A versatile open-source analysis of the limiting efficiency of photo electrochemical water-splitting

Understanding the fundamental thermodynamic limits of photo-electrochemical (PEC) water splitting is of great scientific and practical importance. In this work, a ‘detailed balance’ type model of solar quantum energy converters and non-linear circuit analysis is used to calculate the thermodynamic limiting efficiency of various configurations of PEC design. This model is released as freely accessible open-source (GNU GPL v3) code written in MATLAB with a graphical user interface (GUI). The capabilities of the model are demonstrated by simulating selected permutations of PEC design and results are validated against previous literature. This tool will enable solar fuel researchers to easily compare experimental results to theoretical limits to assess its realised performance using the GUI. Furthermore, the code itself is intended to be extendable and so can be modified to include non-ideal losses such as the over-potential required or complex optical phenomena

Photo-electrochemical engineering of solar water splitting devices

In order to scale up photo-electrochemical (PEC) water splitting, there are a number of engineering challenges that must be overcome. In order to logically approach this task, the PEC device was deconstructed into a classification framework comprising of two parts: the fundamental conceptual design and the engineering PEC device design. The first is used to investigate the fundamental limiting efficiencies of permutations of conceptual design, and the second to study the common elements of photo-electrode reactor design and critically compare examples from literature. After identifying the engineering challenges of scaling PEC devices, two specific issues were explored: 1) the optical losses from light scattered by bubbles at the gas-evolving photo-electrode under ‘front side’ illumination 2) the resistive losses in large area low conductivity substrate such as fluorine doped tin oxide (FTO). For both, the realistic losses were quantified and mitigation strategies evaluated. Finally, a new class of photo-electrochemical reactor was proposed: a membrane-less flow cell reactor that employs hydrodynamic separation of dissolved gaseous products. It is theoretically demonstrated that current densities typical of current PEC devices can be supported whilst suppressing bubble formation through operation at high pressure. The general operational design space was assessed with non-dimensional analysis and numerical modelling. Positive preliminary experimental results are displayed and a future work plan outlined. In summary, this thesis defines the role of photo-electrochemical engineering for solar to hydrogen devices, outlines the most pertinent challenges and begins to tackle a number of them, demonstrating that careful engineering design is required in order to realise any photo-electrochemical water splitting process.Open Acces

Electrochemical techniques for photoelectrode characterisation

Photoelectrodes enable simultaneous light absorption and catalysis of water splitting reactions. Their performance is established using electrochemical characterisation methods. Besides basic characterisation techniques such as voltammetry and chronoamperometry, employed in the dark or under illumination, more advanced techniques, including (photo-)electrochemical impedance spectroscopy, intensity-modulated impedance spectroscopy and transient absorption spectroscopy, can be used to evaluate key parameters and processes. For some of these techniques, data is often interpreted using over-simplified models, leading to the calculation of unreliable parameters. The values of the flat band potential and charge transfer efficiency depend heavily on the methods used to determine them, and it is recommended that the values are corroborated using multiple techniques. Lastly, certain ‘efficiencies’ defined in the literature for electrically biased systems should be revised

Membrane-less photoelectrochemical cells: product separation by hydrodynamic control

A key step in order to realise photo-electrochemical (PEC) water splitting to produce hydrogen sustainably, is reactor design. Good engineering will minimise energy losses (both optical and ohmic) due to reactor construction, whilst ensuring the H2 and O2 produced are separated, and this can subsequently relax the requirements on the photo-absorber material and/or electro-catalysts. In this paper we show that separation of the products through hydrodynamic flow alone would negate the need for the conventionally used membrane, which has an associated ohmic drop and cost. This is demonstrated to be possible using a ‘laminar flow between two parallel plates’ reactor design and AR/Pe and AR are found to be the two key dimensionless numbers that predict product cross-over (where AR, Pe are aspect ratio and Péclet number respectively). Supersaturation was used as an indicator of bubble formation, which disrupts the laminar flow required for separation and it is shown that by increasing the reactor pressure, higher current densities can be tolerated before supersaturation occurs. Removal of the dissolved hydrogen and oxygen from electrolyte is discussed. A multi-physics model, which employs an optical transfer matrix method, is used to validate the previous conclusions. Experimental data for hematite and Pt deposited on FTO was used as the anode and cathode respectively. Parasitic optical losses and efficiency improvement with stacking are shown for the example reactor configuration. Additionally, the concept of stacking this reactor design in order to absorb light in multiple passes is introduced. This approach relaxes a classical constraint on photo-absorber materials: large absorption length compared to small diffusion length of charge carriers in the semiconductor

Assessing the scalability of low conductivity substrates for photo-electrodes via modelling of resistive losses.

