Transport Overpotentials Research Articles

The Influence of Tin Addition on the Electrochemical Performance of IrO2 and Ir Black Oxygen Evolution Catalysts in Polymer Electrolyte Membrane Electrolysis

Polymer electrolyte membrane (PEM) electrolysis is widely considered a promising technology for energy storage and clean hydrogen production, due to the advantages it has over state-of-the-art alkaline water electrolysis.1 Typically, PEM electrolysers can operate at higher current densities than their alkaline counterparts, have a more robust design, and due to the zero-gap design, exhibit smaller Ohmic losses. Furthermore, PEM electrolysers can yield hydrogen at elevated pressures, eliminating the need for compression. Nevertheless, a series of challenges hinder the large-scale commercial implementation of PEM electrolysers. One of the most critical drawbacks is the need for noble metal electrocatalysts, especially at the anode of the cell, such as iridium or ruthenium, to catalyze the oxygen evolution reaction (OER) (1).2H2O → O2 + 4H+ + 4e- E° = 1.23 V vs NHE (1)These metals are used due to their high catalytic activity and long-term stability in the corrosive acidic environment of the PEM cells and their replacement by more available materials is difficult. Over the years, the main efforts of the scientific community towards solving this challenge were focused on increasing the noble metal dispersion, by using various high surface area catalyst supports, using core-shell catalysts, with the aim to minimize the amount of noble metal, modifying the morphology of the electrode, to improve mass transport limitations and electric contact between components and maximize noble metal utilization.2-4 Antimony-doped tin oxide has been reported as a stable OER catalyst support, offering sufficient electronic conductivity at the same, in order to minimize Ohmic losses.5 Furthermore, recent reports indicate that SnO2 does not only act as a catalyst support but takes part in catalysis.6 Building upon literature reports, we prepared a series of Sn/Ir OER catalysts by a two-step sodium borohydride reduction method, in which Sn nanoparticles were deposited over the surface of iridium black particles, decorating it. Catalysts with varying Sn content were incorporated in membrane electrode assemblies (MEAs) and their cell performance was evaluated in a single cell PEM electrolyser. We found that catalysts with up to 20 wt% tin content exhibited an improved cell performance compared to MEAs with similar iridium black loading. We modeled Ohmic, kinetic and mass transport overpotential contributions to experimentally acquired polarization curves and identified that the better cell performance arises mainly from the higher intrinsic catalytic activity, reflected by a higher apparent exchange current density of Sn/Ir OER catalysts compared to Ir black catalysts. The Sn/Ir MEAs also displayed lower mass transport overpotentials, which indicate improved transport in the catalyst layer. This can be due to a more porous structure and less agglomeration of iridium particles induced by the presence of tin over their surface.Interestingly, this performance improvement was only observed for iridium black catalysts. We prepared a series of IrO2-SnO2 catalysts by solution combustion synthesis method, which is known to yield high surface area disperse oxide materials. Comparing the performance of these catalysts to commercial iridium oxide catalysts in the PEM electrolysis cell, the MEAs with SnO2-containing catalysts displayed worse cell performance than MEAs with commercial iridium oxide. Modelling the overpotential contributions showed a lower apparent exchange current density, indicating a lower intrinsic catalytic activity of IrO2-SnO2 catalysts, as well as a higher Ohmic resistance, due to SnO2 addition. These observations indicate the importance of iridium oxidation state for harnessing the promotion effect of tin addition and give important insights into designing more efficient OER catalysts for PEM electrolysis cells.

Composite polymer electrolyte membrane decorated with porous titanium oxide nanotubes for fuel cell operating under low relative humidity

A block copolymer composite membrane of sulfonated poly(arylene ether sulfone ketone) (SPESK) decorated with porous hygroscopic titanium oxide nanotubes (TNT) is designed and fabricated for low relative humidity (RH) operating polymer electrolyte fuel cells (PEFCs). The SPESK-TNT composite membrane in a PEFC operated at 100% RH, and 80 °C resulted in about 1.3 and 1.1-folds higher power density in comparison with the benchmark Nafion (NRE-212) and pristine SPESK membranes at 0.6 V. Further, operating under 30% RH and 80 °C, the SPESK-TNT composite membrane generates 3.8 and 2.8-folds improved power density in comparison with the pristine SPESK and commercial NRE-212 membranes, respectively, at 0.6 V. An outstanding improvement of PEFC performance using SPESK-TNT composite membrane also maintains relative to a SPESK-TiO2 nanoparticles composite membrane, operating under 100 and 30% RH. Moreover, the SPESK-TNT composite membrane shows a stable operating potential of more than 200 h at 30% RH and 80 °C, confirming the durable PEFC operation. The enhanced PEFC performance under dry conditions is mainly the result of improving water management by the TNT filler in the membrane and cathode catalyst utilization, yielding significantly suppressing both ohmic and mass transport overpotentials.

