Ohmic Overpotential Research Articles

Bioinspired Hierarchical Porous Architecture for Enhanced Kinetics and Mechanical Integrity in Thick Cathode.

Increasing the thickness of the electrodes is considered the primary strategy to elevate battery energy density. However, as the thickness increases, rate performance, cycling performance, and mechanical stability are affected due to the sluggish ion transfer kinetics and compromised structural integrity. Inspired by the natural hierarchical porous structure of trees, electrodes with bioinspired architecture are fabricated to address these challenges. Specifically, electrodes with aligned columns consist of tree-inspired vertical channels, and hierarchical pores are constructed by screen printing and ice-templating, imparting enhanced electrochemical and mechanical performance. Employing an aqueous-based binder, the LiNi0.8Mn0.1Co0.1O2 cathode achieves a high areal energy density of 15.1 mWh cm-2 at a rate of 1C at mass loading of 26.0mg cm-2,benefitting from the multiscale pores that elevated charge transfer kinetics in the thick electrode. The electrodes demonstrate capacity retention of 90% at the 100th cycle at a high current density of 5.2mA cm-2. To understand the mechanisms that promote electrode performance, simplified electro-chemo-mechanical models are developed,the drying process and the charge-discharge process are simulated. The simulation results suggested that the improved performance of the designed electrode benefits from the lower ohmic overpotential and less strain gradient and stress concentration due to the hierarchical porous architecture.

A gradient porous transport layer enabling a high-performance proton-exchange membrane electrolysis cell

Large-scale green hydrogen production through proton-exchange membrane electrolysis cells (PEMECs) is constrained by high costs. Operating PEMECs at high current densities while minimizing overpotentials, particularly the mass transport overpotential due to intense water-oxygen two-phase flow, is crucial. The porous transport layer (PTL) plays a key role in facilitating water-oxygen two-phase flow and affecting ohmic resistance and catalyst utilization. In this study, a gradient PTL has been developed by introducing titanium mesh with tunable pores between the titanium felt and the flow field. The woven mesh structure with large pores facilitates water transport under the ribs, while the titanium felt reduces ohmic resistance and improves catalyst utilization. Experimental results demonstrate that the gradient PTL with a single layer of titanium mesh (pore size: 0.6 mm) can reduce the mass transport overpotential from 0.478 V to 0.206 V at 5 A/cm2 and maintain low ohmic and activation overpotentials compared with the conventional PTLs. Additionally, the gradient PTL with two titanium mesh layers (pore sizes: 0.6 mm and 0.28 mm) can further facilitate water transport, reducing mass transport overpotential to 0.149 V at 5 A/cm2. Numerical simulations reveal that the developed gradient PTL increases the volume fraction of liquid water and ensures uniform water distribution in the catalyst layer. The combined results indicate that the gradient PTL design is promising for enhancing mass transport performance while maintaining low ohmic resistance and high catalyst utilization, making PEMECs more viable for large-scale hydrogen production.

Transport characteristics of gas in an alkaline water electrolyser with the addition of baffles: Analysis of mechanism and performance

The adhesion of gases to the electrodes is a major reason for reductions in energy efficiency, and hinders the production of hydrogen in alkaline water electrolysis (AWE). The development of an effective solution to this problem will be crucial for the design and optimization of AWE. In this paper, we present a novel structure for an electrolyser in which baffles are inserted into the channel. The flow process of the electrolyte and the electrochemical reactions in the electrolyser with the new structure are simulated, and the effects of the added baffles on the distribution of the gas phase production and the efficiency of energy conversion are analysed. Several important structural parameters are considered, such as the angle, number and length of the baffles, and corresponding numerical calculations are performed. In order to carry out a comprehensive evaluation of the performance of each electrolyser, the energy efficiency of AWE is evaluated and the current effect, ohmic overvoltage effect and bubble overvoltage effect caused by gas adhesion are quantitatively investigated. The results show that the flow of the electrolyte can be divided into three phases when baffles are added to the channel of the electrolyser, and that the baffles play a crucial role in both the transition phase and the full development phase. The maximum reduction in gas coverage at the electrodes reaches 60% and the maximum increase in energy efficiency reaches more than 3.0% with the addition of baffles. The addition of baffles has significant effects on the current and ohmic overpotential, and the balance between the gas coverage and ohmic overpotential can allow the maximum energy efficiency of the AWE to be achieved.

