Measurement Of Current Distribution Research Articles

Design and Development of Flow Fields with Multiple Inlets or Outlets in Vanadium Redox Flow Batteries

In vanadium redox flow batteries, the flow field geometry plays a dramatic role on the distribution of the electrolyte and its design results from the trade-off between high battery performance and low pressure drops. In the literature, it was demonstrated that electrolyte permeation through the porous electrode is mainly regulated by pressure difference between adjacent channels, leading to the presence of under-the-rib fluxes. With the support of a 3D computational fluid dynamic model, this work presents two novel flow field geometries that are designed to tune the direction of the pressure gradients between channels in order to promote the under-the-rib fluxes mechanism. The first geometry is named Two Outlets and exploits the splitting of the electrolyte flow into two adjacent interdigitated layouts with the aim to give to the pressure gradient a more transverse direction with respect to the channels, raising the intensity of under-the-rib fluxes and making their distribution more uniform throughout the electrode area. The second geometry is named Four Inlets and presents four inlets located at the corners of the distributor, with an interdigitated-like layout radially oriented from each inlet to one single central outlet, with the concept of reducing the heterogeneity of the flow velocity within the electrode. Subsequently, flow fields performance is verified experimentally adopting a segmented hardware in symmetric cell configuration with positive electrolyte, which permits the measurement of local current distribution and local electrochemical impedance spectroscopy. Compared to a conventional interdigitated geometry, both the developed configurations permit a significant decrease in the pressure drops without any reduction in battery performance. In the Four Inlets flow field the pressure drop reduction is more evident (up to 50%) due to the lower electrolyte velocities in the feeding channels, while the Two Outlets configuration guarantees a more homogeneous current density distribution.

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 .

Degradation in parallel-connected lithium-ion battery packs under thermal gradients

Practical lithium-ion battery systems require parallelisation of tens to hundreds of cells, however understanding of how pack-level thermal gradients influence lifetime performance remains a research gap. Here we present an experimental study of surface cooled parallel-string battery packs (temperature range 20–45 °C), and identify two main operational modes; convergent degradation with homogeneous temperatures, and (the more detrimental) divergent degradation driven by thermal gradients. We attribute the divergent case to the, often overlooked, cathode impedance growth. This was negatively correlated with temperature and can cause positive feedback where the impedance of cells in parallel diverge over time; increasing heterogeneous current and state-of-charge distributions. These conclusions are supported by current distribution measurements, decoupled impedance measurements and degradation mode analysis. From this, mechanistic explanations are proposed, alongside a publicly available aging dataset, which highlights the critical role of capturing cathode degradation in parallel-connected batteries; a key insight for battery pack developers.

Operando analysis of in-plane heterogeneity for the PEM electrolyzer cell: Mappings of temperature and current density

The internal multi-physical fields of a proton exchange membrane electrolyzer cell (PEMEC) are affected by various factors, exhibiting an intricate coupling state. During electrolysis process, the complexity manifests as heterogeneous distribution of in-plane temperature and current density, potentially leading to local hot spots and performance degradation. Such challenges pose threats to the lifespan and operational safety of PEMEC. This study developed temperature measurement endplates and segmented printed circuit board (PCB), enabling operando measurement of in-plane temperature and current density distribution inside a 25 cm2 lab-scale PEMEC. The mappings of temperature and current density were plotted, and the in-plane heterogeneity was assessed by the introduced indexes. The results indicate that variations in operational parameters affect the balance among cell heat production, heat dissipation, and heat input in the flow area, leading to temperature distribution heterogeneity. The migration mechanism of high-temperature zone is elucidated. Factors such as water-gas flow and local conduction contribute to the heterogeneity in current density distribution. Additionally, specific operating conditions including cold and warm start-up, and water starvation are taken as cases, and a detailed analysis is conducted on the dynamic behaviors of current and temperature distribution and the heterogeneity evolution. This work not only provides experimental evidence for the electrical-thermal characteristics inside the PEMEC but also offers insights for local performance evaluation and operation fault diagnosis in practical applications.

