Wilson A. Smith

Selective and Efficient Reduction of Carbon Dioxide to Carbon Monoxide on Oxide‐Derived Nanostructured Silver Electrocatalysts

In this work, the selective electrocatalytic reduction of carbon dioxide to carbon monoxide on oxide-derived silver electrocatalysts is presented. By a simple synthesis technique, the overall high faradaic efficiency for CO production on the oxide-derived Ag was shifted by more than 400 mV towards a lower overpotential compared to that of untreated Ag. Notably, the Ag resulting from Ag oxide is capable of electrochemically reducing CO2 to CO with approximately 80 % catalytic selectivity at a moderate overpotential of 0.49 V, which is much higher than that (ca. 4 %) of untreated Ag under identical conditions. Electrokinetic studies show that the improved catalytic activity is ascribed to the enhanced stabilization of COOH(.) intermediate. Furthermore, highly nanostructured Ag is likely able to create a high local pH near the catalyst surface, which may also facilitate the catalytic activity for the reduction of CO2 with suppressed H2 evolution.

Controllable Hydrocarbon Formation from the Electrochemical Reduction of CO2 over Cu Nanowire Arrays

In this work, the effect of Cu nanowire morphology on the selective electrocatalytic reduction of CO2 is presented. Cu nanowire arrays were prepared through a two-step synthesis of Cu(OH)2 and CuO nanowire arrays on Cu foil substrates and a subsequent electrochemical reduction of the CuO nanowire arrays to Cu nanowire arrays. By this simple synthesis method, Cu nanowire array electrodes with different length and density were able to be controllably synthesized. We show that the selectivity for hydrocarbons (ethylene, n-propanol, ethane, and ethanol) on Cu nanowire array electrodes at a fixed potential can be tuned by systematically altering the Cu nanowire length and density. The nanowire morphology effect is linked to the increased local pH in the Cu nanowire arrays and a reaction scheme detailing the local pH-induced formation of C2  products is also presented by a preferred CO dimerization pathway.

Interfacial band-edge energetics for solar fuels production

Photoelectrochemical (PEC) water splitting has received growing attention as a potential pathway to replace fossil fuels and produce a clean, renewable, and sustainable source of fuel. To achieve overall water splitting and the associated production of solar fuels, complex devices are needed to efficiently capture light from the sun, separate photogenerated charges, and catalyze reduction and oxidation reactions. To date, the highest performing solar fuels devices rely on multi-component systems, which introduce interfaces that can be associated with further performance loss due to thermodynamic and kinetic considerations. In this review, we identify several of the most important interfaces used in PEC water splitting, summarize methods to characterize them, and highlight approaches to mitigating associated loss mechanisms.

Photocharged BiVO4 photoanodes for improved solar water splitting

Bismuth vanadate (BiVO4) is a promising semiconductor material for the production of solar fuels via photoelectrochemical water splitting, however, it suffers from substantial recombination losses that limit its performance to well below its theoretical maximum. Here we demonstrate for the first time that the photoelectrochemical (PEC) performance of BiVO4 photoanodes can be dramatically improved by prolonged exposure to AM 1.5 illumination in the open circuit (OC) configuration. Photoanodes subjected to such light treatment achieve a record photocurrent for undoped and uncatalysed BiVO4 of 3.3 mA cm−2 at 1.23 VRHE. Moreover, photoelectrochemical tests with a sacrificial agent yield significantly enhanced catalytic efficiency over the whole operating potential range, suggesting elimination of major losses at the semiconductor–electrolyte interface. Finally, we demonstrate that this so-called ‘photocharging’ technique induces a considerable cathodic shift in the photocurrent onset potential and increases the photovoltage extracted from BiVO4 photoanodes.

