Electrolyzer Anodes Research Articles

Reactivity and Stability of Reduced Ir-Weight TiO2-Supported Oxygen Evolution Catalysts for Proton Exchange Membrane (PEM) Water Electrolyzer Anodes.

Reducing the iridium demand in Proton Exchange Membrane Water Electrolyzers (PEM WE) is a critical priority for the green hydrogen industry. This study reports the discovery of a TiO2-supported Ir@IrO(OH)x core-shell nanoparticle catalyst with reduced Ir content, which exhibits superior catalytic performance for the electrochemical oxygen evolution reaction (OER) compared to a commercial reference. The TiO2-supported Ir@IrO(OH)x core-shell nanoparticle configuration significantly enhances the OER Ir mass activity from 8 to approximately 150 A gIr-1 at 1.53 VRHE while reducing the iridium packing density from 1.6 to below 0.77 gIr cm-3. These advancements allow for viable anode layer thicknesses with lower Ir loading, reducing iridium utilization at 70% LHV from 0.42 to 0.075 gIr kW-1 compared to commercial IrO2/TiO2. The identification of the Ir@IrO(OH)x/TiO2 OER catalyst resulted from extensive HAADF-EDX microscopic analysis, operando XAS, and online ICP-MS analysis of 30-80 wt % Ir/TiO2 materials. These analyses established correlations among Ir weight loading, electrode electrical conductivity, electrochemical stability, and Ir mass-based OER activity. The activated Ir@IrO(OH)x/TiO2 catalyst-support system demonstrated an exceptionally stable morphology of supported core-shell particles, suggesting strong catalyst-support interactions (CSIs) between nanoparticles and crystalline oxide facets. Operando XAS analysis revealed the reversible evolution of significantly contracted Ir-O bond motifs with enhanced covalent character, conducive to the formation of catalytically active electrophilic OI- ligand species. These findings indicate that atomic Ir surface dissolution generates Ir lattice vacancies, facilitating the emergence of electrophilic OI- species under OER conditions, while CSIs promote the reversible contraction of Ir-O distances, reforming electrophilic OI- and enhancing both catalytic activity and stability.

Porous and defective NiCo2O4 spinel derived from bimetallic NiCo-based Prussian blue analogue for enhanced hydrogen production via the urea electro-oxidation reaction

The efficient hydrogen production via overall water splitting is seriously restrained by the slow kinetics of the oxygen evolution reaction (OER). The substantially low potential for the urea electro-oxidation reaction (UOR) can tackle this problem by replacing the OER at the electrolyzer anode. Herein, the NiCo2O4 spinel with rich oxygen vacancies (Ov) and embedded within mesoporous carbon network (Ov-NiCo2O4@mC) was derived from NiCo-based Prussian blue analogous (NiCo PBA) and exploited as the bifunctional electrocatalyst of the UOR and OER for boost the hydrogen production via the overall water splitting. Benefiting to the regular skeleton and abundant dual metal sites of NiCo PBA, the derived Ov-NiCo2O4@mC comprises abundant oxygen vacancies, lattice defects, and adjustable electron structure. These features afford Ov-NiCo2O4@mC the improved UOR ability, showing the potential of 1.33 V versus reversible hydrogen electrode (RHE) at the current density of 10 mA cm−2, along with a small Tafel slopes of 31 mV dec-1, remarkably lower than that of the OER (1.51 V versus RHE, Tafel slope = 85 mV dec-1). The UOR performance of Ov-NiCo2O4@mC substantially outperforms to most of the reported Co and/or Ni-based electrocatalysts. Impressively, the assembled two-electrode urea-assisted overall water splitting device shows the small voltage for the hydrogen production (1.43 V versus RHE) and good long-term stability. The present work can provide a new alternative for the efficient hydrogen production using the MOFs-derivatives.

