Water electrolysis: from textbook knowledge to the latest scientific strategies and industrial developments

International audienceReplacing fossil fuels with energy sources and carriers that are sustainable, environmentally benign, and affordable is amongst the most pressing challenges for future socio-economic development. To that goal, hydrogen is presumed to be the most promising energy carrier. Electrocatalytic water splitting, if driven by green electricity, would provide hydrogen with minimal CO2 footprint. The viability of water electrolysis still hinges on the availability of durable earth-abundant electrocatalyst materials and the overall process efficiency. This review spans from the fundamentals of electrocatalytically initiated water splitting to the very latest scientific findings from university and institutional research, also covering specifications and special features of the current industrial processes and those processes currently being tested in large-scale applications. Recently developed strategies are described for the optimisation and discovery of active and durable materials for electrodes that ever-increasingly harness first-principles calculations and machine learning. In addition, a technoeconomic analysis of water electrolysis is included that allows an assessment of the extent to which a large-scale implementation of water splitting can help to combat climate change. This review article is intended to cross-pollinate and strengthen efforts from fundamental understanding to technical implementation and to improve the ‘junctions’ between the field's physical chemists, materials scientists and engineers, as well as stimulate much-needed exchange among these groups on challenges encountered in the different domains

Subsecond Morphological Changes in Nafion during Water Uptake Detected by Small-Angle X-ray Scattering

The ability of Nafion® membrane to absorb water rapidly and create a network of hydrated interconnected water domains provides this material with an unmatched ability to conduct ions through a chemically and mechanically robust membrane. The morphology and composition of these hydrated membranes significantly affects their transport properties and performance. This work demonstrates that differences in interfacial interactions between the membranes exposed to vapor or liquid water can cause significant changes in kinetics of water uptake. In-situ small-angle X-ray scattering (SAXS) experiments captured the rapid swelling of the membrane in liquid water with nanostructure rearrangement on the order of seconds. For membranes in contact with water vapor, morphological changes are four-orders-of-magnitude slower than in liquid water, suggesting that interfacial resistance limits the penetration of water into the membrane. Also, upon water absorption from liquid water, a structural rearrangement from a distribution of spherical and cylindrical domains to exclusively cylindrical-like domains is suggested. These differences in water-uptake kinetics and morphology provide a new perspective into Schroeder’s Paradox, which dictates different water contents for vaporand liquid-equilibrated ionomers at unit activity. The findings of this work provide critical insights into the fast kinetics of water absorption of Nafion membrane, which can aid in the design of energy conversion devices that operate under frequent changes in environmental conditions

Morphology of supported polymer electrolyte ultra-thin films: a numerical study

Morphology of polymer electrolytes membranes (PEM), e.g., Nafion, inside PEM fuel cell catalyst layers has significant impact on the electrochemical activity and transport phenomena that determine cell performance. In those regions, Nafion can be found as an ultra-thin film, coating the catalyst and the catalyst support surfaces. The impact of the hydrophilic/hydrophobic character of these surfaces on the structural formation of the films has not been sufficiently explored yet. Here, we report about Molecular Dynamics simulation investigation of the substrate effects on the ionomer ultra-thin film morphology at different hydration levels. We use a mean-field-like model we introduced in previous publications for the interaction of the hydrated Nafion ionomer with a substrate, characterized by a tunable degree of hydrophilicity. We show that the affinity of the substrate with water plays a crucial role in the molecular rearrangement of the ionomer film, resulting in completely different morphologies. Detailed structural description in different regions of the film shows evidences of strongly heterogeneous behavior. A qualitative discussion of the implications of our observations on the PEMFC catalyst layer performance is finally proposed

Theoretische Modellierung der elektro-physikalischen Eigenschaften, der Struktur und Funktion von Niedertemperatur-Ionenaustauschmembranen

