Porous Transport Layers Research Articles

Numerical Investigation on Two-Phase Flow-Electrochemical Reaction Coupling in PEM Water Electrolyzers Porous Transport Layer Under High Operating Pressures

Proton exchange membrane water electrolyzers (PEMWEs) operated at high pressures have advantages such as lower energy consumption, reduced noise, and absence of component slippage. They have a more robust potential for commercial applications than other electrolyzers. The transport characteristics of two-phase flow within the porous transport layer (PTL) and channels are not well understood, especially under high-pressure conditions. The gas bubbles generated in the catalyst layer (CL) pass through the PTL and reach the flow channel. However, oxygen evolution reaction in the anode CL may be limited if these bubbles block the pathway of liquid water, causing reactant starvation. The interaction between electrochemical reactions and gas bubbles thus directly impacts a PEMWE's efficiency. This study aims to investigate the coupling between the two-phase flow and electrochemical reactions under high-pressure conditions.A two-dimensional (2D) CFD model was developed to simulate the bubbly flow through a reconstructed PTL. The volume of fluid (VOF) method was employed to capture bubble growth, movement, and detachment, as illustrated in Figure 1. The coupling of bubble transport and electrochemical reactions was achieved through the application of a weighted averaging method. A cross-section of a stochastically reconstructed titanium felt model was generated as the computational domain; see Fig. 1(a). The computation domain consists of a liquid water channel, a PTL, and a simplified CL interface at the bottom. The CL's reaction rate is modeled as a function of local bubble coverage on the CL interface. Transient simulations of the two-phase flow of the PEMWE anode side were carried out for pressures ranging from 1 bar to 200 bar. The simulation results elucidate the flow characteristics of liquid water in the PTL under high-pressure conditions and quantitatively reveal the coupling between two-phase flow and electrochemical reactions. The behavior of the two-phase flow in PEMWE is found to be sensitive to operating pressure. As the operating pressure increases, the size of bubbles generated on the CL interface decreases, resulting in a higher permeability of liquid water than that under low pressures. When the flow rate of liquid water in the channel is sufficiently high, bubbles do not fully develop and are rapidly removed from the domain. This phenomenon significantly shifts the two-phase flow regime within the channels,. Furthermore, the activation overpotential is substantially reduced under high-pressure conditions. The present study demonstrates a numerical simulation tool for high-pressure PEMWE. A future study of a full 3D PTL model is currently underway. Figure 1

Understanding How Porous Transport Layer Features Affect Proton Exchange Membrane Electrolyzer Performance

PEM Water electrolysis has emerged as a promising technology for the production of green hydrogen. A key component of PEM electrolyzer is the porous transport layer (PTL). The porous transport layer has numerous important roles in water electrolysis. In addition to enabling electrical conduction and water and gas transport, the PTL is necessary for maintaining excellent contact with the cell components. Cell performance is anticipated to be affected by PTL properties such as structure, composition, and thickness.This study delves into the critical contribution of porous transport layer properties to mass transport resistance, activation losses, catalyst layer resistance, and ohmic losses. I will present our experimental framework to deconvolute the impact of PTL properties such as thickness, porosity, and microporous layer on various overpotentials. Voltage loss breakdown analysis was conducted to determine kinetic, ohmic, mass transport, and catalyst layer losses. Theoretical calculations of thermodynamic voltage typically rely on standard reaction potentials adjusted for reactant and product concentrations, operating temperature, and pressure via Nernst corrections. Ohmic voltage, which accounts for contact resistances, bulk resistances, and ohmic losses from the membrane, was determined using high-frequency impedance spectroscopy measurements. To calculate losses from OER kinetics, a Tafel fit is subsequently implemented on the HFR-free voltage. Typically, this process tends to result in residual voltage at elevated current densities, including losses arising from mass transport of water and oxygen, resistance within the catalyst layer, and impacts stemming from ionic impurities. Ionic contamination is omitted from our analysis as a result of the short duration of the experiment.Our detailed analysis reveals that the properties of PTL can have a substantial impact on the performance of the water electrolyzer, which is crucial for enhancing the performance and durability of PEMWE. This study presents a novel framework for forecasting the performance of diverse PTL materials, which may be effectively used to drive the development and optimization of PTL for water electrolyzers. Figure 1

Microstructural Characterization of Porous Transport Layer Coatings

The porous transport layer (PTL) is a key component in polymer electrolyte membrane (PEM) water electrolyzers and serves the role of controlling the transport of reactants and products to and from the anode. Maintaining a low interfacial contact resistance between the titanium (Ti) fiber or sinter PTL and the iridium (Ir) anode catalyst is critical to maintaining the performance of the electrolyzer cell. This requires costly precious metal coatings, typically Pt, to inhibit the formation of native oxide layers during long-term cell operation. Strategies to reducing the thickness of these costly coatings including surface treatments, such as etching and abrasion, alternative low-cost coating materials, and implementation of microporous layers. It is crucial to understanding the morphologies and degradation mechanisms of these new strategies in order to accelerate the development of cheaper, more robust PTL coatings.In this work, correlative ex situ characterization methods, including laser confocal microscopy (LCM), focused ion beam (FIB), scanning electron microscopy (SEM), time-of-flight secondary ion mass spectrometry (ToF-SIMS), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and scanning transmission electron microscopy (STEM), are utilized to explore the impact surface treatments, such as etching and abrasion, on the Pt coatings uniformity and surface roughness. The thickness, local topography, and continuity of the Pt coating were quantified and correlated with interfacial contact resistance and electrochemical cell performance measurements. Changes to the thickness and composition of the coatings and oxide layers after single-cell tests will also be reported. Overall, these findings aim to guide the development of PTL treatments and coatings, ensuring the creation of highly durable and efficient PEM water electrolyzers that align with the U.S. Department of Energy's Hydrogen Shot goal of reducing the cost of clean hydrogen production to $1 per kilogram by 2031.Funding for this work was provided by U.S. Department of Energy Office of Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office through the H2NEW Consortium. Electron microscopy research conducted as part of a user project at the Center for Nanophase Materials Sciences (CNMS), which is a US Department of Energy, Office of Science User Facility at Oak Ridge National Laboratory.

