Prospects of Biochar as a Renewable Resource for Electricity

Abstract

To face the change in energy paradigm, we need to devise technology that utilizes renewable resources and eventually realizes sustainability. Fuel cells generate electricity in a greener way, the efficiency and its cost-effectiveness depend mainly on the electrode material. Biochar serves as the promising electrode material, fuel, and separator membrane for fuel cells by being cheap, renewable, and possessing excellent electrochemical performance. The chapter is expected to provide a database of knowledge on how biochar with diversified physical and chemical features and functionalities can be effectively utilized for the possible application as electrode material for energy systems. The chapter appreciates the immense wealth of choice of biochar available with us for an important application in the area of energy as electrode material, fuel, and separator membrane for fuel cells.

1. Introduction

The enormous usage of fossil fuels leads to harmful environmental damage, viz., global warming, depletion of energy resource, and lack of sustainable growth. To overcome this situation, now the world is in need of pollution-free green energy source. Moreover, the energy source should be available anytime anywhere in order to be a perennial source and to have a sustainable development. In regard of this, waste has to be used in a vast amount as an energy source instead of traditional fossil fuels which liberates huge amount of carbon dioxide, i.e., waste to wealth conversion. The energy derived from biomass termed biomass energy is the fourth largest energy source next to three fossil fuels, viz., coal, petroleum and natural gas. About 5% of the United States’ primary energy need is fulfilled by biomass in 2021. As biomass is a carbon-neutral resource, the energy produced out of it is considered clean green energy.

It is noteworthy to know about the biomass and biochar. Biomass is the matter from biological organisms and biochar is the product obtained by the thermal/chemical processing of biomass. The sources of biomass include forest residue, agricultural crops and residues, domestic waste, municipal waste, marine waste, and industrial waste [1]. Biomass though worthless, but it is a great source for valuable biochar. The biochar finds its multifarious applications such as:

• Adsorbent for toxic pollutants [2]

• Soil amendment to improve soil health

• Catalyst support for electrolysis [3, 4]

• Electrode material for electrochemical energy devices, viz., lithium-ion batteries [5], supercapacitors [6, 7], and fuel cells [8, 9].

The widespread utility of these electrochemical energy devices is hampered due to the high cost of the electrode materials. There arises revolution in the field of energy due to the utility of biochar as electrode material in electrochemical energy devices as it replaces the costlier electrode materials, thereby paves the way to the production of electrical energy at low cost. In addition to that, it gives value-added utilization of biomass in the field of energy.

A brief description of fuel cells is worthwhile here. Fuel cell converts the chemical energy of fuel and oxidizing agent into electricity by electrochemical redox reactions. It serves as an endless power source for space vehicles and submarines. The main components of fuel cells are the cathode, anode, and electrolyte which facilitate the passage of ions. Fuel cells are of many types, such as proton exchange membrane fuel cell (PEMFC), solid oxide fuel cell (SOFC), microbial fuel cell (MFC), and direct carbon fuel cell (DCFC).

In this chapter, detailed survey of utility of biochar as electrode material and separator in MFC and as fuel in DCFC is given in systematic way. This enables us to understand the value-added utilization of biomass in the field of energy and to explore many other biomass for its utility in the near future.

2. Microbial fuel cell (MFC)

Microbial fuel cell (MFC), a bioelectrochemical device, employs organic waste to produce electrical energy [10, 11]. In MFC, proton exchange membrane separates the anode and cathode. Electrodes used in MFC are crucial in determining its efficiency [12]. Electrogenic bacteria on the surface of the anode oxidizes the organic matter to generate electrons and protons [13, 14]. The electrons generated flow reaches the cathode to combine with an electron acceptor via an external circuit [15, 16]. Due to superior electrochemical oxidation capacity, great abundance, and clean reaction product, oxygen is the most widely used electron acceptor [17, 18]. However, the poor cathode oxygen reduction reaction and high oxygen mass transfer resistance significantly lower the performance of MFC [19]. The working mechanism of MFC is given as follows:

Acetate and glucose in organic compounds are oxidized in MFCs, which generate electrons, flow down to an external circuit, and produce electricity, whereas organic compounds are anaerobically oxidized and result in the evolution of protons, electrons, and CO₂. In MFC, water is generated in cathode by the reduction of protons and electrons with the usage of oxygen supplied from outside. The protons and electrons thus liberated reach the cathode through an electrical circuit in presence of electrolyte medium. The formation of water in MFC is represented by the equation given as follows [20]:

$${\mathrm{CH}}_3{\mathrm{COO}}^{-}+4{\mathrm{H}}_2\mathrm{O}\to {{\text{2HCO}}_3}^{-}+9{\mathrm{H}}^{+}+8{\mathrm{e}}^{-}.$$E1

$${\text{2O}}_2+\text{8H}++{\text{8e}}^{-}\to \text{4H}.$$E2

In MFC, oxidation occurs at the anode and reduction at the cathode, thereby creates the potential difference between the electrodes, leading to the generation of bioelectricity as shown in Figure 1.