Bubble growth and departure modes on wettable/non-wettable porous foams in alkaline water splitting

<h2>Summary</h2> Bubble growth and departure are ubiquitous phenomena in gas-evolving reactions, which govern the overall energy and mass transport. However, an in-depth understanding of the relationship between bubble dynamics and the electrochemical processes, in particular, the wettability effect on a gas-evolving porous electrode remains elusive. Here, we report the bubble dynamics and overpotential observed during alkaline water splitting on a polytetrafluoroethylene (PTFE) deposited nickel porous electrode. A slight decrease in hydrophilicity induced a drastic transition of bubble dynamics and a significant increase of the transport overpotential. We show that the porous electrode transitioned from a liquid-filled state to a gas-filled state when varying the wettability, which changed the bubble departure sizes and bubble coverage. As a result, there were substantial changes of the transport overpotential. Our work elucidates the fundamental relationship between wettability and water splitting characteristics, which provides a practical scenario for structuring the electrode for gas-evolving reactions.

Advancements and prospects of thermal management and waste heat recovery of PEMFC

• The thermal management strategies of PEMFC are reviewed. • The waste heat recovery pathways of PEMFC are presented. • The challenges and prospects of the aforementioned areas are discussed. Despite that the Proton Exchange Membrane Fuel Cell (PEMFC) is considered to be an efficient power device; around half of the energy produced from the electrochemical reaction is dissipated as heat due to irreversibility of the cathodic reaction, Ohmic resistance, and mass transport overpotentials. Effective heat removal from the PEMFC, via cooling, is very important to maintain the cell/stack at a uniform operating temperature ensuring the durability of the device as excessive operating temperature may dry out the membrane and reduces the surface area of the catalyst hence lowering the performance of the cell. In addition to cooling, capturing the produced heat and repurposing it using one of the Waste Heat Recovery (WHR) technologies is an effective approach to add a great economic value to the PEMFC power system. Global warming, climate change, and the high cost of energy production are the main drivers to improve the energy efficiency of PEMFC using WHR. This paper presents an overview of the recent progress concerning the cooling strategies and WHR opportunities for PEMFC. The main cooling techniques of PEMFCs are described and evaluated with respect to their advantages and disadvantages. Additionally, the potential pathways for PEMFC-WHR including heating, cooling, and power generation are explored and assessed. Furthermore, the main challenges and the research prospects for the cooling strategies and WHR of PEMFCs are discussed.

Impact of electrochemical and mechanical interactions on lithium-ion battery performance investigated by operando dilatometry

The demand for higher energy densities in lithium-ion batteries leads to an increased utilization of the space within the confinements of the cell housing for the electrodes, resulting in increased electrochemical/mechanical interactions and stress inside the cells. In this study, the correlating effects of externally-induced mechanical stress and dilation of the electrodes on the performance of LIBs were investigated using an operando three-electrode dilation cell. The results demonstrate that most of the initial irreversible dilation in a graphite/LiNi0.33Co0.33Mn0.33O2 (NCM111) cell occurs during the initial lithium intercalation at the anode during the transition from stage 2 to 1, i.e. LiC12 to LiC6, due to SEI formation, particle rearrangement and graphene layer spacing. Moreover, high applied pressure, which leads to a reduction of the porosity inside the separator and therefore increased ionic transport resistance, tortuosity and overpotential, results in a hastened degradation mainly at graphite anode. These effects are more pronounced with an inhomogeneous pressure distribution and lithium plating is identified as the main cause of degradation. This dilation cell is a powerful tool for studying the electrode/active material expansion and electrochemical/mechanical effects for a homogeneous and inhomogeneous pressure distribution.

Microstructure and Pressure-Driven Electrodeposition Stability in Solid-State Batteries

Interfacial deposition stability at the Li-metal-solid electrolyte interface in all solid-state batteries is governed by the stress-transport-electrochemistry coupling in conjunction with the polycrystalline/amorphous solid electrolyte architecture. In this work, we delineate the optimal solid electrolyte microstructure comprising grains, grain boundaries, and voids possessing desirable ionic conductivity and elastic modulus for superior transport and strength. An analytical formalism is provided to discern the impact of external “stack” pressure-induced mechanical stress on electrodeposition stability; the stress magnitudes obtained are in the megapascal range, considerably diminishing the stress-kinetics effects. For experimental stack pressures ranging up to 10 MPa, the impact of stress on reaction kinetics is negligibly small, and electrolyte transport overpotentials dictate electrodeposition stability. High current density operation with stable deposition can be ensured with ample external pressure, high temperature, and low surface roughness operation for a low shear modulus ratio of the solid electrolyte to Li-metal. Solid electrolyte architecture is screened via virtual generation and characterization Large grain size, low void size, and porosity provide favorable conduction and stiffness Stack pressure <10 MPa has negligible impact on stress-kinetics coupling High operating pressure, temperature, and low roughness can aid stable deposition Lithium electrodeposition is dependent on solid electrolyte microstructure, stack pressure, and operating temperature. Verma et al. suggest a method to delineate the architecture and operating conditions that lead to stable deposition.

