Oxygen Research Articles

Operando Neutron Imaging for Improved Understanding of Gas Distributions in an Alkaline Electrolyzer

Optimizing hydrogen and oxygen transport within porous electrodes is essential for improving the efficiency of industrial alkaline electrolyzers. Current commercial electrodes, such as nickel foam, are limited by insufficient gas management. Research suggests that at least 5% efficiency improvements can be obtained by enhancing bubble removal in electrolyzers1,2. This challenge is particularly pronounced in large-scale industrial alkaline electrolyzers, with electrode diameters upwards of two meters3.In this study, we present quasi-dynamic operando neutron radiographic measurements conducted on an alkaline electrolysis cell equipped with commercial nickel foam electrodes. The imaging measurements involved a parametric study examining the impact of various operational parameters including pore density, current density, flow rate, and electrolyzer configuration (comparing zero gap versus non-zero gap assembly). All imaging experiments were conducted at NEUTRA at the Paul Scherrer Institute.Operando neutron imaging offers a unique approach for direct monitoring of the spatial distribution of hydrogen and oxygen gases inside the nickel foam during electrolysis. Utilizing the high contrast in attenuation between the electrolyte and gas, it is possible to identify transport-related bottlenecks, active regions of the electrodes during operation, and immobilized bubbles.Our findings suggest that the hydrogen and oxygen evolution reactions primarily occur near the electrode-membrane interface. Specifically, imaging of the bottom part of the electrolysis cell indicates that approximately 50% of hydrogen and oxygen is produced in the innermost quarters of both the cathode and anode. Additionally, stroboscopic operando neutron imaging reveals that 5 - 10% of the volume within the electrode compartments remains occupied by trapped gas bubbles even after system flush-out, underscoring the critical need for enhanced gas management within electrolyzers.In this talk, we will present the experimental method applied in our study, discuss our findings, and detail how these results can lead to further optimization of electrolyzers. We will also demonstrate how the experimental results can be used as input for numerical modelling, and thus providing a practical framework for improving electrolyzer design and operation. Through this work, we aim to contribute to the development of more efficient and effective electrolyzer technologies by combining neutron imaging with numerical modelling. Acknowledgements This work was supported by Innovation Fund Denmark under Case no. 2077-00021A and Case. No. 2040-00025B.

(Invited) Development of High-Efficiency Transparent Cu2O Top Cells for Tandem Photovoltaics with Efficiency Exceeding 30%

Solar cells with high power generation efficiency are expected to be used in mobility applications that require high power output in a limited footprint, such as electric vehicles (EVs) and high-altitude platform stations (HAPS), as well as in current products for housing and industrial applications. A tandem structure is an effective option for high-efficiency solar cells, and various combinations of solar cells have been proposed as the top and bottom cells, including InGaP/GaAs and perovskite/Si. Our proposed tandem solar cell consists of a transparent cuprous oxide (Cu2O) top cell and a crystalline Si bottom cell and is a promising candidate for the next generation of tandem solar cells because of its combination of high efficiency and low cost. High tandem efficiency of 30% or higher is required for various advanced applications such as mobility. Cu2O has a wide bandgap of 2.1 eV, and its spectral sensitivity complements that of crystalline Si, so high bottom-cell efficiency can be achieved. By using Cu2O in the top cell, the Si bottom cell can be expected to have an efficiency of 20% due to long-wavelength light transmitted through Cu2O, and by generating an efficiency of 10% in the Cu2O top cell, a tandem efficiency exceeding 30% is within sight. Cu2O is a low-cost material because it consists of the abundant elements copper and oxygen. Also, it can be formed on inexpensive glass substrates by low-cost manufacturing processes such as sputtering. We have established a technology to deposit high-quality Cu2O thin films as a single phase on transparent electrodes by reactive sputtering. By precisely adjusting the oxygen gas flow rate and substrate temperature, Cu and CuO present as different phases could be removed and a metastable Cu2O phase selectively deposited. A top cell using this low-defect Cu2O as the optical absorption layer has achieved an efficiency of 8.4% [1]. In this paper, we report on a Cu2O top cell that exhibits the world's highest efficiency of 10.5%, which was achieved by improving the short-circuit current density (Jsc) and the open-circuit voltage (Voc). By increasing the size of the Cu2O top cell by a factor of about 3, the area ratio of the dead area at the cell edge to the power-generating area was reduced. We expect that our Cu2O film has a long carrier diffusion length, and thus when the cell area is small, many carriers diffuse to the cell edge face and recombine. By contrast, a larger cell area increases the number of carriers that can contribute to power generation without carriers reaching the cell edge face. Device simulations showed that there were many interfacial defects between p-type Cu2O and n-type Ga2O3, which reduced Voc. Therefore, a unique passivation layer was introduced at the pn interface. This suppressed the interfacial defects and increased Voc by about 0.1 V compared with the conventional cell. The measured efficiency of 10.5% for the Cu2O top cell exceeds our milestone target of 10% for top-cell efficiency, which is required to achieve tandem efficiency of 30%.This work is based on results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO).[1] S. Shibasaki, Y. Honishi, N. Nakagawa, M. Yamazaki, Y. Mizuno, Y. Nishida, K. Sugimoto, and K. Yamamoto, "Highly transparent Cu2O absorbing layer for thin film solar cells," Appl. Phys. Lett. 119, 242102 (2021).

Catalytic Electrodes for the Oxygen Reduction Reaction Based on Co-Doped Carbon Quantum Dots and Anion Exchange Ionomer

