Electrocatalyst Design for Direct Borohydride Oxidation Guided by First Principles

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• 作者：Gholamreza Rostamikia ; Romesh J. Patel ; Irene Merino-Jimenez ; Michael Hickner ; Michael J. Janik
• 刊名：Journal of Physical Chemistry C
• 出版年：2017
• 出版时间：February 9, 2017
• 年：2017
• 卷：121
• 期：5
• 页码：2872-2881
• 全文大小：543K
• ISSN：1932-7455

文摘

Density functional theory (DFT) calculations are used to propose a Au–Cu binary metal catalyst for the electrochemical borohydride oxidation reaction (BOR), which is evaluated experimentally and observed to show enhanced oxidation activity relative to a pure Au electrode. Our previous work has applied DFT methods to determine the BOR mechanism and elucidate the key reaction steps that dictate catalyst activity and selectivity to complete oxidation. A balanced initial adsorption strength of the borohydride anion is essential for an active and selective catalyst. Adsorption must be strong enough to provide a reasonable coverage of surface species and promote B–H bond dissociation but not so strong as to promote easy dissociation and provide a high coverage of surface H atoms that result in H₂ evolution. Borohydride adsorption energetics were evaluated for a series of close-packed pure metal surfaces. Copper catalysts appear encouraging but are not electrochemically stable under reaction conditions. Gold–copper alloys are predicted to show increased activity compared to a pure gold electrode while maintaining the selectivity to direct oxidation and increasing the stability compared to pure Cu. DFT results suggest an approximately 0.2 V decrease in the overpotential for borohydride oxidation on a Au₂Cu(111) electrode compared to that on a Au(111) electrode. This DFT-predicted reduction in overpotential is realized experimentally. Electrodeposition was used to prepare AuCu electrodes, and their borohydride oxidation electrokinetics were examined by linear sweep voltammetry. An 88.5% gold and 11.5% copper sample demonstrated an overpotential reduction of 0.17 V compared to a pure Au electrode. The binding energy and adsorption free energy of BH₄^– over other surface alloys are also examined to further identify promising BOR electrodes.