Device and system design choices for solar energy conversion and storage approaches require holistic design guidelines which simultaneously respect and optimize technical, economic, sustainability, and operating time constraints. We developed a simulation platform which allows for the calculation of solar-to-hydrogen efficiency, hydrogen price, device manufacture and operation energy demand, and the component degradation and replacement time of photo-electrochemical water splitting devices. Utilizing this platform, we assessed 16 different design types representing all possible combinations of a system: i) operating with or without irradiation concentration, ii) utilizing high-performing and highcost or low-performing but low-cost photoabsorbers, iii) utilizing high-performing and high-cost or low-performing but low-cost electrocatalysts, and iv) operating with or without current concentration between the photoabsorber and the electrocatalyst. Our results show that device types exist with a global optimum (a Pareto point), simultaneously maximizing efficiency, while minimizing cost and the energy demand of manufacture and operation. In our examples, these happen to be the device types utilizing high irradiation concentration, as well as expensive photoabsorbers and electrocatalysts. These device types and designs were the most robust to degradation, exhibiting the smallest price sensitivity for increasing degradation rates. Other device types did not show a global optimum, but rather a set of partially optimized designs, i.e. a Pareto front, requiring a compromise and prioritization of either performance, cost, or manufacture and operation energy demand. In our examples, these happen to be the device types using low-cost photoabsorbers. The targeted utilization of irradiation and current concentration predicted that even device types utilizing expensive components can provide competitive solutions to photo-electrochemical water splitting. The quantification of the influence of component degradation on performance allows the suggestion of best practice for device operational time and component replacement. The framework and findings presented here provide holistic design guidelines for photo-electrochemical devices, and support the decision-making process for an integral and practical approach to competitive solar hydrogen production in the future. SignificanceSolar energy is the most abundant renewable energy source on earth. It is dilute, unequally distributed, and intermittent but can be stored, for example, in an energy-dense and transportable fuel such as hydrogen. Photo-electrochemical water-splitting devices convert solar energy into chemical energy integrating photo absorption, charge generation and separation, and electrocatalysis in a single device. The viability of such a device is only possible if four requirements are simultaneously fulfilled: i) high performance, ii) low cost, iii) sustainability, and iv) robustness. All devices developed up to now provide combinations of these aspects but do not sim...

Water-splitting devices that operate with humid air feeds are an attractive alternative for hydrogen production as the required water input can be obtained directly from ambient air. This article presents a novel proof-of-concept microfluidic platform that makes use of polymeric ion conductor (Nafion®) thin films to absorb water from air and performs the electrochemical water-splitting process. Modelling and experimental tools are used to demonstrate that these microstructured devices can achieve the delicate balance between water, gas, and ionic transport processes required for vapor-fed devices to operate continuously and at steady state, at current densities above 3 mA cm. The results presented here show that factors such as the thickness of the Nafion films covering the electrodes, convection of air streams, and water content of the ionomer can significantly affect the device performance. The insights presented in this work provide important guidelines for the material requirements and device designs that can be used to create practical electrochemical hydrogen generators that work directly under ambient air. Broader contextThe large scale deployment of hydrogen production technologies can be triggered by the development of electrolytic devices that function continuously under simple operation schemes. Water-splitting devices that operate under humid air are an attractive alternative to classic alkaline or proton exchange membrane electrolysis systems. In this regard, the implementation of water splitting technologies can be significantly simplified as the water feed could be obtained directly from the environment. Using polymeric ion-conducting materials in a microfluidic platform, this work balances the transport processes that are inherently limiting in devices operated with diluted water feeds, and demonstrates for the first time a vapor-fed microelectrolyzer capable of generating hydrogen at initial current densities above 10 mA cm −2

Solar irradiation concentration is considered a viable strategy for reducing the energy and financial investment of photo-electrochemical hydrogen generation. We quantified and compared the sustainability benefit of this approach to non-concentrating and conventional approaches using life cycle assessment coupled to device performance modeling. We formulated design guidelines to reduce the environmental impact of a device. Model devices were composed of a concentrator module (with tracking, supporting, and framing components), photoabsorbers, membrane-separated electrocatalysts, and a cooling circuit. We selected eight concentrator types covering five concentrating technologies. For each device we studied the effect of the irradiation concentration ratio, electrode to photabsorber area ratio, manufacturing requirements, incoming irradiance, and efficiency of components on sustainability utilizing two indexes: i) the energy yield ratio, and ii) the greenhouse gas yield ratio. Both indexes combine the performance of the system and its environmental impact. Two design guidelines were formulated based on the analysis: i) any concentration-stable photoabsorber and electrocatalyst is equally feasible at concentrations larger than 55, as their performance prevails over their energy demand, and ii) the system needs to be designed at an optimum concentration which depends on: performance, the relative surfaces of the photoabsorber and electrode, and irradiance. The study quantified and confirmed that concentrating solar irradiation has a beneficial effect on sustainability, energy yield, and greenhouse gas emissions compared to non-concentrated approaches. This was true for all concentrating technologies investigated. Consequently, this study provides an eco-performance-based rationale to further pursue the research and development of concentrated photo-electrochemical devices.Broader context: Solar energy is the most abundant energy source but it is distributed and intermittent requiring its conversion and storage for meaningful use. Photoelectrochemical (PEC) conversion approaches provide a practical and impactful storage approach through the development of devices which efficiently and continuously produce low cost hydrogen for several years. A fundamental requirement for any novel technology is its sustainability, which can be assessed by analysis of greenhouse gas emission and energy requirements during all phases of its lifetime. Recent research on these devices focus not only on material selection for photoabsorbers and electrocatalysts, but also on their design. Concentrated solar irradiation has been suggested as an approach to reduce the cost of PEC devices as it replaces a large fraction of expensive materials by less costly collection and concentrating components. However, this approach needs to ensure that the beneficial effects are not overshadowed by additional energy requirements and emissions, and potential efficiency reduction. This article examines the effects of design, material selection, and operatin...

