
\n\n #\n | \n\n name of products\n | \n\n abbreviated names\n | \n\n Surface area [m2]\n | \n\n membrane materials\n | \n\n hydrophilic agent\n | \n\n pore size info\n | \n\n membrane make\n | \n
1 | \nPS-1.6UW | \nPS | \n1.6 | \nPSf | \nPVP (+++) | \n(Not available) | \nFresenius Medical Care, Badhonburg, Germany | \n
2 | \nFLX-15GW | \nFLX | \n1.5 | \nPEPA | \nPVP (-) | \nstandard | \nNikkiso Co., Tokyo, Japan | \n
3 | \nFDX-15GW | \nFDX | \n1.5 | \nPEPA | \nPVP (+) | \nstandard | \nNikkiso Co., Tokyo, Japan | \n
4 | \nFDY-15GW | \nFDY | \n1.5 | \nPEPA | \nPVP (+) | \nlarger | \nNikkiso Co., Tokyo, Japan | \n
5 | \nFDX-150GW | \nnew FDX | \n1.5 | \nPEPA | \nPVP (++) | \nstandard | \nNikkiso Co., Tokyo, Japan | \n
6 | \nFDY-150GW | \nnew FDY | \n1.5 | \nPEPA | \nPVP (++) | \nlarger | \nNikkiso Co., Tokyo, Japan | \n
Technical specification of investigated ultrafilters
The time courses of the sieving coefficient (s.c.\n \n 4\n ) [9, 10] for albumin of PS-1.6UW dialyzer were shown in Figure 8. Strong time-dependent patterns were found with peak values approximately at 10 minutes after starting experiments. The lower the albumin concentration, the higher the s.c.\n \n 4\n values was found with longer time for achieving steady-state.
\nTime courses of the sieving coefficient for albumin under various concentrations of albumin in PS-1.6UW (PSf membrane) Q\n Bi=200 mL/min, Q\n F=10 mL/min, Volume of test sol’n=2.0 L.
The time courses of s.c.\n \n 4\n\n for albumin of three PEPA filters with albumin concentration of 3.64 mg/mL are shown in Figure 9. The s.c.\n \n 4\n gradually increased in these PEPA with PVP(-) or PVP(+) and never took peak values. Membranes used in FLX and FDX basically have the same pore sizes and the only difference is that the latter contained PVP, which concludes that PVP directly influences the membrane transport of albumin. By enlarging the pore diameter by approximately 5 % in FDY with the same PVP content, the s.c.\n \n 4\n increased with the enlargement accordingly.
\nTime courses of the sieving coefficient for albumin under a fixed albumin concentration (3.64 mg/mL) in three PEPA membrane dialyzers Curves are different from the ones found with PSf membrane. Q\n Bi=200 mL/min, Q\n F=10 mL/min, Volume of test sol’n=2.0 L.
The time courses of s.c.\n \n 4\n\n for albumin of the latest version of PEPA dialyzers are depicted in Figure 10 for albumin concentration of 3.64 mg/mL. It should be noted that the peak values were found in new PEPA membranes that included increased amount of PVP at 6 minutes after starting the experiments. Moreover, time dependent pattern of these curves are different from the ones shown in Figure 9 and are similar to those found with PSf membrane in Figure 8. Then it may be concluded that the time course of s.c.\n \n 4\n for albumin is strongly dependent on the amount of PVP included in the membrane and not on the main material of the membrane.
\nTime courses of the sieving coefficient for albumin under a fixed albumin concentration (3.64 mg/mL) in two new PEPA membrane dialyzers Curves are similar to the ones found with PSf membrane. Q\n Bi=200 mL/min, Q\n F=10 mL/min, Volume of test sol’n=2.0 L.