Investigating Corrosion Degradation of Porous Transport Layers for PEM Water Electrolyzers Using on-Line ICP-MS

The degradation of polymer electrolyte membrane water electrolyzer (PEMWE) is the net result of degradation phenomena across each component, which includes structural changes to the catalyst layer, deactivation of the electrolyte, loss of performance due to impurities in the feed water, and the corrosion and passivation of titanium-based bipolar plates (BPPs) and porous transport layers (PTLs).1, 2 This presentation will focus on PTL degradation, which has received relatively little attention compared to loss of performance related to catalyst and electrode degradation mechanisms. Generally, PTL degradation can be categorized into chemical/corrosion degradation and mechanical degradation. 3 In this work, we focus on corrosion degradation of the anode PTL. Titanium (Ti) is the state-of-the-art material for PTLs due to its stability and intrinsic electrical conductivity. Ti can however, develop an oxide layer in the harsh oxidizing conditions of the anode, thus reducing the effective electrical conductivity and cell performance. Coating the Ti parts with platinum group metals (Pt or Ir) is often used to protect against oxidation.4 An electrochemical flow cell system connected to an inductively-coupled plasma-mass spectrometer (ICP-MS) capable of detecting trace concentrations (<ppb) of dissolved elements in solution has been used to investigate the corrosion rates of the Ti-PTLs with and without coatings toward understanding factors influencing degradation mechanisms. The influence of various parameters such as potential, potentiodynamic profile parameters (e.g., scan rate, upper and lower potential limits), electrolyte type, and pH on the corrosion processes has been investigated. References Feng, X. Yuan, G. Liu, B. Wei, Z. Zhang, H. Li, H. Wang, J. Power Sources 2017, 366, 33.Babic, M. Suermann, F. N. Büchi, L. Gubler, T. J. Schmidt, J. Electrochem. Soc. 2017, 164, F387.Park, H. Oh, T. Ha, Y.I. Lee, K. Min, Appl. Energy 155 (2015) 866-880.Rakousky, U. Reimer, K. Wippermann, M. Carmo, W. Lueke, D. Stolten, J. Power Sources 326 (2016) 120–128. Acknowledgements This research is supported by the U.S. Department of Energy, Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office under the auspices of the H2NEW consortium. Argonne National Laboratory is managed for the U.S Department of Energy by the University of Chicago Argonne, LLC under contract DE-AC-02-06CH11357.

Scanning Electron Microscopy Energy-Dispersive X-Ray Spectroscopy Analysis of Catalyst-Coated Porous Transport Layers

The transition of the United States energy infrastructure towards hydrogen energy involves the optimization of proton exchange membrane water electrolyzers (PEMWEs) for reliable hydrogen generation. PEMWE device typically consists of titanium porous transport layer (PTL) and an iridium-based anode catalyst layer. Several directions are being pursued to improve performance and to reduce the costs. To mitigate naturally occurring titanium passivation, protective coatings, typically platinum or other platinum-group metals (PGMs) are commonly applied to the PTLs. The drive to decrease the cost and the use of scarce materials has led to the development of catalyst layers with lower catalyst loadings. Reducing fabrication time and cost as well as increasing efficiency of the anode has led to the investigation of direct application of the iridium-based catalyst onto the Pt-coated Ti PTL. This motivated our investigation of porous-transport electrodes (PTEs) prepared with a variety of deposition methods, using different ink formulations, and targeting different catalyst loadings. There is need to develop a characterization approach that can be used to assess and compare quality of coatings across large parameter space or to monitor quality during production.Several sets of PTEs samples consisting of a titanium porous layer, platinum protective coating and iridium-based catalyst layer were prepared using a wide range of catalyst layer coating methods and parameters, including spray coating, gravure, airbrush and electrodeposition. All samples were characterized using scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM-EDS) to analyze the elemental composition and assess the distribution of elements of interest (Ti, Pt, and Ir). First, the atomic% of each element was used to calculate elemental ratios for quantitative assessment of the amount of deposited catalyst. However, it is crucial to recognize the limitations of relying solely on this information for assessing catalyst coating since it doesn’t account for heterogeneity of the coatings. Although elemental maps provide valuable insights into spatial distribution, they alone do not offer a complete picture. To better understand the catalyst coating distribution, each individual EDS elemental map was used to calculate the surface area, which served as a measure of PTL area covered by the catalyst. Further, surface area ratios (Pt/Ir) were calculated to determine variations in surface coverage. This poster will highlight differences between fabricated samples and will demonstrate that together, at% and surface area ratio values allow for a comprehensive quantitative assessment of the composition and distribution of catalyst surfaces that were produced through a wide range of fabrication variables. The implementation of proper characterization of these PTEs can help aid future decisions on which parameters should be used moving forward. This approach can also be used for quality control, enabling quick screening of commercially produced samples.