Modulating Carrier Oxygen Vacancies to Enhance Strong Oxide‐Support Interaction in IrO2/Nb2O5‐x Catalysts for Promoting Acidic Oxygen Evolution Reaction

AbstractGiven the pronounced dissolution of electrocatalysts in acidic environments, the quest for effective oxygen evolution reaction (OER) electrocatalysts suitable for proton exchange membrane (PEM) water electrolyzers persists as a formidable challenge. In this investigation, catalysts are synthesized by creating oxygen vacancies within various metal oxides (Nb2O5‐x, Ta2O5‐x, ZrO2‐x, TiO2‐x) through plasma‐assisted method, thereby facilitating the immobilization of IrO2 onto these defect‐rich surfaces. The findings unveil that IrO2/Nb2O5‐x manifests reduced overpotentials during acidic OER, achieving an overpotential down to 225 mV@10 mA cm−2, coupled with outstanding durability at multicurrent densities exceeding 200 h, attributed to strong oxide‐support interaction (SOSI) between the IrO2 catalyst and Nb2O5‐x substrate. Density functional theory (DFT) computations uncover intensified binding affinities between IrO2 and Nb2O5‐x, thus modulating the central energy levels of Ir's d orbitals toward favorable OER conditions, consequently bolstering the electrocatalytic activity and stability of the composite catalyst. Furthermore, employing IrO2/Nb2O5‐x as a PEM electrolyzer anode enables consistent operation at 1000 mA cm−2 for 200 h, with an Ir content of only 0.2852 mg cm−2 and an energy consumption of 4.34 kWh Nm−3 H2. This achievement substantially lowers the cost of hydrogen production to US$ 0.96 per kilogram, underscoring its potential for practical applications.

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.

(Invited) Degradation and Poisoning of Proton-Exchange Membrane Water Electrolyzer Anode Catalysts

Hydrogen production through water electrolysis is a crucial technology to enable the transition to a carbon neutral, hydrogen-based economy. One of the primary types of electrolyzers is the low-temperature proton-exchange membrane water electrolyzer (PEMWE). As with the analogous proton-exchange membrane fuel cells, platinum group metal electrocatalysts are utilized due to their activity and stability in acidic environment. Electrocatalysis R&D efforts for PEMWEs primarily focus on the oxygen evolution reaction (OER) since it is several orders of magnitude kinetically slower than the hydrogen evolution reaction (HER).1 Iridium-based metal oxides (IrOx) are regarded as the best PEM electrolyzer electrocatalysts as they balance activity and stability.2 However, improvements are needed in both areas to enable minimization of Ir loading to meet system cost targets while also meeting challenging performance and lifetime targets.3 An understanding of the factors affecting both the activity and stability of Ir-based OER catalysts is needed to develop strategies to enable both high OER activity and long-term stability of the PEMWE anode catalyst.This presentation will describe our studies of the effects of perfluorosulfonic acid (PFSA) binder on the OER kinetics and the factors influencing the stability of two commercial Ir-based OER catalysts, one comprised of IrOx only and the other also containing TiOx. Measurements of OER kinetics as a function of ionomer to catalyst ratio utilizing rotating disc electrode and cavity microelectrode methods in an acidic aqueous electrochemical environment will be described. Dissolution and loss of Ir under the operating conditions of the PEMWE anode is considered to be one of the main PEMWE anode degradation mechanisms.4 To understand the factors affecting this phenomenon, the potential and time-dependence of Ir (and Ti) were determined using 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. The electrochemical data combined with the ICP-MS data have been used to evaluate the influence of various factors such as potential, potentiodynamic profile parameters (e.g., scan rate, upper and lower potential limits) on the dissolution processes in acidic aqueous electrolyte at room temperature.AcknowledgementsThis 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, also under contract DE-AC-02-06CH11357.References A. Raveendran, M. Chandran, and R. Dhanusuraman, RSC Adv., 13 (2023) 3843.J. Ouimet, J.R. Glenn, D.D. Porcellinis, A.R. Motz, M. Carmo, K.E. Ayers, ACS Catalysis, 12 (2022) 6159."Technical Targets for Proton Exchange Membrane Electrolysis", U.S. Department of Energy, Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office, March, 2023.Zeng, R. Ouimet, L. Bonville, A. Niedzwiecki, C. Capuano, K. Ayers, A.P. Soleymani, J. Jankovic, H. Yu, R. Maric, et al., J. Electrochem. Soc., 169(5) (2022) 054536.

