On the influence of porous transport layers parameters on the performances of polymer electrolyte membrane water electrolysis cells

Anode Alchemy on Multiscale: Engineering from Intrinsic Activity to Impedance Optimization for Efficient Water Electrolysis

The past decade has seen significant progress in proton exchange membrane water electrolyzers (PEMWE), but the growing demand for cost-effective electrolytic hydrogen pushes for higher efficiency at lower costs. As a complex system, the performance of PEMWE is governed by a combination of multiscale factors. This review summarizes the latest progress from quantum to macroscopic scales. At the quantum level, electron spin configurations can be optimized to enhance catalytic activity. At the nano and meso scales, advancements in atomic structure optimization, crystal phase engineering, and heterostructure design improve catalytic performance and mass transport. At the macro scale, innovative techniques in gas bubble management and internal resistance reduction drive further efficiency gains under ampere-level operating conditions. These modifications at the quantum level cascade through meso- and macro-scales, affecting charge transfer, reaction kinetics, and gas evolution management. Unlike conventional approaches that focus solely on one scale-either at the catalyst level (e.g., atomic, or crystal modifications) or at the device level (e.g., porous transport layers design)-combining multiscale optimizations unlocks greater performance improvements. Finally, a perspective on future opportunities for multiscale engineering in PEMWE anode design toward commercial viability is offered.

Elucidating the Effect of the Catalyst Layer Morphology on the Growth and Detachment of Bubbles in Water Electrolysis via Lattice Boltzmann Modeling

The performance of water electrolysis is profoundly influenced by the behavior of the gas bubbles generated at the electrode surface. In order to improve the efficiency of bubble transportation, various nanostructures for the catalyst layer (CL), such as nanorods (NRs) or nanoparticles (NPs), have been proposed. However, there is still a lack of complete understanding about the relationship between the catalyst layer morphology and bubble evolution, which has a considerable impact on bubble transport. This study examines the effect of the catalyst layer morphology on the growth and detachment of bubbles in water electrolysis by employing the Lattice Boltzmann (LB) model. The performance of three different catalyst layer morphologies, namely, nanorods, nanoparticles, and hierarchical nanostructures, on bubble dynamics is estimated. The results suggest that the catalyst layer morphology is characterized by two factors: the bubble contact area and the electrochemically active surface area, which varies the bubble diameter, detachment time, and, subsequently, the bubble coverage. As a result, the energy efficiency of water electrolysis is influenced, aligned with experimental data that show a voltage differential of 148 mV at a current density of 0.65 A/cm2. This study highlights the significance of improving the structure of the catalyst layer to boost the efficiency of water electrolysis systems.

Understanding the Effects of Anode Catalyst Conductivity and Loading on Catalyst Layer Utilization and Performance for Anion Exchange Membrane Water Electrolysis

Anion exchange membrane water electrolysis (AEMWE) is a promising technology to produce hydrogen from low-cost, renewable power sources. Recently, the efficiency and durability of AEMWE have improved significantly due to advances in the anion exchange polymers and catalysts. To achieve performances and lifetimes competitive with proton exchange membrane or liquid alkaline electrolyzers, however, improvements in the integration of materials into the membrane electrode assembly (MEA) are needed. In particular, the integration of the oxygen evolution reaction (OER) catalyst, ionomer, and transport layer in the anode catalyst layer has significant impacts on catalyst utilization and voltage losses due to the transport of gases, hydroxide ions, and electrons within the anode. This study investigates the effects of the properties of the OER catalyst and the catalyst layer morphology on performance. Using cross-sectional electron microscopy and in-plane conductivity measurements for four PGM-free catalysts, we determine the catalyst layer thickness, uniformity, and electronic conductivity and further use a transmission line model to relate these properties to the catalyst layer resistance and utilization. We find that increased loading is beneficial for catalysts with high electronic conductivity and uniform catalyst layers, resulting in up to 55% increase in current density at 2 V due to decreased kinetic and catalyst layer resistance losses, while for catalysts with lower conductivity and/or less uniform catalyst layers, there is minimal impact. This work provides important insights into the role of catalyst layer properties beyond intrinsic catalyst activity in AEMWE performance.