The sluggish kinetics of the oxygen reduction reaction (ORR), more than six orders of magnitude slower than the hydrogen oxidation reaction in acidic conditions, is a major concern for various energy storage and conversion devices, including metal air batteries and fuel cells. The exceptionally high O––O bond energy (~500 kJ/mol) requires generally the use of noble-metal electrocatalysts, including platinum and palladium, for the four-electron reduction in acidic conditions.The ORR is however faster in alkaline medium and non-noble electrocatalysts are applicable, including doped carbon materials. Nanostructured carbons have especially retained the attention due to their solubility in organic solvents that make the drop-casting procedure available. The electroactivity of carbon quantum dots (CQD) for the ORR was demonstrated in 2012.We have recently compared the preparation of CQD by three different methods -pyrolysis, microwave irradiation and hydrothermal synthesis - and reported their respective properties. The hydrothermal method is most promising due to the easily controllable composition and structure via the optimization of precursors and can produce CQD with uniform particle size. We showed especially that the position of nitrogen atoms in the nanostructure, for example in graphitic, pyrrolic or pyridinic sites, changes significantly the properties of the CQD.In this work, we therefore investigate a simple and easily up-scalable synthesis of heteroatom co-doped CQD, using inexpensive starting materials and a fast hydrothermal synthesis procedure, which provides uniform CQD size. The CQD are characterized by various techniques, including XPS, FTIR, and Raman spectroscopy.The overpotential of the ORR contains the activation overpotential that can be reduced by the presence of a potent electrocatalyst improving the charge transfer kinetics, the Ohmic drop and the diffusion overpotential, related to the transport of oxygen gas and of hydroxide ions. Whereas the oxygen diffusion can be facilitated by the microstructure of the electrode, the hydroxide ion transport is enhanced by the presence of an anion exchange ionomer (AEI) in the electrode. The ORR occurs at the triple phase boundary between gas phase (pore), electron conducting catalyst (carbon paper/CQD) and hydroxide ion conductor (AEI), here poly(sulfone trimethylammonium hydroxide) (PSU-TMA).The electrodes investigated in this work are therefore prepared by drop-casting on carbon paper of a slurry containing co-doped CQD and AEI. Impedance spectroscopy and d.c. capacitance measurements are used to determine the electrochemically active surface area of the electrodes. The electrocatalytic activity for the ORR is studied in alkaline conditions (1 M KOH) by cyclic (CV) and linear sweep voltammetry (LSV) at various speeds of a rotating disk electrode (RDE). A.R. Nallayagari, E. Sgreccia, R. Pizzoferrato, M. Cabibbo, S. Kaciulis, E. Bolli, L. Pasquini, P. Knauth, M.L. Di Vona, Tuneable properties of carbon quantum dots by different synthetic methods, J. Nanostruct. Chem. 12 (2022) 565–580. A.R. Nallayagari, E. Sgreccia, L. Pasquini, F. Vacandio, S. Kaciulis , M.L. Di Vona, P. Knauth , Catalytic electrodes for the oxygen reduction reaction based on co-doped (B-N, Si-N, S-N) carbon quantum dots and anion exchange ionomer, Electrochemica Acta 427 (2022) 140861.

Suppression of Hydrogen Peroxide Formation on Pt Using WO3 as Anode Co-Catalysts

Polymer electrolyte fuel cells (PEFCs) are emerging as promising candidates for next-generation energy systems, particularly for hydrogen vehicles and clean energy applications, due to their advanced high-power density production and efficiency. However, the commercialization of PEFCs is hindered by issues related to fuel cell durability, primarily caused by the chemical degradation of proton exchange membranes (PEMs) due to reactive oxygen-related radical attacks, notably generated through the production of hydrogen peroxide (H2O2). Anode catalysts, such as carbon-supported platinum nanoparticles (Pt/C), play a vital role in facilitating the hydrogen oxidation reaction (HOR), which is accompanied by the undesired formation of H2O2 due to oxygen gas (O2) crossover from the cathode. As for the Pt surface, the structure is sensitive to H2O2 production. Hence, minimizing H2O2 generation while maintaining HOR activity by developing anode catalysts is necessary to suppress degradation. Tungsten oxide (WO3) has emerged as an interesting active material due to its stability in acidic solutions and its ability to increase the rate of hydrogen oxidation by facilitating hydrogen spill-over from the Pt surface to WO3 to generate hydrogen tungsten bronze (HxWO3). [1, 2] This process enhances the availability of hydrogen on the Pt surface by effectively transferring hydrogen atoms from Pt to WO3. With more active sites available on the Pt surface, there is a higher likelihood of hydrogen oxidation, reducing the formation of H2O2 as an intermediate product.This study aims to develop an anode catalyst by incorporating WO3 nanoparticles into the Pt/C to suppress H2O2 production during HOR, particularly in the presence of oxygen. The rotating ring-disk electrode (RRDE) technique was utilized to evaluate catalyst performance and detect H2O2 generation during HOR. HOR activity and H2O2 production rate measurements were conducted in H2-sat. and H2/air-sat. 0.1 M HClO4 electrolyte at different temperatures (25, 40, and 60 ℃). The electrochemical measurements demonstrated that both Pt/C and WO3-Pt/C catalysts exhibited enhanced H2O2 formation rates with increasing temperature. This observation underscores the significance of temperature as a modulator of catalytic performance and suggests the existence of temperature-dependent kinetic mechanisms governing H2O2 generation during HOR. Moreover, the comparative analysis between Pt/C and WO3-Pt/C catalysts reveals distinct temperature-dependent behaviors. While both catalysts exhibit temperature-enhanced H2O2 formation, the WO3-Pt/C catalyst demonstrated significantly reduced H2O2 production at 0 V vs reversible hydrogen electrode (RHE) approximately 40% at 60 ℃ as compared with Pt/C, and it exhibited high HOR mass activity and surface activity across a broad temperature range. This suppression is attributed to the synergistic effect between platinum and tungsten oxide species, which facilitates improved HOR kinetics and H2O2 selectivity.This study was supported in part by funds for the “R&D of novel anode catalyst” project in the “Collaborative industry-academia-government R&D project for solving common challenges toward dramatically expanded use of fuel cells” from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. The authors thank Prof. Hiroyuki Uchida (The University of Yamanashi) for his kind advice.

Poly(2,6-dimethyl-1,4-phenylene oxide)-Based Anion Exchange Membranes Enhanced with 3D-Ionic Triptycene Additive for Water Electrolysis

In water electrolysis systems, anion exchange membranes (AEMs) play a crucial role in separating the hydrogen and oxygen gases produced, while selectively transporting hydroxide ions between the cathode and anode. Key characteristics of AEMs include high hydroxide ion conductivity and durable stability under challenging alkaline conditions. We introduce a novel approach aimed at overcoming the hurdles related to achieving high ion conductivity and alkaline stability in AEMs. We propose a simple method that incorporates a 3D ionic triptycene additives in a quaternized poly(phenylene oxide) (PPO) polymer matrix to form a blended anion exchange membrane. Triptycene, known for its bulky and rigid molecular structure formed by three interconnected benzene rings, has traditionally posed challenges in polymer chain packing due to its high free volume. Despite its potential, controlling its polymerization degree and enhancing mechanical properties has been difficult due to its complex three-dimensional structure. In this study, we introduced triptycene molecule as ionic additives to polymer matrix without polymerization process. The triptycene was chloromethylated to obtain the triptycene-Cl additive and PT-x blended membranes were quaternized by immersion in TMA solution, which led to a high quaternary ammonium group. The addition of triptycene provided extra quaternary ammonium groups and increased free volume, enhancing hydroxide ion transport efficiency and dimensional stability, despite increased water uptake. Moreover, the inclusion of the 3D ionic triptycene additive contributed to additional free volume within the packing structure of the polymer chains. These properties facilitate efficient transport of hydroxide ions. As the loading of 3D ionic triptycene increased, the QPQT-x blended membranes demonstrated improvements in experimental IEC value, in-plane hydroxide ion conductivity, and water uptake, while suppressing the swelling ratio. The QPQT-20 membrane exhibited satisfactory mechanical stability and high in-plane hydroxide ion conductivity, making it suitable for water electrolyzer cell tests. Compared to a commercial FAA-3-50 membrane, the QPQT-20 membrane exhibited lower and more stable area specific resistance. The BPPO/3D ionic triptycene blended membrane, dual-quaternized and containing 20% 3D ionic triptycene, demonstrates notably superior water electrolyzer cell performance (0.87 A/ cm2 at 2.0 V), in comparison to the commercial FAA-3-50 membrane (0.54 A/cm2 at 2.0 V).