Exploring Low Temperature Proton Exchange Membrane Water Electrolysis Anode Catalyst Degradation Under Cathode Backpressure Conditions

Hydrogen is expected to play an important role in the decarbonization of the future energy landscape by integrating renewable, nuclear, and fossil fuels and using excess power from the grid to produce H2 that can then be stored and used for a variety of applications ranging from ammonia and steel production, stationary power, buildings, and transportation.1 In order to achieve low-cost hydrogen production that is competitive with steam methane reformation (SMR), it is necessary to decrease the capital cost and this can be done by leveraging variable renewable energy (VRE) as an electricity feedstock for LTE.2 However, cost, and durability are still current challenges in state-of-the art water electrolysis systems. More research is needed to better understand the effects of dynamic operation on electrolyzer degradation. Understanding the degradation mechanisms governing intermittent electrolysis operation can enable future improvements related to component design and system operation.In recent years, electrolyzer anode degradation has been investigated during continual intermittent operation at the single cell level. Square-wave potential cycling demonstrated higher anode degradation compared to the triangle-wave cycling over the same 525 hour test (21.9 d).3 Electrolyzers operating at differential pressures and producing electrochemically compressed hydrogen could decrease the hydrogen production cost by eliminating the need for equipment related to mechanical compression.4 This work investigates the dynamics of low-temperature proton exchange membrane (PEM) electrolyzers, with a specific focus on anode catalyst layer degradation from redox transition when subjected to hydrogen crossover from cathode backpressure conditions. While cathode backpressure and operation requirements are well understood from an industry perspective, its impact on degradation mechanisms and loss rates has been relatively underexplored when low-temperature electrolysis is focused on cost reduction and variable load.In this study, hydrogen cross-over rates were varied by cathode backpressure conditions (ambient to 30 Bar) and were compared to elucidate the impact hydrogen has on anode catalyst layer degradation. The durability test duration varied from 168 – 500+ hrs. Hydrogen cross-over rates were found to correlate to cell potential during operational shutdowns and loss rates during load cycling. Electrochemical impedance spectroscopy further demonstrates higher degrees of kinetic loss and the immergence of additional loss mechanisms. The findings presented in this research contribute to the fundamental understanding of low-temperature PEM electrolysis dynamics under cathode backpressure conditions. The implications of this research offer practical insights for the design and operation of efficient and durable electrolysis systems crucial for the advancement of sustainable hydrogen production technologies.(1) H2@Scale. Hydrogen and Fuel Cell Technologies Office. https://www.energy.gov/eere/fuelcells/h2scale.(2) Mark, R.; Jadun, P.; Gilroy, N.; Connelly, E.; Boardman, R.; Simon, A. J.; Elgowainy, A.; Zuboy, J. The Technical and Economic Potential of the H2@Scale Concept within the United States; NREL/TP-6A20-77610; National Renewable Energy Laboratory: Golden, CO, 2020.(3) Alia, S. M.; Reeves, K. S.; Yu, H.; Park, J.; Kariuki, N.; Kropf, A. J.; Myers, D. J.; Cullen, D. A. Electrolyzer Performance Loss from Accelerated Stress Tests and Corresponding Changes to Catalyst Layers and Interfaces. Journal of The Electrochemical Society 2022, 169 (5), 054517. https://doi.org/10.1149/1945-7111/ac697e.(4) Ragnhild Hancke, Thomas Holm, and Øystein Ulleberg, “The Case for High-Pressure PEM Water Electrolysis,” Energy Conversion and Management 261 (2022): 115642, https://doi.org/10.1016/j.enconman.2022.115642. Figure 1

Modeling of Pore-Scale Capillary-Dominated Flow and Bubble Detachment in PEM Water Electrolyzer Anodes Using the Volume of Fluid Method

Fluid dynamics models complement expensive experiments with limited measurement accuracy that investigate the mass transport in PEM water electrolysis. Here, a first-principle microscale model for oxygen transport is successfully validated that accounts for (1) uncertain transport processes in catalyst layers, (2) numerically challenging capillary-dominated two-phase flow and (3) bubble detachments in channels. We developed algorithms for the stochastic generation of geometries and for the coupling of flow and transport processes. The flow model is based on the volume of fluid method and reproduces experimentally measured pressure drops and bubble velocities within minichannels with a 30% and 20% accuracy, respectively, provided that the capillary number is above 2.1 × 10−7. At lower capillary numbers, excessive spurious currents occur. Correspondingly, two-phase flow simulations within the porous transport layers are stable at current densities above 0.5 A cm−2 and match operando gas saturation measurements within a 20% margin at relevant locations. The simulated bubble detachments occur at pore throats that agree with porosimetry and microfluidic experiments. The presented model allows explaining and optimizing mass transport processes in channels and porous transport layers. These were found to be negligibly sensitive to transport resistances within the catalyst layer, providing information on boundary conditions for future catalyst layer models.

