We report here the non-catalytic benefits of Ni(II) ion binding to a Si|PNP type surface for PEC-HER, caused by band-bending induced by ligand attachment and Ni(II) binding, rather than decreasing the catalytic overpotential. We deconvolute the ("thermodynamic") flat band potentials (VFB) from the kinetic onset potentials (Von). A bis-bidentate (PNP)2 macrochelate enables one-step Ni(II) binding to p-Si(111). Illuminated J–V measurements under catalytic conditions show that Si|BisPNP-Ni exhibits the most positive onset potential for HER (−0.55 V vs Fc/Fc+). Interestingly, thermodynamic flat band potential measurements in the dark reveal that Si|BisPNP-Ni also exhibits the most positive VFB value (−0.02 V vs Fc/Fc+). Electrochemical impedance spectroscopy data generated under illuminated, catalytic conditions demonstrate a surprising lack of correlation evident between Von and equivalent circuit element parameters commonly associated with HER. Overall, the extent of band-bending induced by the metal ion binding is the primary driver of photoelectrochemical (PEC)-HER benefits, while the kinetic (catalytic) effects of the PNP-Ni(II) are minimal. This suggests that dipole and band-edge engineering must be a primary design consideration (not secondary to catalyst efficiency) in semiconductor|catalyst hybrids for PEC-HER.

Importance of band bending (thermodynamics) over catalyst efficiency (kinetics) in silicon based PEC-HER

ACS Fall 2021

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