Energy consumption model of flow-through electrolyzer considering mass transfer overpotential for hydrogen production from alkaline water splitting

Flow-through electrolyzer can improve the efficiency of hydrogen production from alkaline water splitting and determining the optimal electrolyte velocity through electrode pores is crucial for reducing energy consumption. In this study, an energy consumption model of flow-through electrolyzer considering mass transfer overpotential (Emass) has been developed. The model of Emass due to gas hold-up (ϕ) in the electrolyzer accounts for the density, viscosity, surface tension, linear velocity, and current density. The model-predicted value is in good agreement with the experimental results, which indicated that the increase of velocity effectively reduces ϕ and Emass. Then, Voltage-Current curve models combining reversible voltage, activation overpotential, ohmic overpotential, and Emass are developed. Based on this, an energy consumption model, combining both consumption from electrolyzer and pumps, is established to evaluate the overall energy consumption of hydrogen production. It can be calculated the Emass at 400 mA cm−2, 6 M KOH, 353 K can be reduced from 0.363 V to 0.048 V if the circulation velocity rises from 0.001 m s−1 to 0.1 m s−1. The model predicts that total energy consumption is 4.36 kWh Nm−3 with energy consumption for pump being 0.02 kWh Nm−3 at 400 mA cm−2 and 0.063 m s−1.

An Analytical Model for an Anode-Supported Solid Oxide Fuel Cell

This study presents a one-dimensional, non-isothermal, steady-state model to analyze performance of an anode-supported solid oxide fuel cell (SOFC). The modeling domain encompasses components including a NiO-YSZ support and functional layer forming the anode, a YSZ electrolyte, a GDC interlayer, as well as LSCF+GDC and LSCF constituting the cathode. The model distinguishes different types of electrode polarization resistances using experimental impedance data at various temperatures under open circuit voltage (OCV), employing a detailed equivalent circuit model to determine the temperature and partial pressure dependencies of electrode exchange current densities. Moreover, this approach serves as an efficient method for estimating the conductivity of both the anode and cathode. As a result, activation energies and ohmic overpotentials are derived, considering microstructural and geometrical aspects, which allows for accurate quantification of the ohmic resistance associated with ionic conduction.The model incorporates coupled multi-physics elements, such as mass and charge transport, electrochemical reactions, and heat transfer. A notable feature of the model is its consideration of reaction zone layers near the electrolyte, where electrochemical reactions generate electrons, oxide ions, and water vapor. Enhancing prediction accuracy, the model applies multilayer discretization within the electrode and electrolyte to reflect actual material properties and localized conditions more accurately. The model predicts the performance of a single-cell SOFC using pure hydrogen at the anode and varying oxygen concentrations (100%, 50%, 20%, and 10%) at different operating temperatures (700°C, 660°C, 620°C, and 580°C) and design conditions. Simulated results are compared with experimental data, and the impact of various operating and design parameters on SOFC performance is examined. Findings indicate that kinetic and concentration overpotentials in an anode-supported SOFC are lower than ohmic overpotentials, even at higher current densities. Furthermore, ohmic overpotential is identified as the primary contributor to total cell potential loss, highlighting the need for its minimization to improve cell performance.

Modeling of Fluidized Bed Molten Carbonate Direct Carbon Fuel Cell: Effect of Reverse Boudouard Reaction

