Electrochemical impacts of sheet-like hafnium phosphide and hafnium disulfide catalysts bonded with reduced graphene oxide sheets for bifunctional oxygen reactions in alkaline electrolytes

Non-noble metal-based catalysts with efficient catalytic activities for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are critical for energy conversion devices, including fuel cells and metal–air batteries. In this work, novel hafnium phosphide-reduced graphene oxide nanosheets (HfP-rGO NS) and hafnium disulfide-reduced graphene oxide nanosheets (HfS(2)-rGO NS) were synthesized and investigated as bifunctional electrocatalysts for OER and ORR. The prepared HfP-rGO NS and HfS(2)-rGO NS catalysts showed nanosheet structures, where the HfP or HfS(2) nanosheet was closely packed with rGO. A unique methodology was adopted to lodge the non-metal oxide catalytic sheets (i.e., HfP and HfS(2)) over the rGO sheets, which positioned the oxide layer on the catalytic sheet surface for instant oxygen evolution. Low intensity X-ray diffraction patterns and Raman spectra confirmed the sheet-like structure of HfP-rGO NS and HfS(2)-rGO NS. Scanning electron microscope mapping images revealed that all elements (i.e., Hf, P, C and O for HfP-rGO NS and Hf, S, C and O for HfS(2)-rGO NS) were equally distributed in the synthesized heteroatomic nanosheets. Moreover, both the HfP-rGO NS and HfS(2)-rGO NS demonstrated excellent durability for both ORR and OER. This outperforms the most state-of-the-art non-precious-metal-based bifunctional catalysts, which is attributed to the synergistic effect of rGO and Hf-based catalysts. The different ORR and OER reaction potentials in HfP-rGO NS and HfS(2)-rGO NS likely result from the influence of HfP and HfS(2).