Photoelectrochemical Hydrogen Production Using New Combinatorial Chemistry Derived Materials

Solar photoelectrochemical water-splitting has long been viewed as one of the “holy grails” of chemistry because of its potential impact as a clean, renewable method of fuel production. Several known photocatalytic semiconductors can be used; however, the fundamental mechanisms of the process remain poorly understood and no known material has the required properties for cost effective hydrogen production. In order to investigate morphological and compositional variations in metal oxides as they relate to opto-electrochemical properties, we have employed a combinatorial methodology using automated, high-throughput, electrochemical synthesis and screening together with conventional solid-state methods. This report discusses a number of novel, high-throughput instruments developed during this project for the expeditious discovery of improved materials for photoelectrochemical hydrogen production. Also described within this report are results from a variety of materials (primarily tungsten oxide, zinc oxide, molybdenum oxide, copper oxide and titanium dioxide) whose properties were modified and improved by either layering, inter-mixing, or doping with one or more transition metals. Furthermore, the morphologies of certain materials were also modified through the use of structure directing agents (SDA) during synthesis to create mesostructures (features 2-50 nm) that increased surface area and improved rates of hydrogen production

Oxygen electroreduction on gold-cobalt oxide binary nanocluster catalysts

The electroreduction of oxygen was investigated on mixed gold-cobalt oxide (Au CoOx) nanoclusters in potassium hydroxide solution. Au CoOx binary nanoclusters were prepared with different atomic ratios of Au:Co using pulsed-voltage electrodeposition at 20 kHz from electrolytes with varying concentrations of Au and Co cations. The electrocatalytic activities of the different catalysts deposited on rotating disk electrodes were evaluated for the oxygen reduction reaction (ORR) using voltammetry. The Au CoOx binary electrocatalysts with less than 1.2% Co atoms were found to increase the ORR activity compared to those of either the pure Au or pure CoOx. As the Co content in the binary mixture was increased, a maximum in the Au electronegativity, as measured by X-ray photoelectron spectroscopy, was observed which corresponded to a maximum observed rate for oxygen reduction. The Au:Co atomic ratio at the maximum rate was approximately 80:1. The number of electrons transferred in the oxygen reduction decrease from four to two as the Co content increased, suggesting that the electroreduction pathway is altered by the presence of Co

Improved photoelectrochemical performance of Ti-doped α-Fe 2O3 thin films by surface modification with fluoride

CoF3 aqueous solution was used to modify the surface of Ti-doped iron oxide thin film photoanodes to negatively shift the flat-band potential and allow photogenerated electrons to directly reduce water to hydrogen without an external bias; the zero bias performance was further improved by the use of glucose (a biomass analog) to bypass the relatively slow oxygen evolution reaction to provide a source of electrons to rapidly consume photogenerated holes

NiFe-oxide electrocatalysts for the oxygen evolution reaction on Ti doped hematite photoelectrodes

Nickel iron binary oxide electrocatalysts prepared from different precursors were evaluated to facilitate the oxygen evolution reaction (OER) on semiconducting metal-oxide photoelectrodes. The electrocatalysts deposited from Ni(II) and Ni(II)Fe(II) precursors had the highest activity for OER, however, their presence on the surface of the Ti doped hematite photoelectrode decreased the incident photon-to-current efficiency (IPCE) of the photoelectrode for water splitting in an alkaline electrolyte. In contrast, the NiFe-oxide deposited from the Ni(II)-Fe(III) precursor which had a lower OER activity was found to increase the IPCE of the photoelectrode by as much as a factor of 5

Size-dependent activity of gold nanoparticles for oxygen electroreduction in alkaline electrolyte

The dependence of the kinetics of electrocatalytic oxygen reduction in basic electrolyte on the size of Au nanoparticles was determined for 3 and 7 nm clusters supported on carbon. The size-selected nanoparticles were prepared by reverse micelle encapsulation using PS-P2VP diblock copolymer, and the kinetic current for oxygen reduction was measured with a rotating disk electrode (RDE). The kinetic current was found to be 2.5 times higher for the 3 nm gold nanoparticles compared to the 7 nm gold nanoparticles at 23 °C. The 3 nm particles were found to facilitate four-electron electroreduction, whereas a two-electron electroreduction was inferred from the RDE data on the 7 nm particles. From experiments of the temperature-dependent current the apparent activation energy for the 3 nm clusters was found to be half that of the 7 nm clusters (0.1 and 0.2 eV, respectively)