Preparation and Performance Evaluation of Microporous Transport Layers for Proton Exchange Membrane (PEM) Water Electrolyzer Anodes

In this work, ≈25 μm thin titanium microporous layers (MPLs) with ≈2 μm small pores and low surface roughness were coated and sintered on top of ≈260 μm thick commercial titanium-powder-sinter sheets with ≈16 μm pores, maintaining a porosity of ≈40% in both layers. Serving as porous transport layers (PTLs) on the anode side in proton exchange membrane water electrolyzers (PEMWEs), these pore-graded, two-layer sheets (“PTL/MPL”) are compared to single-layer PTLs in single-cell PEMWEs. The PTL/MPL samples prepared here give a 3–6 mΩ cm2 lower high-frequency resistance (HFR) compared to the as-received single-layer PTL, which is attributed to a partial reduction of the TiO2 surface passivation layer during the MPL sintering process. For ≈1 μm thin anodes with an iridium loading of ≈0.2 mgIr cm−2, the use of an MPL leads to a ≈24 mV improvement in HFR-free cell voltage at 6 A cm−2. As no such benefit is observed for ≈9 μm thick anodes with ≈2.0 mgIr cm−2, mass transport resistances within the PTL/MPL play a minor role. Possible reasons for the higher catalyst utilization in ultra-thin electrodes when using an MPL are discussed. Furthermore, an MPL provides superior mechanical membrane support, which is particularly relevant for thin membranes.

Enhanced Corrosion Resistance in Aluminum-Based Electrolyzer Components via Stoichiometry Tuned Atomic Layer-Deposited TiOx Films.

Titanium (Ti) is widely used as anode current collectors in proton exchange membrane (PEM)-based water electrolyzers due to its self-passivated oxide layer, which protects it from corrosion in acidic solutions. However, the cost of the material and machining process for Ti is high. A wider utilization of water electrolyzers to produce hydrogen could be favored by the use of less expensive coated aluminum (Al) substrates, which could potentially replace high-cost Ti-based components. It is shown here by depositing a pinhole-free oxygen vacancy-rich titanium oxide (TiOx) protection layer by atomic layer deposition (ALD), the corrosion resistance of Al substrates in acidic environments at oxygen evolution potentials can be enhanced. The optimization of the oxygen vacancy concentration is accomplished by tuning the ALD parameters to achieve ideal stoichiometry and conformal coating on rough substrates. The robustness of the coatings was evaluated at high potentials (2.4 V vs NHE = normal hydrogen electrode) in low pH conditions. A low TiOx dissolution rate of the order of ∼6 nm year-1 was observed. By testing under industrially relevant conditions, i.e., high applied voltages (2.4 V) and low pH, an Al loss at around the zero ppb level was achieved using optimized ALD parameters. It is proposed that a 40 nm TiOx coating on Al may be adequate to provide 60,000 h of durability in a PEM water electrolyzer anode current collector.

Potential Profiles through the PEM Water Electrolyzer Anode Compartment and Cost-Effective Cell Components

Potential profiles through the PEM water electrolyzer anode compartment have suggested the replacement of Pt-coated Ti components by more cost-effective materials like carbon and stainless steel [1]. Latest modelling results show that the decoupled potential of the current collector components depends on the presence of additional platinum coatings (Figure 1). These additional coatings are commonly applied at every component interface in the anode compartment to lower the contact resistance and, in turn, the electricity consumption of the electrolyzer. However, due to ionic decoupling in ultrapure feedwater the Pt-coatings can be replaced, e.g., by carbon coatings [1]. Therefore, the properties of vacuum-spray coated Ti macroporous layers (MPLs) on 316L stainless steel as demonstrated by DLR in Stuttgart [2] are well suited to protect the stainless-steel component at a distance of only tens of microns from the anode catalyst layer (ACL) especially in the absence of additional Pt coatings (Figure 1 right). Thus, a new path towards cost effective porous transport layers becomes available. Figure 1. Left. Open circuit potentials measured ex-situ (O2 saturated H2SO4 pH 4) and in situ (anode pole plate in a PEM water electrolyzer single cell). Right. Modelling of potential profiles through sprayed MPLs [2] in the absence (brown) and presence (grey) of additional Pt coatings in the anode compartment.