Computational Modeling of Proton Exchange Membrane Electrolyzer

Hydrogen holds great promise to help decarbonize our energy sector. Proton exchange membrane electrolyzers (PEMELs) are undergoing rapid development and deployment for the generation of green hydrogen based on renewable sources of electricity. The inherent capacity of PEMELs to readily respond to fluctuations in electricity input is an added advantage for the generation of hydrogen. PEMEL optimization with regard to materials, component design, and operating conditions is critical for high performance and efficiency. The development of a robust computational model for PEMELs can facilitate not only an improved understanding of the relevant physical and electrochemical phenomena but also help to develop and test effective strategies for performance optimization. The current research is aimed at developing a 3D parallel flowfield channel PEMEL model in COMSOL Multiphysics. To date, results have been obtained for a single-phase model with the baseline operating temperature and relative humidity (RH) of 60°C and 90%, respectively, under the following assumptions: (1) the feed gas (water vapor) is treated as an ideal gas, (2) the flow in the channel is laminar and incompressible, (3) membrane, catalyst layer (CL), and porous transport layer (PTL) microstructures are assumed to be homogeneous and isotropic, and (4) all physical and electrochemical processes are considered to be at steady state. The simulated polarization curve was validated against in-house experimental data for Nafion 115 at 60°C. We then investigated the effect of operating temperature and membrane thickness on current density, overpotentials due to cell resistance, and polarization curves. It was observed that the PEMEL performance improved significantly with temperature, owing primarily to the higher protonic conductivity of the membrane and improved catalyst kinetics. Polarization curves were also obtained for Nafion 212 (50.4 µm) and 117 (178 µm) at 80°C and the thinnest membrane, i.e. Nafion 212, performed better than its thicker counterparts due to reduced ohmic losses. We are currently developing a two-phase flow PEMEL model wherein liquid water is supplied to the anode inlet, and oxygen gas mixed with liquid water emerges from the anode exit. At high current density, oxygen bubbles can obstruct the reactant flow to the anode catalyst layer resulting in suboptimal PEMEL performance. Results will be presented for the dynamics of two-phase flow in a PEMEL system under various operating conditions from which we may draw insights to optimize PEMEL performance.

The Effect of SOFC Flow Rate Conditions on the Composition of Lean Oxygen Gas Used in Trigeneration System

Solid oxide fuel cell (SOFC) is attracting attention for its high efficiency and low environmental impact. By integrating them into cogeneration systems, which harness the waste heat produced during power generation, significant overall efficiency gains can be achieved. Trigeneration systems (TGS), which extend this concept by incorporating exhaust gases alongside cogeneration, hold promise for reducing overall cost and energy required for the process. In this study, TGS was considered to utilize the exhaust gas from the SOFC cathode as a lean oxygen gas. Conventionally, nitrogen used as lean oxygen gas is obtained through energy-intensive deep cooling separation of air, and the process requires significant cost and energy. They are expected to be reduced by using SOFC exhaust gas to obtain lean oxygen gas. However, employing SOFC in TGS requires consideration of the tradeoff between oxygen concentration and power density. Consequently, this study aims to elucidate the impact of operating SOFC at high oxygen utilization on power generation characteristics. The influence of flow conditions on power generation characteristics was examined through experimental investigations under integral reactor conditions, where air was supplied to the cathode at a small flow rate, and under differential reactor conditions, where gas with a different composition was supplied at a larger flow rate. Additionally, simulations of oxygen concentration distribution within the flow channel were conducted.Under integral reactor conditions, power generation characteristics of the SOFC exhibited voltage drops in the low current density region at lower flow rates. To investigate variations in oxygen utilization under different flow conditions, concentration and current density distributions within the cell were obtained from numerical analysis using the finite volume method1). A 1-D model was established for analysis, considering mass balance of gas flow in the channel direction. The distribution was calculated by correlating current density with overvoltage and oxygen concentration, as determined from results obtained under differential reactor conditions. The analysis revealed that as flow rates decreased, current density diminished towards the cell outlet, and at lower flow rates, with slight current flow observed near the outlet, indicating little oxygen is consumed there. Consequently, optimizing operating conditions to minimize the portion of the system where oxygen consumption is low are required for effective utilization of SOFC in TGS.AcknowledgementThis work was supported by F. C. C. Co.References1) Y. Inui et al., IEEJ Trans. PE, 123(1), 66 (2003)

H2 Production Efficiency in PEM Water Electrolysis Cells – Evaluating the Effect of the Electrode-Electrolyte Interface on O2 and H2 Crossover

PEM water electrolyzers (PEMWEs) are a promising candidate to store fluctuating renewable energy due to their wide current density operating regime. This flexibility is a significant advantage compared to alkaline water electrolysis. However, this flexibility in current density is also a challenge to fully understand local and interfacial effects at different operating conditions minding the different scales that have to be considered in PEMWEs – from the nanometer-sized ionomer thin film to millimeter-sized channel structures for water supply and gas removal.Gas crossover is a big challenge in PEMWEs. It does not just limit the operating regime due to possible formation of explosive gas mixtures on the anode side. 1 It also contributes significantly to the overall efficiency of the system by causing some hydrogen that was produced by the electrical energy input to not reach the cathode exhaust. Crossover hydrogen is lost to the anode exhaust and crossover oxygen chemically recombines with hydrogen, which is therewith also lost when considering the overall hydrogen production efficiency. 2 Compared to PEM fuel cells, gas crossover characteristics in PEMWEs under operation are diverging more from steady-state permeation data. 1, 3 This can be caused by operation dependent two-phase flow characteristics of liquid water and gaseous product. While the anode is actively flushed with liquid water as reactant supply, the cathode of a PEMWE also experiences two-phase operation; either due to being flushed with water additionally or caused by the significant electro-osmotic drag at high current densities leading to water droplet formation in the cathode catalyst layer. The exact environment of the electrode-electrolyte interface will dictate the crossover behavior at different operating points but it is not well understood.In this study we shed some light on the effect of the interfacial two-phase characteristics on the gas crossover behavior at different operating conditions. Hydrogen and oxygen steady-state permeation over PEMWE membranes is studied using a mass spectrometer at different single- and two-phase flow conditions that can be decoupled from the electrochemical gas production and electro-osmotic drag effects due to active hydrogen and oxygen gas supply. By a thorough parameter variation we are able to distinguish solubility, diffusivity and fugacity effects on the overall permeability of the active gases. We show that common pure gas-phase measurements lead to gas permeabilities that show a severalfold difference compared to the results from the two-phase measurements.By comparing such steady-state permeation with gas crossover characteristics at different operating points a better understanding of the electrode-electrolyte interface composition during PEMWE full cell operation can be achieved. While the exact interfacial characteristics depend on the catalyst layer and transport media structure, this experimental data shall serve as a starting point for revisiting the tuning of structural parameters in order to achieve PEMWEs with improved hydrogen production efficiencies. Such dynamic effects will be important in large-scale cells where the two-phase characteristics can change along the cell, additionally. Bernt, M.; Schröter, J.; Möckl, M.; Gasteiger, H. A., Analysis of Gas Permeation Phenomena in a PEM Water Electrolyzer Operated at High Pressure and High Current Density. J. Electrochem. Soc. 2020, 167.Schalenbach, M.; Carmo, M.; Fritz, D. L.; Mergel, J.; Stolten, D., Pressurized PEM water electrolysis: Efficiency and gas crossover. Int. J. Hydrogen Energy 2013, 38, 14921-14933.Trinke, P.; Haug, P.; Brauns, J.; Bensmann, B.; Hanke-Rauschenbach, R.; Turek, T., Hydrogen Crossover in PEM and Alkaline Water Electrolysis: Mechanisms, Direct Comparison and Mitigation Strategies. J. Electrochem. Soc. 2018, 165, F502-F513.

