Multi-cell Stack Research Articles

Experimental and modeling study on dynamic characteristics of a 65 kW dual-stack proton exchange membrane fuel cell system during start-up operation

Dynamic characteristics of multi-cell voltage in a non-loaded proton exchange membrane fuel cell (PEMFC) system during the start-up process are critical for the lifetime of a fuel cell stack. The effects of hydrogen flow rate and initial gas species in the anode compartment are experimentally studied with a 65 kW dual-stack PEMFC system. The individual cell voltages are measured using a cell voltage monitor (CVM) device and their statistic characteristics are analyzed. Furthermore, a model comprised of a hydrogen flow network model and an equivalent electrical circuit model is developed to simulate the individual cell voltage variation during start-up. Numerical and experimental results show excellent agreement. Finally, internal current and voltage distributions in a single cell model and multi-cell stack model are investigated and discussed. A large hydrogen flow rate is beneficial to reduce the serious local carbon corrosion caused by capacitance effect. The revealed dynamic characteristics of multi-cell voltage of the dual-stack are beneficial for the PEMFC system integration and optimization of control strategy during start-up operation.

Proton-conducting ceramic fuel cells: Scale up and stack integration

We present our efforts to scale up proton-conducting ceramic fuel cells (PCFCs) from the button-cell level into small, multi-cell stacks. While recent advancements with lab-scale PCFCs are encouraging, there are few reports of scaling PCFC technology to the stack level. The compatibility of protonic-ceramic materials with stack-packaging materials - metallic interconnects, current collectors, glass-ceramic sealants, gaskets - has not been demonstrated. Here we show that through tuning of materials, fabrication procedures, and operating conditions, protonic-ceramic fuel cell stacks can achieve reasonable performance and low degradation.The MEA is based around barium cerate-zirconate perovskites. Better long-term stack durability is found with BaCe0.4Zr0.4Y0.1Yb0.1O3−δ (BCZYYb) electrolyte. The anode support is a nickel-electrolyte composite, while the cathode is BaCo0.4Fe0.4Zr0.1Y0.1O3−δ (BCFZY). Planar MEAs reach 5 cm2 in active area, and are packaged within ferritic-steel interconnects and macor frames to form multi-cell stacks. Our three-cell stack demonstrates encouraging performance, reaching 0.69 and 0.47 W cm−2 under H2 and CH4 fuels, respectively, at 600 ∘C. A gadolinium-doped ceria cathode-electrolyte interlayer reduces degradation rates to 1.5% kh−1 at 0.1 A cm−2 and 3.3% kh−1 at 0.4 A cm−2 at 550 ∘C. No chromium transport is observed. Degradation mechanisms and the role of the GDC interlayer are postulated.

Characterizing Stack Degradation in Proton-Conducting Ceramic Fuel Cells and Electrolyzers

We present our efforts to scale-up proton-conducting ceramic fuel cells (PCFC) for integration into small multi-cell stacks. Proton-conducting ceramics are an exciting new class of materials that are now emerging from research laboratories to address societal challenges in electricity generation, energy storage, and fuels synthesis. While there are many technologies under investigation, intermediate temperature proton-conducting fuel cells (PCFC) have been drawing attention due to their high performance at low temperatures in fuel cell mode (up to ~2 Wcm2 at 600 ºC) and their high efficiency (90 – 98 % Faradic efficiency) when operated in electrolysis mode. However despite the impressive achievements at lab-scale PCFC technology limited investments have been made on the scaling up process due to the challenges that the PCFC fabrication procedure represents. In fact, there are no stack demonstrations reported to date in the archival literature. In this presentation we will review our efforts at Colorado School of Mines to increase the size of PCFCs beyond the button-cell level, and to integrate these cells into multi-cell stack assemblies. The stability of our PCFC stack reached the degradation rates of 3.5% and 2.1% per 1000 hours, respectively, under fuel cell and electrolysis modes.An illustration of our stack design is shown in Figure 1, and includes a photograph of a fully assembled three-cell stack after performance testing. The design is centered on three repeating components: the protonic-ceramic electrolysis cells, the composite-ceramic frames in which the cells are bonded, and the thin metallic interconnect / bipolar plates that conduct electricity between adjacent cells. The cells feature a dense, 20 μm-thick BaCe0.4Zr0.4Y0.1Yb0.1O3- d (BCZYYb4411) electrolyte on a porous Ni-BCZYYb4411 anode fabricated through reactive-sintering methods. The cathode is a novel BaCo0.4Fe0.4Zr0.1Y0.1O3-δ (BCFZY) atop of the BCZYYb4411 electrolyte with the maximum active area of ~ 7cm2 per single unit cell. Reactive gases are fed through fuel and oxidizer ports machined into the frame. The metallic interconnect is a ferritic steel developed for intermediate-temperature solid-oxide fuel cells and electrolyzers, and is compression sealed to the frame. Metallic meshes (not shown) electrically connect the electrodes to the interconnects.High protonic-ceramic cell performance operating at 550 0C has been previously demonstrated at Colorado School of Mines. While the lower operating temperature should present benefits for the long-term stability of the stack it also brings questions regarding the behavior of stack materials such as the interconnects which represent a source of degradation in performance depending on the composition of the scale that is grown on them. In an effort to better understand the root causes behind performance degradation processes in our protonic-ceramic stacks, we have integrated numerous voltage-taps throughout the stack shown in Figure 1. In this presentation, we will review our results for long-term protonic-ceramic stack operation under both fuel cell and electrolysis mode, discuss after-test interconnects analysis showing high-iron-content scale and present strategies for reducing degradation rates and extending stack life. Figure 1

