Shane Ardo

Technical and economic feasibility of centralized facilities for solar hydrogen production via photocatalysis and photoelectrochemistry

Photoelectrochemical water splitting is a promising route for the renewable production of hydrogen fuel. This work presents the results of a technical and economic feasibility analysis conducted for four hypothetical, centralized, large-scale hydrogen production plants based on this technology. The four reactor types considered were a single bed particle suspension system, a dual bed particle suspension system, a fixed panel array, and a tracking concentrator array. The current performance of semiconductor absorbers and electrocatalysts were considered to compute reasonable solar-to-hydrogen conversion efficiencies for each of the four systems. The U.S. Department of Energy H2A model was employed to calculate the levelized cost of hydrogen output at the plant gate at 300 psi for a 10 tonne per day production scale. All capital expenditures and operating costs for the reactors and auxiliaries (compressors, control systems, etc.) were considered. The final cost varied from

Experimental demonstrations of spontaneous, solar-driven photoelectrochemical water splitting

1.60–

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

10.40 per kg H2 with the particle bed systems having lower costs than the panel-based systems. However, safety concerns due to the cogeneration of O2 and H2 in a single bed system and long molecular transport lengths in the dual bed system lead to greater uncertainty in their operation. A sensitivity analysis revealed that improvement in the solar-to-hydrogen efficiency of the panel-based systems could substantially drive down their costs. A key finding is that the production costs are consistent with the Department of Energys targeted threshold cost of

Particle suspension reactors and materials for solar-driven water splitting

2.00–

Photoelectrochemistry of core–shell tandem junction n–p^+-Si/n-WO_3 microwire array photoelectrodes

4.00 per kg H2 for dispensed hydrogen, demonstrating that photoelectrochemical water splitting could be a viable route for hydrogen production in the future if material performance targets can be met.

Non-Nernstian Two-Electron Transfer Photocatalysis at Metalloporphyrin–TiO2 Interfaces

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.

Direct Observation of Photodriven Intermolecular Hole Transfer across TiO2 Nanocrystallites: Lateral Self-Exchange Reactions and Catalyst Oxidation

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.

Slow Cation Transfer Follows Sensitizer Regeneration at Anatase TiO2 Interfaces

Reactors based on particle suspensions for the capture, conversion, storage, and use of solar energy as H2 are projected to be cost-competitive with fossil fuels. In light of this, this review paper summarizes state-of-the-art particle light absorbers and cocatalysts as suspensions (photocatalysts) that demonstrate visible-light-driven water splitting on the laboratory scale. Also presented are reactor descriptions, theoretical considerations particular to particle suspension reactors, and efficiency and performance characterization metrics. Opportunities for targeted research, analysis, and development of reactor designs are highlighted.

Photoelectrochemical Behavior of n‑Type Si(111) Electrodes Coated With a Single Layer of Graphene

Tandem junction (n–p^+-Si/ITO/WO_3/liquid) core–shell microwire devices for solar-driven water splitting have been designed, fabricated and investigated photoelectrochemically. The tandem devices exhibited open-circuit potentials of E_(∝) = −1.21 V versus E^0′(O_2/H_2O), demonstrating additive voltages across the individual junctions (n–p^+-Si E_(∝) = −0.5 V versus solution; WO_3/liquid E_(∝) = −0.73 V versus E^0′(O_2/H_2O)). Optical concentration (12×, AM1.5D) shifted the open-circuit potential to E_(∝) = −1.27 V versus E^0′(O_2/H_2O) and resulted in unassisted H_2 production during two-electrode measurements (anode: tandem device, cathode: Pt disc). The solar energy-conversion efficiencies were very low, 0.0068% and 0.0019% when the cathode compartment was saturated with Ar or H_2, respectively, due to the non-optimal photovoltage and band-gap of the WO_3 that was used in the demonstration system to obtain stability of all of the system components under common operating conditions while also insuring product separation for safety purposes.

Use of Bipolar Membranes for Maintaining Steady‐State pH Gradients in Membrane‐Supported, Solar‐Driven Water Splitting

A long-standing question in the photochemical sciences concerns how to integrate single-electron transfers to catalytic multielectron transfer reactions that produce useful chemical fuels. Here we provide a strategy for the two-electron formation of C-C bonds with molecular catalysts anchored to semiconductor nanocrystallites. The blue portion of the solar spectrum provides band gap excitation of the semiconductor while longer wavelengths of light initiate homolytic cleavage of metal-carbon bonds that, after interfacial charge transfer, restore the catalyst. The semiconductor utilized was the anatase polymorph of TiO(2) present as a nanocrystalline, mesoporous thin film. The catalyst was cobalt meso-5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin chloride, Co(TCPP)Cl. For this catalyst and iron protoporphyrin IX chloride, Fe(PPIX)Cl, two distinct and sequential metal-based M(III/II) and M(II/I) reductions were observed under band gap illumination. Spectroelectrochemical characterization indicated that both reductions were non-Nernstian, behavior attributed to an environmentally dependent potential drop across the molecule-semiconductor interface. Reaction of Co(I)(TCPP)/TiO(2) with organobromides (RBr = 1-Br-hexane or benzyl bromide) resulted in the formation of Co(III)-R(TCPP)/TiO(2). Visible light excitation induced homolytic cleavage of the Co-C bond and the formation of C-C-bonded products. The reactions were catalytic when band gap excitation or an electrochemical bias provided TiO(2) electrons to the oxidized catalyst. Sustained photocurrents were quantified in photoelectrosynthetic solar cells under forward bias.