Non-Pt Cathode Electrocatalysts

The sluggish kinetics of ORR are related to various factors, such as pH of the electrolyte, reaction temperature, etc. The alkaline media is preferred over an acidic one for the use of nonnoble metal catalysts because of their less-corrosive nature. The methanol crossover from the anode to cathode in DMFC may cause internal short circuit and thereby reducing the efficiency of DMFC. Therefore, cathode catalysts should be highly selective to ORR and should be methanol tolerant. In this section, recent findings of Pt-free cathode catalysts for ORR mechanisms are reviewed. The various non-Pt catalysts for ORR in alkaline electrolytes are transition metal-based compounds, nonprecious metal oxides, macrocycle-based catalysts, and carbon- based catalysts.

Transition Metal-Based Electrocatalysts

The transition metals alloyed with precious metals show excellent electrocatalytic activity toward ORR. The transition metal alloys have many advantages over monometallic forms, such as synergistic effects of more than one component, variations in the structure and morphology of the surface, concentration of electrons, and disorder of the lattice structure [19]. Many transition metal alloys, such as CuFe NPs/N- rGO [20], Pd.Fe NPs/CB, Pd,Co NPs/CB. Pd,Ni NPs/CB [21]. Ir-Pd/C [22], NiPd NPs/N-GR [23], and Au-Pd6CoCu/C [24], have been reported as cathode catalysts for ORR. The activation energy of Pt/C is 48 kJ mol1, whereas, for Pd/C, it is 39 kJ mof1 with an overvoltage of 300 mV. Therefore, the ORR performance of palladium (Pd)-based transition metals in alkaline electrolytes is more favorable on Pd/C catalyst than that of Pt/C [25].

Transition metal chalcogenides (TMC) have already been used as a cathode catalyst in acidic medium; however, TMCs have recently been employed in alkaline media. Mutyala et al. [26] reported phosphorus-doped and MoS, encapsulated, interconnected, porous, carbon (MoS,/P-ICPC) catalyst for ORR. The ORR activity (onset voltage and current density) of the catalyst is comparable to commercial Pt/C catalyst. Moreover, MoS2/P-ICPC catalyst exhibits high tolerance to methanol crossover and good durability. Recently, Masud et al. [27] confirmed that electrodeposited Co^eg nanostructures on GCE exhibit high performance for ORR with a higher tolerance to methanol crossover than that of Pt catalysts. The onset voltage at Pt/glassy carbon (GC) is negatively shifted to 0.801 V vs. reversible hydrogen electrode (RHE) from 0.931 V vs. RHE in the presence of methanol due to poisoning of the electrode surface. However, the presence or absence of methanol does not affect the onset voltage of Cc^Seg/GC electrode (0.811 V vs. RHE). The CojSeg/GC electrode also exhibits high stability after 1000 linear sweep voltammetry (LSV) cycles and reproducibility in presence of 0.5 M methanol solution. The enhanced performance of Co7Ses is due to the presence of Se in the lattice, which can easily modify the electronic structure of the active site (Co) of catalyst along with conduction and valence band positions with respect to water oxidation bands. It is noticed that change in transition-metal oxide to transition-metal selenide increases the conduction and valence band edges, as a result water oxidation and reduction levels become closer. The charge transfer between the catalyst and the electrolyte is enhanced because of the closeness of the band positions. Yu et al. [28] reported co-doping of iron (Fe) and nickel on a CoSe, catalyst that was obtained via the solvothermal route. Co07Fe0 ,Se2 shows cathodic peak voltage of 0.564 V and onset voltage of 0.759 V, while Co07Ni03Se2 exhibits cathodic peak voltage of 0.558 V and onset voltage of 0.741 V, indicating that Co07Fe0 3Se2 is a better cathode catalyst than that of Co07Ni0 ,Se2. Moreover, the Co07Fe03Se2 catalyst is more stable and tolerant toward the crossover of methanol, ethanol, and ethylene glycol in comparison to that of commercial Pt/C. As reported by the authors, the improved activity of Co07Fe03Se2 catalyst is due to Fe doping on Co-based chalcogenides, which can enhance the constitution and stability of the active species.

