Comparison between the electrical junction properties of H-terminated and methyl-terminated individual Si microwire/polymer assemblies for photoelectrochemical fuel production

The photoelectrical properties and stability of individual p-silicon (Si) microwire/polyethylenedioxythiophene/polystyrene sulfonate:Nafion/n-Si microwire structures, designed for use as arrays for solar fuel production, were investigated for both H-terminated and CH_3-terminated Si microwires. Using a tungsten probe method, the resistances of individual wires, as well as between individual wires and the conducting polymer, were measured vs. time. For the H-terminated samples, the n-Si/polymer contacts were initially rectifying, whereas p-Si microwire/polymer contacts were initially ohmic, but the resistance of both the n-Si and p-Si microwire/polymer contacts increased over time. In contrast, relatively stable, ohmic behavior was observed at the junctions between CH_3-terminated p-Si microwires and conducting polymers. CH_3-terminated n-Si microwire/polymer junctions demonstrated strongly rectifying behavior, attributable to the work function mismatch between the Si and polymer. Hence, a balance must be found between the improved stability of the junction electrical properties achieved by passivation, and the detrimental impact on the effective resistance associated with the additional rectification at CH_3-terminated n-Si microwire/polymer junctions. Nevertheless, the current system under study would produce a resistance drop of ~20 mV during operation under 100 mW cm^(−2) of Air Mass 1.5 illumination with high quantum yields for photocurrent production in a water-splitting device

Modeling, simulation, and design criteria for photoelectrochemical water-splitting systems

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

Measurement of the Electrical Resistance of n-Type Si Microwire/p-Type Conducting Polymer Junctions for Use in Artificial Photosynthesis

The junction between n-type silicon microwires and p-type conducting polymer PEDOT:PSS (poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate)) was investigated using a soft contact method. Dopant levels within the microwires were varied during growth to give a highly-doped region for improved contact and a low-doped region for light absorption. The low-doped region of the microwires had a dopant density of 5 X 10(17) cm(-3) while the highly-doped region had an increased dopant density of 5 X 10(18) cm(-3) over similar to 20 mu m. Uniform, highly-doped microwires, with a dopant density of 4 X 10(19) cm(3), were used as a comparison. Regions of highly-doped n-type Si microwires (N-D = 5 X 10(18) cm(-3) and 4 X 10(19) cm(-3)) contacted by PEDOT:PSS showed a significantly lower junction resistance compared to the low-doped (3 X 10(17) cm(-3)) regions of the microwire. Junctions incorporating the metal catalyst used during growth were also investigated. Microwires with copper at the interface had similar currentvoltage characteristics to those observed for the highly-doped microwire/conducting polymer junction; however, junctions that incorporated gold exhibited significantly lower resistances, decreasing the iR contribution of the junction by an order of magnitude with respect to the total voltage drop in the entire structure

Enhanced Stability and Efficiency for Photoelectrochemical Iodide Oxidation by Methyl Termination and Electrochemical Pt Deposition of n-Si Microwire Arrays

Arrays of Si microwires doped n-type (n-Si) and surface-functionalized with methyl groups have been used, with or without deposition of Pt electrocatalysts, to photoelectrochemically oxidize I–(aq) to I_3–(aq) in 7.6 M HI(aq). Under conditions of iodide oxidation, methyl-terminated n-Si microwire arrays exhibited stable short-circuit photocurrents over a time scale of days, albeit with low energy-conversion efficiencies. In contrast, electrochemical deposition of Pt onto methyl-terminated n-Si microwire arrays consistently yielded energy-conversion efficiencies of ∼2% for iodide oxidation, with an open-circuit photovoltage of ∼400 mV and a short-circuit photocurrent density of ∼10 mA cm^(–2) under 100 mW cm^(–2) of simulated air mass 1.5G solar illumination. Platinized electrodes were stable for >200 h of continuous operation, with no discernible loss of Si or Pt. Pt deposited using electron-beam evaporation also resulted in stable photoanodic operation of the methyl-terminated n-Si microwire arrays but yielded substantially lower photovoltages than when Pt was deposited electrochemically

A scanning probe investigation of the role of surface motifs in the behavior of p-WSe_2 photocathodes

The spatial variation in the photoelectrochemical performance for the reduction of an aqueous one-electron redox couple, Ru(NH_3)_6^(3+/2+), and for the evolution of H_2(g) from 0.5 M H_2SO_4(aq) at the surface of bare or Pt-decorated p-type WSe_2 photocathodes has been investigated in situ using scanning photocurrent microscopy (SPCM). The measurements revealed significant differences in the charge-collection performance (quantified by the values of external quantum yields, Φ_(ext)) on various macroscopic terraces. Local spectral response measurements indicated a variation in the local electronic structure among the terraces, which was consistent with a non-uniform spatial distribution of sub-band-gap states within the crystals. The photoconversion efficiencies of Pt-decorated p-WSe_2 photocathodes were greater for the evolution of H_2(g) from 0.5 M H_2SO_4 than for the reduction of Ru(NH_3)_6^(3+/2+), and terraces that exhibited relatively low values of Φ_(ext) for the reduction of Ru(NH_3)_6^(3+/2+) could in some cases yield values of Φ_(ext) for the evolution of H_2(g) comparable to the values of Φ_(ext) yielded by the highest-performing terraces. Although the spatial resolution of the techniques used in this work frequently did not result in observation of the effect of edge sites on photocurrent efficiency, some edge effects were observed in the measurements; however the observed edge effects differed among edges, and did not appear to determine the performance of the electrodes