(Invited) Comparison of Catalyst-Coated Electrodes vs. Catalyst-Coated Membranes in PEM Water Electrolysis Cells

The PEM (Proton-Exchange Membrane or Polymer Electrolyte Membrane) cell concept [1] is used in fuel cells and water electrolysis cells. PEM water electrolysis is a compact, efficient and flexible technology for the production of “green-hydrogen” from renewable (intermittent) energy sources. The unit cell (Figure 1) is made of several adjacent functional layers: (i) the proton-conducting polymer membrane (1); (ii) two catalysts layers (CLs) for hydrogen/oxygen evolution containing platinum group metals or PGMs (2;2’); (iii) a porous transport layer used for mass transport of reactants/products and electricity distribution (3;3’); (iv) water flow fields for water circulation (4;4’); bipolar plates (5;5’). From a practical viewpoint, the CLs can be deposited on either side of the interfaces formed between the polymer membrane and the porous transport layer: (i) on the supporting porous transport layers to form catalyst-coated electrodes which are then hot pressed against the bare membrane to form membrane – electrode assemblies; (ii) directly onto the membrane to form catalyst-coated membranes (CCM).The aim of this paper is to evaluate the efficiency and durability of both concepts, drawing on recent advances in manufacturing techniques to reduce the loading of PGM catalysts.[1] W.T. Grubb Jr., Batteries with Solid Ion-Exchange Electrolytes. Journal of the Electrochemical Society. 106 (1959) 275-279.[2] C. Rozain, P. Millet, Electrochemical characterization of Polymer Electrolyte Membrane Water Electrolysis Cells, Electrochimica Acta, 131 (2014) 160-167. Figure 1. Schematic diagram showing the cross-section of a PEM water electrolysis cell [2]. Figure 1

Membrane Electrode Assembly Development for Industrial CO2 Electrolysis

Membrane electrode assemblies (MEAs) are the key components of zero-gap CO2 electrolyzers, having various functions in the electrolysis process. The catalyst layers serve as the reaction interface, the porous transport layers ensure reactant and product transport to and from the catalysts, while the membrane is responsible for the ion transport between the two half cells. Thus, the structure of the MEA has a determining role on the electrolyzer operation. Industrial application requires large surface area electrolyzers and / or assembly of stacks. The scale-up, however, imposes different challenges on MEA preparation and on electrolyzer operation, therefore the optimal operating conditions can not necessarily be translated from small-scale (< 10 cm2) experiments.With the aim to scale-up the CO2 electrolysis to an industrially relevant level (>2500 cm2 active area), we thoroughly study the MEA components in a test electrolyzer cell having 100 cm2 active area. Currently we focus mainly on the optimization of the catalyst layers and the porous transport layers. The used electrolyzer size is already large enough that we can draw relevant conclusions for the operation of the large-scale electrolyzer, however, still offers possibility to screen a wide range of parameters.I will present selected results on how the improvement of the quality of the catalyst layers - through the optimization of the catalyst ink and the coating process- results in enhanced CO2 reduction performance. We also focus on the screening of different porous transport layers on the anode side with the long-term goal to reduce the precious metal consumption of the CO2 electrolysis. In the second part of my presentation, I will show how the operational parameters (e.g. cell temperature, humidity) affect the stability of the electrolysis. We have thoroughly investigated one of the biggest issues of the anion exchange membrane CO2 electrolysis: the carbonate precipitate formation and the related flooding phenomenon. We identified the pressure drop at the cathode side as a key test output parameter, which can serve as an early warning for the start of the undesired precipitation. When it is recognized in time, and the adequate preventive measures are applied, the lifetime of the electrolyzers can be greatly improved.

Diffusion-Bonded Unitized Anode Assemblies for PEM Electrolysis Stacks

Hydrogen generation via proton exchange membrane (PEM) water electrolysis is in the midst of a technological renaissance aimed at achieving cost parity with steam reforming within the decade. Reaching that plateau will require substantial system cost reductions accompanied by increased performance of the cell stack module. Many proposed improvements to the cell stack draw from the established principles of electrochemical and manufacturing engineering, targeting lower component resistances while lowering part count and enhancing ease of assembly, respectively.The PEM electrolyzer anode assembly (comprising the porous transport layer (PTL) and the anode flow field) is suited for just such a revision. In practice, the anode PTL is positioned on a metallic bipolar plate that is machined or embossed with a flow field, or the PTL is joined to an assemblage of thin macroporous media that serves as a flow field1. In both cases, the interfacial resistances between the subcomponents of the assembly are non-negligible and are conventionally mitigated by applying noble metal coatings, typically Pt.In this contribution, we investigate alternatives to conventional assemblies via diffusion bonding. Using thermal treatments, the porous transport layer is joined to the anode flow field, yielding a robust, atomic-scale metallurgical bond. We demonstrate the manufacturing of large-format assemblies in multiple configurations, using either stacked microporous media or grooved plates, meeting tight tolerances for thickness across their span. We also show that the bulk-like character imparted to the assembly obviates the need for antipassivation coatings at the critical interfaces between components, evinced by low series resistance in both ex situ and in situ electrolysis cell tests.References(1) Dristy, M. E. Proton Exchange Membrane Electrochemical Cell System. US7270908B2, September 18, 2007.

