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

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

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

Theory of Chemical Kinetics and Charge Transfer based on Nonequilibrium Thermodynamics

Classical theories of chemical kinetics assume independent reactions in dilute solutions, whose rates are determined by mean concentrations. In condensed matter, strong interactions alter chemical activities and create inhomogeneities that can dramatically affect the reaction rate. The extreme case is that of a reaction coupled to a phase transformation, whose kinetics must depend on the order parameter -- and its gradients, at phase boundaries. This Account presents a general theory of chemical kinetics based on nonequilibrium thermodynamics. The reaction rate is a nonlinear function of the thermodynamic driving force (free energy of reaction) expressed in terms of variational chemical potentials. The Cahn-Hilliard and Allen-Cahn equations are unified and extended via a master equation for non-equilibrium chemical thermodynamics. For electrochemistry, both Marcus and Butler-Volmer kinetics are generalized for concentrated solutions and ionic solids. The theory is applied to intercalation dynamics in the phase separating Li-ion battery material Li$_x$FePO$_4$.Comment: research account, 17 two-column pages, 12 figs, 78 refs - some typos corrected Accounts of Chemical Research (2013

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