Understanding the Performance of NiO Photocathodes with Alkyl-Derivatized Cobalt Catalysts and a Push-Pull Dye.

Kelly L Materna,Anders Thapper,Anna M Beiler,Haining Tian,Leif Hammarström,Sascha Ott

doi:10.1021/acsami.0c05228

Abstract

Mesoporous NiO photocathodes containing the push–pull dye PB6 and alkyl-derivatized cobaloxime catalysts were prepared using surface amide couplings and analyzed for photocatalytic proton reduction catalysis. The length of the alkyl linker used to derivatize the cobalt catalysts was found to correlate to the photocurrent with the highest photocurrent observed using shorter alkyl linkers but the lowest one for samples without linker. The alkyl linkers were also helpful in slowing dye–NiO charge recombination. Photoelectrochemical measurements and femtosecond transient absorption spectroscopic measurements suggested electron transfer to the surface-immobilized catalysts occurred; however, H2 evolution was not observed. Based on UV–vis, X-ray fluorescence spectroscopy (XRF), and X-ray photoelectron spectroscopy (XPS) measurements, the cobalt catalyst appeared to be limiting the photocathode performance mainly via cobalt demetallation from the oxime ligand. This study highlights the need for a deeper understanding of the effect of catalyst molecular design on photocathode performance.

Highlights

Since climate change is actively occurring, fossil fuels must be replaced by clean, renewable fuels.[1]
Photocathodes in WS-DSPECs contain both molecular dyes to harvest sunlight and molecular proton reduction catalysts to perform hydrogen evolution catalysis.[7−9] Both molecules are immobilized on a semiconductor electrode, commonly by a covalent linkage using surface anchoring groups, which are already attached to the molecules
Cobalt loss is still observed when no linker is present on the surface post CA (NiO-PB6-Al2O3-Co) with the majority of the oxime ligand remaining on the electrode; this further suggests that cobalt loss does not result from the use of alkyl linkers but is an inherent instability when surfacebound to the photocathode under photoelectrochemical conditions (Figure S16)

Summary

■ INTRODUCTION

Since climate change is actively occurring, fossil fuels must be replaced by clean, renewable fuels.[1]. Cobalt loss is still observed when no linker is present on the surface post CA (NiO-PB6-Al2O3-Co) with the majority of the oxime ligand remaining on the electrode; this further suggests that cobalt loss does not result from the use of alkyl linkers but is an inherent instability when surfacebound to the photocathode under photoelectrochemical conditions (Figure S16). On the photocathodes, Co is in a less rigid or contained environment, being attached to the metal oxide surface via carboxylic acid anchoring groups on the oxime ligand or alkyl linkers In this architecture, Co likely has greater exposure to the surrounding electrolyte than in the enclosed MOF structure, providing an easier way for the cobalt to demetallate. Cobalt metal center is removed from the ligand, charge accumulation will not occur at the catalyst since it no longer remains intact, which lowers the overall performance of these electrodes

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

Findings

■ REFERENCES