Karl Walczak

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.

Efficient and Sustained Photoelectrochemical Water Oxidation by Cobalt Oxide/Silicon Photoanodes with Nanotextured Interfaces

Plasma-enhanced atomic layer deposition of cobalt oxide onto nanotextured p(+)n-Si devices enables efficient photoelectrochemical water oxidation and effective protection of Si from corrosion at high pH (pH 13.6). A photocurrent density of 17 mA/cm(2) at 1.23 V vs RHE, saturation current density of 30 mA/cm(2), and photovoltage greater than 600 mV were achieved under simulated solar illumination. Sustained photoelectrochemical water oxidation was observed with no detectable degradation after 24 h. Enhanced performance of the nanotextured structure, compared to planar Si, is attributed to a reduced silicon oxide thickness that provides more intimate interfacial contact between the light absorber and catalyst. This work highlights a general approach to improve the performance and stability of Si photoelectrodes by engineering the catalyst/semiconductor interface.

An experimental and modeling/simulation-based evaluation of the efficiency and operational performance characteristics of an integrated, membrane-free, neutral pH solar-driven water-splitting system

The efficiency limits, gas-crossover behavior, formation of local pH gradients near the electrode surfaces, and safety characteristics have been evaluated experimentally as well as by use of multi-physics modeling and simulation methods for an integrated solar-driven water-splitting system that operates with bulk electrolyte solutions buffered at near-neutral pH. The integrated membrane-free system utilized a triple-junction amorphous hydrogenated Si (a-Si:H) cell as the light absorber, Pt and cobalt phosphate (Co–Pi) as electrocatalysts for the hydrogen-evolution reaction (HER) and oxygen-evolution reaction (OER), respectively, and a bulk aqueous solution buffered at pH = 9.2 by 1.0 M of boric acid/borate as an electrolyte. Although the solar-to-electrical efficiency of the stand-alone triple-junction a-Si:H photovoltaic cell was 7.7%, the solar-to-hydrogen (STH) conversion efficiency for the integrated membrane-free water-splitting system was limited under steady-state operation to 3.2%, and the formation of pH gradients near the electrode surfaces accounted for the largest voltage loss. The membrane-free system exhibited negligible product-recombination loss while operating at current densities near 3.0 mA cm−2, but exhibited significant crossover of products (up to 40% H2 in the O2 chamber), indicating that the system was not intrinsically safe. A system that contained a membrane to minimize the gas crossover, but which was otherwise identical to the membrane-free system, yielded very low energy-conversion efficiencies at steady state, due to low transference numbers for protons across the membranes resulting in electrodialysis of the solution and the consequent formation of large concentration gradients of both protons and buffer counterions near the electrode surfaces. The modeling and simulation results showed that despite the addition of 1.0 M of buffering agent to the bulk of the solution, during operation significant pH gradients developed near the surfaces of the electrodes. Hence, although the bulk electrolyte was buffered to near-neutral pH, the electrode surfaces and electrocatalysts experienced local environments under steady-state operation that were either highly acidic or highly alkaline in nature, changing the chemical form of the electrocatalysts and exposing the electrodes to potentially corrosive local pH conditions. In addition to significant pH gradients, the STH conversion efficiency of both types of systems was limited by the mass transport of ionic species to the electrode surfaces. Even at operating current densities of <3 mA cm−2, the voltage drops due to these pH gradients exceeded the combined electrocatalyst overpotentials for the hydrogen- and oxygen-evolution reactions at current densities of 10 mA cm−2. Hence, such near-neutral pH solar-driven water-splitting systems were both fundamentally limited in efficiency and/or co-evolved explosive mixtures of H2(g) and O2(g) in the presence of active catalysts for the recombination of H2(g) and O2(g).

