A series of Ni-based electrocatalysts, [Ni(7P(Ph)2N(C6H4X))2](BF4)2, featuring seven-membered cyclic diphosphine ligands incorporating a single amine base, 1-para-X-phenyl-3,6-triphenyl-1-aza-3,6-diphosphacycloheptane (7P(Ph)2N(C6H4X), where X = OMe, Me, Br, Cl, or CF3), have been synthesized and characterized. X-ray diffraction studies have established that the [Ni(7P(Ph)2N(C6H4X))2](2+) complexes have a square planar geometry, with bonds to four phosphorus atoms of the two bidentate diphosphine ligands. Each of the complexes is an efficient electrocatalyst for hydrogen production at the potential of the Ni(II/I) couple, with turnover frequencies ranging from 2400 to 27,000 s(-1) with [(DMF)H](+) in acetonitrile. Addition of water (up to 1.0 M) accelerates the catalysis, giving turnover frequencies ranging from 4100 to 96,000 s(-1). Computational studies carried out on the [Ni(7P(Ph)2N(C6H4X))2](2+) family indicate the catalytic rates reach a maximum when the electron-donating character of X results in the pKa of the Ni(I) protonated pendant amine matching that of the acid used for proton delivery. Additionally, the fast catalytic rates for hydrogen production by the [Ni(7P(Ph)2N(C6H4X))2](2+) family relative to the analogous [Ni(P(Ph)2N(C6H4X)2)2](2+) family are attributed to preferred formation of endo protonated isomers with respect to the metal center in the former, which is essential to attain suitable proximity to the reduced metal center to generate H2. The results of this work highlight the importance of precise pKa matching with the acid for proton delivery to obtain optimal rates of catalysis.

A series of dipeptide substituted nickel complexes with the general formula, [Ni(P(Ph)(2)N(NNA-amino acid/ester)(2))(2)](BF(4))(2), have been synthesized and characterized (P(2)N(2) = 1,5-diaza-3,7-diphosphacyclooctane, and the dipeptide consists of the non-natural amino acid, 3-(4-aminophenyl)propionic acid (NNA), coupled to amino acid/esters = glutamic acid, alanine, lysine, and aspartic acid). Each of these complexes is an active electrocatalyst for H(2) production. The effects of the outer-coordination sphere on the catalytic activity for the production of H(2) were investigated; specifically, the impact of sterics, the ability of the side chain or backbone to protonate and the pK(a) values of the amino acid side chains were studied by varying the amino acids in the dipeptide. The catalytic rates of the different dipeptide substituted nickel complexes varied by over an order of magnitude. The amino acid derivatives display the fastest rates, while esterification of the terminal carboxylic acids and side chains resulted in a decrease in the catalytic rate by 50-70%, implicating a significant role of protonated sites in the outer-coordination sphere on catalytic activity. For both the amino acid and ester derivatives, the complexes with the largest substituents display the fastest rates, indicating that catalytic activity is not hindered by steric bulk. These studies demonstrate the significant contribution that the outer-coordination sphere can have in tuning the catalytic activity of small molecule hydrogenase mimics.

Electrolytic water splitting offers energy storage and conversion opportunities, yet the slow kinetics of the oxygen evolution reaction requires the incorporation of catalytic materials. Herein, we present a facile method for the synthesis of a low-cost Ni-Fe-Co material that efficiently catalyzes the oxygen evolution reaction. A mixed transition metal electrocatalyst was synthesized using a nickel-plated iron substrate and a low concentration cobalt reagent. The catalyst was able to achieve competitive current densities (>130 mA cm−2) and still exhibit a low overpotential of 0.30 V at the benchmark current density of 10 mA cm−2. The Ni-Fe-Co catalyst was stable and maintained its activity during 24 h of electrolysis in alkaline media. The catalyst was also stable when maintained in ambient conditions for 90 days. This is the first reported oxygen evolution reaction catalyst that exhibits these competitive characteristics and that was synthesized using an environmentally-conscious, one-step process that can be manufactured on an industrial scale.

Introduction The electrolysis of water has been at the forefront of catalysis research for over a decade and has gained tremendous attention since the introduction of metal-air cells and photoelectrochemical (PEC) water splitting devices that bridge the gap between carbon-based fuels and alternative energy resources.1 However, these substantial advancements are still very expensive and inefficient to implement due in part to the non-spontaneous nature of the oxygen evolution reaction (OER). The OER is energy intensive; therefore, developing ways of reducing the overpotential and increasing the current throughput with catalytic materials is required.1,2 The OER is a very complex mechanism that consists of various adsorption interactions at the electrode-electrolyte interface.2 Therefore, efficient catalytic electrodes have a high active surface area for adsorbing intermediates, are stable in the electrolytic medium for long periods of time, and are good at electron transfer processes.2 Mixed metal oxides are supported to be highly catalytic, stable, and can be formed using a variety of different experimental techniques.1 Many of the mixed metal oxides being pursued are abundant and easily accessible.  Mixed metal oxides are also often found as by-products of corrosion in structural applications and high temperature processes. These facts have led to our research in developing highly stable and electrochemically catalytic mixed metal oxides using different experimental techniques.  Nickel alloy-based oxides are of interest due to their promising electrocatalytic capabilities3,4 when compared to the base metals alone. The ability to produce mixed metal oxides through facile techniques in mild conditions is highly desired. Experimental Catalytic electrodes were prepared using a variety of methods. These electrodes were then compared based on their electrochemical activity for catalyzing the OER. Preliminary experiments show promising and interesting results supporting a shift in overpotential and an increase in current density for the OER when the prepared mixed metal oxides are used as the working electrode in a traditional three-electrode configuration.  Surface characterization using XPS, XRD, SEM, and Raman spectroscopy was employed to relate electrochemical response to the surface composition of the different electrodes to aid in determining the overall mechanism of catalytic activity.  It is also suggested that the breakdown of the outer surface oxide layer along with re-deposition throughout the electrochemical cycle might affect the overall electrochemical capabilities of these mixed metal oxide electrodes. Results Preliminary investigations suggest that electrode preparation technique and surface oxide film depth play a significant part in the catalytic activity of the mixed metal oxides. These results have led to further understanding of the OER mechanism and to better methods for preparing mixed metal oxide electrodes.   Figure I. Linear sweep voltammogram of base metal vs. prepared mixed metal oxide electrocatalyst   Acknowledgements: This work was partially funded by Department of Energy under contract DE-NE0008236.   References   C. Yuan., H. B. Wu, et al. Angew. Chem. Int. Ed., 53(6)1488-1504, (2014). I. Katsounaros, S. Cherevko, A. R. Zeradjanin and K. J. J. Mayrhofer, Angew. Chem. Int. Ed., 53, 102-121, (2014). H.-Y. Wang, Y.-Y. Hsu, R. Chen, T.-S. Chan, H. M. Chen and B. Liu, Adv. Energy. Mater.,  5,(2015). H. Zhang, H. Y. Li, H. Y. Wang, K. J. He, S. Y. Wang, Y. G. Tang and J. J. Chen, J. Power Sources, 280, 640-648, (2015). Figure 1