Hydrotalcite-wrapped Co–B alloy with enhanced oxygen evolution activity

Rapid and Energetic Solid-State Metathesis Reactions for Iron, Cobalt, and Nickel Boride Formation and Their Investigation as Bifunctional Water Splitting Electrocatalysts

Metal borides have long-standing uses due to their desirable chemical and physical properties such as high melting points, hardness, electrical conductivity, and chemical stability. Typical metal boride preparations utilize high-energy and/or slow thermal heating processes. This report details a facile, solvent-free single-step synthesis of several crystalline metal monoborides containing earth-abundant transition metals. Rapid and exothermic self-propagating solid-state metathesis (SSM) reactions between metal halides and MgB₂ form crystalline FeB, CoB, and NiB in seconds without sustained external heating and with high isolated product yields (∼80%). The metal borides are formed using a well-studied MgB₂ precursor and compared to reactions using separate Mg and B reactants, which also produce self-propagating reactions and form crystalline metal borides. These SSM reactions are sufficiently exothermic to theoretically raise reaction temperatures to the boiling point of the MgCl₂ byproduct (1412 °C). The chemically robust monoborides were examined for their ability to perform electrocatalytic water oxidation and reduction. Crystalline CoB and NiB embedded on carbon wax electrodes exhibit moderate and stable bifunctional electrocatalytic water splitting activity, while FeB only shows appreciable hydrogen evolution activity. Analysis of catalyst particles after extended electrocatalytic experiments shows that the bulk crystalline metal borides remain intact during electrochemical water-splitting reactions though surface oxygen species may impact electrocatalytic activity.

Recent Progress on Surface Reconstruction of Earth-Abundant Electrocatalysts for Water Oxidation

As one important electrode reaction in electrocatalytic and photoelectrochemical cells for renewable energy circulation, oxygen catalysis has attracted considerable research in developing efficient and cost-effective catalysts. Due to the inevitable formation of oxygenic intermediates on surface sites during the complex reaction steps, the surface structure dynamically evolves toward reaction-preferred active species. To date, transition metal compounds, here defined as TM-Xides, where "X" refers to typical nonmetal elements from group IIIA to VIA, including hydroxide as well, are reported as high-performance oxygen evolution reaction (OER) electrocatalysts. However, more studies observe at least exterior oxidation or amorphization of materials. Thus, whether the TM-Xides can be defined as OER catalysts deserves further discussion. This Review pays attention to recent progress on the surface reconstruction of TM-Xide OER electrocatalysts with an emphasis on the identification of the true active species for OER, and aims at disseminating the real contributors of OER performance, especially under long-duration electrocatalysis.

Non-Precious Electrodes for Practical Alkaline Water Electrolysis

Water electrolysis is a promising approach to hydrogen production from renewable energy sources. Alkaline water electrolyzers allow using non-noble and low-cost materials. An analysis of common assumptions and experimental conditions (low concentrations, low temperature, low current densities, and short-term experiments) found in the literature is reported. The steps to estimate the reaction overpotentials for hydrogen and oxygen reactions are reported and discussed. The results of some of the most investigated electrocatalysts, namely from the iron group elements (iron, nickel, and cobalt) and chromium are reported. Past findings and recent progress in the development of efficient anode and cathode materials appropriate for large-scale water electrolysis are presented. The experimental work is done involving the direct-current electrolysis of highly concentrated potassium hydroxide solutions at temperatures between 30 and 100 °C, which are closer to industrial applications than what is usually found in literature. Stable cell components and a good performance was achieved using Raney nickel as a cathode and stainless steel 316L as an anode by means of a monopolar cell at 75 °C, which ran for one month at 300 mA cm⁻². Finally, the proposed catalysts showed a total kinetic overpotential of about 550 mV at 75 °C and 1 A cm⁻².