Supported Transition Metal Phosphides: Activity Survey for HER, ORR, OER, and Corrosion Resistance in Acid and Alkaline Electrolytes

Carbon-supported MxPy (M = Ni, Co, W, Cr, and Mo) were prepared via pyrolysis using a very simple and scalable method utilizing nontoxic metal and phosphorus precursors. The electrochemical hydrogen evolution (HER), oxygen reduction (ORR), and oxygen evolution (OER) reactions and corrosion resistance under both acidic and alkaline conditions were examined for all these catalysts and compared to those for the benchmark catalysts Pt/C (HER/ORR) and IrO2 (OER). The highest activities were found in alkaline solutions for Co2P for HER and ORR and Ni2P for OER. Good activity was also found in acid for some of these reactions, although the catalysts suffered from susceptibility to corrosion. Co2P was further studied in an alkaline environment, as it shows high catalytic activity toward the oxygen reduction reaction (ORR) without significant hysteresis. The onset potential (at 0.5 mA cm–2) obtained was 0.8 V vs RHE, and a Tafel slope value of 38 mV dec–1 was found with a maximum kinetic mass activity of 2870 A gCo–1 at 0.7 V vs RHE. Utilizing high-resolution transmission electron microscopy, it is possible to observe high-surface-area needle-like single-crystal cobalt oxide structures on the surface of the Co2P particles at the beginning of the ORR. Hence the high rates of initial corrosion of the Co2P appear to be associated with the dissolution and precipitation of cobalt oxide on the particle surface. The as-synthesized Co2P/C also shows good performance in an 8-h stability test for the OER, carried out at 1.6 V vs RHE in alkaline conditions, with negligible drop in current density over time. Interestingly, in an acidic environment the catalyst is very active toward two-electron oxygen reduction, leading to H2O2 with high selectivity (85%). It is intriguing that the pH dependence of this catalyst toward the ORR is similar to that seen for gold.