Enabling Aqueous NiO Photocathodes by Passivating Surface Sites That Facilitate Proton-Coupled Charge Transfer

Abstract

Photoelectrodes consisting of wide-band gap metal oxides functionalized with chromophores and catalysts are a potential low-cost solution to generate solar fuels from aqueous solution. NiO is a common photocathode material used in these systems, yet it generally exhibits poor performance in aqueous conditions. Although the poor performance has been attributed to various surface species, the atomistic nature of these traps and role of water in dictating performance has not been resolved. Here, using first-principles calculations, we find that adsorption of water to a defective NiO(111) surface can result in intraband gap surface electronic states that are associated with hydroxyl and oxygen moieties adjacent to Ni vacancies. Electrochemical measurements on mesoporous NiO exhibit surface-localized faradaic events that give rise to a surface capacitance attributable to these states. Data collected in solutions of acetonitrile and water at various ratios, and in water after surface passivation by target atomic deposition (TAD) of Al, yield large shifts in the surface capacitance and in the open-circuit voltage, short-circuit current, and charge transfer resistance of dye-sensitized solar cell (DSSC) devices. Correlations between the measured surface capacitance and DSSC metrics in various solvent ratios and after TAD treatment suggest that surface states facilitate proton-coupled charge transfer with the iodide/triiodide (I − /I 3 − ) electrolyte and are a primary driver of the dark recombination current in devices. TAD prevents this charge transfer, dramatically improving aqueous DSSC characteristics. This work provides a framework to better understand and tune NiO performance for applications that require aqueous solution.

Dye-sensitized solar cells strike back

“…The thereby enabled protoncoupled charge transfer processes are deleterious to the performance of aqueous NiO photocathodes. 789 The first sensitized photocathode for light-driven hydrogen generation was reported by Sun and co-workers. 790 It consisted of a cobaloxime molecular catalyst in solution and an organic triphenylamine-type dye anchored on nanostructured NiO.…”

“…As a result, proton-coupled charge transfer processes affect the performance of aqueous NiO photocathodes. 789 Other photocathode materials such as CuCrO 2 794,795 and CuGaO 2 797 have shown more promise than NiO but their performance is still lacking compared to the photoanode side. Meyer and co-workers used boron-doped Si as the p-type material, protected by a 10 nm Ti layer with an additional 3.0 nm layer of TiO 2 for anchoring of chromophores.…”

“…The thereby enabled proton-coupled charge transfer processes are deleterious to the performance of aqueous NiO photocathodes. 789 …”

“…As a result, proton-coupled charge transfer processes affect the performance of aqueous NiO photocathodes. 789 Other photocathode materials such as CuCrO 2 794,795 and CuGaO 2 797 have shown more promise than NiO but their performance is still lacking compared to the photoanode side.…”

Influence of Surface and Structural Variations in Donor–Acceptor–Donor Sensitizers on Photoelectrocatalytic Water Splitting

“…To probe for charge injection, the chromophores were adsorbed on thin films of a p-type semiconductor, NiO, an n-type semiconductor, SnO 2 |TiO 2 core–shell, and an insulating MO x , Al 2 O 3 . Electrodes were prepared using previously reported methods. − The chromophore layer was loaded by soaking the MO x films in a 0.5 mM solution of BTD1 or BTD2 in dichloromethane for 4 h. Surface coverages (Γ in nmol cm –2 ) of the chromophores on the MO x surfaces were evaluated by UV–visible absorption measurements using previously established methods . The spectra of the dried films were recorded before and after desorption in basic aqueous solutions (pH 13) to obtain the change in absorbance, Δ A .…”