Application of Hipims Coatings to Enhance the Durability and Performance of Structural Components of PEM Electrolyzer Anodes

Porous transport layers (PTL) and bipolar plates (BPP) are structural elements that transport water, gases, electrons as well as heat within the electrolysis cells. On the anode side, the state-of-the-art material for BPP and PTL is titanium due its higher corrosion resistance under the operating conditions of electrolysis (high electrochemical potential, acidic and oxidizing environment) compared to other materials [1]. However, titanium components tend to form an oxide layer that rises the interfacial contact resistance (ICR) and leads to a poor performance since metal oxides are semiconductors or insulators. This issue is usually addressed by applying precious metal coatings, such as platinum or gold. In this work, we present the potential of high power sputtering techniques (HiPIMS) to manufacture ultra-low loading Pt coatings (<20 nm) with good homogeneity and high performance at an industrial scale. The influence of HiPIMS parameters on coating-substrate interface, as well as the coating properties have been evaluated. SEM, AFM and HRTEM microscopy have been used to evaluate the coating thickness and morphology. XRD analysis has been carried out to evaluate the influence of the HiPIMS parameters on the growth mode of Pt coatings, tailoring the coating texture from [200] to [111]. HiPIMS technique also provide enhanced density compared with other coating techniques due to the higher ionization achieved in the plasma [2]. The deposition of denser and defect-free coatings by HiPIMS enables the application of stainless steel components. HiPIMS prevents the formation of columnar structures and reduces oxygen diffusion. In this work, Nb- and Ti-based protective coatings have been deposited by HiPIMS on stainless steel 316L. A comparative between coated Ti Gr2 and SS316L has been set, showing the potential to reduce costs in the next generation of PEMWE structural components.

Comprehensive Characterization of Porous Transport Electrodes with SEM, XPS, and Tof-SIMS

Advancements towards more efficient design and manufacturability of polymer electrolyte membrane electrolyzers include recent efforts to better integrate the porous transport layer (PTL) and adjacent anodic catalyst layer (CL), forming the porous transport electrode (PTE). It is well known that characterizations of the PTL alone, especially when combined with a protective coating, present numerous challenges. The PTE structure is even more complex, and it is difficult to comprehensively characterize all components and interfaces. In prior work, we established Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS) as a characterization technique for the PTLs coated with protective coatings. This work demonstrates ToF-SIMS as a powerful technique for the characterization of PTLs integrated with CLs. A series of sintered Ti PTLs were first coated with protective Pt coatings and then coated with various IrOx and IrRuOx catalysts via electrodeposition. Prepared PTE samples ranged in catalyst loadings and catalyst composition. Selected samples were also subjected to annealing post-treatments to further modify catalyst composition. X-ray photoelectron spectroscopy (XPS) was conducted to identify surface chemical speciation and states of the catalysts. ToF-SIMS analysis was conducted using Cs in both positive and negative modes to identify chemical species, including Ir, Ru, and their oxides as a function of catalyst layer depth. These two methods were complemented by Scanning Electron Microscopy and Energy Dispersive X-ray Spectroscopy (SEM- EDS) analysis to evaluate morphology and distribution of catalysts. This comprehensive physical and chemical characterization of the PTEs provides insights that are useful for the characterization of as produced, modified, and degraded PTE samples.

New Electrocatalyst with the Content of Ir

Introduction Proton exchange membrane (PEM) water electrolysis (WE) is a promising technology for hydrogen production. However, the local environment of the anode in PEMWE is highly acidic and high potential due to the oxygen evolution reaction (OER; 2H2O → O2 + 4H+ + 4e-), and the available anode materials are limited to precious metal elements such as Pt, Ru and Ir. In the current technology, Ir-based materials are the mainstream anode catalysts, and Ir loading of 1-2 mg/cm2 is necessary to achieve both activity and durability. On the other hand, the annual production of Ir is said to be only 5-7 tons. The demand for hydrogen is expected to increase to the GW scale in the future. For example, if water electrolysis is operated at 2 A/cm2 and 2 V with Ir loading of 2 mg/cm2 (0.5 mgIr/W), it is difficult to meet the demand with the existing Ir resources. Tosoh have been engaged in the electrolytic manganese dioxide (EMD) business. EMD is synthesized thorough anodic oxidation of Mn2+ ion (Mn2+ + 2H2O → MnO2 + 4H+ + 2e-) under acidic conditions, therefore it is potentially suitable for the anode environment of PEMWE. In this study, a very small amount of Ir (<0.1 mg/cm2) was successfully composited with EMD. The resulting composite consisting of Ir and EMD showed more active than commercial Ir oxide with the same Ir content, and operated stably for at least 1000 h. Experiment EMD thin film was electrodeposited anodically on Pt-coated Ti fibers (Pt/Ti) thorough electrolyzing a mixed solution of MnSO4 and H2SO4. The obtained EMD-coated Pt/Ti were immersed in an Ir-containing solution, and then annealed to obtain Ir-containing EMD (hereinafter referred to as "Ir-EMD"). The Pt-coated Ti fibers are used as a porous transport layer (PTL) in PEMWE cells, and our developed material serves as both an OER catalyst and a PTL. OER property was evaluated using a PEMWE cell at 80°C in a two-electrode system. Pt-supported carbon was used as the cathode catalyst and Nafion®-115 as the PEM. See Fig. 1 for PEMWE cell construction flow. Result and discussion The Ir content in Ir-EMD was determined to be 0.1 mg/cm2 by ICP-AES. This Ir content is 10-20 times lower than that used in current PEMWE electrolyzers. The OER activity of Ir-EMD was evaluated by linear sweep voltammetry (LSV) (Fig. 2). For comparison, EMD (with no Ir) and Ir oxide (commercial product) with an Ir content of 0.1 mg/cm2 were evaluated in the same way. The slow OER property of EMD was improved dramatically by introducing Ir into EMD. Also, at the same amount of Ir, Ir-EMD exhibited higher OER activity than that of Ir oxide. The inset in Fig. 2 shows Tafel plots corresponding to LSVs, and the slope of the linear region (Tafel slope) can be used to predict which elementary reaction is the rate-limiting step in the OER. The Tafel slope of Ir-EMD is 58 mV/dec., which is close to the Tafel slope of Ir oxide, 50 mV/dec. This suggests that OER on Ir-EMD proceeds by the same mechanism as Ir oxide. The durability of Ir-EMD and Ir oxide was tested at a current density of 2 A/cm2 (Fig. 3). Ir oxide was deactivated within only 8 days, and it is confirmed that Ir have disappeared after electrolysis. On the other hand, Ir-EMD has operated stably for more than 42 days (≈1000 h), with a degradation rate of only 13 μV/h. In other words, Ir-EMD has practical durability while reducing the amount of Ir by more than 10 times compared to current technology.Acknowledgements : This presentation is based on results obtained from a project, JPNP20003, subsidized by the New Energy and Industrial Technology Development Organization (NEDO). Figure 1

