The generation of renewable electricity is variable, leading to periodic oversupply. Excess power can be converted to hydrogen via water electrolysis, but the conversion cost is currently too high. One way to decrease the cost of electrolysis is to increase the maximum productivity of electrolyzers. This study investigated how nano-and microstructured porous electrodes could improve the productivity of hydrogen generation in a zero-gap, flow-through alkaline water electrolyzer. Three nickel electrodesfoam, microfiber felt, and nanowire felt-were studied to examine the tradeoff between surface area and pore structure on the performance of alkaline electrolyzers. Although the nanowire felt with the highest surface area initially provided the highest performance, this performance quickly decreased as gas bubbles were trapped within the electrode. The open structure of the foam facilitated bubble removal, but its small surface area limited its maximum performance. The microfiber felt exhibited the best performance because it balanced high surface area with the ability to remove bubbles. The microfiber felt maintained a maximum current density of 25,000 mA cm -2 over 100 hrs without degradation, which corresponds to a hydrogen production rate 12.5-and 50-times greater than conventional proton-exchange membrane and alkaline electrolyzers, respectively.Flow-through alkaline electrolysis with a Ni microfiber felt improved the maximum H 2 production rate by 50 times relative to conventional alkaline electrolyzers. Compared to Ni-Cu nanowires and Ni foam, Ni microfiber felt provided the most surface area for water splitting without blocking the removal of gas bubbles.
Control over the shape of a metal nanostructure grants control over its properties, but the processes that cause solution-phase anisotropic growth of metal nanostructures are not fully understood. This article shows why the addition of a small amount (75–100 μM) of iodide ions to a Cu nanowire synthesis results in the formation of Cu microplates. Microplates are 100 nm thick and micronwide crystals that are thought to grow through atomic addition to {100} facets on their sides instead of the {111} facets on their top and bottom surfaces. Single-crystal electrochemical measurements show that the addition of iodide ions decreased the rate of Cu addition to Cu(111) by 8.2 times due to the replacement of adsorbed chloride by iodide. At the same time, the addition of iodide ions increased the rate of Cu addition to Cu(100) by 4.0 times due to the replacement of a hexadecylamine (HDA) self-assembled monolayer with the adsorbed iodide. The activation of {100} facets and passivation of {111} facets with increasing iodide ion concentration correlated with an increasing yield of microplates. Ab initio thermodynamics calculations show that, under the experimental conditions, a minority of iodide ions replaces an overwhelming majority of chloride and HDA on both Cu(100) and Cu(111). While Cu nanowire formation is predicted (and observed) in solutions containing chloride and HDA, the calculations indicate that a strong thermodynamic driving force occurs for {111} facet (and microplate) growth when a small amount of iodide is present, consistent with the experiment.
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