Critical Role of Phosphorus in Hollow Structures Cobalt-Based Phosphides as Bifunctional Catalysts for Water Splitting.

Cobalt phosphides electrocatalysts have great potential for water splitting, but the unclear active sides hinder the further development of cobalt phosphides. Wherein, three different cobalt phosphides with the same hollow structure morphology (CoP-HS, CoP2 -HS, CoP3 -HS) based on the same sacrificial template of ZIF-67 are prepared. Surprisingly, these cobalt phosphides exhibit similar OER performances but quite different HER performances. The identical OER performance of these CoPx -HS in alkaline solution is attributed to the similar surface reconstruction to CoOOH. CoP-HS exhibits the best catalytic activity for HER among these CoPx -HS in both acidic and alkaline media, originating from the adjusted electronic density of phosphorus to affect absorption-desorption process on H. Moreover, the calculated ΔGH* based on P-sites of CoP-HS follows a quite similar trend with the normalized overpotential and Tafel slope, indicating the important role of P-sites for the HER process. Moreover, CoP-HS displays good performance (cell voltage of 1.67 V at a current density of 50 mA cm-2 ) and high stability in 1 M KOH. For the first time, this work detailly presents the critical role of phosphorus in cobalt-based phosphides for water splitting, which provides the guidance for future investigations on transition metal phosphides from material design to mechanism understanding.