Development and assessment of simplified analytical method for current distribution among REBa2Cu3O y parallel conductors in armature windings for fully superconducting rotating machines

Fully superconducting rotating machines employing REBa2Cu3O y (REBCO, RE = rare earth elements or Y) superconducting armature and field coils, are particularly interesting for aircraft applications, owing to their high output power density (kW kg−1). To achieve high current capability in superconducting coils, we have proposed a cabling design for transposed multi-strand parallel conductors. In the parallel conductor design, the REBCO strands are insulated from each other, except for both terminal ends, and transposed during the winding process to achieve uniform current distribution by cancellation of interlinkage magnetic flux between the strands. In this study, a simplified analytical method considering inductances was developed based on Laplace’s equation in cylindrical coordinates to roughly calculate the current distributions of multi-strands under armature coil conditions. The validity of the analytical method was investigated through current distribution measurements of the sample coils wound with two-strand parallel conductors. Consequently, the analytical method was validated with approximately 10% deviation under the experimental coil conditions. To establish a more accurate analysis method, certain improvements are needed.

Simultaneous measurement of local current and temperature distributions in proton exchange membrane fuel cells with dead-ended anode

The dead-ended anode (DEA) operation could cause local flooding and local hydrogen starvation in proton exchange membrane fuel cells (PEMFCs), resulting in drastic changes in fuel cell performance. In order to study the detailed local dynamic characteristics of DEA operation, simultaneous measurements of the dynamic variation of local current and local temperature in a PEMFC with DEA have been performed using a local current measurement gasket and micro thermocouples. The results show that the local fuel cell performance distribution gradually becomes uneven during DEA operation and the evolutions of local current and temperature strongly depend on the locations along the flow channel. The highest local current and local temperature are observed near the anode inlet, posing serious threats for the local fuel cell. Furthermore, with anode purging, the current distribution becomes uniform quickly due to the fast mass transfer capacity of hydrogen, but the local temperatures take much longer time to become uniform and stable because of the thermal capacity of fuel cell components. The anode purging also causes local temperature undershoots and the amplitude is over 4℃ in this study. Finally, the effects of operating conditions are discussed. The local current and temperature distributions become more uneven as overall operating current increases and operating temperature decreases. The local temperatures are all reduced as anode inlet pressure and air stoichiometry increase due to the enhanced convection cooling effect. It is demonstrated that measuring local current and temperature is effective in investigating local dynamic performance of PEMFCs with DEA.

Measurement of current distribution using infrared thermography.

Current distribution measurement methods are widely used in medical examinations, predicting faults in semiconductor devices and assessing structural integrity. Several methods for measuring current distribution are available, such as electrode arrays, coils, and magnetic sensors. However, these measurement methods are unable to obtain images of current distribution with high spatial resolution. Therefore, there is a need to develop a non-contact method to measure current distribution that is capable of capturing images with high spatial resolution. In this study, a non-contact current distribution measurement method based on infrared thermography is proposed. The method uses thermal variations to quantify the current amplitude and reconstructs the current direction based on the passivity of the electric field. For quantification of low frequency current amplitude, the experimental results show that the method can provide accurate current measurement results, for example, at the power frequency (50Hz), in the range of 1.05-3.45A, its relative error can be improved to ±3.66% when the calibration fitting method is used. For the high-frequency current, an effective estimate of the current amplitude is obtained using the first-order derivative of temperature variation. When applied to the eddy current detection (256 KHz), it achieves a high-resolution image of the current distribution, and the effectiveness of the method is verified through simulation experiments. The experimental results show that the proposed method not only measures the current amplitude accurately but also improves the spatial resolution in acquiring two-dimensional current distribution images.

Highly Accelerated Stress Testing of Proton Exchange Fuel Cell Membranes for Heavy Duty Transportation Applications

