Water Electrolysis Technology Research Articles

Feasibility of Placing Large-Scale Electrolyzers in Domestic Power Plants

The aim of the article is to verify the feasibility of placing large-scale (up to 100 MWe) water electrolysis technology unit inside, or in the proximity of a domestic nuclear power plant. The possible location is investigated from technological, logistic and nuclear-safety points of view.

Enhancing hydrogen evolution reaction performance through defect engineering in WTeX (X= S, Se) monolayers: A first-principles study

Finding a novel material to substitute the existing noble metal catalysts for the hydrogen evolution reaction (HER) is essential for the further advancement of water electrolysis technology. Herein, we investigated the feasibility of using defect engineering to improve the hydrogen evolution reaction (HER) catalytic performance of WTeX (X = Se, S) materials in the 2H, 1 T, and 1 T′ phases. The results demonstrate that the introduction of defects significantly enhances WTeX monolayers’ HER performance. As exemplary cases, the Gibbs free energy changes for 2H WTeSe with a Te vacancy (2H WTeSe–VTe) and 2H WTeS with a Te vacancy (2H WTeS–VTe) are 0.029 eV and −0.002 eV, respectively. It was found that the active sites in 2H WTeSe–VTe and 2H WTeS–VTe have a substantial number of empty anti-bonding states, which increases the bond strength between H and the catalyst, leading to exceptional catalytic performance. This work demonstrates the feasibility of defect-engineered WTeX structures as electrocatalysts and provides a reliable reference for the study of non-precious metal catalysts.

Prediction of hydrogen production in proton exchange membrane water electrolysis via neural networks

Advancements in water electrolysis technologies are crucial for green hydrogen production. Proton exchange membrane water electrolysis (PEMWE) is characterized by its efficiency and environmental benefits. The prediction and optimization of hydrogen production rates (HPRs) in PEMWE systems is difficult and still challenging because of the complexity of the system as well as the operational parameters. The integration of artificial intelligence (AI) and machine learning (ML) appears to be effective in optimization within the energy sector. Hence, this work employs the artificial neural network (ANN) to develop a model that accurately predicts HPR in PEMWE setups. A novel approach is introduced by employing the Levenberg–Marquardt backpropagation (LMBP) algorithm for training the ANN. This model is designed to predict HPR based on critical operational parameters, including anode and cathode areas (mm2), cell voltage (V) and current (A), water flow rate (mL/min), power (W), and temperature (K). The optimized ANN configuration features an architecture with 7 input nodes, two hidden layers of 64 neurons each, and a single output node. The performance of the ANN model was evaluated against conventional regression models using key metrics: mean squared error (MSE), coefficient of determination (R2), and mean absolute error (MAE). The findings of this study reveal that the developed ANN model significantly outperforms traditional models, achieving an R2 value of 0.9989 and an MAE of 0.012. In comparison, random forest (R2 = 0.9795), linear regression (R2 = 0.9697), and support vector machines (R2 = − 0.4812) show lower predictive accuracy, underscoring the ANN model's superior performance. This work demonstrates the efficiency of the LMBP in enhancing hydrogen production forecasts and sets a foundation for future improvements in PEMWE efficiency. By enabling precise control and optimization of operational parameters, this study contributes to the broader goal of advancing green hydrogen production as a viable and scalable alternative to fossil fuels, offering both immediate and long-term benefits to sustainable energy initiatives.

Co-production of hydrogen, oxygen, and electricity via an integrated solar-driven system with decoupled water electrolyzer and Na-Zn ion battery

Combining water electrolysis and rechargeable battery technologies into a single system holds great promise for the co-production of hydrogen (H2) and electricity. However, the design and development of such systems is still in its infancy. Herein, an integrated hydrogen-oxygen (O2)-electricity co-production system featuring a bipolar membrane-assisted decoupled electrolyzer and a Na-Zn ion battery was established with sodium nickelhexacyanoferrate (NaNiHCF) and Zn2+/Zn as dual redox electrodes. The decoupled electrolyzer enables to produce H2 and O2 in different time and space with almost 100% Faradic efficiency at 100 mA cm−2. Then, the charged NaNiHCF and Zn electrodes after the electrolysis processes formed a Na-Zn ion battery, which can generate electricity with an average cell voltage of 1.75 V at 10 mA cm−2. By connecting Si photovoltaics with the modular electrochemical device, a well-matched solar driven system was built to convert the intermittent solar energy into hydrogen and electric energy with a solar to hydrogen-electricity efficiency of 16.7%, demonstrating the flexible storage and conversion of renewables.

Hydrogen production from non-potable water resources: A techno-economic investment and operation planning approach

The production of hydrogen through water electrolysis facilitates large-scale energy storage, provides ancillary services, and supports the decarbonization of industries with hard-to-reduce emissions. As renewable energy adoption expands, the demand for these applications is increasing. However, water electrolys is currently faces challenges in cost competitiveness compared to other hydrogen production methods. Additionally, its reliance on potable water sources places additional strain on freshwater reserves. This study addresses these issues by exploring the economically optimal investment and operational strategy for hydrogen production using non-potable water sources. The goal is to maximize net profit through mathematical optimization, utilizing three advanced electrolysis technologies: alkaline electrolysis, proton exchange membrane electrolysis, and solid oxide electrolysis. The study examines three non-potable water sources: seawater, urban wastewater effluent, and rainwater. The model takes into account electricity market values from day-ahead and balancing markets, as well as the market values for hydrogen, oxygen, freshwater, and mixed solid salt. Two scenarios are analyzed, reflecting varying levels of uncertainty in electricity price forecasts. Each scenario includes nine case studies, representing various combinations of water sources and electrolysis technologies. The results indicate that alkaline electrolysis and proton exchange membrane electrolysis are financially viable technologies, with potential annual net profits of up to 6.4 and 6.2 million euros, respectively. In contrast, solid oxide electrolysis incurs a negative net profit across all scenarios and is not economically viable under the given conditions. The main revenue sources are hydrogen sales and combined up-balancing and down-balancing services in electricity markets. Participation in the balancing market constitutes a significant portion (35 to 61%) of the total revenue in all cases with positive net profits, making it critical for economic viability. Rainwater is identified as the most cost-effective water source for hydrogen production. However, the costs associated with water treatment and brine management are minimal, contributing only 0.7 to 3.7% to the total cost of hydrogen production. Thus, differences in net profit are primarily attributed to the type of electrolysis technology used.