Simultaneously improving the pore structure and electron conductive network of the anode catalyst layer via SnO2 doping for proton exchange membrane water electrolysis

Proton exchange membrane water electrolysis (PEMWE) is a promising technology for green hydrogen production. However, its large-scale commercial application is limited by its high precious metal loading, because low catalyst loading leads to reduced electron transport channels and decreased water transportation, etc. Herein, we study the electrode level strategy for reducing Ir loading by the optimization of the micro-structure of the anode catalyst layer via SnO2 doping. The pore structure and electron conductive network of the anode catalyst layer can be simultaneously improved by SnO2 doping, under appropriate conditions. Therefore, mass transfer polarization and ohmic polarization of the single cell are reduced. Moreover, the enhanced pore structure and improved electron conduction network collectively contribute to a decreased occurrence of charge transfer polarization. By this strategy, the performance of the single cell with the Ir loading of 1.5 mg cm-2 approaches the single cell with the higher Ir loading of 2.0 mg cm-2, which means that SnO2 doping saves about 25% loading of Ir. This paper provides a perspective at the electrode level to reduce the precious metal loading of the anode in PEMWE.

Review on Bubble Dynamics in Proton Exchange Membrane Water Electrolysis: Towards Optimal Green Hydrogen Yield

Water electrolysis using a proton exchange membrane (PEM) holds substantial promise to produce green hydrogen with zero carbon discharge. Although various techniques are available to produce hydrogen gas, the water electrolysis process tends to be more cost-effective with greater advantages for energy storage devices. However, one of the challenges associated with PEM water electrolysis is the accumulation of gas bubbles, which can impair cell performance and result in lower hydrogen output. Achieving an in-depth knowledge of bubble dynamics during electrolysis is essential for optimal cell performance. This review paper discusses bubble behaviors, measuring techniques, and other aspects of bubble dynamics in PEM water electrolysis. It also examines bubble behavior under different operating conditions, as well as the system geometry. The current review paper will further improve the understanding of bubble dynamics in PEM water electrolysis, facilitating more competent, inexpensive, and feasible green hydrogen production.

Recent advances in proton exchange membrane water electrolysis

Proton exchange membrane water electrolyzers (PEMWEs) are an attractive technology for renewable energy conversion and storage. By using green electricity generated from renewable sources like wind or solar, high-purity hydrogen gas can be produced in PEMWE systems, which can be used in fuel cells and other industrial sectors. To date, significant advances have been achieved in improving the efficiency of PEMWEs through the design of stack components; however, challenges remain for their large-scale and long-term application due to high cost and durability issues in acidic conditions. In this review, we examine the latest developments in engineering PEMWE systems and assess the gap that still needs to be filled for their practical applications. We provide a comprehensive summary of the reaction mechanisms, the correlation among structure-composition-performance, manufacturing methods, system design strategies, and operation protocols of advanced PEMWEs. We also highlight the discrepancies between the critical parameters required for practical PEMWEs and those reported in the literature. Finally, we propose the potential solution to bridge the gap and enable the appreciable applications of PEMWEs. This review may provide valuable insights for research communities and industry practitioners working in these fields and facilitate the development of more cost-effective and durable PEMWE systems for a sustainable energy future.

Ionomer-free and recyclable porous-transport electrode for high-performing proton-exchange-membrane water electrolysis

Clean hydrogen production requires large-scale deployment of water-electrolysis technologies, particularly proton-exchange-membrane water electrolyzers (PEMWEs). However, as iridium-based electrocatalysts remain the only practical option for PEMWEs, their low abundance will become a bottleneck for a sustainable hydrogen economy. Herein, we propose high-performing and durable ionomer-free porous transport electrodes (PTEs) with facile recycling features enabling Ir thrifting and reclamation. The ionomer-free porous transport electrodes offer a practical pathway to investigate the role of ionomer in the catalyst layer and, from microelectrode measurements, point to an ionomer poisoning effect for the oxygen evolution reaction. The ionomer-free porous transport electrodes demonstrate a voltage reduction of > 600 mV compared to conventional ionomer-coated porous transport electrodes at 1.8 A cm^-2 and <0.1 mg_Ir cm^-2, and a voltage degradation of 29 mV at average rate of 0.58 mV per 1000-cycles after 50k cycles of accelerated-stress tests at 4 A cm^-2. Moreover, the ionomer-free feature enables facile recycling of multiple components of PEMWEs, which is critical to a circular clean hydrogen economy.