Metal Oxide-Based Electrocatalysts

Many nonprecious metal oxides have been reported as cathode catalysts for ORR, such as CoPc/C-Wls049 [29]. Au-MnO,/MWNT. Au-ZnO/MWNT [30], CoO@ NS-CSs [31], Mn-CeO,/rGO [32], Ti,0,/rG0 [33], Co/CoOv@NC-CNTs [34], CoO/ MnO,/RGO [35]. MnOx- Co,04/C [36]. CoO@Co/N-C [37], CeO,/rGO [38]. RGO/ ZnW04/Fe,04 [39], Fe-MFC60-T [40], RGO@Co,04 [41], and different spinel oxide- based materials, such as 3D dandelion-like NiCo204, flower-like NiCo,04 [42], rGO/ CoFe204 [43], hexagonal spinel-type Мп2АЮ4 nanosheets [44], and 3D macroporous NiCo204 sheet [45]. Many studies have been conducted on MnxOy-based cathode catalysts because of their rich redox chemistry and abundance as well as the environmentally friendly nature of manganese [46-48]. Lee et al. [49] reported that rGO/ MnO,/Ag exhibits enhanced current density and electron transfer rate per O, at a voltage of 0.3 V compared to 20 wt% Pt/C. In addition, the results of kinetic analysis confirm that O, has been directly reduced to H20 via a four-electron pathway with strong resistance to fuel crossover in comparison to that of commercial 20 wt% Pt/C. In 2015, Chen et al. [50] investigated the electrocatalytic activity of 3D nitrogen- doped graphene/MnO (3D-N-RGO/MnO) toward ORR. The as-synthesized catalyst exhibited enhanced catalytic activity and more positive potential, high tolerance to methanol crossover, and long-term stability because of the synergistic effects of 3D nitrogen-doped RGO and MnO. Therefore, graphene/metal oxide catalysts have been widely used in batteries, fuel cells, supercapacitors, and biosensors [51-54]. Zuo et al. [55] synthesized porous MnO, by facile sonochemical method (SC-PMO) and studied ORR activity. In Figure 11.5, all the transmission electron microscope (ТЕМ) images at different magnifications (Figure 11.5a and b) and the scanning electron microscope (SEM) image (Figure 11.5c) confirm the porous structure of SC-PMO. The as-synthesized porous SC-PMO shows improved electrocatalytic activity, better stability, and higher methanol tolerance than that of commercial Pt/C catalyst in a basic medium. All the electrochemical analysis confirms that the enhancement in ORR activity at porous SC-PMO catalyst is ascribed to the porosity and well dispersity, which further increases the kinetics of the ion and electron transport mechanism.

Yu et al. [56] investigated the performance of MnO, nanofilms directly grown over nitrogen-doped hollow graphene spheres (MnO,/N-HGSs) as an ORR catalyst in Zn-air batteries. As shown in Figure 11.6a, MnO,/N-HGSs exhibit maximum power density of 82 mW cnv2 at an open-circuit voltage of 1.48 V, which is almost comparable to that of commercial Pt/C catalysts (power density 94 mW cnr2 at an open circuit voltage of 1.49 V). The long-term stability of MnO,/N-HGSs toward ORR is confirmed from the galvanostatic discharge curves shown in Figure 11.6b. Moreover, the specific capacity of MnO,/N-HGSs (744 mAh g1) is also comparable to that of a commercial Pt/C cathode (757 mAh g1) at a current density of 10 mA cnv2.

Morphology of porous MnO, prepared by SC-PMO. (а) ТЕМ micrograph, and (b) FE-SEM image. Adapted and reproduced with permission from reference [55]. Copyright 2017 Elsevier

FIGURE 11.5 Morphology of porous MnO, prepared by SC-PMO. (а) ТЕМ micrograph, and (b) FE-SEM image. Adapted and reproduced with permission from reference [55]. Copyright 2017 Elsevier.

 
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