A Low-Cost Bipolar Plate Comprising a Channeled Porous Transport Layer for Proton and Anion Exchange Membrane Electrolyzers

Hydrogen is gaining acceptance as a viable energy vector for renewable energy and as a green molecule for chemical synthesis, but its production in a sustainable, efficient, and affordable manner requires further optimization. Proton exchange membrane electrolyzers (PEMELs) can be efficient for hydrogen production. Within the PEMEL stack, the repeating unit cell consists of a bipolar plate, two porous transport layers (PTLs), two catalyst layers, and one proton exchange membrane (PEM). The oxygen evolution reaction at the anode creates a corrosive environment, therefore, titanium is used as the anode bipolar plate material. The anode bipolar plate is typically machined from a block of titanium for small volume production or stamped for large volume production, which is time-consuming and challenging due to frequent tool breakage resulting in high cost. Furthermore, ineffective in-plane water transport within the anode PTL can cause inadequate water supply to the catalyst layer resulting in unutilized catalyst. To alleviate these problems, we have designed a novel PEMEL cell containing channeled titanium foam enclosed within a PTFE frame that functions both as the bipolar plate and the PTL, termed channeled porous transport layer (CPTL). In our design, the channels face the current collector such that the water flows through the channels and the pores of the foam in both the through-plane and in-plane directions, thereby maximizing catalyst coverage and utilization. Similarly, the pores and channels of the CPTL provide an effective pathway for the evacuation of oxygen gas evolved at the anode catalyst layer. Most importantly, the channels in the titanium foam can be formed as part of the PTL production process which could greatly reduce PEMWE manufacturing costs and lower the cost of hydrogen. The same CPTL design can also be employed in anion exchange membrane electrolyzers (AEMELs) whose anode environment permits the use of nickel foam which further reduces material costs. We will present experimental results for PEMELs and AEMELs performance with this new design and compare them with those obtained with conventional bipolar plates.

Investigation of Ruthenium and Iridium Metal as Potential Anodes for Oxide Reduction in Molten LiCl-Li2O

The electroreduction of UO2 during pyroprocessing is counterbalanced by the oxidation of dissolved oxygen ions typically at a platinum anode in molten LiCl-Li2O to form oxygen gas. Platinum and previously explored alternative anodes have been found to corrode concurrently with the evolution of oxygen which poses an economic challenge for implementing pyroprocessing industrially. Ruthenium and iridium metal were investigated as potential anodes for the electroreduction step of pyroprocessing. Electrochemical data for the tested anodes will be presented. Additionally, extensive surface characterization including X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy and Raman spectroscopy will be reported for the tested anodes.This work was performed under ARPA-e grant DE-AR0001697. XPS was purchased under NSF MRI award 2117820. C.M. is supported by DOE Fellowship under award DE-NE0009105.

(Invited) Mn-Based High-Energy Cathodes for Lithium-Ion Batteries

Cation-disordered Li-excess rocksalts (DRX) are promising cathode materials for next-generation lithium-ion batteries utilizing sustainable resources. [1] These compounds are typically Co- and Ni-free, and they offer exceptionally large charge storage capacities by activating the redox reactions of both cationic transition-metals and anionic oxygen in the lattice. [2-4] Recent studies showed that significant performance improvement can be achieved by introducing fluorine substitution. Fluorine was found to improve local structural stability and chemomechanical properties, as well as reduce oxygen gas release, impedance rise and capacity fade. [5-7] However, achieving adequate cycle life remains a formidable barrier in DRX cathodes.In this presentation, we will show our recent effort in developing stable DRX oxyfluorides based on Mn-redox chemistry. Mn-rich DRX oxyfluoride cathodes consisting of earth-abundant elements will be presented. These materials experience unique local structural transformations upon charge and discharge, leading to dynamic evolution in voltage profile and significant capacity rise during early cycles. Stable cycling can be achieved after the initial activation process. Our understanding in how chemistry can impact local and long-range structures and their evolution during electrochemical cycling will also be presented, as well as the perspectives on future directions in cathode development. References Chen D.; Ahn J.; Chen, G. An Overview of Cation-Disordered Lithium-Excess Rocksalt Cathodes. ACS Energy Lett. 2021, 6, Lee, J.; Urban, A.; Li, X.; Su, D.; Hautier, G.; Ceder, G. Unlocking the Potential of Cation-Disordered Oxides for Rechargeable Lithium Batteries. Science 2014, 343, 519.Yabuuchi, N.; Takeuchi, M.; Nakayama, M.; Shiiba, H.; Ogawa, M.; Nakayama, K.; Ohta, T.; Endo, D.; Ozaki, T.; Inamasu, T.; Sato, K.; Komaba, S., High-Capacity Electrode Materials for Rechargeable Lithium Batteries: Li3NbO4-based System with Cation-Disordered Rocksalt Structure. Natl. Acad. Sci. 2015, 112, 7650.Chen, D.; Kan, W. H.; Chen, G. Understanding Performance Degradation in Cation-Disordered Rock-Salt Oxide Cathodes. Energy Mater. 2019, 9, 1901255.Lee, J.; Papp, J. K.; Clément, R. J.; Sallis, S.; Kwon, D.-H.; Shi, T.; Yang, W.; McCloskey, B. D.; Ceder, G. Mitigating oxygen loss to improve the cycling performance of high capacity cation-disordered cathode materials. Commun. 2017, 8, 981.Lun, Z.; Ouyang, B.; Kitchaev, D. A.; Clément, R. J.; Papp, J. K.; Balasubramanian, M.; Tian, Y.; Lei, T.; Shi, T.; McCloskey, B. D.; Lee, J.; Ceder, G. Improved Cycling Performance of Li-Excess Cation-Disordered Cathode Materials upon Fluorine Substitution. Energy Mater. 2018, 9,1802959.Ahn, J.; Chen, D.; Chen, G.. A Fluorination Method for Improving Cation-Disordered Rocksalt Cathode Performance. Energy Mater. 2020, 10, 2001671.