Direct carbon fuel cells (DCFCs) are an alternative technology to coal-fired power plants for converting the chemical energy of abundant carbon-rich fuel into electrical energy. DCFCs have higher electrical efficiency and lower greenhouse emissions, which are easier to separate and capture. In the transition to renewable sources of energy, efficient and low-emission technologies need to be explored. Since DCFC technologies are in their early stages of development, modeling and simulation studies are needed to reduce costs and aid in experimental design. In this study, an electrochemical and transport model for a fluidized bed molten carbonate DCFC (FBMCDCFC) is developed. The performance of the fuel cell is evaluated through the polarization losses. This study expounds on recent developments in DCFCs by considering the effect of the reverse Boudouard reaction and the 2-electron oxidation of CO alongside the 4-electron oxidation of carbon at higher temperatures. The electrolyte used is a 62 : 38 mol % Li2CO3/K2CO3 eutectic mixture, while the fuel is activated carbon impregnated by HNO3. The results show that the greatest polarization losses for the FBMCDCFC are from anode activation overpotential and ohmic overpotential. The cathode activation and concentration overpotentials are relatively small. Increasing the reaction temperature results in a greater decrease in anode activation overpotential than ohmic overpotential, despite the increase in CO formation from the reverse Boudouard reaction. The peak power density of the FBMCDCFC is 434.437 W m-2 when operated at 923 K and a gas flow rate of 30 mL min-1. The peak power density of the FBMCDCFC is greater with higher temperatures, where the peak increases to 584.802 W m-2 when operated at 1023 K. The peak power density is greater with lower gas flow rates, where the peak increases to 458.175 W m-2 at a gas flow rate of 25 mL min-1. The mathematical model can aid in optimizing the performance of FBMCDCFCs, such as how increasing the operating temperature favorably increases anode reaction kinetics and reduces ohmic resistances in the cell. Future work will focus on improving the model correlations to close the gap between the results of modeling and actual experiments.Figure 1. a) Power curves at different temperatures, 823 K to 1323 K (30 mL min-1 flow rate, 1 atm total pressure); b) Power curves at different gas (O2 and CO2) flow rates (923 K, 1 atm total pressure) Figure 1

Interleaved Electrode Structures for Enhanced Electrolyte Mass Transport in Li-Ion Batteries : A Numerical Study

The enhancement of energy and power density in Li-ion batteries remains a focal point in battery research. When Li-ion batteries are operated at high C-rates, mass transport limitations cause large concentration gradients of Lithium ions to form in the electrolyte. The resultant concentration overpotentials and underutilization of active material effectively imply a lower capacity for higher current rates, which creates a tradeoff between energy and power density. The integration of 3D interleaved electrode structures has been proposed to augment power density while preserving energy density. This work comprehensively analyzes the capacity improvement achieved by utilizing a wavy interleaved structure using electrochemical simulations. A pseudo-3D (P3D) model based on the Doyle-Fuller-Newman framework is employed to analyze the performance of various depths and frequencies of the wavy interleaved structure. Galvanostatic simulations are performed for NMC/Graphite-SiOx and LMO/Graphite cells at different C-rates, and the resultant voltage is systematically divided into component overpotentials to delineate the contribution of activation, ohmic, and mass-transfer overpotentials in both solid electrode and electrolyte. With the same amount of active material, simulations indicate a performance improvement of 7% in capacity at high C-rates. It is observed that using a wavy structure significantly reduces electrolyte concentration heterogeneities and resulting concentration gradients when compared to a conventional flat structure. The overpotential breakdown confirms that the improvement in voltage is indeed contributed precisely by the concentration overpotential. Concentration overpotential decreases with deeper interleaved structures as well as for more spatially frequent structures.Additionally, the impact of the interleaved electrode structure on cell degradation was also assessed. Lithium plating, which is the most prominent side reaction in the anode, was modeled using a Butler-Volmer kinetic equation, with deposition occurring locally when the solid phase potential drops below that of the electrolyte potential. The interleaved structure alleviates plating significantly due to higher uniformity in Li-ion concentration. With this work, we computationally demonstrate that interleaved structure works well for extracting higher capacity with the same active material, and also aids in suppressing Lithium plating.Figure Caption : Computed Li-ion concentration profile in the solid active material of an interleaved electrode Li-ion cell as a function of location (x, y) and distance from particle center (z) Figure 1

Assessing Nonlinear Polarization in Electrochemical Cells Using AC Impedance Spectroscopy