Ionic Decoupling in PEM Water Electrolyzers and Cost-Effective Cell Components

Potential profiles through the PEM water electrolyzer anode compartment have been found to be highly dependent on feedwater resistivity (see Figure 1). Typical type II industrial feedwater leads to a considerable drop in local potential from the anode catalyst layer (> 1.5 V vs. RHE) to the bipolar plate (1.0 V vs. RHE). At such a potential, cost-effective materials like stainless steel and carbon are stable [1] which expands their use case from PEM fuel cells to PEM water electrolyzers. Moreover, the potential profiles suggest substituting most of the PTL by more cost-effective materials than titanium, e.g., stainless steel [1].[1] H. Becker et al. Assessing potential profiles in water electrolysers to minimize titanium use, Energ. Environmental Sci . 2022, 15, 2508-2518. Figure 1

Potential Dependence of Ir Dissolution Using Time-Resolved Measurements

Electrocatalysis efforts in low temperature, PEM-based electrolysis tend to focus on the oxygen evolution reaction (OER) since it is several orders of magnitude kinetically slower than its counterpart, the hydrogen evolution reaction (HER). 1 In comparison to other OER catalysts, Ir-based metal oxides (IrOx) 2-6 are regarded as the best PEM electrolyzer electrocatalysts as they are both active and are relatively stable.7, 8 However, even Ir-based electrocatalysts slowly undergo dissolution under the operating conditions of the electrolyzer anode. 9-10, 7 Considering the low Earth abundance and high-cost of Ir, understanding the kinetics and mechanism of its electrochemical dissolution is of vital importance to develop strategies targeting high activity, long-term stability, and limited metal dissolution in acidic media. Mechanistic studies of both OER and dissolution at the solid–liquid interface are more challenging, as they typically require detection of reaction intermediates with short lifetimes. An electrochemical cell coupled to a highly-sensitive analytical technique provides a platform for detection of dissolution products and may help to resolve the degradation pathway of Ir and its oxides and the correlation to the OER mechanism.In this work, we aim to understand the structure-durability relation for different Ir oxides. 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 is used to investigate the dissolution processes of Ir from the oxides. The influence of various parameters such as potential, potentiodynamic profile parameters (e.g., scan rate, upper and lower potential limits) and catalyst type, on the dissolution processes in acidic electrolytes at room temperature will be investigated. Fundamental models have been developed to explain the mechanisms of the dissolution processes under various conditions. Moreover, the structural changes before and after catalyst testing will be studied. Further, the electrochemical data combined with the ICP-MS data will be correlated with X-ray absorption spectroscopy (XAS) and small angle X-ray scattering (SAXS) to obtain a comprehensive view of the oxide structure. Acknowledgements This work was supported by the U.S. Department of Energy, Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office (HFTO) under the H2NEW Consortium. This work was authored in part by Argonne National Laboratory, a U.S. Department of Energy (DOE) Office of Science laboratory operated for DOE by UChicago Argonne, LLC under contract no. DE-AC02-06CH11357. References M. Alia, B. Rasimick, C. Ngo, K. C. Neyerlin, S. S. Kocha, S. Pylypenko, H. Xu, B. S. Pivovar, J. Electrochem. Soc., 163, F3105 (2016).M. Alia, M. -A. Ha, G. C. Anderson, C. Ngo, S. Pylypenko, R. E. Larsen, J. Electrochem. Soc., 166, F1243 (2019).Cherevko, S. Geiger, O. Kasian, N. Kulyk, J. P. Grote, A. Savan, B. R. Shrestha, S. Merzlikin, B. Breitbach, A. Ludwig, et al., Catal. Today, 262, 170 (2016).Lee, J. Suntivich, K. J. May, E. E. Perry, Y. Shao-Horn, J. Phys. Chem. Lett., 3, 399 (2012).F. Abbott, D. Lebedev, K. Waltar, M. Povia, M. Nachtegaal, E. Fabbri, C. Copéret, T. J. Schmidt, Chem. Mater., 28, 6591 (2016).N. Nong, T. Reier, H.-S. Oh, M. Gliech, P. Paciok, T. H. T. Vu, D. Teschner, M. Heggen, V. Petkov, R. Schlögl, et al., Nat. Catal. , 1, 841(2018).Danilovic, R. Subbaraman, K.-C. Chang, S. H. Chang, Y. J. Kang, J. Snyder, A. P. Paulikas, D. Strmcnik, Y.-T. Kim, and D. Myers, J. Phys.Chem. Lett., 5, 2474 (2014).Reier, M. Oezaslan, and P. Strasser, ACS Catalysis, 2, 1765 (2012).Cherevko, S. Geiger, O. Kasian, N. Kulyk, J.-P. Grote, A. Savan, B. R. Shrestha, S. Merzlikin, B. Breitbach, A. Ludwig, K. J. J. Mayrhofer, Catal. Today, 262, 170 (2016).Kasian, S. Geiger, P. Stock, G. Polymeros, B. Breitbach, A. Savan, A. Ludwig, S. Cherevko, K. J. J. Mayrhofer, J. Electrochem. Soc., 163, F3099 (2016).