Highly Durable Barium Iridates for Oxygen Evolution Electrocatalysis Via Atomic Ordering of Nb5+/Ta5+(d 0) and Suppression of Lattice Oxygen Participation

Proton exchange membrane water electrolyzers (PEMWEs) play a pivotal role in the journey towards a sustainable hydrogen society [1]. Despite offering remarkable current density and exceptional hydrogen purity (99.99%), the highly acidic nature of sulfonated fluoropolymer membranes hinders material selection for anode catalysts to facilitate the oxygen evolution reaction (OER) [2]. Therefore, iridium-based electrocatalysts are regarded as indispensable choices owing to their strong resistance to corrosion under acidic environments based on Pourbaix diagram [3]. However, despite their remarkable stability and performance in acid, two main unsolved issues remained in iridium-based electrocatalysts. The first is the scarcity of iridium, which hampers the widespread adoption of PEMWEs. Iridium accounts for 25 % of the total cost of membrane electrode assemblies (MEA), making the reduction of iridium content an urgent necessity. The other unsolved issue is the suppression lattice oxygen evolution reaction (LOER) [4]. While LOER offers an opportunity to overcome the theoretical OER overpotential, such lattice oxygen participation also induces oxygen gas evolution from [IrO6] crystals and consequently lattice collapse. Hence, there is a pressing need for alternative catalysts with lower iridium content but limited LOER to achieve comparable activity and stability to commercial IrO2.In this study, using hexagonal 9R-perovskite BaIrO3 as starting materials, we systematically demonstrate the d orbital contribution to structural durability according to lattice oxygen participation, especially d 0 states of Nb5+ and Ta5+. The choice of BaIrO3 as a reference catalyst was governed by its structural robustness from face-sharing connectivity between [IrO6] clusters, which is more difficult to disassemble compared to IrO2 with edge-, and corner-sharing configurations [5]. Two notable features were observed when transition-metals (Mn, Co, Ni, In, Nb, and Ta) were doped into BaIrO3; i) phase transformation occurs from 9R- to 6H- or 12R-type polymorphs and 2) dopants occupy specific sites in a very ordered manner (Figure 1a). We directly observed such ordering natures by atomic-column resolved scanning transmission electron microscopy (STEM). When Nb5+ and Ta5+ added, 12R-Ba4NbIr3O12 and Ba4TaIr3O12 were formed with Nb and Ta cations located in bridging sites between face-sharing [Ir3O12] trimers.Density functional theory (DFT) calculations were conducted to study the electronic structure of Ba4NbIr3O12 (Figure 1b). The electron configuration of Nb5+ consists of fully filled 4p 6 and empty 4d 0 states, thereby Nb 4d orbitals contribute to the upper electronic band above +2 eV. Consequently, in contrast to the large overlap between O 2p states (black arrow) with Ir 5d states in [IrO6], O 2p states (red arrow) of [NbO6] beneath the Fermi level are significantly suppressed. These findings indicate that [NbO6] serves as an OER inactive site and prevents structural disassembly by limiting lattice oxygen participation. We also carried out time-of-flight SIMS to quantitatively compare lattice oxygen contribution between pristine BaIrO3 and Ba4NbIr3O12 during OER using anodic cycling samples in 1 M HClO18 4. As depicted in Figure 1b, substantially lower amount of O18 was detected in Ba4NbIr3O12 than in BaIrO3, which directly demonstrated the significant suppression of LOER.The catalytic longevity of IrO2, BaIrO3, and Ba4NbIr3O12 was evaluated by cyclic voltammetry (CV) and chronoamperometry (CA) tests (see Figure 1c). The relative catalytic behavior was compared in terms of preserving OER activities by normalizing with initial current densities. Both CV and CA results provide straightforward evidence, validating that Ba4NbIr3O12 is prominent among the three catalysts. As a result, we have developed promising electrocatalysts with both low iridium content and higher durability, which stem from the synergism of chemical ordering of d0 cations to suppress lattice oxygen participation and robust face-sharing trimers in the structures. Our research proposes the proper management of chemical ordering in oxides offers a straightforward yet effective approach for developing electrocatalysts with significantly enhanced longevity.

(Invited) Treated Perfluorosulfonic Acid Ionomer Layer with Dual Oxygen Transport Pathways for PEMFC Cathode Catalyst Layer

A hydrophobic treatment was developed to reduce the liquid water saturation level and increase its removal rate from the cathode catalyst layer (CL) of a PEM fuel cell with the expected outcome to be higher performance in the transport-controlled region. However, test results show improvement (> 200% increase in peak power)[1] in both kinetic and transport-controlled regions. Further investigation by mathematical modeling and neutron reflectometry shows the treatment affects the CL in two ways. First, the ionomer/gas phase interface in the pores of the CL becomes more hydrophobic. This facilitates less liquid water coverage of the ionomer layer surface and faster water drainage from the CL that results in better performance at high current densities, which is as expected. Second, it affects the microstructures of the ionomer phase and causes it to separate into two regions, a larger more hydrophobic (or less hydrophilic) region with fewer sulfonic acid groups and therefore lower water hydration levels and a smaller less hydrophobic (or more hydrophilic) region with more ionic groups with higher hydration levels. The more hydrophobic region, which facilitates faster liquid water removal from its interface with the gas pores and direct access to the oxygen gas at higher current densities, provides a higher oxygen solubility pathway to the catalyst active sites in CL at both low and high current densities leading to higher fuel cell performance over the whole polarization curve. See image. The ratio of these two regions can be controlled by the treatment process. This presentation will discuss the hydrophobic treatment process and the major findings of this study.References Regis P. Dowd, Yuanchao Li, Trung Van Nguyen, “Controlling the Ionic Polymer/Gas Interface Property of a PEM Fuel Cell Catalyst Layer During Membrane Electrode Assembly Fabrication,” Appl. Electrochem., 50 (10) 993-1006 (2020).Yuanchao Li and Trung Van Nguyen, “A One-Dimensional Model of the Cathode Catalyst Layer of a PEM Fuel cell Hydrophobically Treated for Water Management,” J. Electrochemical Society, 169 114505 (2022). Figure 1