According to the US DOE’s multi-year Fuel Cell research, development, and demonstration plan, a 30,000 h lifetime, approximately four times that of the light-duty vehicles (LDV), is required for Heavy Duty Vehicle (HDV) applications [1]. Thus, highly accelerated stress tests are necessary in order to assess the viability to novel membrane concepts within a reasonable amount of time. Previously we have reported the development of a highly accelerated stress test (HAST) to generate local stressful conditions that are representative of those in automotive fuel cell stacks [2]. Using a 50-cm2 cell cycled between 0.05 and 1.2 A/cm2 with a low inlet RH in the co-flow configuration, the HAST creates a distribution of combined mechanical/chemical stressors in the membrane with the maximum chemical stress occurring near the gas inlets and the maximum mechanical stress near the outlets. Conducting HASTs using a current distribution measurement tool and a shorting/crossover diagnostic method to track the state of health of a state-of-the-art membrane containing both a mechanical support and a cerium-containing chemical stabilizing additive, the result shows that the membrane location with the most severe thinning coincides with that of the deepest membrane hydration cycling. Upon examination of the cerium redistribution patterns after the test, it was found that the severe humidity cycling generated by the HAST condition near the outlet region not only generated the highest membrane mechanical stress but also resulted in the strongest water flux, which may cause local depletion of the cerium added as chemical stabilizer.In this study, we have focused on determining the root cause of membrane failure in the region of hydration cycling. Two hypotheses were investigated. The first is that chemical degradation can be accelerated in the presence of mechanical stress. The chemical degradation leads to membrane thinning which, in turn, leads to fewer sulfonic acid sites in the membrane available for Ce cation exchange. Two synergistic pathways for membrane degradation by combined mechanical and chemical stressors have been summarized by Kusoglu and Weber [3]. Ex-situ hydrogen peroxide vapor cell tests conducted with a pressure bias across the membrane to create a tensile stress were conducted to see if mechanical stress has an impact on membrane chemical degradation rate [4].The second hypothesis is that local depletion of the mobile Ce stabilizer in regions of high humidity gradients can lead to chemical degradation in those regions. We have shown previously that humidity gradients can cause cations to move with convective forces from wetter to drier regions within a perfluoro-sulfonic acid (PFSA) membrane [5]. To study this hypothesis, we conducted two types of tests. First, we repeated the HAST tests of the state-of-the-art membranes and stopped the tests prior to the onset on membrane thinning to see whether Ce depletion occurs before or after the onset of thinning. We also ran HAST tests on membranes which contained no Ce stabilizer to see if membrane failure still occurred in the region of strongest humidity cycles. In the absence of stabilizer, we expect chemical degradation to occur faster towards the cell inlets where, on average, the MEA is driest.We will show that the HAST test significantly accelerates membrane failure by increasing temperature and frequency of local deep humidity cycles without changing the primary failure mode or location of a state-of-the-art membrane. This work suggests that these HAST tests can be used to accelerate the development of membranes for HD fuel cell applications. DOE H2 Heavy Duty Truck Targets; https://www.hydrogen.energy.gov/pdfs/19006_hydrogen_class8_long_haul_truck_targets.pdf Y.-H. Lai et. al, J. Electrochem. Soc., 165 (6) (2018) F3100-F3103Kusoglu and Weber, J. Phys. Chem. Lett., 6, 4547 (2015)Y.-H. Lai et. al, ECS s 2021; https://iopscience.iop.org/article/10.1149/MA2021-02381128mtgabs F.D. Coms et. al, ECS s 2021; https://iopscience.iop.org/article/10.1149/MA2021-02381130mtgabs

A Robust and Quantified Analysis of Mass Transport and Its Application in Designing Next Generation Architecture for Polymer Electrolyte Water Electrolysis Cells