Stochastic generation of electrolyzer anode catalyst layers

We introduce a stochastic methodology to reproduce the complex pore structure observed in commercial iridium catalyst layers. This method preserves the α pore (pores smaller than 250 nm) and β pore (pores greater than or equal to 250 nm) regions of the catalyst layer. The morphology of the generated materials was validated by comparing the pore size distributions of generated materials against those obtained from commercial materials imaged using x-ray nano computed tomography. We further demonstrate that the pore size distributions of the generated materials are statistically indistinguishable from the imaged catalyst layers, indicating that the stochastic methodology is capable of accurately reproducing catalyst layer morphology. Pore network modelling was conducted on the generated catalyst materials to simulate single-phase permeability, electrical conductivity, and ionic conductivity, and these properties were found to be within experimentally measured ranges for electrolyzer catalyst layers. Additionally, simulations were performed on the generated materials with varying ionomer and iridium catalyst loadings. As the ionomer loading is added, proton conductivity increases exponentially, which demonstrates the importance of optimizing ionomer loading, considering that these effects will be exacerbated in the hydration and temperature conditions of operating electrolyzers. The stochastic material generation method presented in this work is a powerful tool for the development of novel low loading catalyst layers, where the effect of various structural parameters on electrolyzer performance characteristics can be explored.

Toward Understanding Catalyst Layer Deposition Processes and Distribution in Anodic Porous Transport Electrodes in Proton Exchange Membrane Water Electrolyzers

AbstractFinding the optimum structure in porous transport electrodes (PTEs) for proton exchange membrane water electrolyzer anodes is one of the central current technological challenges. Both the structure of the porous transport layer (PTL) and its interaction with the catalyst layer are crucial in finding this optimum structure. In this regard, manufacturing the catalyst layer on top of a PTL as a structure‐building process must be understood to find improved transport electrode structures. This work presents a PTE tomography where the catalyst ink is directly processed on a PTL. The catalyst distribution of anodic PTEs is analyzed and compared via X‐ray microtomography and cross‐sectional imaging of embedded PTE samples. The majority of the catalyst lies within the first 100 µm of the PTE. Considering the penetration depth of the membrane, a maximum of 60% of the catalyst is effectively used. For the first time, a voxel‐based catalyst layer deposition model is created and analyzed that is based on simple assumptions in the deposition process. This deposition model fits very well with the previous tomographic analysis. In the future, this model will allow more profound insight into the manufacturing process and is an important prerequisite for a future optimum design of PTEs.