Since the albumin concentrations of the test solutions were lower by the factor of 1/30-1/10 to the standard albumin concentration in human blood (3.6 – 4.0 g/dL), s.c.\n \n 4\n\n values for albumin shown above do not directly correspond to the clinical results. One should, however, need to consider that the membrane separation characteristics depend on the pore diameter, amount of hydrophilic agent as well as experimental conditions [11].
\nAccording to the Japanese reimbursement system, all the commercial dialyzers are classified into five categories in accordance with the clearances for β2-microglobulin (β2-MG, MW 11800) under Q\n \n B\n =200 mL/min, Q\n \n D\n\n =500 mL/min for dialyzers with surface area of 1.5 m2 (Table 3). Classes IV and V dialyzers, clearances for β2-MG greater or equal to 50 and 70 mL/min, respectively, are the “super-high flux” models and more than 95 % of Japanese dialysis patients are treated with dialyzers of this kind [12]. These dialyzers had been used also for on-line hemodiafiltration (HDF) with considerable amount of albumin removal (> 3 g/treatment) until 2010 before on-line HDF has been officially announced to be included in the reimbursement system.
\nClassification of dialyzers in Japanese reimbursement system
1. Flow conditions: Q\n \n B\n =200 mL/min, Q\n \n D=500 mL/min, Q\n \n F\n =10 mL/min/m2.
2. A\n \n 0\n =1.5 m2\n
3. If A\n \n 0\n is NOT1.5 m2, use of the closest model is recommended. Clearance for β2-MG under A\n \n 0\n =1.5 m2 may be estimated by using the performance evaluation equations with K\n \n o\n \n A as a constant.
Although 99 uremic toxins are compiled by Vanholder [13], clinicians and researchers have different opinions on which solutes to be removed or up to how much albumin may be leaked out. Figure 11 shows the relationship between the reduction rate of β2-MG and albumin loss taken with various dialyzers in different modalities. Only a limited increase in β2-MG reduction was found with the increase of albumin removal. Therefore β2-MG removal may not be directly related to convection transport when super-high flux dialyzers are used. In other words, super high-flux dialyzers are the ones in which β2-MG removal does not correlate with the amount of albumin loss or the convection transport.
\nRelationship between the reduction rate of β2-microglobulin (MW: 11,800) and albumin loss.
\n Figure 12 shows the same relationship between the reduction rate of α1-microglobulin (α1-MG, MW 33,000) and albumin loss. Up to albumin loss of 3 g/session, almost linear relationship was observed, meaning that it may not be possible to remove α1-MG without removing albumin, although the molecular weight of albumin is twice as large as that of α1-MG. There is no such article that reports α1-MG is toxic; moreover, α1-MG is not even included in Vanholder’s list [13]. We, however, experienced fairly good number of patients who have become better with albumin loss of 3 g or more for bone pain, shoulder pain, and improvement of fingertip power, and 5 g or more for finger numbness, restless legs syndrome. Therefore α1-MG may be a possible surrogate marker of HDF treatment for those who have symptoms with normal HD therapy. Relief of clinical symptoms with various treatment modalities is summarized in Figure 13.
\nRelationship between the reduction rate of α1-microglobulin (MW: 33,000) and albumin loss.
Relief of clinical symptoms by employing various protein-losing treatment modes.
On-line HDF is mostly performed in post-dilution mode with high Q\n \n B\n (400 mL/min) in European countries, whereas that is mostly performed in pre-dilution mode with limited Q\n \n B\n\n (250 mL/min) in Japan. Diafilters preferred in post-dilution and pre-dilution HDF must be designed under different concepts. Membrane for the post-dilution HDF requires a limited permeability for albumin, otherwise unexpected large amount of albumin may be leaked out. Therefore relatively large surface area is preferred for achieving large amount of fluid exchange (20 L/session). Since usually higher clearances are expected with post-dilution HDF, membrane for the pre-dilution HDF prefers higher solute permeability that may allow much albumin to penetrate across the membrane. Amount of albumin loss, however, may be relatively easily controlled by changing the amount of ultrafiltration that is usually around 60 L/session. In the recent market, since diafilters specifically designed for either post-dilution or pre-dilution are available, choice of diafilters must be paid much attention not only for effective treatment but also for safety. Moreover, a proposal of technical specifications for the future diafilters is also reported [14].