Boosting alkaline hydrogen evolution via in-plane heterostructure construction with Ultra-Exposed heterointerfaces

Enhancing the catalytic efficiency of molybdenum disulfide (MoS2) towards the hydrogen evolution reaction (HER) is valuable for water electrolysis technology. This study fabricated a novel Co2P@1T-MoS2 “in-plane” heterostructure as a catalyst for alkaline HER. In this heterostructure, the Co2P nanoclusters are dispersed within the 1 T-MoS2 nanosheet. Remarkably, the coexistence of Co2P and 1 T-MoS2 phases within the same plane maximizes the exposure of interfacial active sites. The interfacial interaction between Co2P and 1 T-MoS2 facilitates water dissociation and hydrogen adsorption. This results in outstanding catalytic activity with a small overpotential of 37 mV at a current density of 10 mA cm−2 in 1.0 M KOH. Intriguingly, the Co2P@1T-MoS2 heterostructure exhibits higher activity in alkaline electrolytes compared to acidic ones. The difference in active sites for hydrogen adsorption (Co versus S) in distinct electrolytes is believed to be the underlying reason for this “abnormality”.

Oxygen-Deficient Thermal Treatment Boosts Performance of Co-Ni-Fe Oxide Electrocatalyst in Oxygen Evolution Reaction

The production of green hydrogen through water electrolysis using electricity generated from renewable sources is one of the most promising strategies toward achieving a carbon-neutral economy.[1-3] Water electrolysis is an effective and well-established water splitting process consisting of two half-cell reactions, the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) at cathode and anode, respectively. Although hydrogen as the value-added product is generated at the cathode, the anodic reaction plays an important role in determining the reaction kinetics of the entire water splitting process due to the multiple-electron transfer.[4] Therefore, developing low-cost and highly active OER electrocatalysts is one of the keys to further improve overall efficiency of water electrolysis. Among various water electrolysis technologies, alkaline water electrolysis (AWE) has been commercialized for many decades and standing for medium-scale hydrogen production and using non-noble metals as electrocatalysts.[5]Co-, Ni-, and Fe- oxides or (oxy)hydroxides are reported as auspicious OER electrocatalysts for AWE, as well as abundance on earth.[6] In addition, oxygen vacancies in metallic oxides are reported recently as an effective approach to enhance catalyst activity for OER.[7] Herein, we present trimetallic oxides (CoNiFe oxides) annealed in different atmospheres which are oxygen-rich (A; air), an oxygen-poor (M; 1 vol% air in argon), and oxygen-free (H; 5 vol% H2 in argon), respectively, The CoNiFe (2:2:1)/M with a molar ratio of Co : Ni : Fe = 2 : 2 : 1 treated under an oxygen-poor atmosphere (M) exhibits the best performance in 1 M KOH, with an overpotential of 291 mV at 10 mA cm-2 geo, a mass activity at 1.56 V versus the reversible hydrogen electrode (RHE) of ~345 A g-1 catalyst, and a small Tafel slope of 32.0 mV dec-1. Moreover, CoNiFe (2:2:1)/M shows excellent stability in the OER process for at least 50 hours at 100 mA cm-2 geo at lab scale and ~75 h at 400 mA cm-2 in an alkaline electrolyzer. This work presents a simple and effective method to design OER catalysts based on trimetallic oxides with oxygen deficiencies for alkaline water electrolysis.

Towards High-Performance AEM Water Electrolyzers: Transitioning from PGM to Low-PGM and PGM-Free Electrocatalysts

Hydrogen, known for its high gravimetric energy density and low environmental impact, is widely regarded as a promising low-carbon energy carrier. The production of hydrogen via water electrolysis offers a valuable opportunity to efficiently utilize renewable energy resources – both as a storage and transport medium and to produce hydrogen that is a crucial feedstock for multiple industrial and manufacturing applications. Among various water electrolysis technologies, anion exchange membrane water electrolyzers (AEMWEs) are notable for their distinct and advantageous features. Their alkaline operational environment significantly reduces corrosion, allow for the possibility to use catalysts and cell component materials that are earth abundant and cost effective. Moreover, the use of anion exchange membranes (AEMs) as the electrolyte and zero-gap design enhance operational efficiency and hydrogen production rates compared to traditional alkaline electrolyzers. These benefits position AEMWEs as a compelling solution for efficient, cost-effective large-scale hydrogen production.To date, the development of lower-cost AEMWEs has been hindered by several challenges. A critical issue is the activity and durability of catalysts, particularly those that are platinum group metal (PGM)-free. This leads many researchers and electrolyzer manufacturers to lean on PGM catalysts at both the anode and cathode. These catalysts, while effective, significantly increase the cost and complexity of the system, posing a barrier to widespread adoption and commercialization of AEMEL technology in the burgeoning hydrogen economy. PGM-free anodes have been demonstrated for the AEMWE, but there have been fewer PGM-free options for the cathode.This study assessed the performance and durability of various low-PGM and PGM-free electrocatalysts for the oxygen evolution (OER) and hydrogen evolution (HER) reactions. We focused on Lanthanum Strontium Cobalt (LSC), Nickel Ferrite (NiFeOx), and Lead Ruthenate (PbRuOx) for the OER at the anode, and Nickel Molybdenum (NiMo), Nickel Rhenium (NiRe), and a low-PGM PtNi for the HER at the cathode. Furthermore, the performance of the NiMo HER catalyst was enhanced by optimizing the physical properties of both the catalyst powder and the electrodes. These experimental results offer crucial insights into developing PGM-free AEM water electrolyzer systems, marking a progressive step towards their commercial feasibility.

