Electrocatalytic O₂ Reduction by Covalently Immobilized Mononuclear Copper(I) Complexes: Evidence for a Binuclear Cu₂O₂ Intermediate

Abstract

A Cu I complex of 3-ethynyl-phenanthroline covalently immobilized to an azide-modified glassy carbon surface is an active electrocatalyst for the 4-electron reduction of O 2 to H 2 O. The rate of O 2 reduction is 2 nd order in Cu coverage at moderate overpotential, suggesting that two Cu I species are necessary for efficient 4-electron reduction of O 2 . Mechanisms for O 2 reduction are proposed that are consistent with the observations for this covalently immobilized system and previously reported results for a similar physisorbed Cu I system. Discrete copper complexes are potential catalysts for the 4-electron reduction of O 2 to water in ambient temperature fuel cells as evidenced by Cu-containing fungal laccase enzymes that rapidly reduce O 2 directly to water at a trinuclear Cu active site at remarkably positive potentials. [1][2][3][4][5] Several groups have studied molecular Cu complexes immobilized onto electrode surfaces as an entry into the study of 4-electron O 2 reduction. [6][7][8][9][10][11][12][13][14][15][16][17][18][19] In particular, physisorbed Cu I (1,10-phenanthroline), Cu(phen P ), reduces O 2 quantitatively by 4 electrons and 4 protons to water. [8][9][10] Anson, et al., determined that this reaction was 1 st order in Cu coverage, suggestive of a mononuclear Cu site as the active catalyst. 8,10 In the present study, similar Cu I complexes are covalently attached to a modified glassycarbon electrode surface to form a species denoted Cu(phen C ), and the effect of Cu coverage on the kinetics of electrocatalytic O 2 reduction is investigated. At low overpotentials, we observe a 2 nd order dependence of the O 2 -reduction rate on the coverage of Cu(phen C ), from which we infer that two physically proximal Cu(phen C ) bind O 2 to form a binuclear Cu 2 O 2 species required for 4-electron reduction. We suggest that a similar binuclear species also forms in the case of Cu(phen P ) 8,10 but that rate-limiting binding of O 2 to the first Cu(phen P ) NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript followed by rapid surface diffusion of a second Cu(phen P ) has, until now, obscured the binuclear nature of the reaction.The covalent attachment of 3-ethynyl-1,10-phenanthroline to an azide-modified glassy carbon electrode to form Cu(phen C ) relies on the Cu I -catalyzed cycloaddition of azide and ethynyl groups to form a triazole linker, commonly referred to as the click reaction. 20,21 The electrode is azide terminated by treating a roughly-ground, heat-treated glassy carbon surface with a solution of IN 3 in hexanes, a procedure modified from that first described by Devadoss and Chidsey. 22 An XPS survey of the azide-modified surface shows two N 1s peaks at 399 eV and 403 eV in a 2:1 ratio attributable to the azide nitrogens. [22][23][24] Upon exposure to 3-ethynyl-1,10-phenanthroline under the click reaction conditions 25 , the 403-eV peak disappears and the 399-eV peak broadens, consistent with the formation of the 1,2,3-triazole linker. 22,24 XPS peaks at 934 and ...

A Generic Platform for the Addressable Functionalisation of Electrode Surfaces through Self‐Induced “Electroclick”

“…Indeed, it addresses two issues: First, the possibility of selective surface patterning through electrochemical control; this has been demonstrated in the case of a gold electrode but should be extendable to the more robust vitreous and graphitic electrodes. [33,34] Secondly, thanks to the presence of the N 4 -Cu redox link, it allows electron communication between two different probes to be explored. Indeed, the experimental evidence for electron relay in molecular devices is of great interest in biology (e.g., a model for electron-transfer in DNA [35] ) and biotechnology (design of new biochips ] ).…”

Pyrolyzed Copper-Based Catalyst with High Oxygen Reduction Activity for PEM Fuel Cell Applications

“…McCrory et al 22 investigated the ORR activity, at pH 4.8, of non-pyrolyzed Cu-3-ethynyl-1-10-phenanthroline covalently attached to an azide-modified glassy carbon electrode via a "click" reaction, allowing controlled catalyst coverage on the carbon surface, finding evidence that to achieve O 2 * Electrochemical Society Active Member.…”

“…13,[15][16][17][18][19][20][21] However, in alkaline media Cubased catalysts show higher catalytic activity, making them suitable for a wide range of applications as ORR catalysts. 3,14,[20][21][22] Gewirth's group has adsorbed Cu-based catalysts onto a carbon support and studied the catalytic activity of the non-pyrolyzed samples in acidic and alkaline media, 14,15,21 reporting rotating disk electrode (RDE) onset potentials up to 0.53 V vs. RHE at pH 1. McCrory et al 22 investigated the ORR activity, at pH 4.8, of non-pyrolyzed Cu-3-ethynyl-1-10-phenanthroline covalently attached to an azide-modified glassy carbon electrode via a "click" reaction, allowing controlled catalyst coverage on the carbon surface, finding evidence that to achieve O 2 reduction to water by an adsorbed Cu catalyst, two proximal Cu I sites are required.…”

“…3,14,[20][21][22] Gewirth's group has adsorbed Cu-based catalysts onto a carbon support and studied the catalytic activity of the non-pyrolyzed samples in acidic and alkaline media, 14,15,21 reporting rotating disk electrode (RDE) onset potentials up to 0.53 V vs. RHE at pH 1. McCrory et al 22 investigated the ORR activity, at pH 4.8, of non-pyrolyzed Cu-3-ethynyl-1-10-phenanthroline covalently attached to an azide-modified glassy carbon electrode via a "click" reaction, allowing controlled catalyst coverage on the carbon surface, finding evidence that to achieve O 2 reduction to water by an adsorbed Cu catalyst, two proximal Cu I sites are required. 22 In a previous study we reported 19 the synthesis and RDE characterization of Cu-triazole catalysts adsorbed, and covalently attached (via diazonium coupling) to a carbon black support.…”

“…McCrory et al 22 investigated the ORR activity, at pH 4.8, of non-pyrolyzed Cu-3-ethynyl-1-10-phenanthroline covalently attached to an azide-modified glassy carbon electrode via a "click" reaction, allowing controlled catalyst coverage on the carbon surface, finding evidence that to achieve O 2 reduction to water by an adsorbed Cu catalyst, two proximal Cu I sites are required. 22 In a previous study we reported 19 the synthesis and RDE characterization of Cu-triazole catalysts adsorbed, and covalently attached (via diazonium coupling) to a carbon black support. Immobilized catalysts showed higher stability than adsorbed ones in acidic environment, as demonstrated by RDE cycling in O 2 saturated electrolyte.…”