(Digital Presentation) Electrochemical Synthesis of Chromium Carbide Coatings on the Surface of Carbon Fibres in NaCl-KCl-CrCl3-Cr Melt and Study of Their Electrocatalytic Properties

Composite materials based on carbides of refractory metals are of exceptional interest in various fields of application. They have unique characteristics such as high hardness, catalytic activity [1,2], etc. They are widely used in industry due to their high corrosion and oxidation resistance at high temperatures. Chromium carbide can be produced in various ways, for example, the most common method of production is physical deposition from the gas phase (PVD coating). Other synthesis methods are not so popular, but they are also widely known (electron beam surfacing, plasma arc welding, etc.). These methods have both advantages and disadvantages. Refractory metal carbide coatings obtained by electrochemical methods in molten salts have a number of advantages, such as the possibility of obtaining homogeneous coatings on products with complex shape, low porosity of coatings, high adhesion to the substrate.Coatings of refractory metal carbides on a developed surface can also be used as electrocatalytic systems in reducing and oxidizing reactions. At the same time, the advantage of using electrocatalysis is the possibility of carrying out various reactions in a wide range of conditions from an extremely reducing environment to an extremely oxidizing one, the ability to control the route of the reaction by precisely adjusting the potential. Unlike common catalysis, electrocatalytic reactions do not leave reagents in the form of oxides, hydroxides or salts in the products. In electrochemical processes, an electron acts as a reducing agent, therefore high purity products are formed.The purpose of this study was the electrochemical synthesis of chromium carbide in molten salts on a substrate made of carbon fibres Carbopon-B-22, as well as the study of electrocatalytic properties of the obtained composites.To precipitate chromium onto a carbon substrate, a non-pressure transfer method in a molten equimolar mixture of NaCl and KCl containing 20 wt.% CrCl3, which is in contact with metallic chromium, was used. The synthesis of chromium carbides was carried out at temperatures of 1123 and 1223 K for 8-16 hours.To study the kinetics of the electrocatalytic decomposition of hydrogen peroxide on the surface of carbon fiber–synthesized chromium carbides, a method based on measuring the volume of the released oxygen gas was used [3,4]. The initial carbon fiber Carbopon-B-22 was used as the cathode, and a carbon fiber electrode with Cr3C2 and Cr7C3 chromium carbides coatings was used as the anode.In this study, two integral methods were used to determine the order of the hydrogen peroxide decomposition reaction: the substitution method (calculation of reaction rate constants according to equations for different reaction orders) and graphical (plotting of various concentration dependences on time and determining the linear dependence of one of them). The reaction rate constant was determined by the slope tangent of the straight line in the corresponding coordinates.The activation energy of the process was calculated based on experimental data obtained at different temperatures.References Stulov Yu., Dolmatov V., Dubrovskiy A. and Kuznetsov S. Electrochemical synthesis of functional coatings and nanomaterials in molten salts and their application // 2023. V. 13. 352.Dolmatov V.S., Kuznetsov S.A. Synthesis of Refractory Metal Carbides on Carbon Fibers in Molten Salts and Their Electrocatalytic Properties // Journal of The Electrochemical Society. V. 168. 122501.D. G. Miklashov, V. S. Dolmatov and S. A. Kuznetsov The currentless synthesis of tantalum and niobium carbide coatings on the carbon fibers and their electrocatalytic activity for the hydrogen peroxide decomposition reaction J. Phys.: Conf. Ser. 1281 012054.Yang G., Chen F., Yang Z. Electrocatalytic Oxidation of Hydrogen Peroxide Based on the Shuttlelike Nano-CuO-Modified Electrode // Inter. J. Electrochem. – 2011. – № 2012. – P. 1-6.

Design Rules for Optimizing Microporous Layer Properties and Back-Layer Effect Using Decal Transfer Method in PEMWE

Polymer electrolyte membrane water electrolysis (PEMWE) has the fatal drawback of using expensive noble metal catalysts, especially the rare iridium substances in the anode. Therefore, improvements in catalyst utilization are required to achieve high performance and durability even at a minimized use of catalyst. One option is introducing a titanium-based microporous layer (MPL) at the interface of the catalyst layer and porous transport layer (PTL) to create an increased contact area and form a better electrical connection. However, how the MPL should be designed and manufactured is not fully understand yet. Herein, we present a decal-transfer method, which is a convenient process to manufacture a uniform and thin MPL on a porous substrate layer. Based on this method, we investigated the two-phase transport phenomena of the MPL by varying the particle size and thickness of the MPL and the porosity of the substrate layer. It is concluded that a smaller micrometer-scale titanium particle size is advantageous in both catalyst utilization and water transport, resulting in lower kinetic and mass transport overpotential. Furthermore, the interface property acted as the determinant factor for the overall performance, while the bulk property of the PTL was not a critical factor unless it did not hinder the mass transport of oxygen gas in-plane direction.

Oxide Speciation and Oxy-Fluoro Complexes in Molten Fluoride Salts

The presence and speciation of oxides are relevant to various engineering applications of molten fluoride salts. Here, our studies are directed toward two primary applications: 1) as coolants, fuel salts, or breeding blankets in advanced fission and fusion reactors, and 2) as the reaction medium for the novel direct synthesis of Li-ion battery cathodes from Li-ores.In an advanced nuclear reactor, oxide might be introduced to the molten fluoride coolant/blanket via ingress of oxygen or moisture from air. Oxygen and moisture act as oxidants, reacting with the metal fluoride to produce metal oxides or hydroxides, along with corrosive HF. By these reactions, the presence of oxide is an indicator of leaks as well as of the progression of corrosion. Quantification of oxides by square wave voltammetry (SWV) has been proposed as a technique for electrochemical oxide sensors [1]. The intensity of the peak current for the oxidation of oxide to oxygen gas is proportional to the quantity of oxide present, so comparison to a calibration curve can indicate how much oxide is present in the melt. True engineering systems, however, would likely contain multiple solutes including corrosion products, fission products, and activation products. Multi-component chemistries are further complicated by differences in solvent-solute interactions; different fluoride solvents have varying degrees of polymerization, so they incorporate solutes into their structure differently. Practically, this means that oxides may exist in different complexes depending on the solvent and other solutes, therefore oxide quantification via SWV is complicated by solvent and solute chemistry.The synthesis of Li-ion battery cathodes via reaction in molten fluoride salts is currently being studied within a DOE Office of Science BES project as a lower-carbon emissions alternative to traditional cathode manufacturing. This concept involves the dissolution of spodumene (LiAlSi2O6) in molten fluorides and the electrochemical deposition of disordered transition metal (M) oxyfluoride anion rocksalt (Li1+xTM1-xO2-zFz) for use as a battery cathode. The anion disorder in the rocksalt is relevant due to its relation to cathode performance. Energy storage capacity from the transition metal redox states increases as F is substituted for O. Cathode synthesis, therefore, requires tuning of the chemical potentials and transport properties in melts containing fluoride and oxide anions. In this context, a deeper understanding of the structure and speciation of oxide in molten fluorides is critical to formulating the reaction pathway and required composition and configuration of the rocksalt product.Given these applications, we investigate the speciation of oxide and its existence within oxo-fluoro complexes in molten fluoride salts using electrochemistry. We present results of electroanalytical studies of oxides in molten 2LiF-BeF2 and 46.5LiF-11.5NaF-42KF, using cyclic voltammetry and SWV. We make connections between solute speciation and solvent structure, allowing for mutually enhanced understanding of solubility and fluoroacidity. Broadly, electroanalytical studies provide a deeper understanding of oxide’s speciation in molten fluoride systems which, in turn, informs electrochemical sensor development and electrochemical synthesis processes. By investigating the fundamental science of oxide and oxide complexes, we hope to advance technologies which enable the transition to clean energy.[1] L. Massot, L. Cassayre, P. Chamelot, and P. Taxil, “On the use of electrochemical techniques to monitor free oxide content in molten fluoride media,” J. Electroanal. Chem., vol. 606, no. 1, pp. 17–23, Aug. 2007, doi: 10.1016/j.jelechem.2007.04.005.