\nMany randomized control studies have been done in order to verify superiority or better outcomes of on-line HDF [15-[19]; however, we have not yet come into a conclusion that states on-line HDF is better than other treatment modalities. These studies showed that on-line HDF was at least better than low-flux HD; however, the difference between on-line HDF and high-flux HD was ambiguous [18, 19], in terms of survival rate within a study period of three years or so. Post–hoc analyses and sub-analyses of those studies showed superiority of on-line HDF with large amount of fluid exchange (at least > 15 L) to other treatment modalities in terms of dialysis-induced hypotension, reaction to ESR medications, as well as survival rate. Among them, the ESHOL study [20] greatly encouraged patients on dialysis as well as medical staffs in which on-line HDF showed better clinical outcomes in all the end points than high-flux HD. Many debates, however, still continues also elsewhere including in Japan where the number of patients on on-line HDF is rapidly growing and exceeded 10 % of the total patients [21].
\nBiological consideration of the dialysis membrane is often referred to biocompatibility. Since dialyzers are repeatedly used four hours a session, three times a week, even a small event that repeatedly would occur each time may cause undesired side effects such as chronic inflammation.
\nUp until 1970s, RC membrane dominated over the market, and it was gradually replaced by synthetic polymeric membranes. Transient leukopenia that is an abrupt decease of leukocytes occurs at 15 to 30 minutes after starting the treatment has been one of the best known bio-incompatible events [22]. Reprocessing dialyzers was common in 1970’s and since bio-incompatible events were often found when a dialyzer was used for the first time, this was called the “first use syndrome” [23].
\nCraddock et al. reported that complement activation under the use of RC membrane induced transient accumulation of leukocytes in the blood vessels and in the lung [24]. As shown in section 2, RC has three hydroxyl groups in its backbone, and these hydroxyl groups have been realized to be closely related to undesired complement activation. Then acetate groups was introduced to the one, two, or all three of hydroxyl group(s) to produce cellulose acetate (CA), cellulose diacetate (CDA), and cellulose triacetate (CTA), respectively. Since these semi-synthesized cellulosic membranes have not only better biocompatibility but also higher permeabilities for solutes and water transport, they are still on the market.
\nIt is well known that the Glomerular basement membrane (GBM) in human kidney is negatively charged. Although AN-69® is also a negatively charged membrane, one must pay much attention for the use of this membrane because it may cause anaphylaxy shock soon after starting the treatment [25]. Strong negative charge (-70 mV) would activate Hageman (XII) factor to XIIa that eventually produces bradykinin from kininogen as a substrate. Under normal situation bradykinin may be deactivated by kininase II; however, if the patient takes angiotensin-converting enzyme (ACE) inhibitor, it deactivates kininase II. This would induce the cascade reaction with bradykinin, including NO generation, increased vascular permeability, expansion of blood vessels, suppressing blood pressure, and ending up with shock during the treatment. This is often called “negative charge syndrome” (NCS, Figure 14). Although all dialysis membranes are negatively charged, it is usually a contraindication to prescribe ACE inhibitor to a patient under the use of AN69®.
\nMechanisms of negative charge syndrome (NCS).
Hemophan® was developed by introducing a positively charged substance, diethylaminoethyl (DEAE), to RC membrane in order to improve its surface character (Membrana, Germany). Although only a limited amount of DEAE was introduced relative to entire amount of cellulose, complement activation was greatly suppressed. Hemophan®, however, adsorbed heparin, which induced blood coagulation. Because of this fact, the production of this membrane was ceased. Another trial was made by coating the membrane surface with vitamin E in order to make the RC membrane antioxydative (Terumo, Tokyo, Japan). Later, this technique was applied to PSf membrane and the commercial model is still available (Asahi Kasei Medical Co., Tokyo, Japan).
