Fuel Cells

The fuel cells are alternate energy sources that are developed to account for the increasing energy demand and rising energy prices, and the detrimental effects of fossil fuels such as pollution, global warming, and climate changes. Moreover, the sources of fossil fuels are rapidly deteriorating. The fuel cells use fossil fuel derivatives, hydrogen, and alcohols as fuels to produce electrical energy with high efficiency and reliability. Fuel cells can be classified based on the electrolyte, and the classification of fuel cells and their characteristics are presented in Table 8.6.

The construction of a fuel cell is shown in Figure 8.15. The major parts of a fuel cell are the anode, cathode, and an electrolyte membrane. The fuel is passed through the anode side of the fuel cell and air through the cathode side. The anode is permeable to the fuel where it is oxidized to ions by releasing the subsequent number of electrons, for example, H, gas is oxidized to H+ ions and electrons in polymer electrolyte membrane fuel cell. The H+ ions pass through the anode to the electrolyte, and the electrons flow through an external circuit, generating an electric current. At the cathode, the oxygen from the air is reduced in the presence of H+ ions and electrons to form water molecules. A series of such cells are stacked to meet the high-power production units. The interconnects separate each cell from a chemical short circuit and transfer the electric current to the current collectors in each cell or the external load.

Perovskites Used in Proton Exchange Membrane Fuel Cell

The catalysts based on noble metals, such as Pt or Au, are well known ORR and OER catalysts with high stability. Replacing the noble metal catalysts can significantly reduce the installation cost of proton exchange membrane fuel cells (PEMFC). Many perovskite materials exhibit comparable catalytic activities in alkaline medium towards ORR and OER activities comparable to platinum-based noble metal catalysts. Perovskites with La at А-site and transition metals at B-site demonstrated good ORR activity. Partially substituting the В-site ions with other transition metal elements significantly improves the catalytic behavior as well as the stability of La-based perovskite materials. LaCoO, and LaNiO, perovskites with В-site partial substitution of Mn, Co, Fe, or Ni are the most promising perovskites materials for PEMFCs. These perovskite materials are stable up to a temperature range of 60°C-200°C, and they are promising as cathodes in alkaline PEMFCs (Tarrago et al. 2016).

Perovskites Used in Solid Oxide Fuel Cells

In general, the cations in the perovskite structured materials exhibit different valences, and therefore they can occupy different sites in the structure; as a result, unique physical and chemical properties, high oxygen vacancies, and ionic conductivity can

Fuel Cell

Commercial Electrolytes and Electrodes

Temperature of Operation

Fuel Type

Remarks

Polymer electrolyte membrane/ PEMFC)

Solid polymer electrolyte and porous carbon structures containing platinum as a catalyst

60°C-200°C

Hydrogen at anode and oxygen at the cathode

Deliver high power density

Alkaline fuel cells

Modified PEMFC with potassium hydroxide in water as the electrolyte and a variety of non-precious metals as a catalyst at the anode and cathode

60°C-90°C

Pure hydrogen and oxygen gas.

These fuel cells use an alkaline membrane with demonstrated efficiencies above 60% in space applications.

Solid oxide fuel cell

Solid oxide material as the electrolyte and electrodes.

500°C-1,000°C

Gasoline, diesel, biofuels. Or mixtures of hydrogen, carbon monoxide, carbon dioxide, steam, and methane,

High combined heat and power efficiency, long-term stability, fuel flexibility, low emissions, and relatively low cost. No noble metal catalyst and up to 85% efficiency

DMFC

Platinum and/or ruthenium particles embedded on nanostructured carbons, and polymer as electrolytes

50°C-120°C

Methanol

Transportation of methanol is easier than hydrogen, and energy-density is higher than hydrogen. High power density

Molten carbonate fuel cell

Electrolyte composed of a molten carbonate salt mixture suspended in a porous, chemically inert ceramic lithium aluminum oxide matrix and non-precious metals as catalysts.

550°C-750°C

Natural gas and biogas

Methane and other light hydrocarbon fuels are converted to hydrogen within the fuel cell itself by a process called internal reforming.

Fuel Cell

Commercial Electrolytes and Electrodes

Temperature of Operation

Fuel Type

Remarks

Phosphoric acid fuel cells (PAFCs)

100% phosphoric acid eleclrolyte retained in a silicon carbide matrix and carbon paper coated with a finely dispersed platinum as a catalyst.

150°C-220°C

Hydrogen-rich gases

These fuel cells are used for stationary power generation. PAFCs are more tolerant of impurities in fossil fuels that have been reformed into hydrogen than proton exchange membrane (PEM) cells. PAFCs are expensive due to the high loading of the platinum catalyst.

Microbial fuel cells

Microbes such as bacteria catalyze electrochemical oxidations or reductions at an anode or cathode

~25°C

Organic matter such as wastewater or nutrients

Microbial fuel cells create electricity using electrode reducing microorganisms. Electrode-oxidizing organisms take electrons from the cathode to reduce various substances to acetates.

Schematic of a PEMFC

FIGURE 8.15 Schematic of a PEMFC.

be attained. A large number of oxygen vacancies and the associated ionic conductivity make them suitable as electrode and electrolyte materials in SOFCs. In SOFCs, the perovskite materials are identified as suitable candidates for cathode, anode, electrolyte, and interconnectors. The operating and making cost of SOFCs can be considerably reduced by lowering the operating temperature and replacing noble metal alloys. The oxidizing and reducing atmospheres at high temperatures necessarily require stable interconnectors and current collectors. Therefore, by decreasing the operating temperature, less expensive materials can be used in the above applications, thereby reducing the initial cost of SOFCs.

