Transition metal carbides including chromium, molybdenum, and tungsten are of particular interest as renewable energy catalysts due to their low cost and abundance. While several single metal carbide systems form multiple phases with different compositions and crystal structures, most of these materials are not as well studied due to their limited synthetic approaches and instability. By taking advantage of a low temperature salt flux synthetic method, these unique phases can be more easily synthesized and separated as phase pure materials. As an example, Chromium carbide forms five different crystal structures including three common phases, CrC, CrC, and CrC, and two less studied phases, CrC and CrC. All five compounds were synthesized using the salt flux method and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and electrocatalytic testing for the hydrogen evolution reaction (HER). This low temperature method allows for routine access to multiple compounds in complex phase diagrams and separation of each phase synthetically. This represents a significant step forward in synthesizing rare phases like rocksalt CrC and hexagonal CrC and allows for investigation into their potential catalytic properties and future applications.

No abstract

The discovery of abundant perchlorate (ClO 4 − ) on Mars has prompted renewed interest in the production, accumulation, and transport of ClO 4 − and other oxychlorine species in natural systems. Here, we focus on the role of semiconducting minerals in the photochemical generation and destruction of ClO 4 − and chlorate (ClO 3 − ). Illumination of single-crystal or nanocrystalline films of titanium dioxide polymorphs, rutile and anatase, in chloride (Cl − ) solutions can both generate and destroy ClO 4 − depending upon starting ClO 4 − and Cl − concentrations. For single-crystal anatase, we observe an apparent photostationary state, in which ClO 4− production and destruction reach a near-steady state. We observe more ClO 3 − production and less ClO 4 − at higher Cl − concentrations. An inventory of measured dissolved chlorine (Cl) species indicates that some Cl was lost to a volatile form or dissolved form not measured. Our experiments were performed in an aqueous medium under Earth atmosphere and temperature conditions; further experiments under Mars-like conditions are in progress. Photochemical processes as described here, in which activation energy is provided by photons, are particularly important under cold conditions with limited thermal activation.

Homoepitaxial growth of highly ordered and pure layers of rutile on rutile crystal substrates and anatase on anatase crystal substrates using atomic layer deposition (ALD) is reported. The epilayers grow in a layer-by-layer fashion at low deposition temperatures but are still not well ordered on rutile. Subsequent annealing at higher temperatures produces highly ordered, terraced rutile surfaces that in many cases have fewer electrically active defects than the substrate crystal. The anatase epitaxial layers, grown at 250 °C, have much fewer electrically active defects than the rather impure bulk crystals. Annealing the epilayers at higher temperatures increased band gap photocurrents in both anatase and rutile.

A tandem two photoelectrode cell separated by a proton exchange membrane for simultaneous conversion of solar energy into both chemical and electrical energy was investigated by illumination of both a large bandgap photoanode, to produce a highly oxidized species, and a narrow bandgap photocathode, to produce a highly reduced species. The two photoelectrode configuration provides higher photovoltages than a single semiconducting material while also absorbing complementary portions of the solar spectrum leading to potentially higher energy conversion efficiencies. Utilization of kinetically fast redox couples overcomes the overpotential barriers required for water splitting as well as the difficulties associated with gas collection and transport. Additionally, the photopotentials obtained show that not only does this configuration not require external bias but excess electrical power could also be produced in addition to the storage of chemical energy. These experiments demonstrate the feasibility for highly efficient solar energy conversion by directly photocharging a redox flow battery.