Gas management during electrocatalytic water splitting is vital for improving the efficiency of clean hydrogen production. The accumulation of gas bubbles on electrode surfaces prevents electrolyte access and passivates the electrochemically active surface area. Electrode morphologies are sought to assist in the removal of gas from surfaces to achieve higher reaction rates at operational voltages. Herein, regular arrays of linear ridges with specific microscale separations were systematically studied and correlated to the performance of the oxygen evolution reaction (OER). The dimensions of the linear ridges were proportional to the size of the oxygen bubbles, and the mass transfer processes associated with gas evolution at these ridges were monitored using a high-speed camera. Characterization of the adhered bubbles prior to detachment enabled the use of empirical methods to determine the volumetric flux of product gas and the bubble residence times. The linear ridges promoted a self-cleaning effect as one bubble would induce neighboring bubbles to simultaneously release from the electrode surfaces. The linear ridges also provided preferential bubble growth sites, which expedited the detachment of bubbles with similar diameters and shorter residence times. The linear ridges enhanced the OER in comparison to planar electrodes prepared by electrodeposition from the same high-purity nickel (Ni). Linear ridges with a separation distance of 200 μm achieved nearly a 2-fold increase in current density relative to the planar electrode at an operating voltage of 1.8 V (vs Hg/HgO). The electrodes with linear ridges having a separation distance of 200 μm also had the highest sustained current densities over a range of operating conditions for the OER. Self-cleaning surface morphologies could benefit a variety of electrocatalytic gas evolving reactions by improving the efficiency of these processes.

The development of molecular catalysts and materials that can convert carbon dioxide (CO2) into a value-added product is a great chemical challenge. Molecular catalysts set benchmarks in catalyst investigation and design, but their incorporation into solid-state materials, and optimization of the electrochemical operating conditions, is still needed. For example, rhenium­(I) diimine catalysts show almost quantitative selectivity for the conversion of CO2 to carbon monoxide (CO) in acetonitrile (MeCN), but the modification of diimine backbones can be challenging if the goal is to incorporate such molecules into materials. Presented here is a rhenium­(I) complex with a 2-(2′-quinolyl)­benzimidazole (QuBIm-H) ligand, where N-alkylation with a pyrene derivative allows access to a catalyst that can be adsorbed onto electrodes for aqueous CO2 reduction chemistry. The rhenium­(I) catalysts are inactive for homogeneous CO2 reduction in MeCN. However, when adsorbed on edge-plane graphite, the same complexes show good activity for heterogeneous aqueous CO2 reduction, with 90% selectivity for CO. Comparative electrochemical studies between covalent and noncovalent modification of the graphite surfaces were also carried out for related rhenium­(I) tricarbonyl complexes.

The development of catalysts that can convert carbon dioxide (CO 2 ) to useful reduced products is a pressing and ongoing challenge. Refinement of the designs of molecular electrocatalysts is of great interest, especially for mesotetraarylmetalloporphyrins. Iron porphyrins with hydroxyphenyl groups situated near the active site are good electrocatalysts, and herein, we systematically explore how the position and number of meso-2,6-dihydroxyphenyl groups on iron porphyrins influences CO 2 -to-CO conversion. A series of five iron porphyrins with 2,6-dihydroxyphenyl groups systematically placed at the 5, 10, 15, and 20 porphyrin positions were prepared. The isomer with 5,15-bis(2,6-dihydroxyphenyl) substitution was a superior catalyst for CO 2 reduction electrocatalysis in N,Ndimethylformamide solvent. To our surprise, the previously reported tetrakis(2,6dihydroxyphenyl)porphyrin iron complex was not the best performing catalyst. We use density functional calculations to explore the factors that distinguish each of the catalysts and show how calculated Fe−C vibrational frequencies are related to observed electrochemical properties and catalyst kinetics. A corresponding analysis of optical spectra and relative reduction potentials illustrates the relationship between the placement of dihydroxyphenyl groups and catalyst performance. We conclude that substituents at the 5 and 15 positions are best at improving catalyst performance, leaving other parts of porphyrin macrocycles open for other modifications.

A graphite-adsorbed tricarbonylrhenium(i) terpyridine complex supports CO2 reduction electrocatalysis over a wide range of pH values.

Electrocatalytic water splitting on an industrial-scale for chemical energy storage through hydrogen gas production will require further improvements to the other half-reaction, the oxygen evolution reaction (OER). This half-reaction has a high kinetic barrier due to superoxide bond formation, and four proton-coupled electron transfer steps. Considerable attention has focused on improving the reaction kinetics of the OER by tuning the electronic structure of electrocatalysts. Gas management is, however, still a problematic factor for industrial-scale water splitting and especially for highly active electrocatalysts. Accumulation of bubbles on electrode surfaces can result in blocked active sites and increased resistances. Recent literature has pointed to the importance of the architecture of the electrode surfaces for their influence on effectively releasing bubbles without the need for additional energy input (e.g., high shear flows or higher overpotentials). Electrodes with nanoscale textures have demonstrated superaerophobic qualities with a high degree of wetting during electrocatalytic gas evolving reactions. Higher reactions rates can also be achieved using microscale geometries that can assist in the removal of gas from surfaces. These lessons can be applied to a variety of gas evolving reactions, but are of particular interest here for the OER. Herein, we prepared regular arrays of linear ridges with well-defined dimensions and microscale separations between these features. The influence of feature spacing was systematically studied for a series of electrodes and correlated to the performance of the OER. The mass transfer processes associated with the gas evolution were investigated using a high-speed camera. Characterization of the adhered bubbles enabled the use of empirical methods to determine the volumetric flux of product gas, and the bubble residence times. The linear ridges promoted a self-cleaning effect as one bubble would induce neighbouring bubbles to simultaneously release from the electrode surfaces. The ridges provided preferential bubble growth sites and expedited a synchronous detachment of bubbles with similar diameters. Linear ridges with a separation distance of 200 µm achieved nearly a twofold increase in current density relative to the planar electrode at high operational potentials. Comparison of this series of electrodes with their tuned spacing of the microscale, linear features provided a systematic correlation between feature separation and their gas evolution efficiency.