Activated Migration of Hydroxide Ion in Anolyte for Enhancing Performance of Scaled-up Non-Noble Catalysts in Pure Water-Fed Anion Exchange Membrane Water Electrolysis

The continuous advancement in water electrolysis technologies has illuminated the potential of sustainable energy sources, particularly in the realm of green hydrogen production. Anion exchange membrane water electrolysis (AEMWE) stands out as a promising technology capable of theoretically yielding high-purity hydrogen. However, the commercialization of AEMWE faces challenges arising from the inherent discord between high performance and the elevated costs associated with noble metal catalysts, such as platinum and iridium. Moreover, prevalent studies often focus on a limited electrode area of less than 4 cm2, thereby restricting the exploitation of the high purity hydrogen production capabilities inherent in AEMWE. This research prioritizes the enhancement of performance through a meticulously crafted membrane-electrode assembly (MEA) employing a 25 cm2 Ni-frame caged NiMoO4 catalyst for cathode synthesis. The observed performance improvement is attributed to the augmented active site of the catalyst, consisting of NiMoO4 entrapped in the Ni substrate during the compression process of the electrode. Additionally, the increase in local pH at the interfacial between electrode and bulk electrolyte is facilitated by the capillary phenomenon resulting from the space between the Ni substrate and the NiMoO4 catalyst layer, leading to enhanced migration of the electrolyte containing hydroxide ions. Acknowledgements This research was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resources from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20213030040590).

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

Water electrolysis (WE) is a promising route for carbon-free production of green hydrogen, which is increasingly considered to be an important energy carrier. Among the low-temperature WE technologies, anion exchange membrane water electrolysis (AEMWE) has the potential advantages of high operating current densities and low cost due to the use of zero-gap assembly and platinum group metal (PGM)-free components, respectively [1]. However, significant challenges remain for this technology, including low efficiency, insufficient durability, and poor performance in pure water operation. Recent efforts have yielded improvements in the individual components, particularly the oxygen evolution reaction (OER) catalysts and anion exchange polymers; however, to further improve the performance of the AEMWE, it is also necessary to understand and improve the integration of these materials into the membrane electrode assembly (MEA). Within the MEA, the anode and cathode catalyst layers (CLs) incorporate catalyst and ion-conducting and/or binding polymers, which are deposited onto either the membrane or the porous transport layer. The material properties of each component, as well as the loadings of the catalyst and ionomer and methods of dispersion and deposition, have significant impact on the catalyst layer morphology and uniformity [2]. These CL properties affect the transport of electrons, hydroxide ions, and evolved gases, ultimately controlling the accessibility and utilization of catalyst active sites, with significant consequences for both the performance and durability of AEMWEs.Most of the effort in catalyst development for AEMWE has focused on the OER, aiming to improve sluggish kinetics and improve stability at the highly oxidizing operating potentials. A number of PGM-free catalysts have shown promising activity in rotating disk electrode (RDE) tests, particularly oxides with varying ratios of Ni, Fe, and Co. The activity improvements with tuning the composition are often attributed to the increase in surface area and shift in redox potentials for the in-situ conversion to higher valence sites with higher intrinsic OER activity. However, these activity trends do not always translate into device-level performance because of differences in the operating environment, e.g. hydroxide concentration, reaction rates, etc. In the AEMWE, where typically an order of magnitude higher loadings and current densities are required, the challenges of conductivity and mass transport to allow for full site access are exacerbated. Electrochemical impedance spectroscopy (EIS) is a valuable tool for understanding the origin of voltage losses in the AEMWE, typically broken down into ohmic, kinetic, and mass transport losses. Recently, a transmission line model has been applied to non-Faradaic EIS to isolate another contribution to voltage loss: the catalyst layer resistance (CLR), which corresponds to in-plane and through-plane electronic and ionic resistances in the catalyst layer [3]. Using this approach, the roles of intrinsic catalyst activity and catalyst layer properties can be decoupled, helping to guide materials integration efforts.In this study, we investigate two types of OER catalyst materials, NiFe- and Co-based oxides. Within each composition, the catalysts have similar active sites but vary in electronic conductivity, crystal structure, and surface area. By comparing the AEMWE performance at different catalyst loadings, we are able to determine the role of catalyst properties in determining active site utilization and electronic resistance within the catalyst layer. Figure 1 shows the trends in performance and ohmic and catalyst layer resistance as a function of loading for high-conductivity (Co@Co3O4) and low-conductivity (Co3O4) catalysts. Despite nominally having the same surface chemistry and similar particle morphology, they display both different performance and different trends with loading. Using in-situ EIS diagnostics and ex-situ characterization of the catalyst layer morphology, the relationship between catalyst utilization, CLR, and the physical properties of the catalyst and catalyst layer is elucidated. This work provides insight into the catalyst properties that most strongly affect AEMWE performance, as well as how materials integration strategies must be tailored based on these properties.