Layer Design for PGM-Free Catalysts for Pemfcs: Impact of Electrical Conductivity & Diagnostic Tools

Recently, the notable efforts placed on investigating platinum group metal (PGM)-free catalysts (M-N-C catalysts, M = Fe, Co, & Mn) for the oxygen reduction reaction (ORR) have led to the possible development of low-cost proton exchange membrane fuel cells (PEMFCs).1 However, compared to PGM ORR catalysts, the lower turnover frequency and active site density of PGM-free catalysts generally require high-surface-area structures and high catalyst loadings to achieve reasonable low-current density performance in PEMFCs, thereby resulting in significant ionic, mass, and electronic transport overpotentials. 2 With regards to these transport resistances, an intriguing observation from the PEMFC performance with PGM-free cathode catalysts is the significantly higher electronic overpotential (iR-drop) when compared to PGM-based cathode catalysts (e.g., Pt/C). This increased iR-drop results in an increase of the high frequency resistance (HFR) due to the high electrical resistivity (ρe- in Ω/cm) of the thick PGM-free cathode catalyst layers. While ρe- of conventional Pt/C electrodes is rather low (~0.5-1 Ω/cm) and results in negligible overpotential losses, ρe- of PGM-free cathode layers can be higher by a factor of ~10-20 due to poor electrical conductivity of the active site (e.g., Fe-N-C) and the high-surface area structures of these catalysts. While several studies have revealed the impact of ionic and mass transport resistances of PGM-free cathode electrodes,3,4 the impact of electrical resistance is still unclear. A quantification of this effect is required for the rational design of PGM-free cathode electrodes, whereby the following three points would be of major interest: (1) The values of PGM-free catalysts vary widely, so that in a comparative study catalysts with high and low should be investigated. (2) When preparing electrodes with PGM-free catalysts that have a high value ρe-, conductive-carbon additives are commonly added, but there is no simple diagnostic tool that could be used for optimizing the amount of additive. (3) A quantitative model to predict the cell voltage loss due to the impact of ρe- would be desired. This can be accomplished by in-situ AC impedance analysis based on a transmission line model. 5 However, owing to higher ratio of ρe- to the ionic resistivity (ρH+) in PGM-free electrodes (>0.1/1), a modified approach needs to be developed to precisely quantify the voltage losses.6 In the present work, we present small-active area (5 cm2) single-cell H2/O2 PEMFC performance (differential-flow operation) combined with in-situ AC impedance and ex-situ 4-point-probe resistance measurement in order to elucidate the impact of the electrical resistivity of PGM-free cathode catalyst layer on performance. Two different catalysts with low and high ρe- are investigated. The results indicate that the lower one shows no significant change in cell voltage upon the addition of 7.5 wt% of conductive carbon (Figure 1a). However, with same wt% of carbon additive, the cell voltage for catalyst with high ρe- can be increased by 70 mV at 1 A/cm2, resulting in a ~30% improvement in peak power density (Figure 1b). Furthermore, we will present an in-situ AC impedance technique under blocking condition (N2 feed to cathode) that we developed to quantify ρe- and ρH+ of PGM-free cathode electrodes with and without carbon additive. Using these values, the cell-voltage gains due to the reduction of ρe- of a PGM-free cathode by carbon additive can be predicted (Figure 1c). At the end, a comparison between in-situ AC-impedance and ex-situ 4-point-probe measurements for PGM-free cathode electrodes will be presented and discussed. Overall, the study not only provides a new insight and methodology for electrode optimization of PGM-free cathode catalysts, but also reveals in some cases show significant impact of electrical conductivity on fuel cell performance with PGM-free catalysts.ACKNOWLEDGEMENTSThis work is supported by the Fuel Cells and Hydrogen Joint Undertaking under grant agreement No. 779550 (PEGASUS). This Joint Undertaking receives support from the European Union’s Horizon 2020 research and innovation programme, Hydrogen Europe and Hydrogen Europe Research.REFERENCE T. Thompson, A. R. Wilson, P. Zelenay, D. J. Myers, K. L. More, K. C. Neyerlin, and D. Papageorgopoulos, Solid State Ionics319, 68-79 (2018)Banham, J. -Y. Choi, T. Kishimoto, and S. Ye, Advanced Materials 31, 1804846 (2019)Malko, T. Lopes, E. A. Ticianelli, and A. Kucernak, Journal of Power Sources 323, 189-200 (2016)G. Wang, L. Osmieri, A. G. Star, J. Pfeilsticker, and K. C. Neyerlin, Journal of The Electrochemical Society 167, 044519 (2020)Liu, M. Murphy, D. Baker, W. Gu, C. Ji, J. Jorne, and H. A. Gasteiger, ECS Transactions 11, 473 (2007)Landesfeind, M. Ebner, A. Eldiven, V. Wood, and H. A. Gasteiger, Journal of The Electrochemical Society 165, 469-476 (2018) Figure 1

