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Achieving high current densities while maintaining high energy conversion efficiency is one of the main challenges for enhancing the competitiveness of photo-electrochemical devices. We describe a concept that allows this challenge to be overcome by operating under concentrated solar irradiation (up to 474 kW m −2 ), using thermal integration, mass transport optimization and a close electronic integration between the photoabsorber and electrocatalyst. We quantify the increase in the theoretical maximum efficiencies resulting from thermal integration, and experimentally validate the concept using a III–V-based photoabsorber and IrRuOx–Pt-based electrocatalysts. We reach current densities higher than 0.88 A cm −2 at calculated solar-to-hydrogen conversion efficiencies above 15%. Device performance, dynamic response and stability are investigated, demonstrating the ability to produce hydrogen stably under varying conditions for more than two hours. The current density and output power (27 W) achieved provide a pathway for device scalability aimed towards the large-scale deployment of photo-electrochemical hydrogen production. For photo-electrochemical hydrogen production to become viable on a large scale, not only efficiency but also power density must be optimized. Here, the authors explore the impact of thermal integration on photo-electrochemical devices driven by concentrated solar irradiation and design one that operates with high efficiency and power density output.

The reaction of carbon dioxide with hydroxide to form carbonate in near-neutral or alkaline medium severely limits the energy and carbon efficiency of CO 2 electroreduction. Here we show that by suppression of the otherwise predominant hydrogen evolution using alkali cations, efficient CO 2 electroreduction can be conducted in acidic medium, overcoming the carbonate problem. The cation effects are general for three typical catalysts including carbon-supported tin oxide, gold and copper, leading to Faradaic efficiency as high as 90% for formic acid and CO formation. Our analysis suggests that hydrated alkali cations physisorbed on the cathode modify the distribution of electric field in the double layer, which impedes hydrogen evolution by suppression of migration of hydronium ions while at the same time promoting CO 2 reduction by stabilization of key intermediates. Acidic media provide an opportunity to alleviate carbonate formation in electrocatalytic CO 2 reduction but increase competition from H 2 evolution. This study demonstrates that alkali cations in acidic media suppress H 2 evolution leading to high Faradaic efficiency for carbon-based products and models the physical effects that lead to this result.

Human deep space exploration is presented with multiple challenges, such as the reliable, efficient and sustainable operation of life support systems. The production and recycling of oxygen, carbon dioxide (CO 2 ) and fuels are hereby key, as a resource resupply will not be possible. Photoelectrochemical (PEC) devices are investigated for the light-assisted production of hydrogen and carbon-based fuels from CO 2 within the green energy transition on Earth. Their monolithic design and the sole reliance on solar energy makes them attractive for applications in space. Here, we establish the framework to evaluate PEC device performances on Moon and Mars. We present a refined Martian solar irradiance spectrum and establish the thermodynamic and realistic efficiency limits of solar-driven lunar water-splitting and Martian carbon dioxide reduction (CO 2 R) devices. Finally, we discuss the technological viability of PEC devices in space by assessing the performance combined with solar concentrator devices and explore their fabrication via in-situ resource utilization. Long-term space missions to the Moon and Mars rely on sunlight as an energy source. Here, authors assess the performance of monolithic photoelectrochemical devices for light-assisted O 2 and fuel production on the Moon and Mars as potential complementary technologies to existing life support systems.

A gas diffusion electrode (GDE) based CO 2 electrolyzer shows enhanced CO 2 transport to the catalyst surface, significantly increasing current density compared to traditional planar immersed electrodes. A two-dimensional model for the cathode side of a microfluidic CO 2 to CO electrolysis device with a GDE is developed. The model, validated against experimental data, examines key operational parameters and electrode materials. It predicts an initial rise in CO partial current density (PCD), peaking at 75 mA cm −2 at −1.3 V vs RHE for a fully flooded catalyst layer, then declining due to continuous decrease in CO 2 availability near the catalyst surface. Factors like electrolyte flow rate and CO 2 gas mass flow rate influence PCD, with a trade-off between high CO PCD and CO 2 conversion efficiency observed with increased CO 2 gas flow. We observe that a significant portion of the catalyst layer remains underutilized, and suggest improvements like varying electrode porosity and anisotropic layers to enhance mass transport and CO PCD. This research offers insights into optimizing CO 2 electrolysis device performance. CO 2 electrolyzers with gas diffusion electrodes take advantage of improved mass transport of gaseous CO 2 to the catalyst surface to afford increased current densities, but the complex and coupled multi-phase processes occurring inside the electrolysis device are not fully understood. Here, the authors use a two-dimensional volume-averaged model of the cathode side of a microfluidic CO 2 to CO electrolysis device with a gas diffusion electrode and find that under high cathodic potential, the catalyst layer is prone to forming H 2 and CO bubbles, mirroring observed experimental electrode instability.