Advanced Porous Transport Electrode for High-Performance Proton Exchange Membrane Water Electrolyzers

Proton exchange membrane water electrolyzers (PEMWEs) hold great potential for supplying green hydrogen to support extensive energy storage and mobility in a future energy landscape centered on renewable energy sources.1,2 However, their contribution to global hydrogen production is currently limited, mainly due to their comparatively high cost.3 To improve the economic competitiveness of green hydrogen and foster broader market penetration, the Department of Energy (DOE) has launched the Hydrogen Earthshots, aiming for a substantial cost reduction to $2 per kilogram by 2025 and $1 per kilogram for green hydrogen by 2030.4 The high efficiency and prolonged durability are pivotal for the overall performance and cost reduction of the PEMWEs. This requires careful consideration of the interfacial contact between the porous transport layer (PTL) and the anode catalyst layer.5 One option for PEMWEs is to directly coat the anode catalyst layer onto the PTL, forming porous transport electrodes (PTEs). Recent literature suggests that it is feasible to manufacture PTEs with low precious metal loading, demonstrating both high performance and durability.6,7 In this study, an innovative reactive spray deposition technology (RSDT) is used to fabricate PTEs with low PGM loading (0.2 - 0.3 mgPGM cm-2) in both catalyst layers. The RSDT is a flame-based method that combines the synthesis and deposition of the catalyst in a single step, significantly reducing the fabrication time and cost of the membrane electrode assembly (MEA).8–11 The RSDT-fabricated PTEs are coupled with various membranes, and their performance has been assessed and compared to the state-of-the-art MEAs for PEMWEs. In addition, the performance loss in each cell has been studied and discussed in detail. Furthermore, a standard accelerated stress test (AST) protocol has been applied to assess the durability of the RSDT-fabricated MEAs, with one order of magnitude lower PGM loading in their catalyst layers in comparison to the commercial MEAs for PEMWEs.Reference1. Pham, C. Van, Escalera-López, D., Mayrhofer, K., Cherevko, S. & Thiele, S. Essentials of High Performance Water Electrolyzers – From Catalyst Layer Materials to Electrode Engineering. Adv. Energy Mater. 11, (2021).2. Carmo, M., Fritz, D. L., Mergel, J. & Stolten, D. A comprehensive review on PEM water electrolysis. Int. J. Hydrogen Energy 38, 4901–4934 (2013).3. Buttler, A. & Spliethoff, H. Current status of water electrolysis for energy storage, grid balancing, and sector coupling via power-to-gas and power-to-liquids: A review. Renew. Sustain. Energy Rev. 82, 2440–2454 (2018).4. Styapal, S. et al. DOE Update on Hydrogen Shot, RFI Results, and Summary of Hydrogen Provisions. (2021).5. Peng, X. et al. Insights into Interfacial and Bulk Transport Phenomena Affecting Proton Exchange Membrane Water Electrolyzer Performance at Ultra-Low Iridium Loadings. Adv. Sci. 8, (2021).6. Xie, Z. et al. Ionomer-free nanoporous iridium nanosheet electrodes with boosted performance and catalyst utilization for high-efficiency water electrolyzers. Appl. Catal. B Environ. 341, 123298 (2024).7. Lee, J. K. et al. Ionomer-free and recyclable porous-transport electrode for high-performing proton-exchange-membrane water electrolysis. Nat. Commun. 14, 1–11 (2023).8. Zeng, Z. et al. Advanced nickel-based catalysts for the hydrogen oxidation reaction in alkaline media synthesized by reactive spray deposition technology: Study of the effect of particle size. Int. J. Hydrogen Energy 1–12 (2023) doi:10.1016/j.ijhydene.2023.03.249.9. Zeng, Z. et al. Degradation Mechanisms in Advanced MEAs for PEM Water Electrolyzers Fabricated by Reactive Spray Deposition Technology. J. Electrochem. Soc. 169, 054536 (2022).10. Xing, J. et al. Long-term durability test of highly efficient membrane electrode assemblies for anion exchange membrane seawater electrolyzers. J. Power Sources 558, 232564 (2023).11. Mirshekari, G. et al. High-performance and cost-effective membrane electrode assemblies for advanced proton exchange membrane water electrolyzers: Long-term durability assessment. Int. J. Hydrogen Energy 46, 1526–1539 (2021).