Electrochemical energy storage and conversion devices are used to power a range of electrical loads including portable electronics, electric vehicles, and utility grids. The energy efficiency of these systems decreases with increasing current load due to energy losses (in the form of heat) associated with different processes. Electrochemical impedance spectroscopy (EIS) is a nondestructive technique commonly used to assess rate-limiting steps in electrochemical devices. Most EIS measurements for energy storage devices are only performed around open-circuit and do not probe system behavior under non-zero biases where these devices operate. Despite its widespread use, this approach does not provide information about the relative overpotentials associated with coupled nonlinear processes (e.g., diffusion limited redox reactions) commonly encountered in these systems. Furthermore, ambiguous terms such as equivalent series resistance and interfacial resistance can lead to confusion and data misinterpretation. For example, Ohm’s law cannot be applied to calculate nonlinear overpotentials using a resistance that was measured from a locally linear response.To determine overpotentials in nonlinear systems using EIS, one must integrate the individual resistive elements of the equivalent circuit as a function of steady state current. Recent experimental work has used this method to decouple ohmic, kinetic, and transport overpotentials in redox flow batteries. In these prior studies, impedance elements were derived, but analytical expressions for the corresponding overpotentials were not presented.While integrated resistance measurements are a powerful experimental tool, a more rigorous mathematical framework describing the approach is needed. The present work bridges this gap by: (i) deriving a general impedance expression for coupled charge transfer and diffusion processes and (ii) integrating the individual resistive elements as a function of steady-state current to obtain global polarization relationships. Analytical expressions for diffusion overpotentials are obtained for special cases, and numerical methods are applied when closed-form expressions do not exist. The work considers three model electrode geometries (planar, cylindrical, and spherical) and two diffusion boundary types (reflective and transmission). Finally, this treatment is extended to macroscopically homogenous porous electrodes which are relevant for describing practical devices.This presentation will highlight how measuring impedance at different steady-state biases provides critical insights on the rate-limiting processes in energy storage devices. While this work focuses on a simple Faradaic charge transfer process, the method can be used to evaluate other nonlinear phenomena in electrochemical power supplies. For example, the resistance of passive films (e.g., solid electrolyte interphase layer) in Li-ion batteries is typically only probed near open-circuit. Similarly, literature on solid-state batteries commonly uses ambiguous terms such as interfacial resistance to describe rate-limiting processes. For these more complex systems, integrating the resistive elements can provide insights on how these phenomena influence device efficiency. Overall, combining these experimental observations with physical models derived using the approach illustrated herein represents a powerful tool to guide component/system design and improve device performance. Acknowledgements This research was conducted at Oak Ridge National Laboratory, managed by UT Battelle, LLC, for the U.S. Department of Energy (DOE). This work is sponsored by the U.S. Department of Energy in the Office of Electricity through the Energy Storage Research Program and the Office of Energy Efficiency and Renewable Energy (EERE) in the Vehicle Technologies Office (VTO) through the Advanced Battery Materials Research (BMR) Program.

(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 compression on PEM Electrolyzer membrane electrode Assemblies

Reliable operation of polymer electrolyte membrane electrolysers requires a sound understanding of the effects surrounding the cell assembly. This work focues on the compression of the membrane electrode assembly (MEA) during cell assembly, which strongly influences cell performance, safety and lifetime of the electrolyser. In this work, we analysed the effect of increasing MEA compression on membrane deformation under the flow-fields land and channel regions and correlated structural changes to changes in cell performance. Via an overpotential break-down analysis, the main losses caused by membrane deformation due to cell compression were elucidated, supported by a structural analysis of cryogenic cross-sections of the used catalyst coated membranes (CCMs). Based on these results, an optimal cell compression between 30 and 40 % for the cell design used in this work to balance opposite trends of ohmic vs. mass transport overpotentials. In addition it was shown that the compressed membrane does not flow into the channel areas of the flow field, but rather into the pores of the porous transport layer.

Comprehensive evaluation of the influence of PEM water electrolyzers structure on mass transfer performance based on entropy weight method

The proton exchange membrane water electrolyzer (PEMWE) has broad prospects in hydrogen production due to its green and efficient advantages. This paper comprehensively analyzes the performance and water-gas transport characteristics of PEMWE with different cell structures, in which polarization curves, flow velocity distribution, oxygen concentration, temperature, liquid water saturation, and pressure are investigated. Simultaneously, the study explores the effects of variations in inlet velocity, inlet temperature, porosity, and thickness of the anode porous transport layer (APTL) on the four flow fields. Finally, the technique for order preference by similarity to ideal solution (TOPSIS) based on entropy weight method is employed for a comprehensive evaluation of the flow fields. The results indicate that, compared to parallel flow field (PFF), sinusoidal wave flow field (SWFF), intercepted flow field (ICFF), and single-serpentine flow field (SSFF) exhibit stronger exhaust capabilities, with temperature uniformity increasing by 83.65 %, 59.24 %, and 70.15 %, respectively. However, the longer flow channel of SSFF results in a high pressure drop and lower liquid water saturation. An increase in inlet velocity leads to a decrease in temperature, causing an increase in voltage, as higher temperatures reduce activation and ohmic overpotentials. Additionally, an increase in the porosity and thickness of APTL deteriorates the polarization performance of PEMWE. The research results of this study can provide valuable assistance for the future design of PEMWE.