Correlating nanostructure features to transport properties of polymer electrolyte membrane electrolyzer anode catalyst layers

In this work, polymer electrolyte membrane water electrolyzer catalyst layers are stochastically generated with α pore regions (defined as regions with pores <250 nm in size), β pore regions (defined as regions with pores >250 nm in size) and an ionomer layer of uniform thickness. Pore network modelling simulations are performed to correlate structural characteristics (porosity, mean pore diameter, and β pore volume fraction) to single and two-phase mass transport properties. Porosity and two-phase mass permeability are well correlated, but order of magnitude variations are observed due to inhomogeneous pore size distributions at the same porosity. The effect of varying ionomer and iridium oxide material mass fractions on ion and electron transport properties is also explored. Electrical conductivity is well correlated with catalyst volume fraction, while ionic conductivity is less correlated, likely due to changes in connectivity resulting from the inhomogeneous structure of the porous phase. Due to the non-homogeneity of the pore sizes in the catalyst layer, existing correlations for porous media are insufficient predictors. As such, we recommend that both porosity and mean pore diameter are considered when estimating transport properties. It is additionally recommended that both α pore and β pore regions must be incorporated into catalyst layer designs to allow both enhanced connectivity and mass transport characteristics.

(Invited) Electrochemical Gas Separation

Electrochemical gas separators are electrically powered systems that selectively remove a targeted gas species from a mixture of gases by virtue of electrochemical reactions. Electrochemical separation is an attractive option owing to its higher efficiency and lower cost compared to incumbent technologies like pressure swing adsorption, cryogenic processes, and selective permeation. This work focuses on two specific electrochemical separation processes – (1) separation of hydrogen from a mixture of gases to help develop the hydrogen distribution infrastructure, and (2) nitrogen separation from air for aircraft fuel tank inerting.Reductions in CO2 emission since the beginning of this century by the US electric power sector are mainly attributed to the replacement of coal by natural gas in power plants. Nevertheless, the combustion of natural gas still contributes heavily to global warming and climate change. It is imperative to find alternatives to combustion-based energy technologies and nurture the growth of renewable energy systems. In this scenario, hydrogen is a leading candidate as a carbon-free fuel with high energy density and is expected to play a key role in future energy systems. However, hydrogen faces serious obstacles in its distribution due to the lack of a nationwide hydrogen pipeline network. Developing a dedicated hydrogen pipeline network will be quite expensive, therefore, it is worthwhile to examine whether existing natural gas pipelines could be effectively deployed for hydrogen distribution. This would be accomplished by directly injecting a prescribed amount of hydrogen at the point of production into a natural gas pipeline. Such a mixture of hydrogen and methane is labeled as hythane. While this enables the convenient transport of hydrogen across large distances, the process can only be completed by separating hydrogen from methane at the destination point. Electrochemical hydrogen separation (ECHS) systems built around proton-selective polymer electrolyte membranes (PEMs) represent an effective platform to separate and simultaneously compress hydrogen in a continuous operation. Furthermore, ECHS ensures that the resulting gas is not contaminated by lubrication oil as observed in conventional systems. In ECHS, the hythane mixture enters the anode compartment wherein the hydrogen is selectively dissociated to protons and electrons. The protons are then driven across the PEM by an externally applied voltage to recover hydrogen at the cathode.The first part of this study demonstrates hydrogen purification using low-temperature PEM-based ECHS from various gas mixtures including methane/hydrogen, carbon dioxide/hydrogen, water gas shift effluent, and hythane. ECHS performance is first investigated for pure hydrogen as a function of membrane thickness, cell temperature, and relative humidity of the anode stream. In the second set of experiments, various ratios of methane/hydrogen and carbon dioxide/hydrogen are introduced to examine the effect of hydrogen concentration in the feed gas mixture on ECHS performance. Finally, experiments are performed for hydrogen purification from a water gas shift (WGS) effluent mixture as well as a practical hythane gas feed. ECHS performance for all gas mixtures was benchmarked against the pure hydrogen case. The purity of the separated hydrogen gas was measured to confirm the effectiveness of the method. The results show that ECHS represents a good solution to separate hydrogen from the hythane mixture at the downstream end of the pipeline.Pertinent to the second electrochemical separation process examined here, after the TWA flight 800 disaster due to a fuel tank explosion in 1996, inerting of aircraft fuel tanks became a priority. During fuel tank inerting, an inert gas like nitrogen is supplied to the tank to reduce its flammability. An electrochemical gas separation and inerting system (EGSIS) is a device that generates nitrogen enriched air (NEA) from ambient air by the application of electrical power. EGSIS combines a PEM electrolyzer anode wherein water is dissociated to release oxygen, and a PEM fuel cell cathode where atmospheric air is converted to NEA. Aircraft tank inerting requires varying NEA flowrates (low during takeoff and ascent, and high during descent). In conventional hollow fiber membrane air separation modules typically used in current aircraft, the total membrane surface area is determined by the maximum required NEA flow rate which results in large and expensive modules. On the other hand, the NEA flow rate can be easily controlled in EGSIS by simply adjusting the applied voltage. This portion of the study focuses on results for a single EGSIS cell and its optimization. Various EGSIS stack configurations are also described in order to develop a practical system. Finally, a techno-economic analysis of EGSIS is presented to show that EGSIS can compete favorably with incumbent technologies in terms of fuel usage and cost.