\nPSf and the ones whose chemical structures are similar to PSf have the highest market share among all dialysis membranes. They usually include polyvinylpyrrolidone (PVP) as a hydrophilic agent since they are hydrophobic in nature. PVP was once used as a supplement of plasma in medicine. Anaphylaxy shock, however, was reported, the cause of which was strongly doubted to be the PVP included in the membrane. Then we performed the following clinical investigation by using dialyzers with PSf and the ones with PEPA membrane with different amount of PVP [11].
\nTime course of C3a change during four hr treatment. The same PSf dialyzers with PVP(+++) were used in the 1-st and last (7-th) weeks. The same FDY dialyzers with PVP(+) were used from the 2nd to the 6th weeks.
The time course of C3a concentration profile in clinical study is shown in Figure 15. PSf with PVP(+++) showed three times higher concentration 15 minutes after the start of treatment. The C3a elevation was slightly lower at the first use of PEPA with PVP(+) and the peak concentrations were approximately halved or even less from the second to the fifth week. The peak concentration, however, returned back to three folds in the first use of PSf after five-week use of PEPA with PVP(+).
\nAccording to another clinical data shown in Figure 16, PSf with PVP(+++) showed highest C3a elevation, followed by PEPA with PVP(++), PVP(+), and PVP(-). The degree of C3a elevation was a function of amount of PVP included in the membrane regardless of the main material of the membrane.
\nTime course of C3a change during four hr treatment in 1 patient. Symbols are arranged in the chronological order from the top to the bottom.
From these results, we learned that PVP may not be the best choice as a hydrophilic agent in terms of blood compatibility.
\nWith above mentioned technique, we will be able to expect an even better dialysis membrane to come into the market. Several futuristic functions desired for dialysis membrane is also introduced, expecting a new era to come. Followings are the problems to be solved in the future perspectives of dialysis membrane.
\nSince on-line HDF has been gaining popularity in European countries as well as in Japan, HDF with much larger amount of fluid exchange has to be more easily performed for the further success of this modality. Standard on-line HDF in European countries is performed in post-dilution system with Q\n \n B\n =400 mL/min, Q\n \n D\n =700 mL/min, Q\n \n F\n =90 mL/min, Q\n \n S\n =80 mL/min=19.2 L/4hr in single patient dialysis machine (SPDM) system, whereas that in Japan is performed in pre-dilution system with Q\n \n B\n =250 mL/min, Q\n \n D\n =500 mL/min, Q\n \n F\n =260 mL/min, Q\n \n S\n\n =250 mL/min=60 L/4hr in central dialysis fluid delivery system (CDDS) [26] (Figure 17). In terms of solute removal, the difference between these two methods is the largest target solute to be removed, i.e., “European HDF” is targeted to remove β2-MG (MW 11,800) with little loss of albumin (some ten mg/treatment), whereas “Japanese HDF” is targeted to remove α 1-MG (MW 33,000) or even greater ones with albumin “removal” less than 4 g/treatment because enough removal of α1-MG cannot be possible without removing considerable amount of albumin (Figure 13). Although ultra-“super-high flux” dialyzers are commercially available in Japan, termed class V in Japanese reimbursement system, which remove α1-MG to achieve clinically effective reduction rate (> 30 %) [\n \n 26\n \n ], they also remove considerable amount of albumin (> 5 g/treatment) as well as amino acids, important small solutes from the nutritional point of view. Therefore when more precise prescriptions are necessary, on-line pre-dilution HDF is preferred because it removes α1-MG more than 30 % with albumin loss of 4 g/treatment or less and with considerably reduced clearance for small solutes, including amino acids, due to reduced net dialysis fluid flow rate (net Q\n \n D\n =500-250=250 mL/min).