Tuning Oxygen Evolving Catalytic Activity of a Precious Metal-Free Nickel Boride by a Cost-Effective Method

The production of hydrogen (H2) by electrochemical water splitting [Green Hydrogen (Green H2)], if driven by green electricity, has attracted considerable interest as an alternative sustainable energy carrier, e.g., in fuel cells for electricity or power and heat generation [1]. However, the lack of satisfactory and cheap electrocatalysts (ECs) to drive the vital electrochemical reactions of hydrogen evolution (HER) and oxygen evolution (OER) in water splitting is the biggest challenge for this technology.Currently, the most advanced electrocatalysts for the two half-reactions of water electrolysis (HER and OER) are noble metal-based materials [2]. Despite the excellent catalytic properties of these noble metal catalysts, their high cost, and scarcity make their commercial use both uneconomical and impractical in low temperature water electrolysis (LT-WE) systems at industrial scale for green H2 as a clean energy alternative to cheaper fossil fuels[3]. In addition to addressing this issue by the development of precious-material-based with a low-loading of precious metal elements, the development of desirable and high-efficiency electrocatalysts (ECs) based on earth-abundant metal elements materials is paramount. However, their catalytic activity is often limited by poor electron transfer, low conductivity, low stability, and small surface area of these materials. Thus, it is crucial to develop earth-abundant metal elements-based materials that are more cost-effective to reduce the high cost of active catalysts for water electrolysis. Although several low-cost materials have already been designed, the potential required for the practical application of water splitting is still very high compared to the theoretical potential (1.23 V), which means that much energy is consumed, which theoretically could be reduced. Due to their remarkable advanced charge transfer and durability for water oxidation in alkaline media, electrocatalysts derived from metal borides-based materials have received a lot of attention as new high-performance catalysts for water oxidation.[4] These properties stem from the multi-dimensional covalent bonding of the metalloid with the surrounding metal atoms.We present here an approach to tune the OER of transition metal borides (TMBs) using the cost-effective and less energy-consuming method that is nevertheless a highly efficient material for OER (Fig. 1). We have developed a low-cost, rational and easily scalable method to enhance the catalytic activity of nickel boride (Ni–B), which requires an annealing process to optimize the catalytic activity. However, this method may have limited applicability on a large scale as it requires a highly inert environment and high-temperature treatment of Ni–B catalysts. We also elucidate a structure-activity relationship using scale-bridging techniques such as TEM and electrochemical characterization. The developed based on non-noble electrocatalysts are prepared by a simple method of chemical reduction of transition metal elements with boron in aqueous solution. The production is a one-step process and the material does not require any further treatment, yet this material shows comparable activity to the state of the art (precious metal-based materials) for OER in alkaline solution at a selected current density. This improved catalytic performance is further corroborated by the microstructural investigations in the TEM. The nanoparticles obtained have an improved porous structure compared to Ni–B and thus provide more available sites for the surface reactions and hence for the catalytic performance of the material. Furthermore, it is shown that activation enables the morphological and structural changes, while some transition metal elements act as sacrificial elements to provide more accessible and stable sites for oxygen-forming centers. This paves the way for a better understanding of metal boride-derived electrocatalysts for water oxidation, a crucial chemical reaction in water electrolysis and other electrochemical energy technologies. Acknowledgements: This project has received funding from the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No 945422. We acknowledge the use of the DFG-funded Micro-and Nanoanalytics Facility (MNaF) at the University of Siegen (INST 221/131-1).

(Invited) Cost Reduction Strategies in Low Temperature Water Electrolysis with Improved Catalytic Activity

Proton exchange membrane water electrolysis (PEMWE) technology is especially advantageous for green H2 production as a clean energy vector. During the water electrolysis process, the oxygen evolution reaction (OER) requires a large amount of iridium (2-3 mgIr cm−2) as catalyst. This material is scarce and expensive, representing a major bottleneck for large-scale deployment of electrolyzers. Recently, Retuerto et al. have synthesized different Ir double perovskites (Sr2CaIrO6) with the Ir atoms in a high oxidation state (Ir6+/5+) 1 and their catalytic activity was extensively investigated. Specifically, a Ca-based double perovskite (Sr2CaIrO6) has showed the highest OER activity similar to those of Ir or Ir oxides-based catalysts while being stable for the OER in half cell measurements. We report the development of a membrane electrode assembly taking advantage of the high activity of the Sr2CaIrO6 catalyst that has only 0.2 mgIr cm−2, around 10 times lower compared to what it is used in the anodes of commercial PEMWE, showing superior performance and stability. The morphology/structure of the developed anode and Sr2CaIrO6 catalyst, before and after operation in PEMWE, were extensively characterized by X-ray diffraction (XRD, high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and X-ray absorption near edge structure (XANES). We discuss in this contribution how the finding of this work can provide a strategy to lower the Ir content in commercial PEMWE.Anion exchange membrane water electrolysis (AEMWE) combines the advantages of proton exchange membrane water electrolysis (PEMWE), i.e., high current density while maintaining high efficiency and fast dynamic response with the advantages of alkaline water electrolysis (AWE), i.e., low-cost materials. However, AEMWE technology is still depends on a supporting electrolyte and its durability is low. In this regard, the German Aerospace Center (DLR) is developing novel approaches and cell components. Raney-nickel electrodes were developed via vacuum plasma spraying, reaching performance and stability comparable to SoA MW size PEMWE. The electrodes were further refined, switching to lower cost atmospheric plasma spraying while maintaining high performance. Studying the effect of supporting electrolyte concentration it was found, that lowering the ohmic resistance is by far the largest challenge for high performance AEMWE, especially at low KOH concentrations and electrode kinetics are less impactful. Moreover, a 17% efficiency uplift was achieved by applying a carefully tuned, thermally sprayed, nickel-based macro porous layer (MPL) onto a traditional mesh porous transport layer. Both ohmic and mass transport resistances were found to be lowered by the MPL. FIB-SEM and µCT analysis revealed an ideal structure, i.e., 30-40% porosity, almost exclusively open pores, and a broad pore size distribution2. In a fundamental study3, the mechanism of the OER activity was investigated as well as the effect of trace amounts of iron in Ni(OH)2. X-ray diffraction (XRD) and Thermogravimetric analysis (TGA) TGA analysis lead to the conclusion, that iron stabilized the α-Ni(OH)2which is the most active. CV analysis showed that an important redox switching step is also accelerated by Fe-doping. M. Retuerto, L. Pascual, J. Torrero, M. A. Salam, Á. Tolosana-Moranchel, D. Gianolio, P. Ferrer, P. Kayser, V. Wilke, S. Stiber, V. Celorrio, M. Mokthar, A. S. Gago, K. A. Friedrich, M. A. Peña, J. A. Alonso, D. G. Sanchez, S. Rojas, Highly Active and Stable OER Electrocatalysts Derived from Sr2MIrO6 for Proton Exchange Membrane Water Electrolyzers. Nat. Commun. 2022, 13, 7935. Razmjooei, F.; Morawietz, T.; Taghizadeh, E.; Hadjixenophontos, E.; Mues, L.; Gerle, M.; Wood, B. D.; Harms, C.; Gago, A. S.; Ansar, S. A.; Friedrich, K. A., Increasing the performance of an anion-exchange membrane electrolyzer operating in pure water with a nickel-based microporous layer. Joule 2021, 5 (7), 1776-1799. Wang, L.; Saveleva, V. A.; Eslamibidgoli, M. J.; Antipin, D.; Bouillet, C.; Biswas, I.; Gago, A. S.; Hosseiny, S. S.; Gazdzicki, P.; Eikerling, M. H.; Savinova, E. R.; Friedrich, K. A., Deciphering the Exceptional Performance of NiFe Hydroxide for the Oxygen Evolution Reaction in an Anion Exchange Membrane Electrolyzer. ACS Applied Energy Materials 2022, 5 (2), 2221-2230.