Figure 1.

Working of MFC.

Biochar can act as electrodes (anode/cathode), electrocatalysts, and proton exchange membranes b [21]. Biochar acting as electrodes should possess high porosity, rich carbon content, excellent electrical conductivity, large surface area, and cost-effective. Moreover, it should be nonbiodegradable, biocompatible, and pave way to waste to wealth conversion. Table 1 lists the biochar derived from various biomass, its method of preparation, processing temperature, its utility as electrode material for MFC, and the power density derived from it.

2.1 Biochar as separator

Proton exchange membrane (PEM) in MFC is superior in its performance when it possesses large proton conductivity, minimal oxygen, and substrate crossover, decreased biofouling rate, and low cost. As the source of biochar is abundant and easily available, the biochar possesses strong cation exchange properties, a high concentration of surface-active sites, and excellent porous nature supports its use in PEMs. In PEM fuel cell, the biochar acts as the unique separator which usually replaces the Nafion polymer membrane. Moreover, the biochar acts as a porous membrane having expanding sustainability than the other membrane and cost-effective eco-friendly catalyst material. The biochar-built PEM fuel cell was applicable for the Industrial and lab scale preparation.

2.2 Biochar based catalyst

The basic mechanism of oxygen reduction reaction (ORR) is the absorption of proton from the electrolyte by the oxygen molecule at the cathode. Followed by this, the transfer of electrons takes place from anode to metallic wire. Reaction requires more energy for the production of fuels. For the good performance of MFC, the anode and cathode play the catalyst role. For enhancing the sustainability, stability, and activity, the cathode fabrication is very important. The necessity of good cathodic material is to reduce the activation potential of ORR reaction and the cost-effective process. Initially, the fuel reactions were carried out using a platinum catalyst which serves as the catalyst for the reduction of oxygen and reduction. The economic preparation of the material was not affordable for the large-scale preparation as well as not suitable for the domestic purpose application. For the replacement of notable platinum catalyst, the non-transition metal, 2D material, carbon material, and porous material are used for the fuel reaction. In recent research, the usage of biochar material from natural sources acts as the cathodic material for the ORR reduction and showed the good performance than the other catalyst materials. The mechanistic reactions are explained in various ways.

List of biochar derived from various biomass and its utility as separator, PEM, and catalyst for ORR are given in Table 2.

S.No	Biomass	Electrode	Method of preparation	Processing temperature; (°C)	Power Density (mW/m^2)	Ref
1	Balsa wood	Cathode	Pyrolysis	800	200.0	[22]
2	Banana	Cathode	Thermal treatment	550	393.7	[23]
				900	483.7
			KOH activation	550	424.6
				900	528.2
3	Biodigester plant waste	Cathode	KHCO3 activation	850	NA*	[24]
4	Pinus resinosa	Cathode	Pyrolysis	850	356.0	[25]
5	Coconut shell	Anode	Pyrolysis	400	283.4 ± 9.6	[26]
6	Pinewood chips	Anode	Carbonization	1000	457.0	[27]
	Pine sawdust				532.0
7	Peanut shells	Anode	Pyrolysis	800	NA*	[28]
8	Maple wood	Anode	Pyrolysis	350–600	41.4	[29]
9	Rubber tree sawdust	Anode	Pyrolysis	500	326.0	[30]
10	Waste wood	Anode and cathode	Pyrolysis	1000	600.0	[31]
11	Wood	Cathode	KOH activation	1000	146.7	[32]
12	Corn straw	Anode and cathode	KOH activation	900	889.0	[33]
13	Giant cane stalk	Cathode	NA	900	NA*	[34]
14	Watermelon rind	Cathode	KOH activation	180	26.2	[35]

Table 1.

Sources of biochar and its utility in MFC.

NA*—not available.

