Fuel-cell systems are of interest for a wide range of applications, in part for their utility in power generation from nonfossil-fuel sources. However, the generation of these alternative fuels, through electrochemical means, is a relatively inefficient process due to gas passivation of the electrode surfaces. Uniform microstructured nickel surfaces were prepared by photolithographic techniques as a systematic approach to correlating surface morphologies to their performance in the electrochemically driven oxygen evolution reaction (OER) in alkaline media. Hexagonal arrays of microstructured Ni cylinders were prepared with features of proportional dimensions to the oxygen bubbles generated during the OER process. Recessed and pillared features were investigated relative to planar Ni electrodes for their influence on OER performance and, potentially, bubble release. The arrays of cylindrical recesses were found to exhibit an enhanced OER efficiency relative to planar nickel electrodes. These microstructured electrodes had twice the current density of the planar electrodes at an overpotential of 100 mV. The results of these studies have important implications to guide the preparation of more-efficient fuel generation by water electrolysis and related processes.

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 incorporation of catalytic nanocrystals into macroporous support materials has been very attractive due to their increased catalyst mass activity. This increase in catalytic efficiency is attributed in part to the increased surface area to volume ratio of the catalysts and the use of complementary support materials that can enhance their catalytic activity and stability. A uniform and tunable coating of nanocrystals on porous matrices can be difficult to achieve with some techniques such as electrodeposition. More sophisticated techniques for preparing uniform nanocrystal coatings include atomic layer deposition, but it can be difficult to reproduce these processes at commercial scales required for preparing catalyst materials. In this study, catalytic nanocrystals supported on three dimensional (3D) porous structures were prepared. The demonstrated technique utilized scalable approaches for achieving a uniform surface coverage of catalysts through the use of polymeric sacrificial templates. This template assisted technique was demonstrated with a good control over the surface coverage of catalysts, support material composition, and porosities of the support material. A series of regular porous supports were each prepared with a uniform coating of nanocrystals, such as NaYF4 nanocrystals supported by a porous 3D lattice of Ti1-xSixO2, Pt nanocrystals on a 3D porous support of TiO2, Pd nanocrystals on Ni nanobowls, and Pt nanocrystals on 3D assemblies of Au/TiO2 nanobowls. The template assisted preparation of high surface area macroporous supports could be further utilized for optimizing the use of catalytic materials in chemical, electrochemical, and photochemical reactions through increasing their catalytic efficiency and stability.

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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.