Exploring the Performance of Alkaline Water Electrolyzers Under Dynamic Operational Conditions

Green hydrogen, produced through water electrolysis driven by Renewable Energy Sources (RES), is a compelling solution for storing renewable energy. Directly coupling RES with water electrolyzers enhances system efficiency and cost-effectiveness. Alkaline water electrolysis, a mature and cost-effective technology, faces challenges operating under fluctuating RES conditions using AC/DC convertor , which supply rippled DC. A comprehensive understanding of electrolyzer performance in dynamic conditions is crucial for developing durable, efficient systems.In the present experimental study, we extensively explored the performance of alkaline water electrolyzers and the key factors influencing it under dynamic operational conditions. The electrolytic cell was constructed using a zero-gap configuration, comprising a NiCoOx-based anode deposited on a Ni-mesh and an Ln-doped RuOx cathode deposited on a fine mesh of Ni metal. The Zirfon Perl UTP500 membrane served as a separator between the two cell compartments [1]. A 7.0 Molar KOH electrolyte at a room temperature of approximately 30°C was employed. The cell underwent dynamic operational mode through the application of a square wave cell voltage, shown in Fig.1 (a), utilizing the NF BP4620 power supply. Square waves with different frequencies, specifically 1, 100, 1000, and 3000 Hz, were employed. Steady-state operations were conducted while applying various DC voltages, serving as a reference study. The resulting current and the actual cell voltage waveforms were monitored and recorded using a high-resolution oscilloscope (YOKOGAWA DL850E). The flow of H2 gas was measured using a film flow meter (HORIBA STEC, SF series VP-3). The apparent power, real power, and reactive power components were calculated using formulas from a previously published report [2]. Subsequently, the specific energy consumption was determined for each condition.The applied cell voltages were categorized into two groups. In the first category, the maximum cell voltage (Vmax) was set at 2.0 V, a value sufficiently high and suitable for industrial cell operation when applied in a DC mode. Within this category, the minimum cell voltage (Vmin) took various values, namely 1.8, 1.6, and 1.4 V. In the second category, the average cell voltage was maintained at a constant 2.0 V, while the amplitude of the waveform was adjusted to 0.2 and 0.4 V by modifying both Vmax and Vmin. Figure 1(b) depicts the resulting current of the voltage waveform at 100 Hz, illustrating three different Vmin values. The figure reveals that at a high Vmin of 1.8V, the current consistently maintained positive values during the voltage cycle, indicating that the double-layer capacitance remained charged throughout the operational period. However, with Vmin set at 1.6V, a relatively small capacitive current emerged, suggesting partial discharge of the double-layer capacitance. Moreover, at a lower Vmin of 1.4V, the cell voltage became insufficient to drive water electrolysis in the current cell. Consequently, a relatively large negative (capacitive) current manifested during the Vmin period, denoting a deep discharge of the double-layer capacitance.The obtained results revealed a profound influence of the charge and discharge status of the double-layer capacitance on the AWE cell's performance throughout the voltage cycles. Notably, when the double-layer capacitor remained fully charged during the voltage cycles, minimal impact of the frequency was observed on key performance parameters, including H2 flow rate that shown in figure 1 (c), specific energy consumption that shown in figure 1 (d), and power factor. However, under conditions of partial discharge within the cell voltage cycles, a noticeable correlation between the size and frequency of the voltage waveform and the AWE cell's performance emerged. Deep discharge of the double-layer capacitance led to a frequency-dependent reduction in H2 flow rate, attributed to incomplete charge/discharge cycles. Hence, the critical factor influencing the performance of the electrolytic cell during dynamic operation is the condition (charging/discharging) of the double-layer capacitance. These findings offer valuable insights into optimizing the dynamic operation of AWE cells for enhanced efficiency and performance. Acknowledgments: This research was partially supported by New Energy and Industrial Technology Development Organization (NEDO).

