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

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

Numerical Monte Carlo Simulations to Evaluate the Influence That Spherical Nanoparticle Size and Arrangement Have on Interparticle Charge Transport across the Surface of Dye- and Cocatalyst-Modified Materials

Mechanistic information underlying the function of illuminated mesoporous thin films of nanomaterials that contain distinct light-absorbing and electrocatalytic units can be gleaned from discrete-time random walk Monte Carlo simulations. These simulations bridge the length and time scales between individual electron-transfer events and ensemble behavior observed from bulk thin films. Most simulations are performed using models that simplify random mesoporous networks as isolated spherical nanoparticles. However, these simplifications may not provide sufficient detail to capture macroscopic experimental observations, especially when mesoporous nanomaterials consist of various geometries. Herein, we examine the role that the structure of the mesoporous thin film plays on the ability of photogenerated charges to accumulate on surface-confined redox-active electrocatalysts that require two redox events for turnover. We observe that the structure has a dramatic influence on the expected spectroscopic absorption anisotropy signal over time. We also observe that the yield for electrocatalyst turnover, as a function of the ratio of the electron-transfer time constant for self-exchange reactions and charge recombination time constant between the semiconducting mesoporous thin film and an oxidized/reduced surface-confined dye or electrocatalyst, is influenced by the total surface coverage of redox-active species. Structures consisting of spherical nanoparticles that are barely touching or partially necked are more effective at electrocatalytic turnover in the presence of vacant sites than discrete spherical nanoparticles or those that are arranged in a rodlike structure. Moreover, we show that the yield for electrocatalyst turnover is nearly independent of whether the simulations are performed on complex three-dimensional structures or simple two-dimensional planar grids. This discovery suggests that the added complexity of three-dimensional models may not be necessary to explain differences in electrocatalytic turnover yield in photoelectrochemical constructs containing surface-confined light-absorbing and electrocatalytic units

Numerical Monte Carlo simulations of charge transport across the surface of dye and cocatalyst modified spherical nanoparticles under conditions of pulsed or continuous illumination

Solar fuel constructs consisting of discrete light-absorbers and distinct redox-active electrocatalysts are well suited for numerical modeling of their charge-transfer processes. Herein by several series of Monte Carlo simulations employing spherical nanoparticle molecular supports, we identify conditions that result in the largest yield for forming specific redox states of electrocatalysts. In general, the yield for electrocatalyst oxidation/reduction increased as the self-exchange electron-transfer time constant decreased and/or the recombination time constant increased. When the number of electrocatalysts increased to more than one per nanoparticle, yields for oxidation/reduction of electrocatalysts decreased because oxidative/reductive equivalents were diluted among the larger number of electrocatalysts. As the light intensity increased the yield for oxidation/reduction of electrocatalysts increased both in absolute number and yield per absorbed photon. However, at extreme photon fluences the yield per absorbed photon decreased due to significantly faster recombination which is clear from the equal-concentration second-order nature of the recombination reaction in the number of oxidized/reduced molecules per nanoparticle. Results obtained using electrocatalysts that only required a single oxidation/reduction event for turnover were within error the same irrespective of whether optical excitation was simulated to occur as an initial pulse, to mimic pulsed-laser spectroscopic measurements, or with repeated photoexcitation events, to mimic conditions of solar illumination. However, when electrocatalysts required multiple oxidations/reductions for turnover the intensity of pulsed light required to obtain the same electrocatalyst turnover yield that was observed using repeated photoexcitation depended greatly on the electron-transfer time constants. In addition, at solar-relevant fluences the equal-concentration second-order kinetic process for recombination exhibited a first-order dependence on the number of nanoparticles that contained an oxidized/reduced molecule. The rate of electrocatalyst turnover after two redox events was also determined to have a first-order dependence on the concentration of oxidized/reduced molecules over most of the time that oxidized/reduced molecules were present. Collectively the solar-simulated data showed that even under the assumption of ideal kinetic processes and molecular and semiconductor densities of states, the observed kinetic behavior can be complex and change as a function of time and fluence. These observations suggest that results from pulsed-laser spectroscopic measurements are not always accurate predictors of the expected behavior of sunlight-illuminated dye-sensitized photoelectrochemical cells that drive multiple-charge-transfer reactions

Experimental demonstrations of spontaneous, solar-driven photoelectrochemical water splitting

Laboratory demonstrations of spontaneous photoelectrochemical (PEC) solar water splitting cells are reviewed. Reported solar-to-hydrogen (STH) conversion efficiencies range from 10% STH efficiency using potentially less costly materials have been reported. Device stability is a major challenge for the field, as evidenced by lifetimes of less than 24 hours in all but a few reports. No globally accepted protocol for evaluating and certifying STH efficiencies and lifetimes exists. It is our recommendation that a protocol similar to that used by the photovoltaic community be adopted so that future demonstrations of solar PEC water splitting can be compared on equal grounds

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

Funneling solar energy and charge at sensitized mesoporous films

The capture and transfer of energy in visible light is fundamentally important for solar energy conversion. Time-resolved polarization spectroscopy was used to quantify two separate self-exchange processes that occur following visible light absorption by RuII-polypyridyl compounds anchored to mesoporous, nanocrystalline (anatase) TiO2 thin films commonly used in dye-sensitized solar cells. Excited-state energy transfer: Under conditions of unfavorable excited-state injection, rapid Ru* + Ru → Ru + Ru* was observed. The resulting anisotropy kinetics were successfully simulated by Monte Carlo methods. Lateral hole hopping: Subsequent to electron injection, the resulting RuIII state of the sensitizer was capable of RuIII + RuII → RuII + RuIII reactions across the surface of TiO2 prior to recombination. The absorption anisotropy kinetics for this process varied greatly with sensitizer and electrolyte. These mechanisms could be utilized for solar fuel formation as alternative means of funneling potential energy from sensitizers to multiple-charge-transfer catalysts