Chengxiang Xiang

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.

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).

A monolithically integrated, intrinsically safe, 10% efficient, solar-driven water-splitting system based on active, stable earth-abundant electrocatalysts in conjunction with tandem III–V light absorbers protected by amorphous TiO2 films

A monolithically integrated device consisting of a tandem-junction GaAs/InGaP photoanode coated by an amorphous TiO2 stabilization layer, in conjunction with Ni-based, earth-abundant active electrocatalysts for the hydrogen-evolution and oxygen-evolution reactions, was used to effect unassisted, solar-driven water splitting in 1.0 M KOH(aq). When connected to a Ni–Mo-coated counterelectrode in a two-electrode cell configuration, the TiO2-protected III–V tandem device exhibited a solar-to-hydrogen conversion efficiency, ηSTH, of 10.5% under 1 sun illumination, with stable performance for >40 h of continuous operation at an efficiency of ηSTH > 10%. The protected tandem device also formed the basis for a monolithically integrated, intrinsically safe solar-hydrogen prototype system (1 cm2) driven by a NiMo/GaAs/InGaP/TiO2/Ni structure. The intrinsically safe system exhibited a hydrogen production rate of 0.81 μL s−1 and a solar-to-hydrogen conversion efficiency of 8.6% under 1 sun illumination in 1.0 M KOH(aq), with minimal product gas crossover while allowing for beneficial collection of separate streams of H2(g) and O2(g).

Discovering Ce-rich oxygen evolution catalysts, from high throughput screening to water electrolysis

We report a new Ce-rich family of active oxygen evolution reaction (OER) catalysts composed of earth abundant elements, discovered using high-throughput methods. High resolution inkjet printing was used to produce 5456 discrete oxide compositions containing the elements nickel, iron, cobalt and cerium. The catalytic performance of each of these compositions was measured under conditions applicable to distributed solar fuels generation using a three-electrode scanning drop electrochemical cell. The catalytic activity and stability of representative compositions (Ni0.5Fe0.3Co0.17Ce0.03Ox and Ni0.3Fe0.07Co0.2Ce0.43Ox) from 2 distinct regions were verified by resynthesizing these compositions on glassy carbon rods for electrochemical testing. The activity of the new Ce-rich catalysts was further verified using an unrelated synthetic method to electrodeposit a pseudo-ternary composition Ni0.2Co0.3Ce0.5Ox, which produced a catalyst with 10 mA cm−2 oxygen evolution current at 310 mV overpotential. The unique Tafel behavior of these Ce-rich catalysts affords the opportunity for further improvement.

An electrochemical engineering assessment of the operational conditions and constraints for solar-driven water-splitting systems at near-neutral pH

The solution transport losses in a one-dimensional solar-driven water-splitting cell that operates in either concentrated acid, dilute acid, or buffered near-neutral pH electrolytes have been evaluated using a mathematical model that accounts for diffusion, migration and convective transport, as well as for bulk electrochemical reactions in the electrolyte. The Ohmic resistance loss, the Nernstian potential loss associated with pH gradients at the surface of the electrode, and electrodialysis in different electrolytes were assessed quantitatively in a stagnant cell as well as in a bubble-convected cell, in which convective mixing occurred due to product-gas evolution. In a stagnant cell that did not have convective mixing, small limiting current densities (<3 mA cm^(−2)) and significant polarization losses derived from pH gradients were present in dilute acid as well as in near-neutral pH buffered electrolytes. In contrast, bubble-convected cells exhibited a significant increase in the limiting current density, and a significant reduction of the concentration overpotentials. In a bubble-convected cell, minimal solution transport losses were present in membrane-free cells, in either buffered electrolytes or in unbuffered solutions with pH ≤ 1. However, membrane-free cells lack a mechanism for product-gas separation, presenting significant practical and engineering impediments to the deployment of such systems. To produce an intrinsically safe cell, an ion-exchange membrane was incorporated into the cell. The accompanying solution losses, especially the pH gradients at the electrode surfaces, were modeled and simulated for such a system. Hence this work describes the general conditions under which intrinsically safe, efficient solar-driven water-splitting cells can be operated.

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.

High-throughput bubble screening method for combinatorial discovery of electrocatalysts for water splitting.

Combinatorial synthesis and screening for discovery of electrocatalysts has received increasing attention, particularly for energy-related technologies. High-throughput discovery strategies typically employ a fast, reliable initial screening technique that is able to identify active catalyst composition regions. Traditional electrochemical characterization via current-voltage measurements is inherently throughput-limited, as such measurements are most readily performed by serial screening. Parallel screening methods can yield much higher throughput and generally require the use of an indirect measurement of catalytic activity. In a water-splitting reaction, the change of local pH or the presence of oxygen and hydrogen in the solution can be utilized for parallel screening of active electrocatalysts. Previously reported techniques for measuring these signals typically function in a narrow pH range and are not suitable for both strong acidic and basic environments. A simple approach to screen the electrocatalytic activities by imaging the oxygen and hydrogen bubbles produced by the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is reported here. A custom built electrochemical cell was employed to record the bubble evolution during the screening, where the testing materials were subject to desired electrochemical potentials. The transient of the bubble intensity obtained from the screening was quantitatively analyzed to yield a bubble figure of merit (FOM) that represents the reaction rate. Active catalysts in a pseudoternary material library, (Ni-Fe-Co)Ox, which contains 231 unique compositions, were identified in less than one minute using the bubble screening method. An independent, serial screening method on the same material library exhibited excellent agreement with the parallel bubble screening. This general approach is highly parallel and is independent of solution pH.

Principles and implementations of electrolysis systems for water splitting

Efforts to develop renewable sources of carbon-neutral fuels have brought a renewed focus to research and development of sunlight-driven water-splitting systems. Electrolysis of water to produce H2 and O2 gases is the foundation of such systems, is conceptually and practically simple, and has been practiced both in the laboratory and industrially for many decades. In this Focus article, we present the fundamentals of water splitting and describe practices which distinguish commercial water-electrolysis systems from simple laboratory-scale demonstrations.

Scanning droplet cell for high throughput electrochemical and photoelectrochemical measurements

High throughput electrochemical techniques are widely applied in material discovery and optimization. For many applications, the most desirable electrochemical characterization requires a three-electrode cell under potentiostat control. In high throughput screening, a material library is explored by either employing an array of such cells, or rastering a single cell over the library. To attain this latter capability with unprecedented throughput, we have developed a highly integrated, compact scanning droplet cell that is optimized for rapid electrochemical and photoeletrochemical measurements. Using this cell, we screened a quaternary oxide library as (photo)electrocatalysts for the oxygen evolution (water splitting) reaction. High quality electrochemical measurements were carried out and key electrocatalytic properties were identified for each of 5456 samples with a throughput of 4 s per sample.

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.