Extracting large photovoltages from a-SiC photocathodes with an amorphous TiO2 front surface field layer for solar hydrogen evolution

A thin film heterojunction photocathode is fabricated by depositing an n-type amorphous titanium dioxide (TiO2) onto a p-type/intrinsic hydrogenated amorphous silicon carbide (a-SiC). Using this configuration, the photovoltage of the photocathode increases from 0.5 V to 0.8 V under open circuit conditions, indicating the change in band-edge energetics from the semiconductor–liquid junction to the isolated solid p–i–n junction. The p–i–n structure produces an internal electric field that increases the operating photovoltage, and subsequently improves the drift mechanism of photogenerated charge carriers across the intrinsic layer. The enhancement of the photovoltage leads to a very positive photocurrent onset potential of +0.8 V vs. RHE and exhibits a photocurrent density of 8.3 mA cm−2 at 0 V vs. RHE with only a 100 nm absorber layer. The a-SiC photocathode with a front surface field amorphous TiO2 layer shows a high stability for 12 hours of operation under photocatalytic conditions. This high performance, very thin, and earth-abundant photocathode is very promising for integration with smaller band gap solar absorbers to form a multijunction system for highly efficient bias-free solar water splitting devices.

Photo-assisted water splitting with bipolar membrane induced pH gradients for practical solar fuel devices

Different pH requirements for a cathode and an anode result in a non-optimal performance for practical solar fuel systems. We present for the first time a photo-assisted water splitting device using a bipolar membrane, which allows a cathode to operate in an acidic electrolyte while the photoanode is in alkaline conditions. The bipolar membrane dissociates water into H+ and OH−, which is consumed for hydrogen evolution at the cathode and oxygen evolution at the anode, respectively. The introduction of such a bipolar membrane for solar fuel systems provides ultimate freedom for combining different (photo)cathodes and -anodes. This paper shows that photo-assisted water splitting at both extreme pH gradients (0–14) as well as mild pH gradients (0–7) yields current densities of 2–2.5 mA cm−2 using a BiVO4 photoanode and a bipolar membrane. The membrane potentials are within 30 mV of the theoretical electrochemical potential for low current densities. The pH gradient is maintained for 4 days of continuous operation and electrolyte analysis shows that salt cross-over is minimal. The stable operation of the bipolar membrane in extreme and mild pH gradients, at negligible loss, contributes to a sustainable and practically feasible solar fuel device with existing photoactive electrodes operating at different pH.

Plasmonic nanoparticle-semiconductor composites for efficient solar water splitting

Photoelectrochemical (PEC) water splitting is a promising technology that uses light absorbing semiconductors to convert solar energy directly into a chemical fuel (i.e., hydrogen). PEC water splitting has the potential to become a key technology in achieving a sustainable society, if high solar to fuel energy conversion efficiencies are obtained with earth abundant materials. This review article discusses recent developments and discoveries in the mechanisms by which the localized surface plasmon resonance (LSPR) in metallic nanoparticles can increase or complement a neighbouring semiconductor in light absorption for catalytic water splitting applications. These mechanisms can mitigate the intrinsic optical limitations of semiconductors (e.g., metal oxides) for efficient solar water splitting. We identify four types of enhancement mechanisms in the recent literature: (i) light scattering, (ii) light concentration, (iii) hot electron injection (HEI), and (iv) plasmon-induced resonance energy transfer (PIRET). (i) Light scattering and (ii) light concentration are light trapping mechanisms that can increase the absorption of light with energies above the semiconductor optical band-edge. These two mechanisms are ideal to enhance the absorption of promising semiconductors with narrow bandgap energies that suffer from limited absorption coefficients and bulk charge recombination. On the other hand, (iii) HEI and the recently discovered (iv) PIRET are mechanisms that can enhance the absorption also below the semiconductor optical band-edge. Therefore, HEI and PIRET have the potential to extend the light utilization to visible and near-infrared wavelengths of semiconductors with excellent electrochemical properties, but with large bandgap energies. New techniques and theories that have been developed to elucidate the above mentioned plasmonic mechanisms are presented and discussed for their application in metal oxide photoelectrodes. Finally, other plasmonic and non-plasmonic effects that do not increase the device absorption, but affect the electrochemical properties of the semiconductor (e.g., charge carrier transport) are also discussed, since a complete understanding of these phenomena is fundamental for the design of an efficient plasmonic NP-semiconductor water splitting device.