Efficient Energy Storage via Methane Production Using Protonic Ceramic Electrochemical Cells

Protonic ceramic electrochemical cells (PCECs) are promising energy storage technologies due to their high performance and ability to convert CO2 into value-added chemicals. However, there are many fundamental aspects of this technology that require investigation to better understand its interactions and opportunities. Thus, this paper focuses on the development of a steady-state PCEC model, encompassing intricate details of reactive porous-media transport, elementary catalytic chemistry, and electrochemistry within unit cells. The model is calibrated and validated using experimental data. The model presents the rate of methane production at different temperatures and revealed that efficient PCEC for co-electrolysis of CO2 and H2O is operated in the temperature range of 420–470 °C and the optimum being 450 °C at the rate of 0.0391 moldm−3 min−1. It provides insights into the effects of overpotentials on cell performance. At a current density of 3000 A/m2 the ohmic, concentration, and activation overpotentials amount to 0.65, 0.004, and 0.19 V, respectively. The results obtained from the model highlight the potential of PCECs as efficient CO2 sinks for decarbonization purposes and as a means of methane production. Furthermore, the model's findings offer valuable guidance for the design, selection of stacks, and choice of building materials in PCEC systems. The potential applications of PCECs as real-life fuel utilization technologies are justified with improved material development to reduce cell overpotentials, opening doors for scale-up and eventual commercialization. These findings contribute to the development of sustainable energy systems and advance the pursuit of decarbonization goals.

Influence and Improvement of Membrane Electrode Assembly Fabrication Methods for Proton Exchange Membrane Water Electrolysis

The performance of proton exchange membrane water electrolysis (PEMWE) is crucial for its commercialization. The membrane electrode assembly (MEA) preparation process determines the catalyst layer’s (CL) structure, thereby influencing PEMWE performance. Herein, the effects of conventional preparation methods, i.e., direct spray deposition and decal transfer, on the CL were investigated. It was found that the MEA prepared via the decal process exhibits lower activation and Ohmic overpotential. For decal transfer CL, this is due to the improved electrochemically active surface area and proton conduction, derived from the improved catalyst-ionomer agglomerates interconnection and CL-membrane interfacial contact. For direct spray deposition CL, the crack and larger pores in CL facilitate its water-gas transport. On this basis, a hierarchical CL was designed in order to combine the advantages of direct spray deposition and decal transfer. As a result, the hierarchical CL shows better performance than both direct spray deposition and decal transfer CL. The Ohmic and mass transfer losses are reduced by 13% and 15% at 4 A cm−2, respectively. This work provides valuable insights for MEA development, crucial for the large-scale application of PEMWE.

A Redox-Electrodialysis Model with Zero Fitting Parameters: Insights into Process Limitations, Design, and Material Interventions

Redox-Electrodialysis (r-ED) is an electrochemical desalination cell architecture that has recently received considerable interest, due to its low energy demand relative to electrochemical desalination technologies that rely on electrode-based ion removal. To further improve the energy efficiency of r-ED, we developed a lumped mathematical model with no adjustable parameters to investigate the various sources of overpotential within the cell. Existing models of electrodialysis and r-ED cells either do not accurately incorporate all phenomena contributing to the overpotential or utilize empirical fitting parameters. The model developed here indicates that ohmic overpotentials, especially in the diluate chamber, are the most significant contributors to energy losses. Based on this insight, we hypothesized that adding an ion exchange resin wafer in the diluate compartment would increase the ionic conductivity and decrease the energy demand. Experimental results showed an 18% reduction in specific energy use while achieving the same degree of salt removal (20 mM to 12 mM). Furthermore, the resin wafer enabled complete desalination to potable drinking levels at a current density previously unachievable within practical operating voltage limits (4.93 mA cm−2). We also expanded the model to explore differences in r-ED energy use between configurations using multiple cells and a single cell with increased area.