Recent Advances in Photo-Initiated Electron Transfer at the Interface of Anatase TiO2 Nanocrystallites and Transition-Metal Polypyridyl Compounds

Molecular control of solar light harvesting and interfacial charge transfer at mesoporous, nanocrystalline semiconductor thin films are described. Light absorption by transition-metal coordination compounds anchored to wide band-gap semiconductors can initiate electron-transfer processes that ultimately reduce the semiconductor and oxidize the coordination compound. Such photo-induced charge separation is a key step for solar energy conversion. Three different interfacial charge-separation mechanisms are discussed in addition to regeneration processes wherein a mobile donor donates an electron to the oxidized coordination compound. Inorganic chemistry plays a central role in this approach to solar energy conversion, which may ultimately be optimized for practical applications

Elementary Reaction Steps That Precede or Follow a Unimolecular Reaction Step Can Obfuscate Interpretation of the Driving-Force Dependence to Its Rate Constant

Assessing the validity of a driving-force-dependent kinetic theory for a unimolecular elementary reaction step is difficult when the observed reaction rate is strongly influenced by properties of the preceding or following elementary reaction step. A well-known example occurs for bimolecular reactions with weak orbital overlap, such as outer-sphere electron transfer, where bimolecular collisional encounters that precede a fast unimolecular electron-transfer step can limit the observed rate. A lesser-appreciated example occurs for bimolecular reactions with stronger orbital overlap, including many proton-transfer reactions, where equilibration of an endergonic unimolecular proton-transfer step results in a relatively small concentration of reaction products, thus slowing the rate of the following step such that it becomes rate limiting. Incomplete consideration of these points has led to discrepancies in interpretation of data from the literature. Our reanalysis of these data suggests that proton-transfer elementary reaction steps have a nonzero intrinsic free energy barrier, implying, in the parlance of Marcus theory, that there is non-negligible nuclear reorganization. Outcomes from our analyses are generalizable to inner-sphere electron-transfer reactions such as those involved in (photo)electrochemical fuel-forming reactions

Detailed-balance limits for sunlight-to-protonic energy conversion from aqueous photoacids and photobases based on reversible mass-action kinetics

Detailed-balance limits to energy conversion efficiency critically inform design rules for photochemical power conversion devices. Herein we simulate efficiencies for sunlight-to-protonic power conversion for liquid water, which serves as the protonic semiconductor and is sensitized to visible-light absorption by reversible photoacids or photobases. Our model includes proton-transfer processes based on the Förster cycle with rate constants that follow the empirical Brønsted relation, where bimolecular reactions are encounter controlled. Based on physically relevant model parameters, simulations of steady-state concentrations of H+(aq) and OH−(aq) indicate that for defect-free water the maximum possible protonic quasi-chemical potentials result in a photovoltage of ∼330 mV and a power conversion efficiency of ∼10%. Conditions of maximum power conversion occur when photoacid (photobase) dyes exhibit acidities (basicities) of pKa (pKb) ≥ 14 and , an outcome that is nearly independent of equilibrium pH. These conditions are optimal because under standard-state conditions they result in rates for protonation of water to form H+(aq) and deprotonation of water to form OH−(aq) that are faster than other proton-transfer processes, due to their isoergic/exoergic nature. Simulations also indicate that longer excited-state lifetimes result in an increase in the range of pK* values that lead to significant protonic quasi-chemical potentials. This occurs because longer excited-state lifetimes up to ∼1 μs afford more time to perform excited-state proton transfer. Simulation outcomes are affected little by the inclusion of empirical nonzero activation free energies for isoergic/exoergic proton-transfer reactions or rate constants for equilibrium radiative generation and recombination of excited-state species under thermal detailed balance. Only when local electric fields are assumed present to increase rate constants for proton-transfer reactions do protonic quasi-chemical potentials suffer. Simulations also indicate that the total concentration of photoacid or photobase dyes exhibits two competing effects on protonic quasi-chemical potentials. Decreasing dye concentration results in less sunlight absorption and therefore smaller changes in steady-state concentrations of H+(aq) and OH−(aq), while also increasing the range of pK values that results in significant changes in quasi-chemical potentials. Overall, these results define parameters for effective sunlight-to-protonic power conversion and will help guide researchers in the design and development of photoacid and photobase dyes for light-driven proton pumps

Communication—Electrochemical Characterization of Commercial Bipolar Membranes under Electrolyte Conditions Relevant to Solar Fuels Technologies

Water electrolysis using a catholyte and anolyte at different pH values requires a bipolar membrane to sustain the pH difference and 1.23 V to electrolyze water. Bipolar membranes that separated concentrated aqueous acid and base exhibited an open-circuit potential consistent with the Nernst equation and rapid transport of protons and hydroxide ions. When excess supporting electrolyte was added to both solutions the membrane potential was measured to be ~0 V, which suggested that water electrolysis occurred at ≈1.23 V and therefore, protons and hydroxide ions were not the majority ionic charge carriers