Electrophysical properties of polymer-electrolyte membranes (PEM), used as proton conductors and separators in polymer electrolyte fuel cells (PEFC), were studied and overpotential losses due to coupled transports of water and protons were calculated. The models focus or1 the perfluorinated sulfonic acid ionomers, which hitherto are the material of choice in PEFC. The properties of these PEM are determined by their phase-separated morphology, consisting of water containing pathways for proton and water transport and hydrophobic parts which provide mechanical stability and elasticity. In order to rationalize the water distribution in the porous polymer membrane and its effect an the proton conductivity, a random network model of proton transport was proposed, which takes into account the main features of the water distribution and of the specific swelling behavior. The specific bulk conductivity and capacity were calculated as functions of the water content within the effective medium approach. The obtained proton conductivity shows, in certain cases, a quasi-percolation behavior with a strong increase above a critical water content and a smail residual conductivity below this value (the one for the residual conductivity along pore Walls in the dry membrane) . The calculated geometric capacity possesses a sharp maximum at the percolation threshold. A comparison with experimental conductivity data shows, that the low percolation thresholds, obtained in the model for Nafion-type membranes, can be explained by the existence of a well connected network of pores (of a few nm diameter) in which water is homogeneously distributed already at low water contents. A serious problem for low temperature fuel cells is the partial dehydration of the membrane under working conditions . A model, which takes into account the electroosmotic drag of water molecules from anode to cathode counteracted by a backflow in a hydraulic pressure gradient, was considered . A balance between these fluxes is established in the stationary state, determining the gradient in water content across the membrane. Local values of proton conductivity, hydraulic permeability and electroosmotic coefficient are functions of the local water content . The latter is a function of the local capillary pressure in membrane pores . This function was measured, using a Standard porosimetry metho

Interface Properties of the Partially Oxidized Pt(111) Surface Using Hybrid DFT–Solvation Models

This article reports a theoretical–computational effort to model the interface between an oxidized platinum surface and aqueous electrolyte. It strives to account for the impact of the electrode potential, formation of surface-bound oxygen species, orientational ordering of near-surface solvent molecules, and metal surface charging on the potential profile along the normal direction. The computational scheme is based on the DFT/ESM-RISM method to simulate the charged Pt(111) surface with varying number of oxygen adatoms in acidic solution. This hybrid solvation method is known to qualitatively reproduce bulk metal properties like the work function. However, the presented calculations reveal that vital interface properties such as the electrostatic potential at the outer Helmholtz plane are highly sensitive to the position of the metal surface slab relative to the DFT-RISM boundary region. Shifting the relative position of the slab also affects the free energy of the system. It follows that there is an optimal distance for the first solvent layer within the ESM-RISM framework, which could be found by optimizing the position of the frozen Pt(111) slab. As it stands, manual sampling of the position of the slab is impractical and betrays the self-consistency of the method. Based on this understanding, we propose the implementation of a free energy optimization scheme of the relative position of the slab in the DFT-RISM boundary region. This optimization scheme could considerably increase the applicability of the hybrid method

Oxygen desorption – Critical step for the oxygen evolution reaction

The oxygen evolution reaction (OER) has been widely investigated in computational electrocatalysis. Recent studies suggest that the final oxygen desorption step could be rate-limiting, or even inhibiting, for the classical OER mechanism on the benchmark IrO2 electrocatalyst, and a novel reaction mechanism has been proposed circumventing this bottleneck. In this review, we provide an overview of recent progress in OER electrocatalysis with a concise focus on computational studies that explicitly accounted for the elementary step of O2 desorption. We highlight the computational and methodological intricacies that led to not considering this step as crucial by earlier OER studies. Key suggestions are provided for future studies to open new directions in OER electrocatalysis

Properties of the Pt(111)/electrolyte electrochemical interface studied with a hybrid DFT–solvation approach

Self-consistent modeling of the interface between solid metal electrode and liquid electrolyte is a crucial challenge in computational electrochemistry. In this contribution, we adopt the effective screening medium reference interaction site method (ESM–RISM) to study the charged interface between a Pt(111) surface that is partially covered with chemisorbed oxygen and an aqueous acidic electrolyte. This method proves to be well suited to describe the chemisorption and charging state of the interface at controlled electrode potential. We present an in-depth assessment of the ESM–RISM parameterization and of the importance of computing near-surface water molecules explicitly at the quantum mechanical level. We found that ESM–RISM is able to reproduce some key interface properties, including the peculiar, non-monotonic charging relation of the Pt(111)/electrolyte interface. The comparison with independent theoretical models and explicit simulations of the interface reveals strengths and limitations of ESM–RISM for modeling electrochemical interfaces

Theory of microstructured polymer-electrolyte artificial muscles

Ionic electroactuator beams are promising systems for artificial muscles in microrobotics. Here a theory is developed to investigate one promising class of such systems, which employs flexible volume-filling electrodes impregnated with polymer–electrolyte. The theory provides analytical formulae for the equilibrium beam curvature as a function of voltage and structure-related operational parameters. It predicts a possible enhancement of beam curvature by orders of magnitude over that of flat electrodes. Volume-filling electrodes thus constitute one of the 'strongest' architectures for voltage-induced movement. Approximate expressions for the dynamics of tandem pore charging and beam deflection are developed to determine the maximum pore length that still warrants a sufficiently fast response time (up to 1 s). Upper bounds on applied voltage and response time constrain the maximal device thickness and curvature, and therefore, the resulting work such a device can perform