The primary function of a cathode in SOFC is to provide active sites for the electrochemical reduction of oxygen with high catalytic activity. The ideal cathode materials also must have a high electronic and ionic conductivity, a compatible thermal expansion coefficient, thermal and chemical properties of other components of SOFCs such as electrolyte and interconnect materials, porosity, and low cost. Perovskite structured La^S^MnO,^ (LSM) (Tian et al. 2008) and La^S^CoO,^ (LSC) (Gwon et al. 2014) are the promising high-temperature cathodes in SOFC due to their high electronic conductivity despite their low ionic conductivity. However, tuning the oxygen vacancies and cation vacancies can significantly affect the transport properties of these materials. The substitution of rare-earth ions at A-site lowers the energy barriers for adsorption and diffusion of oxygen species; at the same time, they exhibit excellent compatibility with yttria-stabilized zirconia (YSZ) electrolyte and good electronic conductivity. The cobalt-ferrite perovskites such as Ba05Sr05CottgFe02O, (Li et al. 2019) and LaUvSrvFe^vCov04 (Ma et al. 2018) are also promising candidates as cathode materials. The rare-earth orthoferrites exhibit high catalytic activity and reasonable ionic and electronic conductivities at lower temperatures (600°C-800°C). Doping Ca in these materials increases the electrical conductivity and lower cathodic overpotential, with a compatible thermal expansion coefficient. Recent studies show that double perovskite materials with the general formula ABaCo205+t (A = Gb, Pr, Sm, and Nd) exhibit higher catalytic activity than single perovskites (Hong et al. 2017).

The ideal anode for SOFCs should be a reducing catalyst with good electronic and ionic conductivity, thermal and chemical compatibility, porosity, and low cost. Perovskite structured materials are identified as a promising material as anodes in SOFCs due to their stability under reducing atmospheres and high temperatures, SrTiO, is one among them. The doping of elements in А-site with La and B-site with Nb tremendously increases their electronic conductivity; however, the ionic conductivity is poor (Tiwari and Basu 2015). Substitution of cations like Al, Fe, Ga, Mg, Mn, or Sc significantly changes the redox catalytic properties and the conductivities. Substituting transition metals in the В-site reduces the energy required for the formation of vacancies so as conductivities. La^Sr^Cr^Fe^O, is also identified as a promising anode candidate in SOFCs, due to its high redox stability, conductivity, and electrochemical activity. Double perovskite structured materials also exhibit interesting properties favorable for SOFCs (Aliotta et al. 2016).

The electrolyte of SOFCs is also potentially replaced with perovskite oxide materials. A typical SOFC electrolyte should have high ionic conductivity and low electronic conductivity, in addition to the necessary characteristics of the electrodes such as chemical stability and compatible thermal expansion coefficients. Perovskites with gallium, zirconium, and cerium at the В-site are identified as suitable candidates for electrolytes, e.g., LaGeO, (Ishihara et al. 1995). Substituting the А-site cations with Sr or Ba can alter the conductivities to a certain level. However, doping Mg at B-site can significantly affect the favorable properties as an electrolyte by increasing the concentration of oxygen vacancies. The proton-conducting oxides such as BaZrO, (Iguchi et al. 2007) and BaCeO, (Naeem Khan et al. 2017) are also promising as electrolyte materials. Moreover, these materials allow doping with precious metals to increase oxygen mobility.

The interconnects of SOFCs must be stable to oxidizing and reducing atmospheres, highly electronic conductive, low ionic conductive, gas-tight, and compatible thermal and chemical behavior with other components of SOFCs. Perovskite structured lanthanum and yttrium chromates (Weber et al. 1987; Duran et al. 2004), both /Муре semiconductors, are identified as suitable candidates for SOFC interconnects. The А-site dopants such as Sr, Mg, and Ca and the B-site dopants Co or Fe can improve the conductivity and thermal stability of chromite perovskites.

Perovskite-Based Catalysts for Direct Methanol Fuel Cells

In Direct Methanol Fuel Cells (DMFC), methanol is used as a fuel, which is oxidized electrochemically at the anode and generates electricity, and oxygen is reduced (ORR) at the cathode. The perovskites with transition metals at В-sites can be a promising non-noble metal cathode in DMFCs. Lanthanum-based perovskites with the general formula LaMO, (M = Co, Mn, Ni, Fe) are promising candidates for ORR in alkaline medium with the characteristics of that of noble metal catalysts. The partial substitution of А-site cations with Ca and Sr enhances the current densities at a lower overpotential; however, the transitions metals at B-site plays a significant role in improving the ORR catalysis any further. The presence of two transition elements in the B-site exhibits better activity than the one with the single element at the B-site. The ORR current density of La-based perovskites is in the following order: LaCoO, > LaMnO, > LaNiO, > LaFeO, > LaCrO,. Moreover, the perovskite materials on carbon-based support materials such as graphene and CNTs are more active than the pure perovskites due to the synergic activation mechanism in the composites (Zhu et al. 2015).

Perovskite-type materials can be potentially used as anodes in DMFCs. Pt-containing DMFC anodes exhibit poor reaction kinetics and can be contaminated by carbon species such as CO. Metal oxides are an anticipated alternative as anode due to their chemical stability. The presence of Ru in the В-site of La -or Sr-based perovskites is the most efficient catalyst for methanol oxidation reaction (MOR) (Lan and Mukasyan 2008). The activity towards MOR is further improved by controlling the particle size and doping noble metal, such as Pd or depositing Pt on the surface of the catalysts.

 
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