When scaling up photo-electrochemical processes to larger areas than conventionally studied in the laboratory, substrate performance must be taken into consideration and in this work, a methodology to assess this via an uncomplicated 2 dimensional model is outlined. It highlights that for F-doped SnO2 (FTO), which is ubiquitously used for metal oxide photoanodes, substrate performance becomes significant for moderately sized electrodes (5 cm) under no solar concentration for state of the art Fe2O3 thin films. It is demonstrated that when the process is intensified via solar concentration, current losses become quickly limiting. Methodologies to reduce the impact of substrate ohmic losses are discussed and a new strategy is proposed. Due to the nature of the photo-electrode current–potential relationship, operation at a higher potential where the photo-current saturates (before the dark current is observed) will lead to a minimum in current loss due to substrate performance. Crucially, this work outlines an additional challenge in scaling up photo-electrodes based on low conductivity substrates, and establishes that such challenges are not insurmountable

Dynamic system modeling of thermally-integrated concentrated PV-electrolysis

Understanding the dynamic response of a solar fuel processing system utilizing concentrated solar radiation and made of a thermally-integrated photovoltaic (PV) and water electrolyzer (EC) is important for the design, development and implementation of this technology. A detailed dynamic non-linear process model is introduced for the fundamental system components (i.e. PV, EC, pump etc.) in order to investigate the coupled system behaviour and performance synergy notably arising from the thermal integration. The nominal hydrogen production power is 2 kW at a hydrogen system effciency of 16-21 % considering a high performance triple junction III-V PV module and a proton exchange membrane EC. The device operating point relative to the maximum power point of the PV was shown to have a differing influence on the system performance when subject to temperature changes. The non-linear coupled behaviour was characterised in response to step changes in water flowrate and solar irradiance and hysteresis of the current-voltage operating point was demonstrated. Whilst the system responds thermally to changes in operating conditions in the range of 0.5-2 minutes which leads to advantageously short start-up times, a number of control challenges are identified such as the impact of pump failure, electrical PV-EC disconnection, and the potentially damaging accentuated temperature rise at lower water flowrates. Finally, the simulation of co-generation of heat and hydrogen for various operating conditions demonstrates the significant potential for system effciency enhancements and the required development of control strategies for demand matching is discussed

Effects of low temperature annealing on the photo-electrochemical performance o tin-doped hematite photo-anodes

The effects of post-deposition annealing at 400 and 500 °C on the photo-electrochemical performance of SnIV-doped α-Fe2O3 photo-anodes are reported. Samples were fabricated by spray pyrolysis on fluorine-doped tin oxide (FTO) and on titanium substrates. Photo-electrochemical, morphological and optical properties were determined to explain the shift in photocurrent densities to lower electrode potentials and the decrease of maximum photocurrent densities for alkaline water oxidation after annealing. Annealing at 400 and 500 °C in air did not affect significantly the morphology, crystallinity, optical absorption or spatial distributions of oxygen vacancy concentrations. However, XPS data showed a redistribution of SnIV near SnIV-doped α-Fe2O3 | 1 M NaOH interfaces after annealing. Thus, electron-hole recombination rates at photo-anode surfaces decreased after annealing, shifting photocurrents to lower electrode potentials. Conversely, depletion of SnIV in the α-Fe2O3 bulk could increase recombination rates therein and decrease photon absorption near 550 nm, due to an increased dopant concentration in the semiconductor depletion layer. This accounted for the decrease of maximum photocurrents when electron-hole recombination rates were suppressed using HO2− ions as a hole scavenger. The flat band potential of SnIV-doped α-Fe2O3 remained relatively constant at ca. 0.7 V vs. RHE, irrespective of annealing conditions