Studying the cation dependence of CO2 reduction intermediates at Cu by in situ VSFG spectroscopy.

The nature of the electrolyte cation is known to have a significant impact on electrochemical reduction of CO2 at catalyst|electrolyte interfaces. An understanding of the underlying mechanism responsible for catalytic enhancement as the alkali metal cation group is descended is key to guide catalyst development. Here, we use in situ vibrational sum frequency generation (VSFG) spectroscopy to monitor changes in the binding modes of the CO intermediate at the electrochemical interface of a polycrystalline Cu electrode during CO2 reduction as the electrolyte cation is varied. A CObridge mode is observed only when using Cs+, a cation that is known to facilitate CO2 reduction on Cu, supporting the proposed involvement of CObridge sites in CO coupling mechanisms during CO2 reduction. Ex situ measurements show that the cation dependent CObridge modes correlate with morphological changes of the Cu surface.

Monitoring interfacial electric fields at a hematite electrode during water oxidation.

To understand the mechanisms of water oxidation on materials such as hematite it is important that accurate measurements and models of the interfacial fields at the semiconductor liquid junction are developed. Here we demonstrate how electric field induced second harmonic generation (EFISHG) spectroscopy can be used to monitor the electric field across the space-charge and Helmholtz layers in a hematite electrode during water oxidation. We are able to identify the occurrence of Fermi level pinning at specific applied potentials which lead to a change in the Helmholtz potential. Through combined electrochemical and optical measurements we correlate these to the presence of surface trap states and the accumulation of holes (h+) during electrocatalysis. Despite the change in Helmholtz potential as h+ accumulate we find that a population model can be used to fit the electrocatalytic water oxidation kinetics with a transition between a first and third order regime with respect to hole concentration. Within these two regimes there are no changes in the rate constants for water oxidation, indicating that the rate determining step under these conditions does not involve electron/ion transfer, in-line with it being O-O bond formation.

Hybrid photocathode based on a Ni molecular catalyst and Sb2Se3 for solar H2 production.

We report a H2 evolving hybrid photocathode based on Sb2Se3 and a precious metal free molecular catalyst. Through the use of a high surface area TiO2 scaffold, we successfully increased the Ni molecular catalyst loading from 7.08 ± 0.43 to 45.76 ± 0.81 nmol cm-2, achieving photocurrents of 1.3 mA cm-2 at 0 V vs. RHE, which is 81-fold higher than the device without the TiO2 mesoporous layer.

Strong Impact of Intramolecular Hydrogen Bonding on the Cathodic Path of [Re(3,3'-dihydroxy-2,2'-bipyridine)(CO)3Cl] and Catalytic Reduction of Carbon Dioxide.

Herein, we present the cathodic paths of the Group-7 metal complex [Re(3,3'-DHBPY)(CO)3Cl] (3,3'-DHBPY = 3,3'-dihydroxy-2,2'-bipyridine) producing a moderately active catalyst of electrochemical reduction of CO2 to CO. The combined techniques of cyclic voltammetry and IR/UV-vis spectroelectrochemistry have revealed significant differences in the chemistry of the electrochemically reduced parent complex compared to the previously published Re/4,4'-DHBPY congener. The initial irreversible cathodic step in weakly coordinating THF is shifted toward much less negative electrode potentials, reflecting facile reductive deprotonation of one hydroxyl group and strong intramolecular hydrogen bonding, O-H···O-. The latter process occurs spontaneously in basic dimethylformamide where Re/4,4'-DHBPY remains stable. The subsequent reduction of singly deprotonated [Re(3,3'-DHBPY-H+)(CO)3Cl]- under ambient conditions occurs at a cathodic potential close to that of the Re/4,4'-DHBPY-H+ derivative. However, for the stabilized 3,3'-DHBPY-H+ ligand, the latter process at the second cathodic wave is more complex and involves an overall transfer of three electrons. Rapid potential step electrolysis induces 1e--reductive cleavage of the second O-H bond, triggering dissociation of the Cl- ligand from [Re(3,3'-DHBPY-2H+)(CO)3Cl]2-. The ultimate product of the second cathodic step in THF was identified as 5-coordinate [Re(3,3'-DHBPY-2H+)(CO)3]3-, the equivalent of classical 2e--reduced [Re(BPY)(CO)3]-. Each reductive deprotonation of the DHBPY ligand results in a redshift of the IR ν(CO) absorption of the tricarbonyl complexes by ca. 10 cm-1, facilitating the product assignment based on comparison with the literature data for corresponding Re/BPY complexes. The Cl- dissociation from [Re(3,3'-DHBPY-2H+)(CO)3Cl]2- was proven in strongly coordinating butyronitrile. The latter dianion is stable at 223 K, converting at 258 K to 6-coordinate [Re(3,3'-DHBPY-2H+)(CO)3(PrCN)]3-. Useful reference data were obtained with substituted parent [Re(3,3'-DHBPY)(CO)3(PrCN)]+ that also smoothly deprotonates by the initial reduction to [Re(3,3'-DHBPY-H+)(CO)3(PrCN)]. The latter complex ultimately converts at the second cathodic wave to [Re(3,3'-DHBPY-2H+)(CO)3(PrCN)]3- via a counterintuitive ETC step generating the 1e- radical of the parent complex, viz., [Re(3,3'-DHBPY)(CO)3(PrCN)]. The same alternative reduction path is also followed by [Re(3,3'-DHBPY-H+)(CO)3Cl]- at the onset of the second cathodic wave, where the ETC step results in the intermediate [Re(3,3'-DHBPY)(CO)3Cl]•- further reducible to [Re(3,3'-DHBPY-2H+)(CO)3]3- as the CO2 catalyst.