Electrocatalyst Design for Direct Borohydride Oxidation Guided by First Principles

Abstract

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.

Microhydration of BH4–: Dihydrogen Bonds, Structure, Stability, and Raman Spectra

“…Dihydrogen bonding has been shown to play an important role in the stabilization of this hydrogen storage system . Hydrolysis is an important pathway to liberate H 2 from NaBH 4 , and dihydrogen bonds also contribute notably to NaBH 4 hydrolysis. ,− Direct borohydride fuel cell (DBFC) technology is being actively investigated by a number of research groups. , Further progress has suffered from a lack of research on the poor understanding of the complex electrode mechanism and electrode kinetics of the BH 4 – oxidation. Previous studies show that the solvation structure of the electrolyte is critical to solute electrode oxidation in solution. , A comprehensive, fundamental understanding of molecular-level BH 4 – hydration, and especially the dihydrogen bonding therein, may shed new light on DBFC engineering.…”

“…1,12−15 Direct borohydride fuel cell (DBFC) technology is being actively investigated by a number of research groups. 16,17 Further progress has suffered from a lack of research on the poor understanding of the complex electrode mechanism and electrode kinetics of the BH 4 − oxidation. Previous studies show that the solvation structure of the electrolyte is critical to solute electrode oxidation in solution.…”

Development of Bimetallic PdNi Electrocatalysts toward Mitigation of Catalyst Poisoning in Direct Borohydride Fuel Cells

“…High anode potential favors BOR activity unless the electrocatalyst is poisoned by reaction intermediates [H*, OH*, BH x * ( x = 1–4), BH x O y * ( x = 1–4, y = 1–3), and BO y * ( y = 1–2)] or passivated by metal oxides/hydroxides/oxyhydroxides. , d-block metals such as Pt, Pd, and Ir strongly promote the dissociative adsorption of BH 4 – , leading to the formation of borane (BH x ) and hydroxyborane (BH x O y ) intermediates on the surface of the electrocatalyst. , However, the aforementioned metals bind with hydrogen moderately (Δ G H* = ( G H* – G * –0.5 G H2 ∼ 0.0 eV (* = active site)), leading to a parallel hydrolysis reaction with remarkably high rates. ,,, The high rate of the hydrolysis reaction results in the reduction of the selectivity of the BOR and the Faradaic efficiency of the DBFC . Coinage metals such as Au, Ag, and Cu bind with BH 4 – through molecular adsorption rather than dissociative adsorption. ,− These metals adsorb hydrogen weakly, resulting in a low hydrolysis rate. ,,, In fact, Au shows the highest selectivity toward BOR over hydrolysis, but it exhibits very low activity because of the less-favorable adsorption energy of BH 4 – vis-à-vis the water molecule. , Furthermore, these coinage metals show low electrocatalytic activity toward the BOR owing to their inability to activate B–H bond breaking. ,, Despite not producing all the eight electrons during BH 4 – oxidation, Pd electrocatalysts enable large current densities with high Faradaic efficiencies. ,,− However, full utilization of the active sites is not possible because of the high rate of the parasitic hydrolysis (hydrogen evolution) reaction, which blocks access to the active sites. Alloying or mixing d-group metals (e.g., Ir, Sn, Ni, Ni-Cu, , or Ni-Co) with Pd metal has resulted in an increase in the activity and selectivity of BOR.…”

“…Alloying or mixing d-group metals (e.g., Ir, Sn, Ni, Ni-Cu, , or Ni-Co) with Pd metal has resulted in an increase in the activity and selectivity of BOR. Among the earth-abundant metals, Ni favors a strong dissociative adsorption of BH 4 – . ,, However, the dissociation energy of BH 4 – on Ni is higher (more positive) than that of Pd, which leads to a lower hydrolysis rate and consequently to a lower probability of active-site poisoning on Ni surfaces . Furthermore, Ni exhibits a low BOR overpotential, although its activity varies depending upon the oxidation state of surface Ni atoms. − Preservation of metallic states on the surface of Ni yield high BOR activity, whereas surface oxidation leads to a lowering of the BOR activity. ,, This suggests that a bimetallic electrocatalyst composed of Pd and Ni will yield a high current density with a high Faradaic efficiency because of a lower hydrolysis reaction rate and a lower probability of electrocatalyst poisoning.…”

New Mathematical Formulation for the Deposition Potential and Atomic Radius: Theoretical Background and Applications to Sn–Ln Intermetallic Compounds

“…Another important issue concerning rare earths and molten salts is the pyrochemical reprocessing of nuclear fuel, which is considered a promising option for advanced fuel cycles. − For achieving nuclear fuel closed circulation and sustainable development, efficient pyroprocessing of spent fuel becomes the core concern . In addition, molten salt electrolysis is the key step in pyroprocessing technology; therefore, it is necessary to investigate the electrochemical properties of lanthanides and actinides.…”