S.No	Biomass	Function	Method of preparation	Processing temperature; (°C)	Power Density (mW/m^2)	Refs.
1	Giant cane-clay composite	Solid separators	Pyrolysis	350	40.0	[36]
			Nitrogen flow	900
2	Coconut shell biochar blended with metal (20%)	Anode; CS-Si0.2	Pyrolysis	500	16.8	[37]
		CS-Zn0.2			22.9
		CS-Cu0.2			38.7
3	Sewage sludge	Catalyst (ORR)	Carbonization	900	500.0	[38]
4	Banana peel	PEM	H2SO4 activation	600	41.1	[39]

Table 2.

Sources of biochar, its preparation, functions in MFC, and the power obtained.

3. Direct carbon fuel cell

Global energy demand depends mainly on conventional sources such as coal, petroleum, and natural gas since earlier days. The excess use of coal as an energy source in the past is due to its low cost, abundance, and extensive distribution throughout the world [40]. As these sources are nonrenewable, its continuous usage leads to scarcity. Moreover, continuous usage of conventional sources leads to environmental damage, thereby exploration of clean energy source is the need of the hour. This put forward the steps to initiate energy generation from renewable sources like biomass. The direct carbon fuel cell (DCFC) employs carbon as anode operates on a high-temperature range of 700–900°C. It is superior over other fuel cells by attaining 80% efficiency (for power generation) [41, 42]. The carbon used as fuel in DCFC may be coal, biomass, and organic waste which are abundantly found in nature. DCFC transforms chemical energy trapped in the solid carbon fuel into electrical energy. DCFC accounts for green energy generation as it does not require any gasification processes and other conventional electric generators [43, 44]. The elemental carbon act as fuel contains high-energy density and is oxidized electrochemically at the electrodes [43, 44].

Based on the working electrolyte, there are three main categories of DCFCs, namely, molten hydroxide, molten carbonate, and solid oxide DCFCs. Molten hydroxide and molten carbonate DCFC utilizes NaOH/KOH and carbonates, respectively, as their electrolyte. The electrolyte is filled in a metal vessel, which acts as the cathode. The carbon materials function both as fuel and the anode and are dipped into the electrolyte. Solid oxide DCFC resembles the other two DCFCs except using an oxygen ion (O²⁻) conducting ceramic electrolyte. The most commonly used electrolyte is Y₂O₃ stabilized zirconia due to high ionic conductivity, better stability, chemical and thermal compatibility, mechanical robustness, easy fabrication, and low cost [45, 46, 47]. Solid oxide DCFC though favorable for its simple design, but it has the limitation of low output [48]. This low output is explicitly due to the limited reaction zone at the carbon fuel and the electrolyte interface.

A DCFC consumes solid carbon and oxygen to produce electrical energy through electrochemical anodic and cathodic reactions [49, 50]. The overall reaction involved in DCFC is the simple combination of carbon and oxygen to form carbon dioxide. Electrooxidation of carbon occurs at the anode, whereas electroreduction of oxygen occurs at the cathode [40, 49, 50]. Working of different types of DCFC is shown in Figure 2.

Figure 2.

Working of different types of DCFC.

Reaction at the anode (oxidation of carbon).

$$\mathrm{C}+\left(\text{electrolyte}\right){\text{4OH}}^{-}\to {\mathrm{CO}}_2+{\text{2H}}_2\mathrm{O}+{\text{4e}}^{-}.$$E3

$$\mathrm{C}+\left(\text{electrolyte}\right){{\text{2CO}}_3}^{2-}\to {\text{3CO}}_2+{\text{4e}}^{-}.$$E4

Overall anodic reaction.

$$\mathrm{C}+{\text{2O}}^{2-}\to {\mathrm{CO}}_2+{\text{4e}}^{-}.$$E5

Reaction at the cathode(reduction of oxygen).

$${\mathrm{O}}_2+{\text{2H}}_2\mathrm{O}+{\text{4e}}^{-}\to {\text{4OH}}^{-}.$$E6

$${\text{2CO}}_2+{\mathrm{O}}_2+{\text{4e}}^{-}\to {{\text{2CO}}_3}^{2-}.$$E7

$${\mathrm{O}}_2+{\text{4e}}^{-}\to {\text{2O}}^{2-}.$$E8

If DCFC is supposed to operate at a high temperature (above 700°C), carbon electrooxidation is overlooked by the reverse Boudouard reaction (electrooxidation of CO). So direct electrooxidation of solid carbon is overlooked by direct electrooxidation of CO, termed as CO shuttling mechanism [51] which leads to low carbon fuel utilization [52].

$$\mathrm{C}+{\mathrm{CO}}_2\leftrightarrow \text{2CO}.$$E9

Being reverse Boudouard reaction is endothermic, decreasing the working temperature of DCFC will enhance the CO reduction in the anode exhaust, thereby increase carbon fuel usage [49, 53]. DCFC attracts the scholarly attraction owing to its low-maintenance cost, simple cell structure, and rich availability of carbon feedstocks, i.e., biomass. List of biochar derived from biomass which finds its utility in various types of DCFC is given in Table 3.