Interface Engineering: On the Impact of Micro-Porous Layers on Polymer Electrolyte Water Electrolysis Performance

In polymer electrolyte water electrolysis (PEWE), operational cost represents the main cost fraction. Losses induced by kinetic, ohmic as well as mass transport overpotentials limit cell efficiency and in turn lead to lower volumetric power density and higher energy consumption.The interplay of porous transport layer bulk/surface properties1,2, providing thermal and electrical conductivity as well as fluid management between bipolar plates and catalyst layer, and cell efficiency is a disputed question in literature. Governing design parameters for porous transport layers, as well as fundamental limiting process haven’t been addressed so far This study aims to close the gap between fundamentals and highly efficient PTL microstructure design.A series of four different PTL materials with different MPL types was analysed3. Structural properties, characterized by X-ray tomographic microscopy (XTM) were correlated to in-depth electrochemical analysis. Performance of the novel PTLs was probed by comparison to state-of-the-art PTL materials and stack technology. The superior surface properties of the microporous-layer enables the use of thin membranes with Ti PTLs for the first time. References S. A. Grigoriev, P. Millet, S. A. Volobuev and V. N. Fateev, International Journal of Hydrogen Energy, 2009, 34, 4968-4973.T. Schuler, R. De Bruycker, T. J. Schmidt and F. N. Büchi, Journal of The Electrochemical Society, 2019, 166, F270-F281.T. Schuler, J.M. Ciccone, B. Krentscher, F. Marone, C. Peter, T.J. Schmidt, and F.N. Büchi, Hierarchically Structured Porous Transport Layers for Polymer Electrolyte Water Electrolysis, Advanced Energy Materials, 2020, 10, 1903216. Figure 1

Spatially-Resolved Current Distribution Measurements in Polymer Electrolyte Water Electrolyzers

Operational cost, primarily power consumption, is one of the main cost obstacles for hydrogen produced by polymer electrolyte water electrolyzers (PEWE). Roughly 70% of the hydrogen production cost stems from electricity [2]. Thus, overall process efficiency has a direct influence on the hydrogen price. Dominating sources of electrochemical loss in PEWE cells are the kinetic, electric, and mass transport overpotentials. Ex-situ and modeling work have yielded insights into the physical processes occurring within an operating PEWE, but there is still need for in-situ measurements for verification [3]. Work carried out by Zhang et al. [4, 5] has successfully shown the bubble formation of water electrolysis inside a PEWE by means of an in-situ, high-speed micro-scale visualization system (HMVS). However the HMVS does not explicitly capture the local electrochemical behavior of the cell. Therefore, complex CFD models have been implemented to shed light on the inner workings of PEWE; such models need to be validated with experimental data. This work aims to provide a reliable source of experimental data by developing a technique that allows for in-situ spatial and temporal current distribution measurements, which can be used to validate existing modeling efforts.Current distribution measurements are described extensively in the works of Bender et al. [6, 7] and Mench et al. [1, 8]; a major innovation was the inclusion of a printed-circuit board (PCB) with an array of shunt resistors attached to each segment as shown in Figure 1a (4.5 mm x 4.5 mm segments in this work). The measured voltage drop across each shunt resistor is used to calculate the current in each segment. Passive data acquisition on the distributed current board is done simultaneously to electrochemical control of the electrolyzer, without interrupting its operation. This apparatus is capable of using flow fields of both 5cm2 (16 segments) and 25cm2 active area (100 segments). The distributed current is then interpolated to obtain smoothed contours as shown in Figure 1b. Preliminary current distribution experiments were carried out at 80⁰C with a water flow rate of 0.5 to 15 mL/min/cm2. Experiments were also carried out with two types of porous transport layers from Zhang and co-workers [4, 5]: thin titanium foil LGDLs with 80 nm pore size and Ir-coated titanium felt. Two types of flow field where also used – parallel and triple serpentine. Current distributions were found to respond to transport layer porosity and thickness, flow field design, flow rate, and current density. A non-uniform current distribution is suspected to be ultimately dominated by mass transport characteristics and these experiments support that. Additionally, conditions that yielded near-identical polarization performance were found to have disparate current distributions. This work demonstrates the benefit of a spatially-resolved current distribution measurement technique and the insights it offers which would otherwise be impossible to probe. This work is part of DOE project #DE-EE0008426 "Developing novel electrodes with ultralow catalyst loading for high-efficiency hydrogen production in proton exchange membrane electrolyzer cells." References Clement, J.T., D.S. Aaron, and M.M. Mench, In Situ Localized Current Distribution Measurements in All-Vanadium Redox Flow Batteries. Journal of The Electrochemical Society, 2015. 163(1): p. A5220-A5228.Grigoriev, S., V. Porembsky, and V. Fateev, Pure hydrogen production by PEM electrolysis for hydrogen energy. International Journal of Hydrogen Energy, 2006. 31(2): p. 171-175.Schuler, T., T.J. Schmidt, and F.N. Büchi, Polymer Electrolyte Water Electrolysis: Correlating Performance and Porous Transport Layer Structure: Part II. Electrochemical Performance Analysis. Journal of The Electrochemical Society, 2019. 166(10): p. F555-F565.Mo, J., et al., Discovery of true electrochemical reactions for ultrahigh catalyst mass activity in water splitting. Science Advances, 2016. 2(11): p. e1600690.Li, Y., et al., Wettability effects of thin titanium liquid/gas diffusion layers in proton exchange membrane electrolyzer cells. Electrochimica Acta, 2019. 298: p. 704-708.Bender, G., M.S. Wilson, and T.A. Zawodzinski, Further refinements in the segmented cell approach to diagnosing performance in polymer electrolyte fuel cells. Journal of Power Sources, 2003. 123(2): p. 163-171.Reshetenko, T.V., et al., A segmented cell approach for studying the effects of serpentine flow field parameters on PEMFC current distribution. Electrochimica Acta, 2013. 88: p. 571-579.Ertugrul, T.Y., et al., In-situ current distribution and mass transport analysis via strip cell architecture for a vanadium redox flow battery. Journal of Power Sources, 2019. 437: p. 226920. Figure 1