Investigations to better understand the nature of mass transport and two-phase flow within a polymer electrolyte water electrolyzer (PEWE) have gained popularity as researchers strive to improve system performance and efficiency. Along with traditional polarization experiments, the use of in-situ spatial and temporal current distribution measurement techniques has proven to be very attractive. Different methods for spatial current distribution measurements include the use of resistor networks, potential probes, and printed circuit boards (PCB) [1–3]. Using such techniques, Mench and co-workers have previously demonstrated [4–7] that, under mass transport limited conditions for any combination of flow-field and diffusion media, the current density distributions consistently exhibit a gradient – with high current towards the inlet and low towards the outlet. These contour plots showed certain dissimilarities; however, they serve as a qualitative picture to gauge cell performance and characterize mass transport. Therefore, in this work a robust analysis was developed that provides a quantitative understanding and better characterization of the mass transport limitation within the electrolyzer. A mass transport number (MTN) was defined and used to further characterize the influence of mass transport limitation experienced within an electrolyzer.Applying this new analysis to our the investigations revealed that the PEWE system performance using high-performance micro-patterned Ti-foil liquid-gas-diffusion-layer (LGDL) [8,9] was highly sensitive to the cell architecture and exhibited poor mass transport behavior using traditional channel based flow-fields. Compared with the conventional porous transport layer (PTL) morphology, the lack of in-plane permeability of the LGDLs caused faster onset of mass transport limitations at lower flowrates, local increase in HFR, and poor area utilization. Therefore, a unitized single flow-field-LDGL component that mitigates these problems has been designed. Through this work it will be shown that the flow-field is designed to have pins that allow for greater in-plane transport. The staggered pattern reduces pressure drop and promotes exchange in-between channels that assist in efficient bubble removal. Additionally, the cross-section of the pin, the pitch, and aspect-ratios are adjustable to maximize porosity for different LGDLs. Finally, making this into a single unitized component having a permanently attached LGDL will decrease interfacial contact resistances and improve cell performance (as shown in Figure 1).[1] T. V. Reshetenko, G. Bender, K. Bethune, R. Rocheleau, A segmented cell approach for studying the effects of serpentine flow field parameters on PEMFC current distribution, Electrochim. Acta. 88 (2013) 571–579. https://doi.org/10.1016/j.electacta.2012.10.103.[2] A. Phillips, M. Ulsh, J. Porter, G. Bender, Utilizing a Segmented Fuel Cell to Study the Effects of Electrode Coating Irregularities on PEM Fuel Cell Initial Performance, Fuel Cells. 17 (2017) 288–298. https://doi.org/10.1002/fuce.201600214.[3] C. Immerz, B. Bensmann, P. Trinke, M. Suermann, R. Hanke-Rauschenbach, Local Current Density and Electrochemical Impedance Measurements within 50 cm Single-Channel PEM Electrolysis Cell, J. Electrochem. Soc. 165 (2018) F1292–F1299. https://doi.org/10.1149/2.0411816jes.[4] F.H. Roenning, A. Roy, D. Aaron, M.M. Mench, Spatially-Resolved Current Distribution Measurements in Polymer Electrolyte Water Electrolyzers, ECS Meet. Abstr. MA2020-02 (2020) 2457–2457. https://doi.org/10.1149/MA2020-02382457mtgabs.[5] A. Roy, F. Roenning, D. Aaron, M.M. Mench, Quantifying Lateral Current Spread While Measuring Performance Using a Segmented Polymer Electrolyte Water Electrolysis Cell, ECS Meet. Abstr. MA2020-02 (2020) 2458–2458. https://doi.org/10.1149/MA2020-02382458mtgabs.[6] A. Roy, F. Roenning, D. Aaron, M.M. Mench, Local Two-Phase Flow and Performance in Polymer Electrolyte Water Electrolysis Cells, ECS Meet. Abstr. MA2021-01 (2021) 1190–1190. https://doi.org/10.1149/MA2021-01381190mtgabs.[7] A.Z. Weber, R.L. Borup, R.M. Darling, P.K. Das, T.J. Dursch, W. Gu, D. Harvey, A. Kusoglu, S. Litster, M.M. Mench, R. Mukundan, J.P. Owejan, J.G. Pharoah, M. Secanell, I. V. Zenyuk, A Critical Review of Modeling Transport Phenomena in Polymer-Electrolyte Fuel Cells, J. Electrochem. Soc. 161 (2014) F1254–F1299. https://doi.org/10.1149/2.0751412jes.[8] J. Mo, Z. Kang, S.T. Retterer, D.A. Cullen, T.J. Toops, J.B. Green, M.M. Mench, F.-Y. Zhang, Discovery of true electrochemical reactions for ultrahigh catalyst mass activity in water splitting, Sci. Adv. 2 (2016) e1600690. https://doi.org/10.1126/sciadv.1600690.[9] Y. Li, Z. Kang, X. Deng, G. Yang, S. Yu, J. Mo, D.A. Talley, G.K. Jennings, F.-Y. Zhang, Wettability effects of thin titanium liquid/gas diffusion layers in proton exchange membrane electrolyzer cells, Electrochim. Acta. 298 (2019) 704–708. https://doi.org/10.1016/j.electacta.2018.12.162. Figure 1

Understanding Behavior of Cathode Active Materials with Different Particle Sizes in Mixed Electrode through Current Distribution Measurement

