Identifying Activity and Selectivity Trends for the Electro-synthesis of Hydrogen Peroxide via Oxygen Reduction on Nickel-Nitrogen-Carbon Catalysts

Abstract

Hydrogen peroxide (H2O2) is a valuable, environmentally friendly oxidizing agent with a wide range of applications. The on-site electrocatalytic production of H2O2 through the 2-electron ORR would bring the chemical to uses beyond its present reach, but relies upon the availability of cost-effective catalysts with high selectivity, activity, and stability. Herein we report the synthesis and electrocatalytic assessment of inexpensive and earth abundant nickel-nitrogen-carbon (Ni-N-C) electrocatalysts to gain insight into the activity and selectivity for the ORR towards H2O2 using the rotating ring disk electrode (RRDE) technique. We found that the activity and selectivity of the catalysts depended on the amount of nickel added during synthesis as well as the pH of the electrolyte. Based on characterization by various microscopy and spectroscopy techniques, the materials were found to be heterogeneous in nature, and the presence of Ni during catalyst synthesis was identified as imperative for ORR performance in acidic electrolyte but had minimal impact on the performance in alkaline electrolyte. By correlating the results of electrochemical and material characterization, we postulate that atomically dispersed Ni-Nx/C sites are responsible for the ORR performance in acidic electrolytes, with an activity of -0.3 mA cm-2 and H¬2O2 selectivity of 43% measured for the best Ni-N-C catalyst at an electrode potential of 0.5 V vs RHE. In alkaline media, whereby the ORR is a more facile process, Ni-centered active sites, Ni3S2 particles formed during the catalyst synthesis, and nitrogen-doped carbon sites present in the carbon matrix of the catalyst structure were the likely active sites. This work highlights the potential and generates scientific insight into the use of Ni-N-C catalysts for the electrochemical synthesis of H2O2, which will provide guidance towards the design of improved performance metal-nitrogen-carbon catalysts based on inexpensive precursors and simplistic synthesis techniques.