Life-cycle net energy assessment of large-scale hydrogen production via photoelectrochemical water splitting

Here we report a prospective life-cycle net energy assessment of a hypothetical large-scale photoelectrochemical (PEC) hydrogen production facility with energy output equivalent to 1 GW continuous annual average (1 GW HHV = 610 metric tons of H2 per day). We determine essential mass and energy flows based on fundamental principles, and use heuristic methods to conduct a preliminary engineering design of the facility. We then develop and apply a parametric model describing system-wide energy flows associated with the production, utilization, and decommissioning of the facility. Based on these flows, we calculate and interpret life-cycle net energy metrics for the facility. We find that under base-case conditions the energy payback time is 8.1 years, the energy return on energy invested (EROEI) is 1.7, and the life-cycle primary energy balance over the 40 years projected service life of the facility is +500 PJ. The most important model parameters affecting the net energy metrics are the solar-to-hydrogen (STH) conversion efficiency and the life span of the PEC cells; parameters associated with the balance of systems (BOS), including construction and operation of the liquid and gas handling infrastructure, play a much smaller role.

Robust production of purified H2 in a stable, self-regulating, and continuously operating solar fuel generator

The development of practical solar-driven electrochemical fuel generators requires the integration of light absorbing and electrochemical components into an architecture that must also provide easy separation of the product fuels. Unfortunately, many of these components are not stable under the extreme pH conditions necessary to facilitate ionic transport between redox reaction sites. By using a controlled recirculating stream across reaction sites, this work demonstrates a stable, self-regulating and continuous purified solar-hydrogen generation from near neutral pH electrolytes that yield continuous nearly pure H2 streams with solar-fuel efficiencies above 6.2%.

Modeling, Simulation, and Fabrication of a Fully Integrated, Acid-stable, Scalable Solar-Driven Water-Splitting System

A fully integrated solar-driven water-splitting system comprised of WO3 /FTO/p(+) n Si as the photoanode, Pt/TiO2 /Ti/n(+) p Si as the photocathode, and Nafion as the membrane separator, was simulated, assembled, operated in 1.0 M HClO4 , and evaluated for performance and safety characteristics under dual side illumination. A multi-physics model that accounted for the performance of the photoabsorbers and electrocatalysts, ion transport in the solution electrolyte, and gaseous product crossover was first used to define the optimal geometric design space for the system. The photoelectrodes and the membrane separators were then interconnected in a louvered design system configuration, for which the light-absorbing area and the solution-transport pathways were simultaneously optimized. The performance of the photocathode and the photoanode were separately evaluated in a traditional three-electrode photoelectrochemical cell configuration. The photocathode and photoanode were then assembled back-to-back in a tandem configuration to provide sufficient photovoltage to sustain solar-driven unassisted water-splitting. The current-voltage characteristics of the photoelectrodes showed that the low photocurrent density of the photoanode limited the overall solar-to-hydrogen (STH) conversion efficiency due to the large band gap of WO3 . A hydrogen-production rate of 0.17 mL hr(-1) and a STH conversion efficiency of 0.24 % was observed in a full cell configuration for >20 h with minimal product crossover in the fully operational, intrinsically safe, solar-driven water-splitting system. The solar-to-hydrogen conversion efficiency, ηSTH , calculated using the multiphysics numerical simulation was in excellent agreement with the experimental behavior of the system. The value of ηSTH was entirely limited by the performance of the photoelectrochemical assemblies employed in this study. The louvered design provides a robust platform for implementation of various types of photoelectrochemical assemblies, and can provide an approach to significantly higher solar conversion efficiencies as new and improved materials become available.

Modeling, Simulation, and Implementation of Solar-Driven Water-Splitting Devices.

An integrated cell for the solar-driven splitting of water consists of multiple functional components and couples various photoelectrochemical (PEC) processes at different length and time scales. The overall solar-to-hydrogen (STH) conversion efficiency of such a system depends on the performance and materials properties of the individual components as well as on the component integration, overall device architecture, and system operating conditions. This Review focuses on the modeling- and simulation-guided development and implementation of solar-driven water-splitting prototypes from a holistic viewpoint that explores the various interplays between the components. The underlying physics and interactions at the cell level is are reviewed and discussed, followed by an overview of the use of the cell model to provide target properties of materials and guide the design of a range of traditional and unique device architectures.