\nComparison of post-dilution and pre-dilution on-line HDF with typical European and Japanese flow rates, respectively.
According to the Italian study [17], on-line HDF/HF is a useful tool for treating patients with dialysis induced hypotension. Then diafilters with higher hydraulic permeability with little albumin loss that do not aim to achieve higher solute removal may be useful for those patients. Not to mention, design specifications of dialyzer/diafilter is as important as the membrane permeability in terms of solute removal under given therapeutic conditions.
\nMany classic problems with biocompatibility in the past such as transient leukopenia, complement activation, negative charge syndrome, etc., have already been dissolved by modifying physical and chemical structures of the dialysis membrane. Most currently available synthetic polymeric membranes, however, employ PVP as a hydrophilic agent as well as a pore-forming agent. Study shows that many symptoms including abrupt decrease of blood pressure or shock right after starting treatments could be induced most probably due to PVP included in the membrane, and it is sometimes called “PVP intolerance”. Novel hydrophilic agents may be studied for the purpose of replacing PVP. Alternately, novel casting technique in which no hydrophilic agent is necessary has to be studied, knowing that PMMA, EVAL, and PEPA are cast with no additives although they are also originated from petroleum.
\nSurface modification with the third substances is another way to obtain membranes with preferred permeability as well as biocompatibility. For example, PSf membrane coated with vitamin E showed a great success for reducing reactive oxygen species (ROS) as well as free radicals that also showed preferable clinical results (Terumo, Asahi-Kasei). Toray introduced a novel technique with NV polymer to their PSf membrane to reduce adsorption of cells as well as protein molecules on the membrane. Although both two membranes work well clinically, they still utilize PVP in the same amount as previously included before. Then it should be noted since surface modification is closely related to solute transport as well as biocompatibility, biomimicry situations under dialysis must be further taken into consideration.
\nSince hemodialysis experiments with canines were first reported, many membranes, either natural or synthetic polymeric ones, have been developed and the latter have been the main stream due to higher solute and hydraulic permeabilities as well as better biocompatibility. The mass transport mechanism across the membrane can be expressed by the Fick’s 1-st law of diffusion; however, not only the membrane permeability but also the design specifications are important for assembling dialyzers with better performances.
\nThe chemical structure of the dialysis membrane determines the hydrophilicity and hydrophobicity of the membrane. Since all synthetic polymeric membranes are made from petroleum, they are hydrophobic in nature. Most of these membranes include a hydrophilic agent such as PVP for anti-thrombosis purpose. According to the in vitro experiments and clinical observations, it was proved that PVP was closely related to the sieving coefficient for albumin and had big influence on the complement activation. Then we must pay much attention on additives in addition to the main material(s) of the membrane.
\nPhysical structure of the dialysis membrane can be discussed in two ways, i.e., direct observations by taking microscopic views (SEM) and the theoretical analysis by using a mathematical model. There are two kind of dialysis membranes, “homogeneous” and “asymmetry”, among which the latter is gaining popularity because of the much smaller thickness that contributes to the resistance of solute and water transport. The pore theory is a useful tool for analyzing mass and water transport across the membrane and for designing a physical structure of the membrane.
\nSince the number of on-line hemodiafiltration (HDF) is growing these days not only from the solute removal point of view but also from the improvement of dialysis-induced hypotension during the treatment, membranes specifically designed for performing HDF has to be more extensively studied both clinically and fundamentally. Importance of biocompatibility of the membrane should be more carefully taken into account for selecting a device, considering membrane characteristic such as adsorption, especially in the field of acute kidney injury (AKI).
\nSeveral 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.
In our mission to support the dissemination of knowledge, we travel throughout the world to present our publications, support our Authors and Academic Editors at international symposia, conferences, and workshops, as well as to attend business meetings with science, academic and publishing professionals. We are always happy to meet our contributors in our offices to discuss further collaborations. Take a look at where we’ve been, who we’ve met and where we’re going.
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