Hybrid water electrolysis (HWE) is an emerging field that aims to overcome some of the limitations of conventional water electrolysis (CWE) for the production of green hydrogen. In CWE, two reactions take place at each of the electrodes (anode and cathode): one reaction produces hydrogen at the cathode (the hydrogen evolution reaction) and the other produces oxygen at the anode (the oxygen evolution reaction, OER). The concept of hybrid water electrolysis revolves around replacing the anode reaction of CWE (the OER), which is inefficient and requires a large amount of energy, with an alternative anode reaction that is more efficient. In this review, the team focuses on the electrooxidation of alcohols as an alternative anode reaction. Replacing the OER offers three advantages: (1) reducing the energy costs and increasing the efficiency for hydrogen production, (2) making use of abundant resources such as bio-based alcohols (like ethanol or glycerol), and (3) converting these resources into value-added products (for example, converting glycerol to lactic acid).
"In our review paper, we examine the current state-of-the-art of hybrid water electrolysis in which the electrooxidation of alcohols is the anode process. In particular, we review studies in which the alcohol oxidation reactions were conducted under conditions relevant for industrial alkaline water electrolysis according to standards and targets set by the International Renewable Energy Agency (IRENA) and the European Clean Hydrogen Partnership (EHCP)," said Dulce M. Morales, an Assistant Professor at University of Groningen, The Netherlands. Three specific operating conditions are emphasized in this review: current density (which is proportional to how much hydrogen is produced), electrolyte composition and temperature. Additionally, the researchers identify and discuss important aspects in the field of HWE including selectivity (forming preferably one product instead of a mixture of products), circularity (in the case of the alcohol oxidation reactions, where do the alcohols come from and what happens with the products formed during HWE?), and reactor design (including novel approaches to enhance performance). This work is published in Industrial Chemistry & Materials on 03 July 2025.
Currently, the literature shows that there is a tradeoff between activity and selectivity in alcohol oxidation reactions in alkaline media. This means that current electrochemical systems can either achieve a fast reaction rate (or fast generation of products) or a high selectivity (forming preferably one product instead of a mixture of products), but not both simultaneously. This means that researchers either have to find a way to overcome this tradeoff, or find uses for mixtures of products generated from the oxidation of alcohols (for example, as feed in bioreactors). If these mixtures cannot be used as they are, the components could be separated, but separation processes are usually costly and energy-intensive. Future research should focus on separation processes that have low costs and/or low energy requirements.
Furthermore, there is a scarce number of reports that test catalytic materials for alcohol oxidation reactions under the industrially relevant conditions of alkaline CWE, which makes it difficult to assess if these reactions could replace the conventional OER in the same devices. Research on the alcohol oxidation (and HWE in general) under these conditions is urgently needed to advance the field, as identified in this review. Only under rigorous testing, which includes industrial parameters, can possible knowledge gaps be identified. Such as catalyst stability issues due to the high alkalinity of the electrolyte and high temperature compared to typical lab-scale conditions, which use diluted electrolytes and ambient temperature. Future design of new alcohol oxidation catalysts should consider possible stability issues under such conditions.
HWE, particularly with the oxidation of alcohols as the anode reaction, may offer a realistic pathway to remediate the major downsides of CWE by lowering the cost of green hydrogen production and co-generating additional valuable products. However, as HWE is an emerging field, research efforts are still needed to understand reaction mechanisms, catalyst development, and process optimization under industrially relevant conditions. While such conditions are still unknown for HWE, for CWE these are well-defined. "With our review, we aim to encourage researchers to further investigate HWE employing the industrially relevant conditions of CWE to assess whether it is feasible to implement alternative anode reactions in conventional water electrolyzers," Morales said. This review provides a summary of the current state-of-the-art in this topic, and an overview of challenges and opportunities for further advancement of the HWE field.
The research team includes Floris van Lieshout and Dulce M. Morales from the University of Groningen (the Netherlands), and Eleazar Castañeda-Morales and Arturo Manzo-Robledo from the National Polytechnic Institute (Mexico).
Industrial Chemistry & Materials is a peer-reviewed interdisciplinary academic journal published by Royal Society of Chemistry (RSC) with APCs currently waived. ICM publishes significant innovative research and major technological breakthroughs in all aspects of industrial chemistry and materials, especially the important innovation of the low-carbon chemical industry, energy, and functional materials. Check out the latest ICM news on the blog .