Pore Uniformity Effect of Porous Transport Layers on Bubble Behaviors in PEM Water Electrolysis

Hydrogen, a clean energy source with high energy density and long-term storage potential, is gaining attention as a viable alternative to fossil fuels. Among various water electrolysis methods, Proton Exchange Membrane Water Electrolysis (PEMWE) is suitable for green hydrogen production due to its ability to operate efficiently at high current densities and low temperatures. A key component of PEMWE is the Porous Transport Layer (PTL), which facilitates oxygen gas discharge and water supply. However, to enable practical use of PEMWE at high current densities, effective oxygen removal without accumulation in the PTL is critical. While previous studies have explored the structural characteristics of PTL, research quantifying the effect of PTL pore uniformity on PEMWE performance remains limited. This study compared PTL samples with different pore uniformities using X-ray CT, investigated how these differences affect PEMWE performance, and analyzed bubble detachment characteristics on the PTL surface using high-speed imaging.A specially designed cell with a transparent acrylic end plate was used to observe internal flow in the PEMWE system. The bubble detachment characteristics, including the number, frequency, and diameter of detached bubbles, were analyzed. The homogeneous PTL followed a normal distribution, while the inhomogeneous PTL exhibited a broader pore size range, deviating from this distribution. These structural differences led to significant performance variations. The homogeneous PTL had a bubble detachment rate of 9.49 bubbles/mm2, with an average detachment frequency of 30.5 Hz and a detachment diameter of 2.21 mm. In contrast, the inhomogeneous PTL exhibited a significantly lower detachment rate of 0.30 bubbles/mm2, a higher detachment frequency of 78.2 Hz, and a larger detachment diameter of 22.4 mm. Furthermore, when comparing the departure velocity, calculated by multiplying the detachment frequency by the bubble diameter, the homogeneous PTL achieved a velocity of 289.3 mm/s, approximately 3.2 times faster than the 89.6 mm/s observed in the inhomogeneous PTL. These findings suggest that a more uniform pore structure enables faster and more efficient gas removal, leading to enhanced PEMWE performance. Additionally, the high departure velocity in the homogeneous PTL indicates a more effective two-phase flow, which contributes to the improved water supply and gas discharge, thereby optimizing overall cell efficiency.The homogeneous pore structures promote efficient gas discharge and water supply, leading to improved cell performance. These findings show the importance of designing PTLs with uniform internal pore structures to optimize PEMWE operation. Acknowledgment This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. RS-2023-00208497, RS-2023-00219369).

Performance Recovery in Irreversibly Degraded Polymer Electrolyte Membrane Fuel Cell for Enhanced Water Management

Polymer electrolyte membrane fuel cells (PEMFCs) offer numerous advantages, such as low operating temperatures, ease of start-up and shutdown, and high power density. Recent researches have focused on enhancing their performance for various industrial applications, particularly in transportation fields including vehicles and drones. The gas diffusion layer (GDL) in PEMFC plays a key role in the transport of fuel, oxygen, and water. The physical and chemical degradation of GDL adversely affects the water management resulting in performance deterioration.A major cause of GDL degradation in PEMFCs is the generation of hydrogen peroxide (H₂O₂) during the electrochemical reaction. H₂O₂ lead PTFE loss and carbon corrosion, and this degradation is irreversible. Although several studies demonstrate techniques for recovering performance for the reversible degradation, limited research exists on restoring performance in irreversibly degraded PEMFCs.This study proposes a novel approach for the performance recovery in irreversibly degraded PEMFCs by superimposing acoustic pressure waves on the oxygen gas flow which enhances the water management ability [1]. To investigate the effect of proposed recovery technique, performances were compared with or without the superimposed acoustic pressure waves using a unit cell having 25 cm² active area. The 19 % decrease in performance with aged GDL was observed due to the reduction in the water management capability caused by the decrease of hydrophobicity.The superimposed acoustic pressure waves led to a performance recovery of up to 9. This recovery is attributed to the enhanced removal of accumulated water on the GDL surface through external excitation, which facilitated oxygen supply and improved water discharge. The proposed the water management method can offer a promising solution for recovering performance in irreversibly degraded PEMFCs and has the potential to improve both performance and durability. Acknowledgement This work was supported by the National Research Foundation of Korea(NRF) grant funded by the Korea government(MSIT) (No. RS-2023-00208497, RS-2023-00219369). Reference [1] Kim, J. Y., Mortazavi, M., & Jung, S. Y. (2024). Improving polymer electrolyte membrane fuel cell performance and preventing flooding by exciting gas flow. Journal of Power Sources, 617, 235181.