Advancing Green Hydrogen Production with Innovative Materials and Coatings

Green hydrogen is a vital solution for reducing greenhouse gas emissions in various sectors of the global economy. As proton exchange membrane (PEM) electrolyzer deployments scale to meet hydrogen production demands, it is critical that raw materials and coatings used in stack designs are carefully engineered for a balance of cost, efficiency, and durability. In this presentation, we will describe technical approaches to assist in the customization of porous transport layer (PTL) components for optimal properties such as mass transport, conductivity, and corrosion resistance. In particular, the design of expanded metal sheets as PTL flow-fields will be described. Computational fluid dynamics (CFD) modeling is used to predict key hydrodynamic properties and inform the engineering of these components. With a specific focus on scalable technologies that can achieve GW/y production volumes, we will demonstrate how PTL assemblies can be economically optimized for performance, under specific PEM electrolyzer form factors and operational conditions.

Investigating the Effect of KOH Feed Concentration on a Stainless-Steel Anode Using a 3-Electrode Anion Exchange Water Electrolyser Cell Set-up

Anion-exchange membrane water electrolysers (AEM-WE) have the potential to lower the capital cost of hydrogen production compared to proton-exchange membranes water electrolysers (PEM-WE). The alkaline conditions of AEM-WE result in: (i) a kinetically more favourable oxygen evolution reaction (OER), (ii) the ability to use platinum group metal (PGM)-free metal catalysts, and (iii) reduced cell component costs because of the less corrosive operating conditions. However, AEM-WE is held back from commercialisation by limited current density and durability.1 Compared to conventional alkaline water electrolysis, AEM-WEs have the advantage of lower ohmic resistance thanks to a solid polymer electrolyte which provides intrinsic ionic conductivity instead of relying on 6 M KOH and a porous separator. Most studies show, however, that a dilute KOH feed (< 1 M), rather than pure water, is critical to reaching a high current density in AEM-WE. Varying the concentration of KOH in the feed solution impacts catalyst activity and membrane resistance thereby making performance measurements in AEM-WE more complex. Moreover, the overpotential for the alkaline hydrogen evolution reaction (HER) is significant due to its sluggish kinetics. This means full-cell measurements are insufficient for understanding the behaviour of OER and HER catalysts in AEM-WE. It is therefore crucial to make half-cell measurements in AEM-WEs to decouple the effect of the anode and cathode electrocatalysts from the overall cell performance.In this context, our group recently demonstrated a 3-electrode, 5 cm2 membrane electrode assembly electrolyser cell2 that uses a reference electrode to decouple the anode and cathode contributions to the overall water splitting reaction. The technique was demonstrated for AEM-WEs and is soon to be published by Malone et al. (Figure 1A)3. In this study, we aimed to investigate the effect of KOH concentration in the feed solution on OER by using this 3-electrode cell setup. We used a stainless steel (SS)-felt at the anode as both catalyst and porous transport layer (PTL) in different concentrations of KOH feed while maintaining Pt/C as the cathode catalyst. We also repeated the tests with various anode catalyst layers (IrOx, NiFeOx) in combination with the SS PTL. For each test, full and half-cell polarisation curves and impedance spectroscopy were recorded before and after a sixteen-hour durability test at 1 A cm-2 at 40 °C. The results revealed the catalytic significance of the SS-felt in 1 M KOH as recently shown by Chen et al.4 Additionally, we demonstrated how the SS masks the performance of an IrOx catalyst layer in 1 M KOH. Reducing the KOH concentration to 0.18 M resulted in ≈ 7 % increase in the overall cell voltage at 1 A cm-2 (from 1.88 V to 2.00 V at 40 °C in Figure 1B). Aside from the ≈ 35 % increase in membrane resistance (e.g. 180 to 240 mΩ cm2 at 0.9 A cm-2) in 0.18 M KOH, the higher overpotential arose from the anode (Figure 1C), which could be attributed to the higher OER overpotential of SS-felt in a lower pH solution. Meanwhile, there was no appreciable change in cathode overpotential when the KOH concentration was reduced. These findings demonstrate the importance of 3-electrode cell measurements for understanding the impact of each component in an AEM-WE cell. Furthermore, the results reveal that both the choice of PTL material as well as the concentration of feed solution should be considered when testing different catalysts in AEM-WE.

Development of a 4-in-1 Device to Measure Component Properties of Gas Diffusion Layer/Porous Transport Layer for Both Proton Exchange Membrane Fuel Cell and Water Electrolyzer

This work aims at developing a 4-in-1 device to simultaneously measure parameters under compression (UC), including thickness (TUC), resistivity (RUC, through-plane), and in-plane permeability (IPP-UC), and to separately measure through-plane permeability (TPP). The simultaneous measurement of TUC/RUC/IPP-UC is accomplished through an annulus sample by measuring the flow rate–pressure relationship in a radial flow [1]. By adding a two-piece adaptor, TPP can also be measured using a disk sample. Typical TUC/RUC/IPP-UC/TPP curves of a gas diffusion layer (GDL) are shown in Figure 1. Upon the design requirements, the TUC/RUC/IPP-UC/TPP tester is calibrated and validated.The 4-in-1 device was originally designed for the characterization of the GDL, a key component of the membrane electrode assembly (MEA) for a proton exchange membrane (PEM) fuel cell. The GDL properties are greatly associated with the cell performance, especially under compression. To ensure the quality of GDLs, it is of great importance to characterize GDL properties with respect to the component standardization and specification. The application of the 4-in-1 device was later extended for the characterization of the porous transport layer (PTL), a crucial component for the anode of PEM water electrolyzer in responding to the increased research and efforts made in the development of, mostly, Ti-based PTLs for PEM water electrolysis [2]-[4] , as commercially available PTLs are basically materials borrowed from other applications, e.g., filtration.The developed TUC/RUC/IPP-UC/TPP device has been proven to be a useful tool for the quality control (QC) of GDLs and PTLs, facilitating the component development, complementing the component supply chain, contributing to cell reliability, and thus reduction of cost, for both the PEM fuel cell and PEM water electrolysis technologies. Acknowledgements This work is financially supported by the Office of Energy Research and Development (OERD) and the Advanced Clean Energy (ACE) Program of the National Research Council Canada (project #NRC-23-140).

