Arthur J. Esswein

Wireless Solar Water Splitting Using Silicon-Based Semiconductors and Earth-Abundant Catalysts

An artificial water-splitting system was built using abundant materials and sunlight. We describe the development of solar water-splitting cells comprising earth-abundant elements that operate in near-neutral pH conditions, both with and without connecting wires. The cells consist of a triple junction, amorphous silicon photovoltaic interfaced to hydrogen- and oxygen-evolving catalysts made from an alloy of earth-abundant metals and a cobalt|borate catalyst, respectively. The devices described here carry out the solar-driven water-splitting reaction at efficiencies of 4.7% for a wired configuration and 2.5% for a wireless configuration when illuminated with 1 sun (100 milliwatts per square centimeter) of air mass 1.5 simulated sunlight. Fuel-forming catalysts interfaced with light-harvesting semiconductors afford a pathway to direct solar-to-fuels conversion that captures many of the basic functional elements of a leaf.

Highly active cobalt phosphate and borate based oxygen evolving catalysts operating in neutral and natural waters

A high surface area electrode is functionalized with cobalt-based oxygen evolving catalysts (Co-OEC = electrodeposited from pH 7 phosphate, Pi, pH 8.5 methylphosphonate, MePi, and pH 9.2 borate electrolyte, Bi). Co-OEC prepared from MePi and operated in Pi and Bi achieves a current density of 100 mA cm−2 for water oxidation at 442 and 363 mV overpotential, respectively. The catalyst retains activity in near-neutral pH buffered electrolyte in natural waters such as those from the Charles River (Cambridge, MA) and seawater (Woods Hole, MA). The efficacy and ease of operation of anodes functionalized with Co-OEC at appreciable current density together with its ability to operate in near neutral pH buffered natural water sources bodes well for the translation of this catalyst to a viable renewable energy storage technology.

Three-Coordinate, Phosphine-Ligated Azadipyrromethene Complexes of Univalent Group 11 Metals

Tetraarylazadipyrromethenes are Lewis basic, red-light absorbing dyes with optical properties conducive to sensing and therapeutic applications. Recently, transition metal complexes of these ligands have been described. Here, we report a series of three-coordinate Group 11 complexes of unsubstituted and methoxy-substituted tetraarylazadipyrromethenes. In each, two pyrrole nitrogens chelate a d(10) metal ion; triphenyl- or triethylphosphine occupies a third coordination site. New complexes are characterized by multinuclear NMR, X-ray crystallography, optical absorption and emission spectroscopy, and elemental analysis. Solid-state structures show trigonal planar geometries about the metal centers, and reveal pervasive intra- and intermolecular pi-stacking interactions. Visible light absorption intensifies with metal binding, in some cases shifting to longer wavelengths. The complexes weakly luminesce in the red region; emission wavelengths and quantum yields are similar to those of free azadipyrromethenes. Methoxy-substitution on the ligand red-shifts optical features, whereas substitution of triethylphosphine for triphenylphosphine in the third coordination site has minimal structural or spectral consequences.