Optimization of an Electrochemical Gas Separation and Inerting System

Aircraft fuel tank inerting is typically accomplished by supplying nitrogen enriched air (NEA) into the ullage (volume of air above the fuel level in the tank). We have developed a novel on-board electrochemical gas separation and inerting system (EGSIS) to generate NEA for fuel tank inerting. EGSIS is an electrically powered system that functionally combines a proton exchange membrane (PEM) fuel cell cathode with an electrolyzer anode. Water management is important in such a PEM-based system because proton transfer requires proper hydration of the membrane. Extremes of both dryout and flooding conditions should be avoided for optimal EGSIS performance. Previous single-cell EGSIS experiments revealed that supplying liquid water at the anode will maintain sufficient membrane hydration even when the system is operated under dry cathode conditions. However, it was difficult to avoid flooding at low cathode air stoichiometries when parallel flow field channels were employed. Here, we implement various strategies to optimize EGSIS performance such as using serpentine and interdigitated flow field channels, as well as a double-layer gas diffusion layer with graded hydrophobicity to mitigate flooding and improve water management. We also present a theoretical analysis of various stack configurations for a practical EGSIS module.

Methods— A Simple Method to Measure In-Plane Electrical Resistance of PEM Fuel Cell and Electrolyzer Catalyst Layers

Optimizing the catalyst layer of polymer electrolyte membrane fuel cells and water electrolyzers requires a good understanding of its properties. The in-plane electrical resistance of the catalyst layer is a key property, which impacts the overall cell performance. In this work, we present a simple method to measure the in-plane electrical resistance of catalyst layers under various conditions based on the transfer length method. The applicability of the method was demonstrated on four examples: 1) Placing the compact setup in a climate chamber, showed that reducing the relative humidity from 95% to 40% yields a reduction of the resistivity of 15% in a fuel cell cathode catalyst layer; 2) graphitizing CNovel™ carbon support reduces the resistivity by 98% in a fuel cell cathode catalyst layer; 3) adding an electrically conductive polymer as electrode binder lowers the in-plane resistivity of a water electrolyzer anode by 50%; 4) adding IrO2-nanofibers to a low-loaded IrO2-nanoparticle anode lowers its resistivity by 60%. The broad range of applications in this work confirms the versatility of the setup enabling widespread application. The method hence contributes to an improved deconvolution of different loss mechanisms including electrical in-plane resistivity.

Low precious metal loading porous transport layer coating and anode catalyst layer for proton exchange membrane water electrolysis

A mixed metal oxide-coated Ti felt was constructed for use as a porous transport layer (PTL) in a proton exchange membrane (PEM) water electrolyzer. The PTL was fabricated by coating Ti felt with 0.43 mgTaOx cm−2 and 1 mgIr + Ru cm−2 IrO2–RuO2-TaOx using a thermal decomposition method. The coated Ti felts exhibited high conductivity, mass transport performance, stability and oxygen evolution reaction (OER) catalytic activities. The stability of IrO2–RuO2-TaOx coating obviously improved than traditional electroplated Pt coating. Using the PTL, a single cell performance of 1.836 V @ 2000 mA cm−2 was achieved at 80 °C under ambient pressure with 1 mg cm−2 of precious metal in anode CL. However, the precious metal loading is about 2 mg cm−2 in common PEM electrolyzer anode catalyst layer (CL). The IrO2–RuO2-TaOx-coated Ti felt proved to be a promising low-cost PTL for PEM water electrolysis with high performance.