Full Water Splitting Electrolyzed by Cu-Co Bimetallic Phosphides

Forming complex structures of functional materials in a controlled and reproducible fashion is a well-known challenge. Specifically, bimetallic phosphides are of interest as electrocatalysts for energy-related applications; however, a satisfactory structure-function relationship has not yet been fully deciphered yet. Here, we show that a colloidal chemistry approach produces bimetallic phosphides of Co and Cu, where segregation and phase transformation induce significant changes in morphology compared with solid solutions. Their complexity permits tuning of the catalytic sites to the hydrogen and oxygen evolution reactions (HER and OER), allowing the bimetallic phosphides to catalyze the full water-splitting reaction. The experimental results show that water cleavage, H-OH + e^- → H* + OH (Volmer), is particularly favorable on Cu_xCo_yP catalysts (and especially when x = 50%), enhancing their HER performance. As for the OER enhancement, the results show that the bimetallic phosphides undergo a surface transformation during the OER, whereby (oxy)hydroxides form at anodic potentials in alkaline solutions and serve as the actual electrocatalysts. Thus, the use of Cu_xCo_y phosphides as starting materials can lead to optimized oxide growth. In summary, the higher catalytic activity of the bimetallic phosphides is attributed to their altered morphology, surface area, adsorption sites, and valence state, which also make them efficient bifunctional electrocatalysts for the overall water-splitting reaction.