Statistical Investigation of the Reproducibility of Stack Components in a PEM Water Electrolyzer

To achieve homogeneous conditions throughout a single cell of a stack and within a stack and furthermore within several stacks, the components assembled in a stack should be reproducible to achieve a homogeneous performance. To evaluate how reproducible the stack components are, the porous transport layers, the expanded metals sheets, the perforated metal sheets, and the flow field plates from a 5-cell stack were examined.Eight samples were cut out of each of the 450 cm² layers, and a height profile (area: 10 x 10 mm) was recorded using a laser scan. From the height profile, the roughness values Ra (arithmetic average) and Rz (maximum peak to valley height) could be derived as well as the curvature of the samples (see Figure 1). Figure 1a) shows a contour plot of a perforated metal sheet. There is a height difference of up to 20 µm from the edge to the center of the sample. For the stack assembly this means an initial force is required to flatten the sheet before the actual compression of the stack takes place. Figure 1b) shows the roughness values Ra and Rz. The values vary within the sample (Ra: ± 3.8 µm, Rz: ± 5.7 µm) and throughout the samples (Ra: ± 1.4 µm, Rz: ± 2.5 µm). Further, the thickness of the components was measured, and the variation was calculated. In terms of stacking the components, as done in the stack, the best and worst case were calculated. The aim of this study was to determine input parameters for follow-up tests for the mechanical and electrical characterization of the components as especially the roughness of the components may have a significant impact on the contact resistance. Figure 1

Enhanced Local Ion Transport in High-Performance Anion Exchange Membrane Water Electrolyzers Via Quaternary Ammonium-Functionalized Metal-Organic Framework Electrocatalysts

Anion-exchange membrane water electrolyzers (AEMWEs) present a multitude of advantages, particularly in the utilization of non-platinum group (non-PGM) catalysts. While diverse catalysts have demonstrated notable electrocatalytic activity in alkaline conditions, the integration of the ionomer with the electrocatalyst at the active catalyst layer is crucial for enhancing overall AEMWE performance in practical device demonstrations by improving mass transport. However, the intrinsic instability of polymeric ionomers, leading to physical detachments or alkaline (chemical) degradation, poses a challenge that can potentially diminish performance. In this study, drawing inspiration from the aforementioned ionomer chemistry, we introduce non-PGM NiFe-based metal-organic frameworks (MOFs) incorporating quaternary ammonium (QA) cation moieties covalently functionalized to benzenedicarboxylic acid (BDC) ligands as promising electrocatalysts for the oxygen evolution reaction (OER) in alkaline environments. Three specifically designed functional groups with varying ion exchange capacities (IEC) and alkaline stability—trimethylamine (TMA), N-methylpyrrolidine (C4N), and N-methylpiperidine (C5N)—were directly tethered to BDC ligands. The alkaline OER activity of Ni2Fe1-QA-MOF was meticulously assessed using rotating disk electrodes and AEMWE devices. These directly tethered QA groups markedly facilitated improved ionic transport of OH- ions at the catalyst/electrolyte interfaces compared to BDC ligands, ultimately leading to a significant enhancement in half-cell activity and AEMWE performance. In-situ IR/RAMAN spectroscopy revealed that QA moieties highly promoted local ionic transport at the interfaces, as evidenced by characteristic peaks indicating a robust interaction between QA and OH- ions. Unexpectedly, QA moieties not only contributed to ionic transport in local environments but also induced oxidation states of Ni metal sites, while Fe sites exhibited no changes during the applied potential in the OER region, as comprehensively revealed by synchrotron X-ray absorption spectroscopy (XAS). In comparison with conventional NiFe layered double hydroxide (LDH) structures, we posit that the substantial structural transformation of Ni moieties can be attributed to an increased degree of M-O bond interaction, enhanced by the local interaction between QA and OH- ions. Density functional theory (DFT) calculations also validate the balanced adsorption of OER intermediates on QA-MOFs. Finally, QA-NiFe-MOF was directly synthesized on a porous transport layer (PTL), and its AEMWE performance was evaluated without the use of any ionomer. At 1.0 M KOH, C4N, C5N, and TMA-based NiFe MOFs exhibited current densities of 9.35 A/cm2, 9.11 A/cm2, and 7.6 A/cm2, respectively. These current densities surpassed those of bare BDC-NiFe MOF and commercial IrO2 anode catalysts, which exhibited only 7.39 A/cm2 and 6.4 A/cm2, respectively. Consequently, we emphasize that the facilitation of local ionic transport could yield remarkable AEMWE performance, even in the absence of conventional polymeric ionomers.