Impact of Varying Bipolar Plates Having Flow-Fields on the Performance of Redox Flow Battery at High Current Density

Redox flow Cell (RFC) stores electricity in the form of chemical energy into the electrolyte. The interface of the electrode surface and electrolyte is responsible for redox reaction and resultantly increase in the state-of-charge (SOC) during charging of the cell. Dispersion of the electrolyte into the electrode (porous carbon felt) and flowrate are important parameters for the performance of the cell. A bipolar plate having flow-field is responsible to disperse electrolyte. Two different configurations of interdigitated flow-field (IFF) and serpentine flow-fields (SFF) were fabricated on the impregnated graphite bipolar plates. The performance was compared with RFC having a conventional bipolar plate without any flow-field.The as-received Fumatech FAP-450 anion exchange membrane was used. All membranes were cleaned in 1M H2SO4 solution at 80oC and stored in DI water. The as-received commercial electrolyte consisted of vanadium solution (1.6 M) was used. Graphite felt electrode (GFE) having a thickness of 4.6 mm was used with a compression ratio of 25%. GFE was boiled in 1M H2SO4 for 12 hours and kept in the same solution for 12 hours. These were washed with DI water and dried at 100oC. Afterward, GFE was thermally treated at 390oC for 12 hrs inside the furnace injected by air and then cooled down in the furnace up to room temperature. Membranes and GFE were used from the same batch for each experiment.The performance of RFC was characterized using charge-discharge cycling (CDC), impedance curves (EIS), and polarization (iV-curve). EIS performed in a range of 100mHz to 50kHz having AC perturbation of 10mV. RFC was charged from SOC of 25% to 75%. It was ensured by recording open-circuit-potential (OCP) after each cycle and the measurement of OCP was co-related with SOC values computed based on concentrations. This allows us to set the cut-off potential from 0.8V to 2.0V and it helps to remain in the safe SOC limits to avoid hydrogen evolution.EIS results of conventional (CFF), IFF, and SFF at the flowrate of 40ml.min-1 were shown in Figure 1(a). The introduction of the new bipolar plates having flow-fields has a significant effect on the area-specific resistance (ASR) which was decreased and charge transfer resistance (CTR) was almost similar. The equivalent circuit analysis of the impedance curve was shown in Figure 1(b). Ohmic & charge transfer resistance of the cell having IFF and SFF is less than conventional whereas capacitance highlighted by CPE is almost double. The effect of varying flowrate on EIS results was tested on the IFF as shown in Figure 1(c) and have a negligible effect. CDC was performed using each flow-field to assess the performance of RFC. Impact of flow-field on the capacity as a function of potential at 120mA.cm-2 having 40ml. min-1 is shown in Figure 1(d) and inset showed the average efficiency of 10 cycles. RFC having IFF reached to higher capacity and overall energy efficiency (EE) of this arrangement is also higher than SFF. However, it was observed that columbic efficiency (CE) is low, but it was the effect of membrane aging after repeated cycles. This observation was eliminated in future studies by using a fresh membrane. The impact of varying flowrate was on the capacity and EE of the RFC having IFF at 80 mA.cm-2 was shown in Figure 1(e). Best capacity and EE results were obtained by using IFF at 20 ml.min-1.Therefore, CDC was performed to check the impact of varying current density having IFF at 20ml.min-1 as shown in Figure 1(f). The current density ranging from 40 to 155mA.cm-2 was tested and EE was decreased from 85% to 60% after increasing the current density up to four times. The CE remains the same which means that no side reactions and least crossover. The voltaic efficiency (VE) was kept on decreasing gradually as the current density was increasing. To further investigate this effect polarization analysis was performed to get limiting current density and peak power as shown in Figure 1(g). It highlights that this arrangement can reach up to a higher current density of 410mA.cm-2 and peak power of 56.25W.l-1 for an RFC having 80ml of electrolyte. Overpotentials were shown in Figure 1(h). It signifies that activation overpotential is negligible, ohmic overpotential is minimum at 10ml.min-1, and mass transport overpotential is minimum at 20ml.min-1.This study highlights that optimization of bipolar plate is significant and has a considerable impact on the performance. It is not a straight-forward isolated process and future researchers need to decide about many trade-offs while optimizing even a single component of RFC. Figure 1