Net primary energy balance of a solar-driven photoelectrochemical water-splitting device

A fundamental requirement for a renewable energy generation technology is that it should produce more energy during its lifetime than is required to manufacture it. In this study we evaluate the primary energy requirements of a prospective renewable energy technology, solar-driven photoelectrochemical (PEC) production of hydrogen from water. Using a life cycle assessment (LCA) methodology, we evaluate the primary energy requirements for upstream raw material preparation and fabrication under a range of assumptions of processes and materials. As the technology is at a very early stage of research and development, the analysis has considerable uncertainties. We consider and analyze three cases that we believe span a relevant range of primary energy requirements: 1550 MJ m−2 (lower case), 2110 MJ m−2 (medium case), and 3440 MJ m−2 (higher case). We then use the medium case primary energy requirement to estimate the net primary energy balance (energy produced minus energy requirement) of the PEC device, which depends on device performance, e.g. longevity and solar-to-hydrogen (STH) efficiency. We consider STH efficiency ranging from 3% to 10% and longevity ranging from 5 to 30 years to assist in setting targets for research, development and future commercialization. For example, if STH efficiency is 3%, the longevity must be at least 8 years to yield a positive net energy. A sensitivity analysis shows that the net energy varies significantly with different assumptions of STH efficiency, longevity and thermo-efficiency of fabrication. Material choices for photoelectrodes or catalysts do not have a large influence on primary energy requirements, though less abundant materials like platinum may be unsuitable for large scale-up.

Opportunities to improve the net energy performance of photoelectrochemical water-splitting technology

The hydrogen energy provided by solar-driven photoelectrochemical (PEC) water splitting must be greater than the energy used to produce and operate the technology, as a fundamental system requirement to enable energetic benefits to society. PEC H2 production will require significant advances from both basic scientific research and applied technology development, prior to manufacturing and field deployment. To identify opportunities and priorities, here we use prospective life cycle system modeling to investigate the net-energy significance of six characteristics describing the PEC life cycle: (1) embodied energy of active cell materials, (2) embodied energy of inactive module materials, (3) energy intensity of active cell fabrication, (4) energy intensity of PEC module assembly, (5) initial energy use for production of balance-of-system (BOS), and (6) ongoing energy use for operation and end-of-life of BOS. We develop and apply a system model describing material and energy flows during the full life cycle of louvered thin-film PEC cells and their associated modules and BOS components. We find that fabrication processes for the PEC cells, especially the thin-film deposition of active cell materials, are important drivers of net energy performance. Nevertheless, high solar-to-hydrogen (STH) conversion efficiency and long cell life span are primary design requirements for PEC systems, even if such performance requires additional energy and material inputs for production and operation. We discuss these and other system dynamics, and highlight pathways to improve net energy performance.

Suppression of Photoanodic Surface Oxidation of n-Type 6H-SiC Electrodes in Aqueous Electrolytes

The photoelectrochemical characterization of silicon carbide (SiC) electrodes is important for enabling a wide range of potential applications for this semiconductor. However, photocorrosion of the SiC surface remains a key challenge, because this process considerably hinders the deployment of this material into functional devices. In this report, we use cyclic voltammetry to investigate the stability of n-type 6H-SiC photoelectrodes in buffered aqueous electrolytes. For measurements in pure Tris buffer, photogenerated holes accumulate at the interface under anodic polarization, resulting in the formation of a porous surface oxide layer. Two possibilities are presented to significantly enhance the stability of the SiC photoelectrodes. In the first approach, redox molecules are added to the buffer solution to kinetically facilitate hole transfer to these molecules, and in the second approach, water oxidation in the electrolyte is induced by depositing a cobalt phosphate catalyst onto the semiconductor surface. Both methods are found to effectively suppress photocorrosion of the SiC electrodes, as confirmed by atomic force microscopy and X-ray photoelectron spectroscopy measurements. The presented study provides straightforward routes to stabilize n-type SiC photoelectrodes in aqueous electrolytes, which is essential for a possible utilization of this material in the fields of photocatalysis and multimodal biosensing.