PGM-Free Electrocatalytic Layer Characterization by Electrochemical Impedance Spectroscopy of an Anion Exchange Membrane Water Electrolyzer with Nafion Ionomer as the Bonding Agent

Low-cost anion exchange membrane (AEM) water electrolysis is a promising technology for producing “green” high-purity hydrogen using platinum group metal (PGM)-free catalysts. The performance of AEM electrolysis depends on the overall overvoltage, e.g., voltage losses coming from different processes in the water electrolyzer including hydrogen and oxygen evolution, non-faradaic charge transfer resistance, mass transfer limitations, and others. Due to the different relaxation times of these processes, it is possible to unravel them in the frequency domain by electrochemical impedance spectroscopy. This study relates to solving and quantifying contributions to the total polarization resistance of the AEM water electrolyzer, including ohmic and charge transfer resistances in the kinetically controlled mode. The high-frequency contribution is proposed to have non-faradaic nature, and its conceivable nature and mechanism are discussed. The characteristic frequencies of unraveled contributions are provided to be used as benchmark data for commercially available membranes and electrodes.

On the Operational Conditions' Effect on the Performance of an Anion Exchange Membrane Water Electrolyzer: Electrochemical Impedance Spectroscopy Study

The performance of an anion exchange membrane water electrolyzer under various operational conditions (including voltage, KOH-supporting electrolyte concentration, and flow rate) is studied using conventional time-domain technics and electrochemical impedance spectroscopy (EIS). The water electrolyzer EIS footprint, depending on the variation in operational conditions, is studied and discussed, providing valuable data on the faradaic and non-faradaic processes in MEA, considering their contribution to the total polarization resistance. The distribution of the AEMWE cell voltage contributions is valuable to accessing the key directions in the system performance improvement.

Advances in Selective Electrochemical Oxidation of 5-Hydroxymethylfurfural to Produce High-Value Chemicals

The conversion of biomass is a favorable alternative to the fossil energy route to solve the energy crisis and environmental pollution. As one of the most versatile platform compounds, 5-hydroxymethylfural (HMF) can be transformed to various value-added chemicals via electrolysis combining with renewable energy. Here, the recent advances in electrochemical oxidation of HMF, from reaction mechanism to reactor design are reviewed. First, the reaction mechanism and pathway are summarized systematically. Second, the parameters easy to be ignored are emphasized and discussed. Then, the electrocatalysts are reviewed comprehensively for different products and the reactors are introduced. Finally, future efforts on exploring reaction mechanism, electrocatalysts, and reactor are prospected. This review provides a deeper understanding of mechanism for electrochemical oxidation of HMF, the design of electrocatalyst and reactor, which is expected to promote the economical and efficient electrochemical conversion of biomass for industrial applications.

Modeling and Analysis of Mass Transport Losses of Proton Exchange Membrane Water Electrolyzer

Proton exchange membrane water electrolyzers (PEMWEs) coupled with renewable energy resources are considered to be a key technology for producing green hydrogen. However, the high current density PEMWE operation features remarkable voltage losses. A significant part of these losses is due to the mass transport resistance in the PEMWE. Even though the importance of mass transport resistance is widely recognized, it is still poorly understood. Currently, the two-phase transport through the anode porous transport layer (PTL) and catalyst layer is considered to be the main cause of the mass transport losses. In this work, a dynamic macroscopic mathematical model, coupling electrochemical reaction with mass transport through the PTL and flow channels, has been developed to study the two-phase flow in the PTL and mass transport losses of a PEMWE. The influence of the current density, inlet water flow rate, PTL structural parameters, and capillary pressure curve was evaluated. The existence of the critical current density was observed, as well as its dependence on the operating parameters and PTL structure. Even though the results show that the PTL structure has a significant influence on the PEMWE performance, they indicate that a better mathematical description of the two-phase flow in the PTL is necessary.