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

Herein, the authors produce Co-based (Co 3 (PO 4 ) 2 , Co 3 O 4 , and Co 9 S 8 ) 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 (PO 4 ) 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, Co 9 S 8 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 ≈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 (PO 4 ) 2 facilitate the OER, while Co(OH) 2 facilitate the HER. Notably, the fabricated two-electrode electrolyzers using Co 3 (PO 4 ) 2 , Co 3 O 4 , and Co 9 S 8 electrocatalysts deliver the cell potentials ≈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.