Hui Li1 Peng Wen2 Dominique Itanze1 Shiba Adhikari3 Chang Lu1 Lin Jiang4 Pamela Lundin5 Yejun Qiu2 Scott Geyer1

1, Wake Forest University, Winston-Salem, North Carolina, United States
2, Harbin Institute of Technology, Shenzhen, Guangdong, China
3, Oak Ridge National Laboratory (ORNL), Oak Ridge, Tennessee, United States
4, Soochow University, Suzhou, Jiangsu, China
5, High Point University, Highpoint, North Carolina, United States

Developing earth-abundant and efficient electrocatalysts for photoelectrochemical water splitting is critical to realize high performance solar-to-hydrogen energy conversion process. Herein, we report a novel electrocatalyst of colloidal cobalt phosphide nanocrystals (CoP2 NCs) by a modified “hot injection” method. Benefiting from the high phosphide content, size-uniformity, and appreciable electronic conductivity, the CoP2 NCs exhibit superior Pt-like hydrogen evolution reaction (HER) electrocatalytic activity in acidic solution with a small overpotential of 39 mV to achieve -10 mA cm-2, and also demonstrates long-term stability with negligible activity degradation within 36 h at an overpotential of 100 mV. Theoretical DFT calculation results reveal that the free energy of hydrogen adsorption on highly exposed P-rich (211) surface of CoP2 is close to zero, leading to the desirable balance between hydrogen adsorption and desorption for Pt-like HER behavior. Further integration of the colloidal CoP2 NCs with an atomic layer deposition (ALD) protective planar p-Si electrode to form a hybrid p-Si/20 nm AZO/10 nm TiO2/CoP2 photocathode. Under simulated solar illumination (AM-1.5G, 100 mW/cm2), an onset potential of photocurrent is observed at as positive as 0.48 V vs. RHE on this hybrid photocathode. Notably, the as-deposited p-Si/20 nm AZO/10 nm TiO2/CoP2 photocathode shows a remarkable photocurrent density of -16.7 mA cm-2 at the reversible hydrogen potential (0 V vs. RHE), much higher than that of p-Si/20 nm AZO/10 nm TiO2 without cocatalyst decoration (-5.62 mA cm-2), and also exhibits excellent durability in acidic solution. The high performance and stability are ascribed to the intimate junction between p-Si and n-type AZO for fast interfacial electron transfer and high-photovoltage output, good corrosion-resistance of pinhole-free ultrathin TiO2 protective layer, and favorable Fermi level position and fast HER kinetics of CoP2 cocatalyst.