The performance of a lithium-ion battery can be improved by mixing two or more kinds of cathode active materials in one electrode. In particular, energy density can be increased by mixing active materials having different particle sizes because the electrode can be filled with more active materials. As the performance of a battery is determined by the combination of the properties of the material itself and the electrode design conditions, it is most important to understand the characteristics of the material and the electrode independently. However, since the potential in a mixed electrode is measured as the mixed potential of each active material, it is not easy to identify how each active material behaves electrochemically under the current load condition.In this experiment, we attempted to separate only the effects of active materials while minimizing the influence of the electrode density, which affects the mass transfer in the porous electrode. For this purpose, two electrodes consisting of small and large particles are placed in one pouch cell and connected in parallel with Li metal as a counter electrode. By measuring the current flowing through each electrode individually, we observed how small and large particles work in mixed electrode.Since the influence of mass transfer inside the electrode was minimized, it was assumed that the mass transfer did not affect the currents divided into each electrode. Thus, current flowing to each electrode is determined by the relative contributions of charge transfer and solid diffusion kinetics. Large particles have smaller charge transfer resistance and slower solid diffusion than small particles. If rate-liming step is charge transfer, more currents flow to large particles by small charge transfer resistance. If rate-limiting step is solid diffusion, more currents flow to small particles due to the fast diffusion in active materials. Through this experiment, we could analyze the behavior of active materials having different particle sizes in mixed electrode by detecting the current respectively.

A Segmented Cell Measuring Technique for Current Distribution Measurements in Batteries, Exemplified by the Operando Investigation of a Zn-Air Battery

A transimpedance amplifier circuit as well as an instrumental amplifier circuit were used to measure current densities of a zinc-air battery with an integrated segmented current collector foil. Error calculation showed that the transimpedance amplifier is superior to the used instrumental amplifier, but both methods provide valuable and consistent results. They both showed comparable results with operando insight into the current distribution of the battery. The knowledge about those distributions is essential to avoid fast degradation of battery materials and irreversible capacity loss due to heterogeneous dissolution of the anode during discharge. In this work we showed that oxygen starvation as well as gas flow rate leads to large current gradients. It was also demonstrated that heterogeneous current distributions on cathode side induces also a heterogenous dissolution behavior on the anode, resulting in irreversible capacity loss.

Phase slips dynamics in gated Ti and V all-metallic supercurrent nano-transistors

The effect of electrostatic gating on metallic elemental superconductors was recently demonstrated in terms of modulation of the switching current and control of the current phase relation in superconducting quantum interferometers. The latter suggests the existence of a direct connection between the macroscopic quantum phase (φ) in a superconductor and the applied gate voltage. The measurement of the switching current cumulative probability distributions is a convenient and powerful tool to analyze such relation. In particular, the comparison between the conventional Kurkijärvi–Fulton–Dunkleberger model and the gate-driven distributions give useful insights into the microscopic origin of the gating effect. In this paper, we summarize the main results obtained in the analysis of the phase slip events in elemental gated superconducting weak-links in a wide range of temperatures between 20 mK and 3.5 K. Such a large temperature range demonstrates both that the gating effect is robust as the temperature increases, and that fluctuations induced by the electric field are not negligible in a wide temperature range.

Current density and temperature distribution measurement and homogeneity analysis for a large-area proton exchange membrane fuel cell

Homogeneous distribution of electro-chemical reaction rates among the activation surface is critical for improving the performance and durability of automotive proton exchange membrane fuel cells (PEMFCs). Segmented measurement technology is commonly used to characterize the local physical-parameter distribution in the PEMFC. In this study, the local current density (LCD) and temperature distributions of a PEMFC with the activation area of 108 cm2 and various cathode flow fields are experimentally investigated. A homogeneity parameter is introduced to evaluate the homogeneity of LCD distribution. The results show that the performance and LCD distribution uniformity of the cell with dot-parallel flow field are much better than that with parallel and parallel-serpentine flow fields. The temperature distribution is generally positively correlated with LCD distribution. For the LCD distribution, the high LCD region firstly appears in the cathode downstream region, and gradually transfers upstream with increasing the current load. With the increase of inlet humidity, the LCD near the cathode inlet is improved due to the improvement of membrane hydration. Increasing the cathode stoichiometry can effectively improve the uniformity of LCD distribution, and mitigate the local oxygen starvation, especially at high loads.

