Activity, Stability and Degradation of Carbon Supported Palladium (Pd/C) Fuel Cell Electrocatalysts for the Oxygen Reduction

Abstract

For large-scale commercialization of polymer electrolyte membrane fuel cells (PEMFCs), a significant reduction or, ultimately, replacement of platinum as cathode catalyst for the oxygen reduction reaction (ORR) is of great interest. As a first step, the similarity of palladium and platinum makes Pd an often-discussed alternative material for the electrocatalytic reduction of oxygen. Moreover, apart from Pt, Pd is located on the very top of the well-known volcano plot for the ORR activity of pure metals published by Nørskov et al. (1). However, materials for the oxygen reduction in acidic media have to address both high catalytic activity and long-term stability. Especially voltage cycling stability is an important factor for technical applications. Here, a high tolerance towards dynamic fuel cell operating conditions is required. Although Pd has been known to be less stable against dissolution compared to Pt, e.g., shown in a recent study by Cherevko et al.(2), a comparative study about the effect of Pd particle size is yet outstanding. In this work, a systematic study of both ORR activity and stability against degradation during voltage cycling is conducted on Pd nanoparticles supported on carbon black (Pd/C), on unsupported Pd-black, and on conventional Pt/C as comparison, using commercial catalysts with various particle sizes. First, the activity for the ORR is evaluated in a rotating (ring) disk electrode experiment, indicating a 5-7 fold lower ORR mass activity at 0.85 V vs. the reversible hydrogen electrode (VRHE) of morphologically similar Pd/C compared to Pt/C. However, the ORR mass activity is a function of the initial electrochemically active surface area (ECSA) of the studied Pd catalysts, with a rather strong dependency for small ECSAs and little dependence at higher ones. Accelerated voltage cycling between 0.5 and 1.0 VRHE is conducted in order to correlate catalyst degradation properties with respect to initial ECSA. To monitor the degradation process, the electrochemically active surface area of the Pd catalysts is evaluated frequently during the accelerated voltage cycling. Additional information on the ECSA degradation is gathered by comparing particle size distributions, determined by transmission electron microscopy (TEM), of pristine catalyst material to that of cycled electrodes. Figure 1 shows cyclic voltammograms taken at various times during accelerated voltage cycling of a 40 wt.% Pd/C catalyst. A clear trend towards degradation in both Hupd (<0.4 VRHE) and (hydr-)oxide (>0.55 VRHE) region can be observed. Adversely, currents in the electrochemical double layer region (0.4 VRHE ≤ E ≤ 0.55 VRHE) stay unaffected, indicating a loss of active Pd surface without significant corrosion of the carbon support over the measured timescale. Both the ORR activity and the voltage cycling stability depend on the initial ECSA in opposing directions. In this work, we thus address the question, whether Pd catalysts with initially large particle size may offer substantially better corrosion stability without significant loss in ORR mass activity.