High-Voltage All-Solid-State Lithium Battery with Sulfide-Based Electrolyte: Challenges for the Construction of a Bipolar Multicell Stack and How to Overcome Them

Solid electrolytes can be the key for the desired goal of increased safety and specific energies of batteries. On a cell and battery pack level, the all-solid nature and the absence of liquid elect...

A validated dynamical model of a kW-class Vanadium Redox Flow Battery

The development of redox flow batteries depends on the research on new materials as well as on the technological development, but also on appropriate models which allow to simulate their performance in operative conditions. Very few investigations are reported in the literature concerning the technology, modeling and simulation of large-scale Vanadium Redox Flow Battery systems, built around multi-cell stacks. This paper regards the modeling of an industrial-sized 9 kW test facility. In particular, a complete dynamic model is presented, that takes into account all thermal effects occurring inside the stack, resulting in a complex non-linear coupled formulation, that allows to simulate the battery operation in any realistic conditions. The model is able to simulate the thermal behavior both in standby, i.e. without power and reactant flow, as well as in load operation, i.e. in charge and discharge. The numerical implementation of the model is described in detail. The model validation is also described, consisting in comparing computed data with experimental measurements taken on the available test facility.

A novel diagnostic methodology for fuel cell stack health: Performance, consistency and uniformity

Internal heterogeneity is a significant challenge to the service life of large-format commercial fuel cell stacks. Methods for diagnosing fuel cell stack health, however, are still lacking. This paper proposes a novel diagnostic methodology for analyzing the performance, the consistency between cells and the uniformity within individual cells of fuel cell stacks. Specifically, this three-step methodology can comprehensively analyze the internal heterogeneity of large-format multi-cell stacks. First, a polarization curve test is conducted to provide a quick qualitative performance analysis. Then, the quantitative parameters consistency decline is analyzed by galvanostatic charging and electrochemical impedance spectroscopy methods. Finally, a novel multi-point voltage monitoring method is applied to rule out possible misdiagnoses and to qualitatively analyze cell uniformity. This degradation diagnostic methodology is applied to analyze degradation in several fuel cell stacks, revealing high potential in practice.

Hysteresis of output voltage and liquid water transport in gas diffusion layer of polymer electrolyte fuel cells

The hysteresis of PEM fuel cell output voltage of the single cell and multi-cell stack is presented by experiments, during which the in-situ liquid water visualization at certain point of gas channel, quantitative measurement of the RH (relative humidity) and HFR (high frequency resistance) by EIS (electrochemical impedance spectroscopy) were carried out. It is found that HFR makes little contribution to the hysteresis of voltage. Instead, the hysteresis of liquid water saturation s in GDL is believed to dominant this process. Although the hysteresis of capillary pressure of liquid water in GDL has already been the presented in previous works, it is for the first time that a simplified one-dimensional model on liquid water saturation is updated considering this effect to understand the possible relation between them. Two different liquid water saturation s distributions during imbibition and drainage are obtained by simulation and related hypothesis of different water morphology in GDL is proposed. Furthermore, according to the simulation results, the GDL consisting of material with high resistance to water penetrating is found to have the better performance in the flooding condition, but weak capability to maintain residual water during drainage of PEM fuel cell.

