No abstract

Silicon wire arrays, though attractive materials for use in photovoltaics and as photocathodes for hydrogen generation, have to date exhibited poor performance. Using a copper-catalyzed, vapor-liquid-solid–growth process, SiCl4 and BCl3 were used to grow ordered arrays of crystalline p-type silicon (p-Si) microwires on p+-Si(111) substrates. When these wire arrays were used as photocathodes in contact with an aqueous methyl viologen2+/+ electrolyte, energy-conversion efficiencies of up to 3% were observed for monochromatic 808-nanometer light at fluxes comparable to solar illumination, despite an external quantum yield at short circuit of only 0.2. Internal quantum yields were at least 0.7, demonstrating that the measured photocurrents were limited by light absorption in the wire arrays, which filled only 4% of the incident optical plane in our test devices. The inherent performance of these wires thus conceptually allows the development of efficient photovoltaic and photoelectrochemical energy-conversion devices based on a radial junction platform.

An important approach for solving the world's sustainable energy challenges is the conversion of solar energy to chemical fuels. Semiconductors can be used to convert/store solar energy to chemical bonds in an energy-dense fuel. Photoelectrochemical (PEC) water-splitting cells, with semiconductor electrodes, use sunlight and water to generate hydrogen. Herein, recent studies on improving the efficiency of semiconductor-based solar water-splitting devices by the introduction of surface passivation layers are reviewed. We show that passivation layers have been used as an effective strategy to improve the charge-separation and transfer processes across semiconductor-liquid interfaces, and thereby increase overall solar energy conversion efficiencies. We also summarize the demonstrated passivation effects brought by these thin layers, which include reducing charge recombination at surface states, increasing the reaction kinetics, and protecting the semiconductor from chemical corrosion.These benefits of passivation layers play a crucial role in achieving highly efficient water-splitting devices in the near future. Broader contextSemiconductor interface is one of the most important components/regions in photoelectrochemical (PEC) water splitting devices. The serious surface charge recombination, slow charge transfer kinetics and the poor photoelectrochemical stability are the three main challenges for the practical PEC water splitting devices. Recently, the studies of constructing passivation layer onto the semiconductor to address these challenges have attracted increasing research attentions, due to the potential application of solar to chemical energy conversion. When these layers incorporated onto semiconductor surface, signicant changes of the interface would be found including charge transfer improvement, surface charge recombination, and chemical corrosion, etc. Although various strategies have been suggested for this surface modication, a synergetic effect must be achieved on an advanced photoelectrodes accounting for the improved solar-tohydrogen efficiencies. This review will shed light on the recent PEC water splitting work surface state passivation, corrosion retardation and charge transfer improvement in this passivation layer.

A validated multi-physics numerical model that accounts for charge and species conservation, fluid flow, and electrochemical processes has been used to analyze the performance of solar-driven photoelectrochemical water-splitting systems. The modeling has provided an in-depth analysis of conceptual designs, proof-of-concepts, feasibility investigations, and quantification of performance. The modeling has led to the formulation of design guidelines at the system and component levels, and has identified quantifiable gaps that warrant further research effort at the component level. The two characteristic generic types of photoelectrochemical systems that were analyzed utilized: (i) side-by-side photoelectrodes and (ii) back-to-back photoelectrodes. In these designs, small electrode dimensions (mm to cm range) and large electrolyte heights were required to produce small overall resistive losses in the system. Additionally, thick, non-permeable separators were required to achieve acceptably low rates of product crossover.

Electrochemical reduction of CO2 to fuels and chemicals is currently a focus of significant research effort as a technology that can simultaneously mitigate greenhouse gas emissions while storing renewable electricity for use on demand. Herein, the economics of CO2 reduction to select liquid products is analyzed.

Electrochemical conversion of CO into energy-dense liquids, such as formic acid, is desirable as a hydrogen carrier and a chemical feedstock. SnO is one of the few catalysts that reduce CO into formic acid with high selectivity but at high overpotential and low current density. We show that an electrochemically reduced SnO porous nanowire catalyst (Sn-pNWs) with a high density of grain boundaries (GBs) exhibits an energy conversion efficiency of CO -into-HCOOH higher than analogous catalysts. HCOOH formation begins at lower overpotential (350 mV) and reaches a steady Faradaic efficiency of ca. 80 % at only -0.8 V vs. RHE. A comparison with commercial SnO nanoparticles confirms that the improved CO reduction performance of Sn-pNWs is due to the density of GBs within the porous structure, which introduce new catalytically active sites. Produced with a scalable plasma synthesis technology, the catalysts have potential for application in the CO conversion industry.

Artificial photosynthesis relies on the availability of semiconductors that are chemically stable and can efficiently capture solar energy. Although metal oxide semiconductors have been investigated for their promise to resist oxidative attack, materials in this class can suffer from chemical and photochemical instability. Here we present a methodology for evaluating corrosion mechanisms and apply it to bismuth vanadate, a state-of-the-art photoanode. Analysis of changing morphology and composition under solar water splitting conditions reveals chemical instabilities that are not predicted from thermodynamic considerations of stable solid oxide phases, as represented by the Pourbaix diagram for the system. Computational modelling indicates that photoexcited charge carriers accumulated at the surface destabilize the lattice, and that self-passivation by formation of a chemically stable surface phase is kinetically hindered. Although chemical stability of metal oxides cannot be assumed, insight into corrosion mechanisms aids development of protection strategies and discovery of semiconductors with improved stability.

Several application fields can benefit from solar-hydrogen technologies via specific short-term and long-term pathways.