3D-Printed Hierarchical Electrodes for Ampere-Level Alkaline Water Electrolysis

In the realm of water electrolysis technology, Alkaline Electrolysis Cell (ALK) hydrogen production stands as a pivotal technology, playing a critical role in the pursuit of large-scale green hydrogen production and the achievement of the "dual carbon" objective. Despite its importance, the mass transfer efficiency of commercial porous nickel electrodes currently used in ALK systems is suboptimal. These electrodes, plagued by unstable catalyst loading, typically operate at current densities between 300-500 mA/cm², seldom reaching the desired threshold of 1 A/cm². This limitation hampers the potential of ALK hydrogen production to satisfy industrial-scale hydrogen production demands.In the quest to overcome these limitations, the concept of the Triple Periodic Minimum Surface (TPMS) structure, a cellular architecture mathematically defined and prevalent in natural biological forms such as butterfly wings and beetle shells, is introduced. The TPMS's unique geometry is adept at adapting to mechanical requirements for various applications, including structural and fluidic systems. It allows for versatile manipulation of geometric parameters like aperture size, porosity, and tortuosity, thereby substantially broadening the design possibilities for electrode structures and enhancing the efficiency of gas-liquid and electron transfer.To harness these benefits, we propose the creation of nickel/nickel-iron electrodes using 3D printing technology. These electrodes are characterized by their cross-scale porosity, ordered structure, and adjustable TPMS design. This innovative electrode design significantly augments bubble transport efficiency and mass transfer performance, while also providing additional anchor points for catalysts, thereby enhancing the electrochemical active surface area, intrinsic activity, and stability of the composite catalyst. As a result, these electrodes achieve ampere-level current densities in ALK electrolysis cells, enhancing electrolysis efficiency and reducing energy consumption.Our methodology involves the use of selective laser melting printing technology to fabricate multi-tier nickel/nickel-iron electrodes of varying specifications. These electrodes are then coated with MOOH catalysts using an electrochemical deposition process. Through rigorous testing and comparison of electrochemical performance parameters, we identified the optimal multi-stage nickel/nickel-iron electrode and its composite catalyst.Electrochemical performance tests reveal that, compared to traditional commercial porous nickel electrodes, our 3D printed electrodes decrease overpotential by 14% and improve electrolytic cell efficiency by 10% at a current density of 1 A/cm². Furthermore, these electrodes exhibit minimal degradation after a 500-hour durability test. The demonstrated efficiency and durability of these 3D-printed hierarchical electrodes indicate significant potential for their application in ampere-level alkaline water electrolysis. Figure 1

Investigating the pH-Dependent Activity, Selectivity, and Dynamics of Metal Nitride Electrocatalysts for Fuel Cells and Electrolyzers

To mitigate climate change caused by the use of fossil fuel energy sources, there is an urgent need to decarbonize our energy systems. Developing fuel cell and water electrolysis technologies to support a green hydrogen infrastructure is a promising strategy for this transition. In both hydrogen fuel cells and water electrolyzers, platinum-group metal (PGM) materials such as Pt and IrOx are benchmark electrocatalysts, exhibiting high activity and stability for kinetically-hindered reactions including the oxygen reduction reaction (ORR), the oxygen evolution reaction (OER), and alcohol oxidation reactions (AOR; alternative anodic reactions to OER). Despite their remarkable performance, the high cost and low abundance of PGMs is a key factor limiting the widespread adoption of these technologies.Cobalt- and nickel-based metal and oxide catalysts have emerged in the literature as a promising PGM-alternative class of low-cost, high-performing ORR, OER and AOR catalysts. Substituting oxides with nitrides or other chalcogenides could be a favorable method to improve activity, selectivity, and stability; however, the material stability of these non-oxides as a function of reaction, potential window, and pH is not well-understood. Furthermore, the most active of these materials tend to be carbon-supported nanoparticles with complicated and heterogeneous active sites, which makes deconvoluting the role of the cobalt and secondary element(s) (e.g. N, C, etc.) difficult.With the goal of developing fundamental activity-stability relationships for non-precious metal-based electrocatalysts, we have developed a synthesis procedure for well-defined Co- and Ni-based thin film materials. Focusing on Co, using physical vapor deposition followed by thermal treatments with ammonia and an Sb-based precursor, we have synthesized CoxSbyNz thin films of varying compositions. These synthesis methods can be extended to synthesize a broad class of metallic (Co, Ni, Fe, etc.) or bimetallic X-ide (X = N, S, etc.) thin film materials. We evaluate ORR, OER, and AOR activity with cyclic voltammetry, demonstrating that activity depends on composition and electrolyte pH. Specifically, for Co-based thin films for ORR, we see improvement in activity with the incorporation of N, and improvement in acid-stability with the incorporation of Sb. In addition to ex situ post-characterization with x-ray photoelectron spectroscopy, we utilize in situ techniques to evaluate catalyst stability in real time. We monitor time- and potential-dependent cobalt dissolution with an on-line inductively coupled plasma mass spectrometer (ICP-MS) as well as nitride degradation with electrochemical mass spectrometry (EC-MS). In acidic media, the stability of CoxN is limited by Co dissolution, while in alkaline media stability appears to be limited by nitride degradation. With highly active non-oxide materials it is typically too complicated to disentangle the role of each individual element within an electrocatalyst. With our stepwise material synthesis platform and multifaceted characterization techniques we are able to individually probe the material stability and product-specific activity of each element. These results provide new insights to guide the strategic design of electrocatalysts across a range of reactions and applications.