The uniformity and consistency analysis of a fuel cell stack with multipoint voltage-monitoring method

The non-uniform internal status in one cell and the inconsistency between cells have significant impact on the performance and service life of commercial fuel cell stack. The identification of this phenomenon with in-situ method is the key technology for product development and optimization. This paper presents the corresponding analysis and a practical in-situ method, i.e., the multipoint voltage-monitoring method, to identify the fuel cell stack uniformity and consistency. Cross plate current and voltage variation in bipolar plate can be therefore observed using the proposed method. In addition, an one-dimensional model has been developed to explain this phenomenon. A variable loading rate experiment has been conducted in a four-cell stack to explore the relationship between consistency and uniformity in multi-cell stack. When the loading rate increases from 20 mA/cm2·s to 40 mA/cm2·s, the current concentrating in the cathode outlet also increases. Current non-uniformity is caused by gas distribution inconsistency between cells, which makes cell1 drier than the other cells. Voltage difference between the two monitors of cell1 is about 32mV, and it can be even larger in extreme conditions, like fuel starvation. It means the health condition estimation of fuel cell stack can be easily misdirected, when monitor is placed on different position. Experimental and simulation results show that the multipoint voltage monitoring method can avoid the ‘voltage cheat’ and therefore achieve the real consistency and uniformity performance.

Vanadium redox flow battery with slotted porous electrodes and automatic rebalancing demonstrated on a 1 kW system level

Capacity loss due to electrolyte crossover through the membrane and pump losses due to pressure drop at the porous electrodes are widely known issues in vanadium redox flow batteries during operation. In commercial systems, these losses account for a significant reduction in the overall efficiency. Previous studies have been focused on the development of new membranes to solve the capacity loss, and design modification to reduce the pressure drop. In this work, we propose unique solutions to solve both problems and are demonstrated in a multi-cell stack for the first time. A 20-cell, 1 kW vanadium redox flow battery stack was assembled using thin bipolar plates and porous electrodes featuring interdigitated flow channels. Such a stack design is novel of its kind and can mitigate various problems associated with flow distribution and pump power in flow batteries. In addition, the electrolyte tanks were shunted together to rebalance the electrolyte automatically. The stack showed a very good and stable performance with an energy efficiency of 80.5% at a current density of 80 mA cm−2. The use of hydraulic shunt resulted in a constant capacity over 250 cycles while the use of flow channels on the porous electrodes resulted in ∼40% reduction in pressure drop, compared to a stack with standard felts. The reduction in pressure drop by employing flow channels reduced the pump power proportionally. Overall, capacity retention and utilization of active materials have been improved substantially. These methods are simple and applicable to any size of vanadium redox flow battery.

A cell interaction phenomenon in a multi-cell stack under one cell suffering fuel starvation

Cell interaction is the main factor resulting in a shorter lifespan for multi-cell stacks than for a single fuel cell stack. To explain this mechanism, we propose a cell interaction phenomenon in which one cell experiences fuel starvation. A specific voltage distribution along the straight channel direction has been observed with an innovative multipoint monitoring method. This phenomenon also can be used for fuel starvation diagnosis. This study proposes an ingenious simplified two-chamber model to analyze the current and voltage redistribution mechanism under fuel starvation. The reliability of this model has been validated in this paper. The model shows that current convergence resulted by fuel starvation in one cell can lead to a concomitant local current convergence in nearby normal cells. Based on our calculation, >85% of reaction current concentrates in the anode inlet region of the fuel-starved cell, resulting in a 70% current convergence in the two adjacent normal cells. A serious corrosion of the fuel-starved cell is observed in the post-mortem study. The faulty cell presents a 5° contact angle decrease and a 28% ECSA loss. Scanning electron microscopy and Transmission electron microscopy results show that the decline of anode outlet regions’ cathode catalyst layers are more serious. Some optimal strategies have been proposed to solve this problem.

