(Invited) Activity and Stability of Atomically Dispersed (AD) Fe-C-N ORR Catalyst in Polymer Electrolyte Fuel Cell Environment

Abstract

Activity and stability of (AD)Fe-N-C catalyst with low Fe content (1.5 at%) have been investigated in a polymer electrolyte fuel cell environment. Previous electron microscopy analyses (SEM, EDS and EELS) have revealed the presence of atomically dispersed Fe atoms coordinated with nitrogen (Fe-Nx) and a partially graphitized microporous carbon structure. In this study, 5-cm2 membrane electrode assemblies (MEAs) were prepared by hot pressing anode gas diffusion electrodes (GDEs, 0.2 mgPt/cm2) to N211 membrane, and brush painting the PGM-free cathode catalyst ink; Sigracet SG29BC GDL was used on cathode side without hot pressing. All performance tests were conducted under differential cell conditions with fixed H2, O2 and air flow rates. Polarization curves obtained in H2/O2 show double Tafel slopes which, in conjunction with the position of the redox potential observed in cyclic voltammetry traces, forms the basis for formulating a distributed ORR (oxygen reduction reaction) kinetic model with potential-dependent available sites and extracting reaction order for P(O2) and relative humidity (RH) dependence and activation energy for temperature dependence. Application of this model to polarization data collected in H2/air provides the basis for formulating the oxygen transport model and leads to 7.7±0.6 mA/cm2 modeled catalyst activity (defined as areal current density at 0.9 V IR-corrected cell voltage in H2/O2,1.5 atm, 80oC, and 100% RH), 31.5-34.3 mA/cm2 cell performance at 1.5 atm, 80oC, 100% RH, and 0.8-2 s/cm O2 transport resistance (Rcf) in the cathode catalyst layer (CCL). The derived value of Rcf for ~100-μm thick PGM-free electrode is 2-3 times higher than the measured Rcf for <10-μm thick low-Pt loaded (<0.1 mgPt/cm2) electrodes. As such, Rcf includes resistance to O2 transport in the CCL secondary pores, and the primary pores and ionomer film surrounding the Fe-Nx active sites within the microporous carbon support. Catalyst durability was characterized in accelerated stress tests (AST) with 0.6-0.95 V trapezoid potential cycles in H2/N2. Although the (AD)Fe-N-C catalyst has low initial activity, it exhibits excellent stability in ASTs considering that the measured polarization curves in H2/air show voltage losses of only 5, 15 and 30 mV after 10k, 20k and 30k cycles, respectively, for an average degradation rate of 3.5 mV/h. The voltage losses are nearly independent of the current density, suggesting that the observed performance degradation is mainly due to loss in catalyst activity and the transport resistance is nearly unaffected by AST. The kinetic model indicates a 50% loss in catalyst activity after 30k AST cycles. We observe similar degradation rate on a load cycle (LC) consisting of H2/air polarization curves under different operating pressures (1-2.5 atm), temperatures (60-95oC), O2 mole fraction (2-21%), and relative humidity (30-100%). The degradation rate is faster under a combined AST and LC, with majority of losses occurring in the load cycles. Figure 1 summarizes the modeled loss in active sites for load cycles in H2/O2 (cell 1), load cycles in H2/air (cell 2), AST cycles in H2/N2 (cell 3), and combined AST and load cycles (cell 4). The model indicates <20% increase in the O2 transport resistance in the (AD)Fe-N-C catalyst layer with >75% loss of inactive sites. We have used the coupled kinetic, O2 transport and stability model to project the improvements needed in catalyst activity and electrode structure to approach the automotive target of 1000 mW/cm2 stack power density while meeting the 1.45 kW/oC heat rejection (Q/ΔT) constraint at 2.5 atm inlet pressure, 95oC coolant exit temperature, and 1.5 cathode stoichiometry. The model indicates that we need twelve-fold higher mass activity for reducing the kinetic losses, doubling of active site density and an engineered electrode structure for 50% lower sheet resistance, as well as a 50% reduction in electrode thickness to limit the O2 transport losses. Such an automotive competitive PGM-free catalyst will exceed 22.6 mA/mg mass activity for >43.3 mA/cm2 specific activity at 1.9 mg/cm2 catalyst loading in an electrode structure that is <50-μm thick and engineered for <50 mΩ.cm2 sheet resistance. It will reach 1150 mA/cm2 current density at 656 mV for 750 mW/cm2 power density at Q/ΔT relevant operating conditions, after incurring voltage losses of 405 mV for ORR kinetics, 70 mV for combined proton transport in membrane and CCL, and 35 mV for O2 transport across the gas channel, GDL and CCL. Figure 1