S.No	Biomass	Electrolyte	Processing temperature; (°C)	Open circuit voltage (V)	Power Density (mW/m^2)	Refs.
1	Corn cob	Samarium doped ceria (SDC) with eutectic carbonate phase	750	1.05	185.0	[54]
2	Almond shell	Ceria-carbonate composite	700	1.07	127.0	[55]
3	Olive wood	Ceria-doped samarium (SDC) combined with molten carbonate	700	1.02	105.0	[56]
4	Beech wood chips	Yttria-stabilized zirconia	800	1.00	100.0	[57]
5	Acacia wood chips	Yttria-stabilized zirconia	800	0.80	90.0
6	Waste coffee grounds	Yttria-stabilized zirconia	900	1.10	87.2	[58]
7	Miscanthus straw	Yttria-stabilized zirconia	800	1.15	70.0	[59]
8		Molten Carbonate		0.88	12.0
9	Coconut	Yttria-stabilized zirconia	800	0.87	60.0	[60]
10	Wheat	ScSZ electrolyte layer	800	1.18	67.0	[61]
11	Spruce	ScSZ	800	1.16	57
12	Wood	Molten carbonate	700	1.00	25	[62]
13	Refuse plastic/; paper fuel	Molten carbonate	700	1.20	22	[63]
14	Refuse derived; fuel	Molten carbonate	700	1.00	17
15	Sunflower; husks	Molten hydroxide	450	1.01	22	[64]
16	willow shavings			0.87	20
17	Pine			1.05	18
18	Black liquor	Yttria-stabilized zirconia	700–800	0.80	122	[65]
19	Eucalyptus leaves	Molten carbonate	700	0.84	NA*	[66]
20	Neem leaves			0.73
21	Mast leaves			0.71
22	Melon seed husk	Molten carbonate	450	0.71	550.0	[67]
23	Palm kernel shell	Yttria-stabilized zirconia	850	0.81	330.0	[68]
24	Pine pellets	Molten hydroxide	800	0.84	766.0	[69]
25	Pine bark pellets	Molten hydroxide	800	1.07	450.0	[70]
26	Pistachio shells	Yttria-stabilized zirconia	800	0.94	155.0	[71]
27	Pecan shells				140.0
28	Sawdust				100.0
29	Pomelo peel	Yttria-stabilized zirconia	850	1.02	3090.0	[72]
30	Reed	Ceria-doped samarium (SDC) combined with molten carbonate	750	0.96	3780.0	[73]
31	Rice husk	Yttria-stabilized zirconia	750	0.81	1790.0	[74]
32	Rubber wood	Yttria-stabilized zirconia	850	0.77	294.0	[75]
33	Walnut shell	YSZ	800	0.97	1470.0	[76]
34	Wheat straw	YSZ	800	1.0	1870.0	[77]
35	Corncob	Miscanthus straw biomass		0.98	2040.0
36	Bagasse			0.99	2600.0

Table 3.

List of Biochar served as fuel in DCFC.

NA*—not available.

4. Challenges and future perspective

To understand the mechanism behind the electrochemical reaction kinetics and carbon oxidation at the anode/electrolyte interface is still a challenge for us. In addition to that, metallic components in the biochar inhibit the electrochemical performance of the fuel cell. Determining the amount of ash accumulation is also a key factor and should be researched into as well to determine the lifetime of DCFC. Technological expertise in the cell design along with the clear understanding of kinetics will solve the issues in near future.

5. Conclusion

Biochar an inexhaustive renewable resource solves many environmental issues arised in recent decades, viz., pollution, remediation in soil, and water. Research studies in recent years advocate the multifarious utility of biochar as fuel in DCFC and as electrodes, separator, and catalyst for ORR in MFC. Moreover, biochar-based MFCs remove hazardous chemicals from wastewater, DCFC utilizes carbon from zero-cost sources as fuel along with the generation of electricity.