High-resolution X-ray computed tomography is employed to obtain the exact 3D geometrical configuration of porous anisotropic ceria applied in solar-driven thermochemical cycles for splitting H2O and CO2. The tomography data are, in turn, used in direct pore-level numerical simulations for determining the morphological and effective heat/mass transport properties of porous ceria, namely: porosity, specific surface area, pore size distribution, extinction coefficient, thermal conductivity, convective heat transfer coefficient, permeability, Dupuit-Forchheimer coefficient, and tortuosity and residence time distributions. Tailored foam designs for enhanced transport properties are examined by means of adjusting morphologies of artificial ceria samples composed of bimodal distributed overlapping transparent spheres in an opaque medium.

A multiscale methodology for the determination of the macroscopic optical properties of snow is presented. It consists of solving the coupled volume‐averaged radiative transfer equations for two semi‐transparent phases – ice and air – by Monte Carlo ray tracing in an infinite slab via direct pore‐level simulations on the exact 3D microstructure obtained by computed tomography. The overall reflectance and transmittance are computed for slabs of five characteristic snow types subjected to collimated and diffuse incident radiative flux for wavelengths 0.3–3 μm. The effect of simplifying the snow microstructure and/or the radiative transfer model is elucidated by comparing our results to (i) a homogenized radiation model and considering a particulate medium made of optical equivalent grain size spheres (DISORT), or (ii) a multiphase radiation model considering a packed bed of identical overlapping semi‐transparent spheres. The calculations are experimentally validated by transmittance measurements. Significant differences in the macroscopic optical properties are observed when simplifying the snow morphology and the heat transfer model (i.e., homogenized versus multiphase). The proposed approach allows – in addition to determine macroscopic optical properties based on the exact morphology and obtained by advanced heat transfer model – for detailed understanding of radiative heat transfer in snow layers at the pore‐scale level. Key Points Snow's optical properties heavily rely on exact microstructure Direct pore‐level radiation modeling leads to accurate snow's optical properties In‐dept investigaiton of absorption in snow on the pore‐level scale is achieved

AbstractAtomic layer deposition (ALD) was used to modify two different types of carbon black-based Pt oxygen reduction catalysts with protective TiO2 nanostructures to increase catalyst durability. Rates of ALD growth and the structure of deposited TiO2 were observed to be highly dependent on oxygen content of the catalyst substrate. Electrochemical durability was enhanced with the addition of TiO2 ALD nanostructures, with up to 70% retention in mass activity measured over accelerated durability testing. High-temperature treatment of the top-performing ALD catalyst, which was found to promote structural rearrangement of the TiO2 and Pt phases into hybrid nanoparticles, yielded a twofold increase in activity but was detrimental to durability.Graphical Abstract

Solar driven water splitting can be achieved by coupling electrolyzers with PhotoVoltaics (PV). Integration of both functions in a compact PhotoElectroChemical (PEC) cell is an attractive option but presents significant scientific challenges. In this work, the design of single- and dual-compartment PEC cells for research purposes is discussed. The fabrication of separator-electrode assemblies is an important aspect, and upscaling of these architectures even to centimeter scale is not trivial. The layout of a new dual-compartment compact PEC cell with in-situ monitoring of pH, temperatures, and oxygen and hydrogen evolution for research purposes is presented. Finally, a prospect of future PEC cells for practical applications is presented. La décomposition de l’eau en utilisant la lumière du soleil s’effectue par couplage d’un électrolyseur aux cellules PhotoVoltaïques (PV). L’intégration des deux fonctions dans une seule cellule PhotoElectroChimique (PEC) compacte est envisagée, mais présente un très grand défi scientifique. Cet article traite la conception des cellules PEC de recherche avec ou sans compartimentation. L’assemblage de séparateurs et d’électrodes est un aspect important, et la fabrication de ces architectures, même à l’échelle centimétrique et n’est pas évidente. La disposition d’une nouvelle cellule compacte PEC à deux compartiments permettant de contrôler in situ l’évolution du pH, de la température, et les concentrations d’oxygène et d’hydrogène est présentée. Enfin, une perspective de futures cellules PEC pour des applications pratiques est présentée.

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. 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 gas diffusion electrode (GDE) based CO electrolyzer shows enhanced CO transport to the catalyst surface, significantly increasing current density compared to traditional planar immersed electrodes. A two-dimensional model for the cathode side of a microfluidic CO to CO electrolysis device with a GDE is developed. The model, validated against experimental data, examines key operational parameters and electrode materials. It predicts an initial rise in CO partial current density (PCD), peaking at 75 mA cm at -1.3 V vs RHE for a fully flooded catalyst layer, then declining due to continuous decrease in CO availability near the catalyst surface. Factors like electrolyte flow rate and CO gas mass flow rate influence PCD, with a trade-off between high CO PCD and CO conversion efficiency observed with increased CO gas flow. We observe that a significant portion of the catalyst layer remains underutilized, and suggest improvements like varying electrode porosity and anisotropic layers to enhance mass transport and CO PCD. This research offers insights into optimizing CO electrolysis device performance.