Superior Inorganic Ion Cofactors of Tetraborate Species Attaining Highly Efficient Heterogeneous Electrocatalysis for Water Oxidation on Cobalt Oxyhydroxide Nanoparticles

Abstract

A heterogeneous catalyst incorporating an inorganic ion cofactor for electrochemical water oxidation was exploited using a CoO(OH) nanoparticle layer-deposited electrode. The significant catalytic current for water oxidation was generated in a NaBO solution at pH 9.4 when applying 0.94 V versus Ag/AgCl in contrast to no catalytic current generation in the KSO solution at the same pH. HBO and BO ions were indicated to act as key cofactors for the induced catalytic activity of the CoO(OH) layer. The NaBO concentration dependence of the catalytic current was analyzed based on a Michaelis-Menten-type kinetics to provide an affinity constant of cofactors to the active sites, K = 28 ± 3.6 mM, and the maximum catalytic current density, I = 2.3 ± 0.13 mA cm. The I value of HBO and BO ions was 1.4 times higher than that (1.3 mA cm) for the previously reported case of CO ions. This could be explained by the shorter-range proton transfer from the active site to the proton-accepting cofactor because of the larger size and more flexible conformation of HBO and BO ions compared with that of CO ions.

Facile Templateless Fabrication of a Cobalt Oxyhydroxide Nanosheet Film with Nanoscale Porosity as an Efficient Electrocatalyst for Water Oxidation

“…The other band (relatively weak) at 532.5 eV is attributed to the oxygen from the adsorbed hydroxyl contamination on the film surface. These XPS spectra in Co 2p and O 1s regions for the CoO(OH)‐NS film those were nearly the same as those for CoO(OH)‐NP film, (Figure S1a and b), ensuring that the chemical state of CoO(OH) is close between the CoO(OH)‐NS and CoO(OH)‐NP films.…”

“…Electrocatalytic properties of the CoO(OH)‐NS electrode for water oxidation were investigated in a 0.1 M Na 2 B 4 O 7 solution at pH 9.4. Our recent study demonstrated that HB 4 O 7 − and B 4 O 7 2− ions in solution perform as excellent cofactors, acting as proton acceptors for accelerated O−O bond formation to induces efficient electrocatalytic water oxidation by the CoO(OH)‐NP film . Figure A shows the cyclic voltammograms (CVs) of the CoO(OH)‐NS electrodes with different Γ cov .…”

“…The catalytic current was observed above 0.90 V vs. Ag/AgCl (1.65 V vs. RHE) of the onset potential (defined as the potential for 500 μA cm −2 current generation), meaning the overpotential (η=0.42 V) for water oxidation. Although LSV of CoO(OH)‐NP showed an anodic wave at 0.64 V assigned to Co III /Co IV redox pair,, the onset potential (0.91 V vs. Ag/AgCl) was nearly the same as that for CoO(OH)‐NS. However, Tafel slope (108 mV dec −1 ) for CoO(OH)‐NS was lower than that (159 mV dec −1 ) for CoO(OH)‐NP (Inset of Figure ).…”

“…has attracted much attention due to the leading catalytic performance and facile preparation ,. Recently, oxyhydroxide species such as amorphous CoO(OH), FeO(OH), NiO(OH) and CoFeO(OH) have been also proven as efficient catalysts for water oxidation ,,. In fact, Co−Pi and related catalysts are composed of small fragments/layers of CoO(OH) being responsible for water oxidation catalysis ,.…”

“…This fabrication technique provides a stable and uniform porous film composed of interlaced CoO(OH) nanosheets and a facile control of Γ cov and thickness of the CoO(OH)‐NS layer deposited on the electrode. The nanostructure and electrocatalytic properties are characterized to demonstrate that the large accessible surface of CoO(OH)‐NS film improves the performance for electrocatalytic water oxidation relative to earlier reported CoO(OH)‐NP electrode ,…”