(Invited) Development of Polymer Electrolyte Membrane Fuel Cell Models for Transportation Applications Using Sequence of Direct Simulation Methods

Augmentation of fuel cell performance at high current densities, particularly under automotive application, is important to improve the overall power density and to reduce the cost of proton exchange membrane fuel cell (PEMFC) systems. Primarily, the fundamental non-linearity of the equations governing PEMFC performance on a three-dimensional model necessitates iterative solutions. As of now, mass transport over-potential is a major barrier to achieving high performance at a high current density. Condensed water, specifically in the gas diffusion layer, decreases oxygen transport to the oxygen reduction reaction area. Experimental investigations of oxygen transport are limited by an inability to resolve the water saturation-dependent properties.The alternative approach to comprehend and overcome transport resistances, predominantly inside the gas diffusion layer, is to use advanced mathematical modeling. The timeline of direct simulation techniques using in PEMFC will be presented including the Lattice Boltzmann method (LBM) which is an alternative advanced modeling technique [1-12]. This LBM can be implemented into existing models to visualize the transports inside the structure of microscale geometries, such as: a gas diffusion layer (GDL), micro porous layer (MPL), and catalyst layer (CL). The study where the goal was to couple the conventional computational fluid dynamics (CFD) model in the macroscale geometry and LBM in the microscale geometry using the technique called co-simulation will be presented. The understanding of local kinetic activity inside nanoscale CL implemented into the model will also be discussed. The completion of this model development can enhance the potential capability of a model-based investigation of mass and heat transports to find solution of designs and operational conditions in the PEMFC for automotive applications.

Electrospinning as a route to advanced carbon fibre materials for selected low-temperature electrochemical devices: A review

Electrospinning is a versatile fabrication method for synthesising mesostructured nanofibrous mats and has been explored in this review for the following electrochemical energy storage devices: redox flow batteries, fuel cells, metal air batteries and supercapacitors. Electrospinning has been proven as a highly versatile fabrication method for producing nano-structured fibres with controllable morphology, of both the fibres themselves and the void structure of the mats. Additionally, it is possible to use heteroatom doped polymers or to include catalytic precursors in the electrospinning solution to control the surface properties of the fibres. These factors make it an ideal method for the production of electrodes and flow media for a variety of electrochemical devices, enabling reduction in mass transport and activation overpotentials and therefore increasing efficiency. Moreover, the use of biomass as a polymer source has recently gained attention for the ability to embed sustainable principles in the materials of electrochemical devices, complementing their ability to allow an increase in the use of renewable electricity via their application. In this review, the historical and recent developments of electrospun materials for application in redox flow batteries, fuel cells, metal air batteries and supercapacitors are thoroughly reviewed, including an overview of the electrospinning process and a guide to best practice. Finally, we provide an outlook for the emerging use of this process in the field of electrochemical energy devices with the hope that the combination of tailored microstructure, surface functionality and computer modelling will herald a new era of bespoke functional materials that can significantly improve the performance of the devices in which they are used.

Spatially graded porous transport layers for gas evolving electrochemical energy conversion: High performance polymer electrolyte membrane electrolyzers

Decarbonizing society’s energy infrastructure is foundational for a sustainable future and can be realized by harnessing renewable energy for clean hydrogen and on-demand power with fuel cells. Here, we elucidate how graded porous transport layers (PTLs) are instrumental for high performance gas evolving electrochemical energy conversion devices, with an emphasis on polymer electrolyte membrane (PEM) electrolyzers. Spatially graded PTLs fabricated by vacuum plasma spraying are examined via in operando neutron imaging, electrochemical characterization, and pore network modelling. The results reveal the staggering benefits of positioning the lower porosity region adjacent to the catalyst layer and the higher porosity region adjacent to the flow field, which lead to current densities up to 4.5 A/cm2 with a 29 % reduction in cell potential, 38 % reduction in mass transport overpotential, and 50 % reduction in PTL gas saturation. The liquid water permeability of the PTL also enhances by an order of magnitude, with a drastic reduction in gas saturation adjacent to the catalyst layer. Custom graded PTLs have the potential to transform performance levels for a broad array of gas evolving electrochemical energy conversion devices.