Direct measurement of current distribution in lithium-ion cells by magnetic field imaging

Inhomogeneous current distribution in lithium-ion cells is a major reason for degradation. A method to measure the locally resolved current distribution is to map the magnetic field caused by the current flowing in the cell. This paper presents an unshielded measurement setup using anisotropic magnetoresistive (AMR) sensors to map the magnetic field of a cell during operation. Moreover, it proposes an algorithm to calculate the two-dimensional current distribution from magnetic field data. The achieved accuracy of the current density is 227 mA cm −2 at a local resolution of 4 mm 2 . The mathematical problem is reduced to a single matrix multiplication and takes into account specific characteristics of the measurement setup. Simulated and measured data are used to evaluate the proposed algorithm. Two cells with different geometries, capacities and tab designs are tested and compared. The paper provides guidance for the interpretation of the measurement data and introduces a measure to calculate the validity and accuracy, which can also be used to trim the parameters for the inverse problem. Possible fields of applications include end of line tests or state of health investigations where non-destructive and in-situ measurements are needed. • Nondestructive, locally resolved current distribution measurement. • Setup to precisely measure the magnetic field above the cell. • Algorithm to reconstitute current distribution from magnetic field image. • Current distribution measured for cells with different geometries. • Enables inhomogeneity analysis and defect detection in cells.

The Real Current Density Distribution on Mg Surface

To quantitatively analyze the real current density distribution and electrochemical activity on a Mg electrode, the electronic current densities distributed on a multi-Mg wire array electrode were compared with scanning vibration electrode technique (SVET) mapped ionic current density distributions. The SVET measured current densities were found to be nearly ten times lower than the electronic ones. The cathodic current densities detected in SVET mapping could be a result of hydrogen bubbles trapped on the electrode surface. The measurement of the real current density distribution on Mg surface helps deepen the understanding of Mg anode dissolution.

Simulative Investigation on Local Hydrogen Starvation in PEMFCs: Influence of Water Transport and Humidity Conditions

Durability targets of automotive polymer electrolyte membrane fuel cells (PEMFCs) could be crucially threatened by local hydrogen starvation, typically induced by local blockage of gas channels. To gain a deep insight on the evolving of such starvation events and related carbon corrosion losses, we have developed a numerical model with transient nature that includes detailed transport phenomena and electrochemistry. Special focus is on water transport and sensitivity of relative humidity (RH) on both anode and cathode sides, whose influences were commonly neglected in starvation-related modeling studies. Utilizing the model, we show the dominating effect of in-plane hydrogen convection within the anode gas diffusion layer, which is again determined by the accumulation of other gas species including water vapor. We demonstrate how this is again linked with the water management throughout the fuel cell. Furthermore, water transport is shown to affect local current density and membrane oxygen permeability, both being critical influential factors regarding the severity of a local starvation event. The developed model is validated by conducting transient current density distribution measurements. As RH levels are crucial operational conditions within automotive PEMFCs, this work serves as useful input towards development of future operation strategies for better PEMFC durability.

A pluggable current collector for in-operando current measurements in all-vanadium redox flow batteries with flow field

Understanding current distribution in operating Vanadium redox flow batteries (VRFBs) is vital for improving battery design and performance as the non-uniform current density distribution is known as a major cause of the corrosion and side reactions in VRFBs. However, the existing method for current density measurement in the VRFB with flow field is complex and the rib conductive structure of graphite plate is changed which reduces the measurement accuracy and reliability. In this study, we propose a novel pluggable current collector for in-operando current distribution measurements which greatly simplifies the current density distribution measurement process in VRFBs with flow field. The new method relies on eight pluggable graphite units in a flow plate frame to replicate the actual serpentine flow field structure, thereby providing higher measurement reliability compared to the existing methods which are mostly invasive. Using the new method, current density distributions at different electrode compression ratios (CR) and inflow electrolyte rates are investigated. It is found that insufficient supplemental reactant in electrode border region is the main reason for the uneven current density distribution and the current density non-uniformity is intensified in the initial and final stages of cycling process. Increasing CR can improve the current density in the electrode core region. The overall current density uniformity in a VRFB with serpentine flow field can be enhanced by increasing the inflow electrolyte rate. The developed method is believed to facilitate the understanding of the coupled local reactions and transport processes and guide on the future design of VRFBs.