Development of an Experimentally Validated Computational Fluid Dynamics Model for a PBI Membrane-Based Anhydrous HCl Electrolyzer

We report the development of an experimentally-validated computational fluid dynamics (CFD) model for simulation of an anhydrous HCl electrolyzer. The experimental data from 3 membrane variants was used to provide kinetic and membrane parameters for the model. The model not only accurately simulates overall electrolyzer performance, but it also provides key insights into the transport phenomena within the electrolyzer. The model allows simulation of experimental parameters like high HCl flowrates and increased cell pressure that pose a high safety risk to researchers. The model shows hotspots in the temperature distribution that will need to be addressed by flow field modification when scaling up the electrolysis process. The increasing of cell pressure reduces the gradient of current distribution throughout the electrolyzer and lowers the cell voltage required for a given current density. Increasing electrolyzer temperature reduces cell voltage by decreasing losses due to kinetic overpotential and ohmic overpotential. The implications of the simulated results are discussed, including potential limitations in our experiments and how the model can be used effectively when considering important steps like industry scale-up.

Electrospun catalyst layers using short-side-chain ionomer for low platinum and high performance PEMFCs

Proton exchange membrane fuel cells (PEMFCs) should further reduce the platinum consumption to lower their manufacturing costs and life-cycle carbon emissions. However, the performance of the catalyst layers with low-Pt loading needs to be improved. Short-side-chain (SSC) ionomers have been proven to enhance the catalytic activity of conventional catalyst layers. In this work, the SSC ionomer was applied to the electrospun catalyst layer, and achieved a remarkable improvement in the performance. With the use of commercially available catalyst, the membrane electrode assemblies (MEAs) prepared in this work exhibited a low total-Pt-consumption of 0.064 g kW−1 (stoichiometric ratios of 1.5/2.5 for H2/air, 80°C, 100 kPa) and achieved the 2025 target proposed by the US Department of Energy. Separating activation, ohmic, and concentration overpotentials, the performance enhancement of the electrospun catalyst layer mainly came from the decrease of ohmic overpotential and concentration overpotential. The electrospun ionomer membrane without catalyst was used as a research model to explore the proton transport properties inside the electrospun catalyst layer. The results showed that in the range of PEMFC operating temperature, the proton conductivity of the electrospun catalyst layer could be higher, but meanwhile the proton-conduction activation energy was also elevated. Compared with the conventional catalyst layer, the electrospun catalyst layer showed obviously better performance before and after accelerated stress test, thus verifying the practicality of the electrospun catalyst layers. This work provided a reference for the development of low platinum and high performance catalyst layers.

EWTN: Quantifying Mass Transport to Enable Water Electrolyzer Architectures with Low Flow-Rate Sensitivity

To develop cost-effective and high-performing polymer electrolyte water electrolyzers (PEWEs) for gigawatt-scale applications, researchers have focused on reducing precious metal catalyst loadings and optimizing porous transport layers. However, the performance of PEWEs is also affected by system architecture. Mass transport losses are dependent on localized architecture and material interactions. In-situ measurements, such as current density distribution maps have demonstrated advantages in understanding the intricate characteristics and influence of two-phase flow within PEWEs. This study proposes the parameter of effective water transport number (EWTN) as a quantitative tool to investigate such current density distribution (CDD) measurements for PEWEs. Results show that higher flow-rates have EWTN values of 0.95 and above, indicating no mass transport limitations; while lower flow-rates with large gradients CDD have EWTN values between 0.6-0.8, indicating mass transport limited conditions. The new analysis also identified a correlation between mass transport losses due to bubble accumulation, membrane hydration, and ohmic overpotentials. To address these limitations, an unitized pin-type LGDL/flow-field design was developed, which effectively prevents local gas phase accumulation, resulting in improved mass transport characteristics. The results of this work show reduced flow-rate sensitivity with the pin-type architecture and ∼13% increased performance at 0.24ml/min/cm2 .

Failure mechanism of dielectric breakdown in electrochemical hydrogen pumps

Electrochemical hydrogen pump (EHP) cells, employing ultra-thin (This article refers specifically to films with a thickness of 12μm) proton exchange membranes (PEMs), are used in different fields such as hydrogen purification, hydrogen compression, and the rapid activation of hydrogen fuel cells. However, under certain extreme conditions, the voltage of an EHP cell with an ultra-thin proton exchange membrane can rise rapidly and trigger dielectric breakdown of the PEM, resulting in a PEM perforation and cell failure due to an internal short circuit. In order to investigate the failure mechanism, this paper comprehensively analyzes the electrochemical failure process leading to the breakdown of EHP cells. This investigation is accomplished by monitoring characteristic parameters such as voltage and resistance. The research outcomes indicate that the failure process in EHP cells can be divided into three stages: (I) Normal operation stage, (II) Carbon corrosion stage, and (III) Dielectric breakdown stage. In stage-I, the main process is the EHP reaction itself, the cell is undamaged and the cell voltage is about −0.22 V. In stage-II, the localized water shortage at the anode leads to a rapid increase in the anode ohmic overpotential, and the cell voltage is around −1.9 V, which reaches the carbon corrosion threshold. As a result, the anode catalyst layer undergoes a severe carbon corrosion reaction, generating CO and CO2 gases, and the cell performance decreases but the PEM is not seriously damaged. As substantial amount of anode water is consumed, the cell enters into stage-III. At this stage, the anode overpotential further increases, and the cell voltage reaches above −14 V, resulting in dielectric breakdown of the PEM, internal short circuit of the cell, and the cell cannot work normally. This study can enhance the understanding of the electrochemical failure process in EHP cells, thereby providing a basis for improving their durability and reliability.