Large-Scale Coupling Numerical Simulation of Two-Phase Flow and Electrochemical Phenomena in Alkaline Water Electrolysis

The alkaline water electrolysis is one of large-scale hydrogen production methods. While a lot of experiments and numerical simulations have been conducted to elucidate the internal phenomena and to improve the efficiency of the alkaline water electrolysis, there still is room for improvement in the efficiency. In this study, large-scale coupling numerical simulations of two-phase flow and electrochemical phenomena are conducted to elucidate the influence of the bubble-bubble interactions on ion transport and cell overpotential. The results showed that the interaction between bubbles affects cell overpotential, and cell overpotential can be suppressed when bubbles are optimally dispersed.

Critical Current Density as a Performance Indicator for Gas-Evolving Electrochemical Devices

Reaching high current densities is absolutely imperative for electrochemical energy conversion, from fuel cells to CO2 reduction. Here, we identify the existence of a performance indicator for gas-evolving electrochemical energy conversion devices: the critical current density. The critical current density pinpoints a performance inflection point whereby both the gas saturations and mass transport overpotentials suddenly dominate cell performance and exacerbate failure. We elucidate the mass transport behavior of a polymer electrolyte membrane (PEM) water electrolyzer using in operando neutron imaging at operating current densities as high as 9 A ⋅ cm−2. Product gases become heterogeneously distributed in the porous transport layer adjacent to the catalyst layer and promote disastrous local hotspots. Optimizing new materials and cell architectures with this performance indicator may unlock higher than previously reported performances for electrochemical energy conversion.

Interface Engineering: On the Impact of Micro-Porous Layers on Polymer Electrolyte Water Electrolysis Performance

In polymer electrolyte water electrolysis (PEWE), operational cost represents the main cost fraction. Losses induced by kinetic, ohmic as well as mass transport overpotentials limit cell efficiency and in turn lead to lower volumetric power density and higher energy consumption.The interplay of porous transport layer bulk/surface properties1,2, providing thermal and electrical conductivity as well as fluid management between bipolar plates and catalyst layer, and cell efficiency is a disputed question in literature. Governing design parameters for porous transport layers, as well as fundamental limiting process haven’t been addressed so far. This study aims to close the gap between fundamentals and highly efficient PTL microstructure design. A novel class of hierarchically structured porous transport layers is fabricated based on sintering of economically viable Ti powder, see Figure 1..A series of four different PTL materials including three different MPL types was analysed. Structural properties, characterized by laboratory and synchrotron based X-ray tomographic microscopy (XTM) were correlated to in-depth electrochemical analysis. Performance of the novel PTLs was probed by comparison to state-of-the-art PTL materials and stack technology. The superior surface properties of the microporous-layer enabled the use of ultra-thin membranes with Ti PTLs for the first time. To elucidate the MPL effect and reveal the mechanism of partial catalyst utilization3,4, the catalyst layer conductivity was studied as function of hydration state and ionomer content.Reference1 S. A. Grigoriev, P. Millet, S. A. Volobuev and V. N. Fateev, International Journal of Hydrogen Energy, 2009, 34, 4968-4973.2 T. Schuler, R. De Bruycker, T. J. Schmidt and F. N. Büchi, Journal of The Electrochemical Society, 2019, 166, F270-F281.3 Z. Kang, J. Mo, G. Yang, S. Retterer, D. Cullen, T. Toops, J. Green Jr, M. Mench and F.-Y. Zhang, Energy and environmental science, 2017, 10, 166-175.4 T. Schuler, T. J. Schmidt and F. N. Büchi, Journal of The Electrochemical Society, 2019, 166, F555-F565.Figure 1. XTM rendering of hierarchically structured porous transport layer. Figure 1

Spatially-Resolved Measurement of Current Distribution in Polymer Electrolyte Water Electrolyzers