-Increasingly vast research efforts are devoted to the development of materials and processes for solar hydrogen production by light-driven dissociation of water into oxygen and hydrogen. Storage of solar energy in chemical bonds resolves the issues associated with the intermittent nature of sunlight, by decoupling energy generation and consumption. This paper investigates recent advances and prospects in solar hydrogen processes that are reaching market readiness. Future energy scenarios involving solar hydrogen are proposed and a case is made for systems producing hydrogen from water vapor present in air, supported by advanced modeling.Résumé -L'hydrogène solaire arrive à maturité -Des efforts toujours plus importants sont consacrés au développement de matériaux et de processus permettant la production d'hydrogène par dissociation d'eau utilisant l'énergie solaire. Le stockage d'énergie solaire par voie chimique résout les problèmes associés à la nature intermittente de cette ressource. La génération et la consommation d'énergie sont ainsi découplées. Cet article examine les récents progrès obtenus sur les processus permettant la production d'hydrogène solaire prêts pour commercialisation. Il propose également des scénarios énergétiques innovants utilisant l'hydrogène solaire. Enfin, un dispositif permettant la production d'hydrogène utilisant la vapeur d'eau présente dans l'air ambiant est étudié avec l'appui de la modélisation numérique.

A combined experimental-numerical approach was used to study transient phenomena occurring in a photoelectrochemical cell using a membrane-separated porous TiO 2 -based photoanode and a dark Pt-based cathode. The effects of three parameters (pH in the anodic compartment, operating cell temperature, and cathode compartment preconditioning with hydrogen) on the photocurrent was systematically investigated using design of experiments and analysis of variance. A theoretical model was developed able to accurately reproduce and predict the measurements. The model indicated that the electrochemical reaction uses two parallel pathways on the anodic interface. The first pathway represented the rapid charging of surface states and the subsequent formation of acidic titanol groups at the TiO 2 /H 2 O interface which, upon illumination, caused an anodic overshoot at a short time scale. These states recombined with the formed O 2 at a longer time scale which resulted in a current decrease after the overshoot. The second pathway was governed by transfer processes of H + ions at the TiO 2 /Nafion interface and caused the observed current increase under illumination and positive relaxation in the dark, both at long time scales. A negative undershoot was observed when the reverse electrolysis reaction was preferred.

Theoretical considerations on the modelling of transport in a three-phase electrode and application to a proton conducting solid oxide electrolysis cell. International Journal of Hydrogen Energy, Elsevier, 2012, 37 (16) The paper presents a complete numerical model for the prediction of mass and charge transfer inside electrolysis cells and fuel cells working at high temperature and using cermets as electrodes. It also discusses some assumptions taken in fuel cells or electrolysis cells modelling.I think that the problems dealt in this paper are in perfect match with the topics covered by the International Journal of Hydrogen Energy. Indeed many articles that were published in the journal served as a basis for this work.Meng Ni, from the University of Hong Kong, M.M Hussain, from the University of Waterloo and Anchasa Pramuanjaroenkija, from the University of Miami, would represent in my view relevant reviewers for this paper. As specialists in fuel cells and electrolysis cells modelling, they can rigorously evaluate the scientific content of this paper. Moreover, several assumptions they use for their models are discussed in the paper.I am fully available from the first of January to deal with the reports of the reviewers.I will be pleased to supply any further information should it be required.Yours sincerely, Mikael DumortierCover LetterDemonstration of continuity equations for mass and charge inside a cermet electrode *Manuscript Click here to view linked References Abstract

The direct conversion of solar energy and water into a storable fuel via integrated photoelectrochemical (PEC) devices is investigated. Particularly, the proposed device uses concentrated solar irradiation in order to minimize the amount of rare and expensive components such as light absorbers and catalysts. Consequently, heat management becomes crucial for device performance. We present a 2D coupled multi-physics model using finite element and finite volume methods to predict the performance of the integrated PEC device. The model accounts for charge generation and transport in the triple junction solar cell and the components of the integrated electrolyzer (polymeric electrolyte and solid electrode), electrochemical reaction at the catalytic sites, fluid flow and species transport in the channels delivering the reactant (water) and removing the products (hydrogen and oxygen), and radiation absorption and heat transfer in all components. The model developed shows to be a valuable design and optimization tool for integrated PEC devices working with concentrated irradiation and at elevated temperatures.

This work focuses on the modelling of thermal processes inside a planar high temperature steam electrolyzer that use cermets as electrodes. While the continuity equation for mass and charge have been demonstrated in a previous publication, energy balance for thermal transfers inside the electrode assembly is established via a control volume method. A non-dimensional number is built from different criterion used in the literature in order to validate the local thermal equilibrium assumption (LTE) inside the porous electrodes. A parametric analysis is carried out on a proton-conducting solid oxide electrolysis cell in galvanostatic mode. The results show that the heat sources are mainly ohmic and that their locations are not dependent on inlet current and inlet velocity of gases. This observation allows us to build an original thermal resistance network in order to analytically evaluate the temperature inside each component of the cell. This modelling strategy reduces computation time, allows reverse physical analysis and gives a precise estimation on the maximum temperatures attained in the components of the cells.