Unraveling the Synergy of Anion Modulation on Co Electrocatalysts by Pulsed Laser for Water Splitting: Intermediate Capturing by In Situ/Operando Raman Studies

Abstract

Herein, the authors produce Co-based (Co-3(PO4)(2), Co3O4, and Co9S8) electrocatalysts via pulsed laser ablation in liquid (PLAL) to explore the synergy of anion modulation on phase-selective active sites in the electrocatalytic hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Co-3(PO4)(2) displays an ultralow overpotential of 230 mV at 10 mA cm(-2) with 48.5 mV dec(-1) Tafel slope that outperforms the state-of-the-art Ir/C in OER due to its high intrinsic activity. Meanwhile, Co9S8 exhibits the highest HER performance known to the authors among the synthesized Co-based catalysts, showing the lowest overpotential of 361 mV at 10 mA cm(-2) with 95.8 mV dec(-1) Tafel slope in the alkaline medium and producing H-2 gas with approximate to 500 mmol g(-1) h(-1) yield rate under -0.45 V versus RHE. The identified surface reactive intermediates over in situ electrochemical-Raman spectroscopy reveal that cobalt(hydr)oxides with higher oxidation states of Co-cation forming under oxidizing potentials on the electrode-electrolyte surface of Co-3(PO4)(2) facilitate the OER, while Co(OH)(2) facilitate the HER. Notably, the fabricated two-electrode electrolyzers using Co-3(PO4)(2), Co3O4, and Co9S8 electrocatalysts deliver the cell potentials approximate to 2.01, 2.11, and 1.89 V, respectively, at 10 mA cm(-2). This work not only shows PLAL-synthesized electrocatalysts as promising candidates for water splitting, but also provides an underlying principle for advanced energy-conversion catalysts and beyond.