Advancing Catalyst Performance in Electrochemical Water Splitting

The transition to renewable energy, vital for a sustainable future, frequently encounters challenges such as low energy density and intermittency. A promising solution lies in converting renewable energy into chemical bonds, notably hydrogen, through electrochemical water splitting. This process, however, is hindered by the sluggish kinetics of the oxygen evolution reaction (OER) at the anode. Consequently, this necessitates the use of rare and expensive iridium-based catalysts. Given that iridium is the only metal resilient enough to withstand the harsh anodic conditions of OER, reliance on it poses significant economic challenges, making water electrolysis less competitive compared to fossil fuel-based technologies.In response to these challenges, our research has two main objectives. First, we aim to understand the often observed trade-off between iridium OER activity and stability. (1) Second, we intent to enhance our understanding of catalyst-support interactions by explore the use of conductive and potentially durable materials as supports for iridium to improve efficiency, a critical yet largely unexplored area.To achieve these goals, we employ advanced techniques, including inductively coupled plasma mass spectrometry coupled with scanning flow cell. This method enables us to perform detailed potential and time-resolved stability analyses.Specifically, we conduct an in-depth investigation of the impact of pH across a range from acidic to alkaline environments. Furthermore, we explore a variety of backing electrodes, investigate accelerated stress tests (AST) parameters, (2) and assessing how catalyst loading and Nafion content affect the performance and stability of iridium catalysts.Our findings provide a deeper understanding of iridium’s stability during OER and highlight the importance of catalyst-support interactions. (3, 4) By addressing the key challenges associated with iridium-based catalysts, this knowledge is crucial for the development of more cost-effective catalysts with minimized use and hence sustainable water electrolysis technologies, marking a step closer to realizing more efficient and economically viable methods for harnessing and storing renewable energy.

(Invited) Understanding the Effect of Cell Assembly and Operation on AEM Electrolyzer Performance and Durability

Water electrolysis to produce hydrogen is gaining significant attention due to a significant decrease in the cost of renewable energy sources such as solar, wind, and tidal etc. Among the existing water electrolysis technologies, anion exchange membrane water electrolysis has recently emerged due to its potential advantages over proton exchange membrane electrolysis and traditional alkaline electrolysis. Anion exchange membrane electrolyzers (AEMELs) can allow for the use of PGM-free electrocatalysts and low-cost component materials due to its less corrosive alkaline environment while also enabling electrochemical H2 compression.An overwhelming majority of the research that is going on related to AEMELs is the synthesis of advanced functional nanomaterials – e.g. new catalysts and membranes. Though these are critically important components of the cell, their function is supported by a multitude of other factors. For example, it is well-known that water and gas transport in the electrodes is highly dictated by the porous transport layers. The electrode fabrication technique controls the electrode morphology. The synthesis method for the catalyst determines its porosity and structure. The cell gasketing controls compression. The operating current density and temperature affect stress at the material and electrode level, and dictate other properties such as exchange current, ionic conductivity, water uptake, diffusivities, etc. Many of these properties are less reported and will be the subject of this presentation.This presentation will focus on understanding how various decisions that are made regarding electrode fabrication, cell assembly and cell operation affect the operating voltage and voltage stability of AEM electrolyzers. The goal of this talk is to combine electrochemical and physical characterization data to develop a series of guiding principles for AEMEL operation that can be used across polymer chemistries and catalyst selection.

PFAS-Free: Robust and Chemically Stable Ion-Solvating Polymer Membrane Derived from Poly(arylene alkylene) for High-Performance Alkaline Water Electrolysis

Polymeric ion-solvating membranes1 derived from poly(arylene alkylene)s2 have emerged as promising candidates for alkaline water electrolysis, offering a PFAS-free, robust, and chemically stable alternative. This work focuses on the development of such a membrane with an emphasis on high resistance to gas crossover, addressing the challenges associated with conventional ion-exchange materials.The polymer membrane presented in this study demonstrated very good performance in alkaline water electrolysis. At 80 degrees Celsius, the membrane exhibits a specific conductivity of 36 mS/cm, underscoring its ion-solvating ability and efficiency in a 20 wt% KOH solution. The electrochemical performance is exemplified by the attainment of a 700-mA current below 2.6V with uncatalyzed nickel foam electrodes, showcasing the superior conductivity and efficiency in facilitating the electrolysis process.One of the key features of these membranes is their longevity, as evidenced by their ability to operate for more than 200 hours without any discernible voltage decay. This extended operational lifespan underscores the robustness and durability of the developed membranes, making them highly suitable for practical applications in alkaline water electrolysis.Furthermore, the membranes exhibit a high resistance to gas crossover, addressing a common challenge in electrolysis systems. This property ensures enhanced selectivity and efficiency by minimizing unwanted reactions and improving the overall electrolysis process.In conclusion, the presented work signifies a significant advancement in the development of ion-solvating polymer membranes derived from poly(arylene alkylene) for alkaline water electrolysis. The membranes not only outperform existing alternatives but also offer a sustainable and environmentally friendly solution by being PFAS-free. The demonstrated high performance, stability, and resistance to gas crossover make these membranes promising candidates for the advancement of alkaline water electrolysis technology, contributing to the development of efficient and environmentally conscious hydrogen production systems. Keywords: Alkaline; electrolysis; Poly (arylene alkylene)s; membranes; ion-solvating

(Invited) Dynamic Modeling of Liquid Alkaline Water Electrolyzers: From Laboratory Cells to Industrial-Sized Stacks