Operational cost, primarily power consumption, is the one of the main obstacles for hydrogen produced by polymer electrolyte water electrolyzers (PEWE). Roughly 70% of the hydrogen production cost stems from electricity [1], thus overall electrochemical efficiency has a direct influence on the hydrogen price. Dominating sources of electrochemical loss in PEWE cells are the kinetic, electric, and mass transport overpotentials. Conventional research and diagnostic techniques, such as polarization curves and EIS measurements, predominantly probe macroscopic performance characteristics without any spatial resolution of electrochemical performance. These offer limited insight into the in-situ behaviors of the PEWE and its overpotentials. The work of Schuler, et al. [2] gives a detailed understanding of transport mechanisms inside a PEWE by drawing extensively on ex-situ techniques. Work carried out by Zhang et al. [3] has successfully shown the bubble formation of water electrolysis inside a PEWE by means of an in-situ, high-speed micro-scale visualization system (HMVS). However the HMVS does not explicitly capture the local electrochemical behavior of the cell. Therefore, complex CFD models have been implemented to shed light on the inner workings of PEWE; while insightful, such models need to be validated with experimental data. This work aims to provide a reliable source of experimental data by developing a technique that allows for in-situ spatial and temporal current distribution measurements which can be used to validate existing modeling efforts.Current distribution measurements are described extensively in the works of Bender et al. [4, 5]; a major innovation was the inclusion of a printed-circuit board (PCB) with an array of shunt resistors attached to each segment as shown in Figure 1a (4.5 mm x 4.5 mm segments in this work). The measured voltage drop across each shunt resistor is used to calculate the current in each segment. Data acquisition on the distributed current board is done simultaneously to electrochemical control of the electrolyzer. Thus the distributed current measurement occurs passively while a potentiostat controls the electrolyzer, without interrupting its operation. This setup is capable of using flow fields of both 5cm2 and 25cm2 active area. The 5 cm2 flow field has 16 segments while the 25cm2 has 100 segments that are electronically isolated, yielding high spatial resolution of the current distribution. The distributed current is then interpolated to obtain smoothed contours as shown in Figure 1b and 1c. Preliminary current distribution experiments were carried out at 85⁰C, water flow rate of 0.5 to 3 mL/min/cm2, at a cell compression under bolt torques of 5 Nm and 10 Nm. Experiments were also carried out with two sets of liquid-gas diffusion layers: iridium-coated titanium and bare titanium felts. The results show a relatively uniform current distribution across the electrolyzer with no significant impact of flow rate. The experiments also revealed that the spatial current distribution is highly sensitive to local cell compression, which is not captured in the overall cell performance of a polarization curve. This work demonstrates the benefit of a spatially-resolved current distribution measurement technique and the insights it offer which would otherwise be impossible to probe.

Reconciling temperature-dependent factors affecting mass transport losses in polymer electrolyte membrane electrolyzers

In this work, we investigated the impact of temperature on two-phase transport in low temperature (LT)-polymer electrolyte membrane (PEM) electrolyzer anode flow channels via in operando neutron imaging and observed a decrease in mass transport overpotential with increasing temperature. We observed an increase in anode oxygen gas content with increasing temperature, which was counterintuitive to the trends in mass transport overpotential. We attributed this counterintuitive decrease in mass transport overpotential to the enhanced reactant distribution in the flow channels as a result of the temperature increase, determined via a one-dimensional analytical model. We further determined that gas accumulation and fluid property changes are competing temperature-dependent contributors to mass transport overpotential; however, liquid water viscosity changes led to the dominant enhancement of reactant water distributions in the anode. We present this temperature-dependent mass transport overpotential as a great opportunity for further increasing the voltage efficiency of PEM electrolyzers.

In-Plane Transport in Water Electrolyzer Porous Transport Layers with Through Pores

The mass transport in porous transport layers (PTLs) with pores exclusively in the through-plane direction was investigated using concurrent in operando X-ray radiography and electrochemical performance analysis. We observed via synchrotron X-ray imaging that through pores situated under the lands are inaccessible to liquid water. We thereby observed the limited in-plane mass transport that takes place in PTLs with pores exclusively in the through-plane direction. Additionally, a higher content of product gas was observed with the use of the PTL with through pores under both the channels and the lands (PTLCh,L) when compared to the PTL with through pores only under the channels (PTLCh). This oxygen gas accumulation behaviour corresponded to the higher mass transport overpotential of the PTLCh,L compared to the PTLCh. Finally, the limited in-plane mass transport in the PTL with through pores led to a relatively dehydrated catalyst layer, which was exhibited through higher ionic resistances as a function of increasing current density.

Method for Comparing Porous Carbon Electrode Performance in Redox Flow Batteries

We present a method for comparing porous electrodes for use in redox flow batteries. By comparing physical and electrochemical properties of six carbon cloth electrodes (CCEs) including hydraulic permeability, Ohmic resistance, and mass transport overpotential, we show how optimal engineering conditions such as compression and electrolyte flow rates can be derived. These results are combined and applied to a quinone-based redox flow battery, leading to a peak power density around 0.4 W cm−2 for several different CCEs. This method can be applied broadly to other electrode architectures and flow battery chemistries, and provides a comprehensive framework for their comparison.