Quantifying Lateral Current Spread While Measuring Performance Using a Segmented Polymer Electrolyte Water Electrolysis Cell

Conventional research and diagnostic techniques for fuel cells and electrolyzers, such as polarization curves and EIS measurements, predominantly probe macroscopic performance characteristics. The works of Schuler, et al. [1] have given a detailed understanding of transport mechanisms inside a polymer electrolyte water electrolyzer (PEWE) by drawing extensively on such ex-situ techniques. However, these techniques cannot be used to probe in-situ behaviors of a PEWE. A need for reliable experimental data that captures in-situ spatial and temporal current distribution is the motivation for the segmented cell used here. Current distribution measurement in PEWEs was performed by Bender et al. [2, 3] while Mench and coworkers pioneered the technique in similar flowing electrochemical systems [4, 5]; a major innovation was inclusion of a printed-circuit board (PCB) with an array of shunt resistors attached to each segment as shown in Figure 1a. The advantage of this design is that, although the PCB and flow-field are segmented and electronically isolated, the other components of the cell are not. However, this design is subject to error since current can spread laterally through the porous transport layer (PTL) as it passes from the catalyst layer to the segmented flow field. Since activity at the catalyst layer is of great interest, understanding current spread is a critical step to utilizing current distribution measurements.This work focuses on characterizing the lateral current spread through PTLs. Work by Philips, Ulsh, and Bender [3], showed that the experimental setup could be modified with the help of masks to characterize lateral current spread through the PTL. A set of experiments was thus designed to investigate current spread through the porous transport layer both “inward” and “outward” from a flow field segment. The goal of these experiments is to quantify current spread, allowing correction of its contribution to obfuscating measurement of current generated in the catalyst layer. High current was detected for the PCB segment corresponding to the unmasked area as expected, but substantial current was measured around the exposed area, as well (Figure 1b). If there was zero outward current spread, no current would be measured anywhere away from the single segment area generating current. Figure 1c shows average current density as a function of radial distance from a generating spot; Figure 1d plots normalized current density as a function of radial distance. This set provides calibration for how much current spread laterally outwards as detected at points located away from a current-generating area. Similarly, inward contributions were also mapped. These experiments were also carried out for two types of PTL – Ti-foil-based thin LGDLs [6] and Ti-based felt PTLs. Preliminary observations indicate that lateral current spread in the Ti-felt-based LGDLs is less than that of the Ti-based foil LGDLs. This work quantifies the lateral current spread seen inside a LGDL and also lays the groundwork to correct current spread recorded in a spatially-resolved current distribution measurement technique. It 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 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. 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. Phillips, A., et al., Utilizing a Segmented Fuel Cell to Study the Effects of Electrode Coating Irregularities on PEM Fuel Cell Initial Performance. Fuel Cells, 2017. 17(3): p. 288-298. 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. 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. Mo, J., et al., Discovery of true electrochemical reactions for ultrahigh catalyst mass activity in water splitting. Science Advances, 2016. 2(11): p. e1600690. 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

Vanadium Flow Battery Electrochemistry and Fluid Dynamics Model with in-Situ Current Distribution Validation

It has been estimated that the high penetration of renewable electricity generation (68%-73% from solar and wind) could be possible in the United States by 2050. To achieve this goal, large-scale energy storage technologies will play important role. Energy storage technologies can help to improve electricity stability, flexibility, reliability and resilience on the grid. Among different energy storage methods vanadium redox flow batteries (VRFBs) have been a recent focus of interest due to distinct advantages, i.e., flexibility and scalability, high columbic efficiency, and long cycle life. However, high system cost is a main challenge for widespread commercialization of VRFBs. Although the component costs (membrane cost and chemical cost) tend to dominate the overall system costs, it is possible to decrease costs by improving electrolyte utilization and system efficiency. This can be achieved through increasing vanadium solubility in the solvent, modifying membrane and minimizing crossover, facilitating electrochemical kinetics, improving mass transport and reducing parasitic pump losses. However, VRFB is a complex system with multiple interrelated parameters (mass transport and electrochemical reactions) affecting electrochemical cell performance; these interrelated parameters are difficult to disentangle experimentally. A comprehensive and suitably validated mathematical simulations can both help to understand complex phenomena inside the VRFB cell and provide practical knowledge for controlling and optimizing VRFB systems. In this study, three dimensional, steady-state multi-physics model is developed for VRFB strip cell architecture under the dilute solution theory assumption. The simulation domain includes membrane, electrodes, flow plates and current collectors. Continuum equations: conservation of mass, momentum, species and charge coupled with Butler-Volmer kinetics are employed. Polarization curve analysis and fully segmented, printed circuit board (PCB)-based, localized current distribution measurements are employed to validate the mathematical model. Good agreement is achieved between the model predictions and the experimental measurements. The model successfully predicts not only the charge-discharge polarization curve but also the current distribution along the channel. Figure 1