The transition of the energy system towards renewable energies creates an enormous demand for green hydrogen produced by water electrolysis [1]. Among the available processes, alkaline water electrolysis (AEL) is already well-developed and cost-effective, as it does not require expensive or critical membrane and electrode materials [2]. If the liquid alkaline electrolyte is maintained at sufficient reaction temperature (60-90 °C), efficient operation of AEL systems is possible with rapid response to fluctuating renewable power supply [3]. However, a critical phenomenon for all low-temperature water electrolysis technologies is the increased gas impurity – especially for hydrogen in oxygen – during part-load operation [4]. This decreases the process efficiency and is safety-relevant as electrolysis systems must shut down when exceeding 50% of the lower explosive limit. These impurities become dominant at low current density since gas production decreases while the contamination mechanisms (diffusion through the separator and convective transport through electrolyte cycles mixing) are almost load-independent. We have developed a comprehensive dynamic model to properly describe the performance of an AEL system, including cell voltage, electrolyte temperature, electrolyte concentration, and gas purity [5]. With this model, it is possible to describe the dynamic operation of a laboratory AEL system with high accuracy. However, if this model is employed for an industry-sized shortstack with more than 1 m2 cell size, predictions and experimental results are in lesser agreement. This is most probably because the present model assumption of perfectly mixed half-cells no longer holds in large cells with a pronounced dependency of the gas fraction over the cell height. Thus, an improved model including this height dependence must be developed.[1] M. Wappler, D. Unguder, X. Lu, H. Ohlmeyer, H. Teschke, W. Lueke, Int. J. Hydrogen Energ. 47 (2022) 33551[2] J. Brauns, T. Turek, Processes 8 (2020) 248[3] J. Brauns, T. Turek, Electrochim. Acta 404 (2022) 139715[4] P. Trinke, P. Haug, J. Brauns, B. Bensmann, R. Hanke-Rauschenbach, T. Turek, J. Electrochem. Soc. 165 (2018) F502[5] J. Brauns, T. Turek, J. Electrochem. Soc. 170 (2023) 064510

Evaluation of Nickel-Iron (Oxy)Hydroxide Catalysts in Varied Electrode Geometries Under Industrially Realistic Conditions for Enhanced Alkaline Water Electrolysis

In pursuit of realizing a hydrogen-based economy, the challenge lies in enhancing the efficiency of crucial water electrolysis technologies. Alkaline water electrolysis (AWE) holds promise as an ecologically friendly method for H2 production, primarily due to its capacity for employing non-precious metals as electrocatalysts, in contrast to more costly options which can only switch to noble metals such as ruthenium or iridium. However, this technology faces a significant challenge arising from the sluggish kinetics of the oxygen evolution reaction (OER), which constrains the overall efficiency. [1] Furthermore, the limited stability of many OER catalysts alternatives developed at lab scale and less harsh conditions poses a substantial obstacle to the commercialization of this technology.[2] To overcome this issue, the development of highly active OER catalysts that maintain their activity under industrial conditions is of major importance explaining the growing attention that this topic is gaining in literature. Nevertheless, most of the reported systems are tested at low current densities under easily manageable conditions in the laboratory (≤ 100 mA cm-2, 1 M KOH, RT). Without disagreeing that this approach is necessary to identify promising options among the numerous catalyst systems, it is not sufficient when competing with existing technologies that already achieve high current densities. In this context, we investigate a low-cost, abundantly available non-precious metal based OER-catalyst under industrially relevant conditions (≤ 1.2 A cm-2, 30 wt.-% KOH, 80 °C). The synthesis is based on a multistep galvanic deposition method introduced by Wu et al [3], receiving a highly porous NiFeOOH catalyst. Addressing the complexity of the multiphase process of AWE that involves gas-liquid-solid interfaces, the catalyst system is transferred to numerous nickel electrodes with different 3D geometries (plain woven mesh, knitted mesh, expanded metal mesh) for the investigation of their impact on bubble formation and detachment. To further optimize the catalyst system a variation of the iron content with respect to its’ molar fraction relatively to nickel has been carried out from x(Fe)=0 to x(Fe)=0.75, which after synthesis were first examined using SEM and EDS. The structure can be considered as pompom-shaped particles, forming a porous framework, as can be seen exemplarily in Figure 1a) on a knitted nickel-wire mesh coated with NiFeOOH. Observing variations in iron content, it becomes evident that a higher iron content results in larger particle sizes, thereby altering pore sizes. However, the surface morphologies of each catalyst until a molar fraction x(Fe)=0.5 is reached, can be considered as very similar. When comparing the SEM images of the mesh with a molar fraction x(Fe)=0.75 to the other molar fractions, a secondary phase becomes apparent on the surface. Notably, the porous structure persists within the crystalline phase. It could be shown by EDS that the target values for the molar fractions set in the galvanic bath and the actual molar fractions nearly show no deviations. Further, a uniform dispersion of Ni, Fe and O is observed.The electrochemical investigation of the prepared catalysts was done in a 3-electrode PTFE beaker cell setup in 30 wt % KOH and at 80 °C. The measurement protocol we introduce consists of galvanostatic steps between 0-1.2 A cm-2. The stability is tested chronopotentiometrically as well by maintaining a current density of 500 mA cm-2 for 100 h. It becomes apparent that no optimum is reached in the tested range of iron content and that the potential continues to drop as the molar fraction of Fe increases, achieving the lowest potential with 1.38 V vs. RHE at 50 mA cm-2 and 1.42 V vs. RHE at 750 mA cm-2 with x(Fe)=0.75. It could be further proved that the NiFeOOH catalyst with x(Fe)=0.75 is a highly stable catalyst, resulting in a potential increase of 27 mV over 100 h of testing at 500 mA cm-2, making it to an appropriate OER catalyst for the application in the AWE. The highly porous structure is maintained after testing, proved by SEM imaging. The iron contents after testing are also very similar, which means that there is almost no iron loss in the coating. For the different tested electrode geometries coated with NiFeOOH (x(Fe)=0.5), similar activity is observed, shown in Figure 1b) with a ranging potential between 1.38 V to 1.41 V vs. RHE at 50 mA cm-2 and 1.42 V to 1.45 V vs. RHE at 750 mA cm-2.[1] a) Cao et al., Energy Environ. Sci. 2012 b) Zhao et al., ACS Chemical Reviews 2023.[2] Zeng et al., Journal of Energy Chemistry 2022.[3] Wu et al., Applied Surface Science 2021. Figure 1