Abstract
We have investigated the durability of a platinum group metal (PGM-)free Fe–N–C catalyst in which the Fe sites are atomically dispersed (AD), and found it to be quite stable in standard accelerated stress test (AST) cycles normally used for low-PGM catalysts: a square wave with 0.6 V lower potential limit (LPL)—0.95 V upper potential limit (UPL) with 3-s holds at UPL and LPL in H2/N2, at 1.5 atm, 80 °C and 100% RH. Considering the metrics normally employed to characterize the durability of the low-PGM catalysts after 30,000 AST cycles, this PGM-free catalyst lost <50% catalyst activity, <50% H2/air performance at 0.8 V, and 40 mV at 1.5 A cm−2. However, it is less stable in H2/air, losing ∼50% catalyst activity after just 7.5 h of polarization measurements (load cycles). In combined cycles, the majority of the loss in catalyst activity occurred during the load cycles in H2/air rather than AST cycles in H2/N2. We have concluded that, unlike low-PGM catalysts that lose electrochemically active surface area (ECSA) through potential cycling-induced processes, (AD)Fe–N–C catalysts degrade by processes associated with the presence of oxygen.
Highlights
Rs T δ σ Ψ Subscripts cefmps catalyst specific area diffusivity Nernst potential current density catalyst loading pressure, Pr = 1 bar mass transfer resistance Ohmic resistance oxygen reduction reaction (ORR) symmetry factor overpotential available sites formation factor concentration cell voltage Faraday constant limiting current density molecular weight gas constant or resistance sheet resistance temperature, Tr = 353 K thickness conductivity active sites cathode electric ionomer film mass transfer pore catalyst surface
Rotating disk electrode (RDE) studies indicate that the ORR activity correlates with (AD)Fe sites, with the Fe clusters acting as spectator sites, and that the activity does not increase even if the Fe content is raised above 1.5 at%
We have investigated the durability of a platinum group metal (PGM-)free (AD)Fe–N–C catalyst in standard accelerated stress test (AST) cycles normally used for low-platinum group metal (PGM) catalysts and during load cycles and have reached the following conclusions:
Summary
Rs T δ σ Ψ Subscripts cefmps catalyst specific area diffusivity Nernst potential current density catalyst loading pressure, Pr = 1 bar mass transfer resistance Ohmic resistance ORR symmetry factor overpotential available sites formation factor concentration cell voltage Faraday constant limiting current density molecular weight gas constant or resistance sheet resistance temperature, Tr = 353 K thickness conductivity active sites cathode electric ionomer film mass transfer pore catalyst surface. H2/O2 between open circuit voltage (OCV) and 0.60 V and by 44% after three polarization cycles in H2/O2 between OCV and 0.20 V.8 Another approach of synthesizing an Fe–N–C catalyst involves heat treating a bimetallic (Fe, Zn) metal organic framework (MOF) to produce the atomically dispersed (AD)Fe–N–C catalyst.[10,11,12,13,14] High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) confirms:[8,15]. Rotating disk electrode (RDE) studies indicate that the ORR activity correlates with (AD)Fe sites, with the Fe clusters acting as spectator sites, and that the activity does not increase even if the Fe content is raised above 1.5 at%.8,15 Recently it has been shown that silica templating can enable higher densities of atomically-dispersed Fe centers.[16]
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