New Advances in Graded PEMFC Catalyst Layers

Ohmic and mass transport overpotentials remain investigation and optimization areas for Proton Exchange Membrane Fuel Cell (PEMFC) applications. One potential optimization approach involves the preferential distribution of the chemical substituents spatially in the catalyst layer. This would ultimately result in graded catalyst layers (GCLs). The spatial grading directions can thereby be in the in plane and/or through plane direction(s). Chemical constituents often graded in both directions include the binder and the catalyst[1, 2]. The ionomer, an essential constituent for PEMFC applications has also been a key focus area for many research institutes and groups. Novel optimization approaches include finding new ionomers with low equivalent weights (EWs).Typically, in a PEMFC, the highest proton conduction rate is at the membrane interface and decreases towards the GDL[3, 4]. Hence, previous studies with gradients containing high ionomer contents close to the membrane and lower ones at the GDL interface resulted in improved performances. The higher ionomer content at the membrane interface resulted in better protonic conductions, and the bigger void spaces in the GDL interface resulted in improved mass transport properties[1, 5]. Though precise electrochemical analysis is lacking, it is commonly agreed that the graded layers have lower mass transport and ohmic resistances. However, the studies encountered in the literature are almost exclusively based on high EW ionomers, such as Nafion. In contrast, low EW ionomers, which have higher protonic conductivity and water uptake capabilities have not yet been a focus of researchers. Additionally, the few reports encountered in the literature focus solely on GCLs while often disregarding other contributing effects to the overall cell voltage such as the equivalent weight.Low EW ionomers offer a wider operation range for PEMFCs in terms of relative humidities. By combining the low EW ionomers with graded catalyst layers, the operation range becomes even more flexible, and the limits are further stretched. In this study, we push the operating range limits of low EW graded catalyst layers and compare them to non-graded ones. We investigate different manufacturing techniques and use suitable scalable coating methods for the manufacturing of the electrodes. Afterwards, suitable in situ and ex situ analysis techniques are used in detail to trace down the improvements for the gradients. Among the key findings, we notice that potential improvements for low EW ionomer graded catalyst layers become apparent at considerable low relative humidities. The improvements are visible in the kinetic, ohmic, and mass transport regions.

Understanding Porous Transport Layer Structure-Property-Performance Relationships in PEM Water Electrolyzers

Low temperature electrolysis (LTE) is a very promising technology to help with the decarbonization of the energy sector through the production of green hydrogen from renewable energy sources. The DOE has recently launched the first Energy Earthshot targeting the reduction of clean hydrogen cost by 80% to $1 per 1 kg H2 produced in 1 decade. To reach such ambitious targets the technology development needs to consider both capital expenditures (CAPEX) such as material and component costs and their lifetimes, as well as operational expenditures (OPEX) such as cell performance and efficiency. One component that touches on either expenditure is the porous transport layer (PTL). The PTL provides various key functions within an electrolysis cell/stack. It provides transport pathways for electrons, water and gas, and mechanical support for the membrane. Its interface with the catalyst layer is crucial with regards to the limitation of ohmic overpotentials, but also heat transport within the cell. PTLs are currently fabricated from sintered Ti materials that are required to be coated with a PGM material when operated with low loading catalyst layers. Multi-layer PTL materials may further improve cell performance and enable the thrifting of catalyst loadings and the utilization of thin membrane materials. This presentation will introduce the various functionalities of the PTL and highlight research and development efforts including those of DOE’s H2New Consortium and NREL. Specifically impacts of PTL morphology, PTL coatings, and PTL/catalyst layer interactions will be discussed.