Engineering Controls for Scaling CO2 Electrolyzers

The electrochemical CO2 reduction (CO2R) to ethylene on copper-based catalysts has garnered significant attention for its potential in sustainable industrial practices. While substantial progress has been achieved in the development of small-scale electrolyzers with geometric areas ranging from 1 to 5 cm2, the imperative to address larger scales for industrial decarbonization necessitates a shift in focus. There are inherent critical risks when scaling a nascent technology like CO2 electrolysis. There can be emergent phenomena observed only at larger scales, that can prevent it from reaching commercialization, such as change in local CO2 concentration, defects in the porous transport layer, and temperature gradients The optimized conditions for operating a 5 cm2 electrolyzer may not work well with a 25 cm2 electrolyzer leading to failures. Therefore, there is a need to develop a systematic way to test for these phenomena to address these challenges to successfully operate a CO2R electrolyzer at an industrially relevant scale. In this work, we highlight the performance of a Cu-based catalyst designed for the selective reduction of CO2 to ethylene across varying scales, ranging from 5 to 100 cm2. This investigation shows the changes in the electrolyzer performance at different scales. Our findings highlight the critical importance of implementing engineering controls to overcome challenges associated with the scaling of electrolyzers. Specifically, we explore the impact of anolyte flow rate, gasket compression, and electrochemical pulsing techniques on maintaining the desired selectivity of the electrocatalyst across different scales. By systematically adjusting these parameters, we demonstrate the ability to mitigate non-linear effects and uphold the efficiency of CO2 reduction to ethylene, even on larger electrode surfaces.This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

(Invited) PEM Water Electrolysis Stack Development and Related Characterization Tools

For decades, proton-exchange membrane (PEM) water electrolysis (WE) has been mainly used for oxygen generation in anaerobic environments. Over the past two decades, however, it has been increasingly used for hydrogen generation in the industrial sectors at various scales. The PEMWE technology is also considered a key in the ongoing energy transition, if the process of hydrogen generation by means of WE is linked to renewable energy sources, such as wind, solar, etc.The following key elements enable the operation of a PEM WE plant for hydrogen production: a PEM-based WE stack and the balance of plant. The related system includes, but not limited to such key modules as the water and oxygen management systems, hydrogen gas management system, water input system, safety system, power electronics and electrolysis cell stack power supply, control system, and some other.A WE stack comprises several cells connected in series with electrically conductive bipolar plats and end plates. General design principles for the stack involve reducing efficiency losses, while aiming at lower costs and increase in durability. WE stacks should be designed in such a way, that an even current distribution is maintained, the water feed is optimized, suitable compression ratios are achieved, and preferably high discharge pressure of hydrogen can be enabled. For the stack level, in simple terms, efficiency is benchmarked by the total applied voltage, which includes the Nernst potential, anode and cathode overpotentials, and ohmic overpotentials due to the membrane ionic resistance and interfacial resistance at given current density. Overpotential represents inefficiencies in a cell stack, and some of stack design efforts focus on reducing these overpotential contributions. However, a compromise has to be found between (a) costs reduction challenges (that are often associated with the reduction of PGM-based catalyst loading, attempts to replace Ti as a key material for the bipolar plates, and the less expensive SPE membrane), (b) sufficient durability, (c) performance of the stack, (d) increase in power density of a stack.This talk will focus on the WE stack, and its subcomponents review, as well as characterization methods for both, a single cell and a stack and also challenges of the large-scale stack manufacturing. The key components of the WE stack include ion-conductive solid polyelectrolyte membranes (SPE), anode and cathode catalyst layers (CL), bipolar plates and current collectors/ gas diffusion layers (porous transport layers).In order to gain fundamental understanding of the relationships between electrical loses, degradation of the stack components and strategies to increase power density of the stacks, advanced characterization and modelling tools need to be used. These include, but not limited to electrochemical impedance spectroscopy (EIS), current interrupt (CI), dynamic compression measurements with piezoelectric senor plates, current mapping, gas cross-over analysis, computational fluid dynamics (CFD) simulations modelling, various visualization tools, etc. For example, CFD could assist in understanding and improving fluid flow dynamics in a WE stack as it is a complex challenge that requires a detailed examination of the geometry, channel design, and operating conditions within the stack. As for EIS, the usage of the distribution of relaxation times (DRT) approach would potentially allow to focus on the direct analysis of the data rather focusing on the equivalent circuit development.Approximately 15 years ago the South African Government approved a national program HySA: Hydrogen South Africa, which resulted in the development of the expertise and capacity to conduct research, development, and earlier commercial activities around green hydrogen production by means of water electrolysis. These activities include the development of local IP at the components, stack, and system levels.

The Effect of PTFE Dip Coating on Enhancing the Durability and Performance of Anion Exchange Membrane Electrolyzers

Anion exchange membrane electrolyzers (AEMELs) are widely regarded as one of the most promising technologies to enable low-carbon hydrogen production. AEMELs can combine the advantages the traditional alkaline electrolyzer (AEL) and proton exchange membrane electrolyzer (PEMEL) – allowing for the use of PGM free electrodes, high operating current density, environmentally friendly materials, and compact design. Although significant progress has been made in the field of AEMELs, there is still a gap in performance compared to PEMELs that can be overcome through electrode design.One of the most critical aspects affecting the overall performance of the AEMEC is the water management. It has a major influence on cell behavior, and specifically mass transfer at the anode side of the cell. To improve AEMEL anode water dynamics, our team modified its porous transport layer (PTLs) through the addition of controlled amounts of hydrophobic polytetrafluoroethylene (PTFE). This study investigated the effect of PTFE loading and coating method on the performance of AEMELs. Ni-based PTLs were used. Experimental results revealed that a low amount of PTFE not only enhanced the performance, but it also enhanced the catalyst layer attachment. On the other hand, higher PTFE content led to a reduction in water availability in the electrode, leading to increased mass transfer overpotentials. In summary, increasing the hydrophobicity of Ni-PTLs appears to improve AEMEC performance, with the best performing electrodes having a PTFE loading of ~1-2% (equivalent to 5-10 wt.% on carbon GDLs).