Gradient dopant profiling and spectral utilization of monolithic thin-film silicon photoelectrochemical tandem devices for solar water splitting

A cost-effective and earth-abundant photocathode based on hydrogenated amorphous silicon carbide (a-SiC:H) is demonstrated to split water into hydrogen and oxygen using solar energy. A monolithic a-SiC:H photoelectrochemical (PEC) cathode integrated with a hydrogenated amorphous silicon (a-SiC:H)/nano-crystalline silicon (nc-Si:H) double photovoltaic (PV) junction achieved a current density of −5.1 mA cm−2 at 0 V versus the reversible hydrogen electrode. The a-SiC:H photocathode used no hydrogen-evolution catalyst and the high current density was obtained using gradient boron doping. The growth of high quality nc-Si:H PV junctions in combination with optimized spectral utilization was achieved using glass substrates with integrated micro-textured photonic structures. The performance of the PEC/PV cathode was analyzed by simulations using Advanced Semiconductor Analysis (ASA) software.

Interfacial engineering of metal-insulator-semiconductor junctions for efficient and stable photoelectrochemical water oxidation

Solar-assisted water splitting can potentially provide an efficient route for large-scale renewable energy conversion and storage. It is essential for such a system to provide a sufficiently high photocurrent and photovoltage to drive the water oxidation reaction. Here we demonstrate a photoanode that is capable of achieving a high photovoltage by engineering the interfacial energetics of metal–insulator–semiconductor junctions. We evaluate the importance of using two metals to decouple the functionalities for a Schottky contact and a highly efficient catalyst. We also illustrate the improvement of the photovoltage upon incidental oxidation of the metallic surface layer in KOH solution. Additionally, we analyse the role of the thin insulating layer to the pinning and depinning of Fermi level that is responsible to the resulting photovoltage. Finally, we report the advantage of using dual metal overlayers as a simple protection route for highly efficient metal–insulator–semiconductor photoanodes by showing over 200 h of operational stability.

Semiconducting properties of spinel tin nitride and other IV3N4 polymorphs

Tin nitride, Sn3N4, is a semiconductor composed of common elements with a band gap in the visible range, making it a candidate for optical and electronic applications. In this work, the semiconducting properties of tin nitride are explored by thin-film experiments and first-principles theory to evaluate the prospects of this material for optoelectronic applications. Calculations of related group IV nitride polymorphs provide additional insight into the properties and challenges associated with this class of semiconductors. Experimentally, in Sn3N4 polycrystalline thin films the electron concentration was found to be 1018 cm−3 with Hall mobility of ∼1 cm2 V−1 s−1 and a minority carrier (holes) diffusion length of 50–100 nm. The optical absorption onset was determined at 1.6 eV and an ionization potential was measured at 5.9–6.0 eV. From theory, a direct band gap of 1.54 eV was determined with weak dipole-forbidden lowest energy transitions and the ionization potential was determined to be 6.5 eV, both in reasonable agreement with the experiments. Calculations also predict an exceptionally small electron effective mass (0.18 m0) and a large hole effective mass (12.9 m0), which may be in part responsible for the short hole diffusion length. To elucidate the origin of the heavy holes in Sn3N4, elemental and crystallographic trends in electronic structure and thermochemical properties are calculated in the IV3N4 polymorphs. It was found that hole effective masses generally increase down the period and are structure-dependent, while electron effective masses decrease down the period and show no strong structural trends. These results suggest that changing composition in the group-IV nitride alloys will have a large impact on the fundamental semiconductor properties such as carrier effective masses, and provide other insight into the materials chemistry of Sn3N4 and the IV3N4 family.