Amorphous outperforms crystalline nanomaterials: surface modifications of molecularly derived CoP electro(pre)catalysts for efficient water-splitting

Abstract

The single source precursor (SSP) approach was used to prepare highly active CoP bifunctional electro(pre)catalysts for the oxygen evolution reaction (OER), hydrogen evolution reaction (HER) and overall water splitting (OWS) reaction starting from a molecular β-diketiminato Co(I) cyclo-P₄ complex. Crystalline or amorphous CoP particles were attained depending on the preparation route. Notably, the amorphous CoP displayed higher activity compared to the crystalline CoP on nickel foam (NF) and fluorinated tin oxide (FTO) substrates due to its unique electronic properties and surface characteristics. During the OER, severe oxidation to Co-oxy(hydroxides)/oxides by the loss of P was found to be crucial to increase the concentration of CoO_x active sites. Interestingly, complete leaching of surface P from CoP and surface Co enrichment occurred during the HER. Finally, an OWS device was fabricated where the amorphous CoP outperformed the crystalline CoP with respect to low OWS cell voltage (with a difference of 130 mV) and enhanced stability for 5 days.