Exploring the Effects of Recombination Interlayers on Hydrogen and Oxygen Crossover in PEM Water Electrolysis: Insights from 1D Model-Based Investigations

Proton exchange membrane (PEM) water electrolysis presents a promising solution for renewable energy storage and transportation through the production of green hydrogen. A major problem during the operation of PEM electrolysis, regarding both efficiency and safety, is the gas crossover. Small amounts of the produced hydrogen and oxygen gases permeate through the cell membrane to the opposite electrode side. This is problematic because the hydrogen that permeates to the anode is no longer available as product gas, resulting in Faradaic efficiencies less than unity [1]. Furthermore, hydrogen in oxygen mixtures at the anode with hydrogen contents above 4 vol.% are explosive and pose a critical safety issue [2,3]. Hence, several mitigation strategies like thicker membranes, external recombiners, or recombination interlayers (ILs) inside of the membrane have been developed to reduce gas crossover [4].In this study, we focus on ILs that are used to recombine permeating hydrogen with oxygen to water at catalyst particles (e.g., Pt) inside of the membrane, thus lowering the permeating fluxes. The advantages of ILs are that they can be easily integrated into standard cells, no external devices are needed, and previous experimental work has shown that the hydrogen in oxygen content can be lowered substantially. Until today, there are only experimental investigations regarding this topic, which fail to resolve local effects and only show the integral behavior of a PEM electrolysis cell with an IL. In this contribution, we propose the first model-based approach to investigate local potential, temperature, and concentration profiles during the operation of PEM electrolysis cells with a recombination IL. This further facilitates the understanding of the underlying local mechanisms through numerical investigations and enables further optimization of this mitigation strategy. The proposed model is a 1D through-plane stationary model that takes into account the transport of protons, electrons, heat, and gases dissolved in the water. The domain is represented as a series of seven adjacent layers from the anode to the cathode as shown in Figure 1a. A unique feature of the model is the simulation of the local profiles of the gases dissolved in the water of the membrane. These local concentrations allow us to model the reactions taking place in the IL. The proposed model is fully operational and validated with experimental data for different pressure conditions.The model results show that the IL has a major influence on the local concentrations and crossover fluxes (see Figure 1b). When using an IL, the hydrogen crossover flux entering the anode can be reduced compared to operations without an IL. However, the hydrogen crossover flux leaving the cathode, which is the relevant measure for calculating the Faraday efficiency, is increased when using an IL. This is due to a larger concentration gradient from the cathode to the IL compared to the concentration gradient without an IL. Analogous observations can be made for the oxygen crossover. In addition to these investigations, a series of unresolved issues regarding ILs are tackled with the proposed model. These are namely i) the influence of ILs on the operational limits of a PEM electrolysis cell under different pressure configurations, ii) the influence of recombination heat on the cell’s local heat distribution, and lastly iii) the influence of ILs on the Faraday efficiency of the cell. With regard to the latter investigation, the results show that despite having a major influence on the crossover fluxes, the Faraday efficiency stays constant when using ILs. This can be explained by two counter-acting loss processes, which are identified by the proposed model. These numerical results are consistent with experimental observations. Currently, additional experimental work to further elucidate the reaction kinetics inside of the IL and the relevant transport effects to update the model is underway.The authors gratefully acknowledge funding by BMBF in the framework of project evolver, FKZ 03SF0765.

Bubble Management through Electrolyte Engineering to Reduce Bubble Induced Resistance in Zero-Gap Water Electrolyzers

Hydrogen production through water electrolysis will play a central role to obtain a sustainable energy carrier. Although the community targets to operate at high current density (>1 A cm− 2) to reduce the production cost, the intense operation forms massive gas bubbles leading to detrimental effects on the resultant cell voltage. Gas bubbles may reduce the electrochemical active surface area leading to an increase in kinetic overpotential. The voids in the electrolyte inhibit the ion conduction and the mass transfer. Surface modifications and structuring are often performed on electrodes to control the wettability.[1] In this study, we demonstrate bubble management through electrolyte engineering. Bubble detachment diameters measured in various electrolytes were analyzed through an analytical equation and multiphase fluid dynamics simulations to clarify the major contributions from the electrolytes.[2] The knowledge was further implemented in a homemade zero-gap water electrolyzer to reduce bubble induced ohmic losses.Ni plate was employed as a model electrode to produce oxygen gas bubbles at 50 mA cm− 2. Electrolyte solutions were various concentrations of alkaline KOH and potassium-based buffer solutions (phosphate, borate, and carbonate) at non-extreme pH levels and room temperature. Bubble detachment diameters measured by a high-speed camera were found to continuously decrease in these electrolyte solutions by increasing the molality. After quantifying the electrolyte properties (density, surface tension and viscosity), the measured bubble diameters were compared with an analytical equation, which considers the force balance between the buoyancy force and the surface tension force on a textured surface,[3] to clarify the contributions from the electrolytes and the electrode surface to determine the diameters. The density of buffer solutions significantly increases in the concentrated solutions, which helps to reduce the diameter due to the buoyancy force. The deconvoluted surface contribution was also found to help reducing the diameter in the dense solutions. The electrode surface may be reconstructed during the oxygen evolution in the concentrated conditions, which was also inferred from contact angle measurements after the reactions. The analytical bubble detachment diameter is just derived from the force balance on a bubble at static condition. To cover the viscous force, which appears during the deformation of fluids, numerical simulations using volume of fluid method were performed. In viscous solutions and at high gas inlet assuming high current density, the diameters were found to increase indicating that the viscous force is working on the bubbles towards the electrode surface.Having clarified the major roles from electrolytes in the bubble detachment diameter, ohmic resistance in a homemade zero-gap water electrolyzer was investigated (schematics in Figure 1). NiMoOx on Ni felt (0.2 mm thick), NiFeOx on Ni felt and hydrophilic polyethersulfone (PES, 0.14 mm thick, porosity of 80%) were used as a cathode, an anode, and a porous separator, respectively, in various molality of potassium carbonate solutions at pH 10.5 and 80 °C. Uncompensated resistances were obtained from high-frequency intersects in Nyquist plots obtained from electrochemical impedance spectroscopies at an open circuit voltage (OCV) and a potential at 800 mA cm− 2. A volcano trend of the resistance at OCV (blue bars in Figure 1) reflects the bulk electrolyte conductivity.[4] Notably, additional resistance (red bars in Figure 1) was observed at 800 mA cm− 2 indicating the inhibition of ion conduction due to the presence of massive gas bubbles in the stacked structure. The increment of resistance at 800 mA cm− 2 also exhibited a volcano trend. The ratio of resistances between OCV and 800 mA cm− 2 reflects how the effective ion conduction path was inhibited by the void fraction due to the gas bubbles.[5] The estimated void fractions also demonstrated the volcano trend as a function of the solute molality. Its decreasing trend agreed with the detachment diameters from the Ni plate model electrode discussed above indicating that the quantitative understanding of the bubble dynamics is also effective to reduce the ohmic losses.[2] Accumulation of gas bubbles were indeed observed by the high-speed camera using a transparent dialyzer separator which was closely stacked with NiMoOx/Ni cathode. Overall, our study demonstrates the significance of the electrolyte engineering to control the gas bubbles and the resultant losses.Reference[1] R. Iwata, et al., Joule, 2021, 5, 887.[2] H. Qiu, et al., Langmuir, 2023, 39, 4993.[3] J. Li, et al., Langmuir, 2022, 38, 3180.[4] T. Nishimoto, et al., ACS Catal., 2023, 13, 14725.[5] D. A. G. Bruggeman, Ann. Phys. 1935, 39, 4993. Figure 1