Equivalent Stiffness Model of a Proton Exchange Membrane Fuel Cell Stack Including Hygrothermal Effects and Dimensional Tolerances

Proton exchange membrane fuel cells (PEMFCs) require mechanical compression to ensure structural integrity, prevent leakage, and to minimize the electrical contact resistance. The mechanical properties and dimensions of the fuel cell vary during assembly due to manufacturing tolerances and during operation due to both temperature and humidity. Variation in stack compression affects the interfacial contact pressures between components and hence fuel cell performance. This paper presents a one-dimensional equivalent stiffness model of a PEMFC stack capable of predicting independent membrane and gasket contact pressures for an applied external load. The model accounts for nonlinear component compression behavior, thickness variation due to manufacturing tolerances, thermal expansion, membrane expansion due to water uptake, and stack dimensional change due to clamping mechanism stiffness. The equivalent stiffness model is compared to a three-dimensional (3D) finite element model, showing good agreement for multicell stacks. Results demonstrate that the correct specification of gasket thickness and stiffness is essential in ensuring a predictable membrane contact pressure, adequate sealing, and avoiding excessive stresses in the bi-polar plate (BPP). Increase in membrane contact pressure due to membrane water uptake is shown to be significantly greater than the increase due to component thermal expansion in the PEMFC operating range. The predicted increase in membrane contact pressure due to thermal and hydration effects is 18% for a stack containing fully hydrated Nafion® 117 membranes at 80 °C, 90% relative humidity (RH) using an eight bolt clamping design and a nominal 1.2 MPa assembly pressure.

Mechanical modelling and simulation analyses of stress distribution and material failure for vanadium redox flow battery

During the operation of vanadium redox flow battery, the cell stack can suffer from electrolyte leakage and material failure that significantly affect the overall performance of the battery, provided that the stack is improperly designed and assembled. In order to manufacture more reliable battery stacks without undergoing electrolyte leakage and mechanical failure, the stress distributions on all key components of the stack need to be known. In this study, three-dimensional mechanical models are developed to perform simulation analyses on stress distribution for the cell stacks. Stress distributions on key cell components under specified sealing gasket designs, assembling forces and number of cells in a stack are investigated for the single cell and multi-cell stacks, while potential material failure and damage for the stack components are also analyzed in accordance with maximum stress criterion and von Mises yield criterion depending on the material of the components. Simulations results successfully demonstrate the stress distribution and magnitude in specified stack design and assembly condition, and highlight the importance of mechanical analyses in developing flow battery stacks with superior sealing and mechanical performance for long-term use.

(Invited) Review of Advances in Solid Oxide Fuel Cells in the Past Four Decades

Solid oxide fuel cell (SOFC) technology has made significant progress in the past 40 years and is currently in the early commercialization stage. The progress of the technology can be seen through review of the papers and presentations at the biennial International Symposium on Solid Oxide Fuel Cells initiated in 1989. This paper highlights key advances in materials development, understanding of electrode reactions and chemical interactions, and design, performance, durability and operation of single cells, multi-cell stacks and power systems. Power generation systems built with various design solid oxide fuel cells are described and their operating experiences reviewed, as well as challenges in reducing cell and system costs are summarized.

Advances in Solid Oxide Fuel Cells: Review of Progress through Three Decades of the International Symposia on Solid Oxide Fuel Cells

Solid oxide fuel cell (SOFC) technology has made significant progress in the past 30 years and is currently in the early commercialization stage. The progress of the technology can be seen clearly through review of the papers and presentations at the biennial International Symposium on Solid Oxide Fuel Cells series initiated in 1989. This paper details the growth and evolution of the Symposium series as the technology progressed, and highlights key advances in materials development, understanding of electrode reactions and chemical interactions, and design, performance, durability and operation of SOFC single cells, multi-cell stacks and power systems as documented in the Symposium proceedings.

Methane-Fueled Proton-Conducting Ceramic Fuel Cell Stacks

We present our recent work on scale-up and stack development of fuel cells based on proton-conducting ceramic materials. Through steady improvement in materials and fabrication methods, protonic ceramics are emerging from the laboratory as potential solutions in electricity generation, energy storage, and chemical synthesis. “Button-scale” protonic ceramic fuel cells (PCFCs) based on barium zirconates have demonstrated high power density at low operating temperatures, reaching 450 mW / cm2 at 500 ºC under hydrogen fuel, and 300 mW / cm2 at 550 ºC under direct methane-steam fuel. While these materials show great promise, scale up of protonic-ceramic fuel cells beyond the “button-cell” level remains limited. In this presentation, we review our progress in extending previous button-cell experiments to small fuel-cell stacks. First, fabrication procedures have been developed to increase PCFC active area by a factor of 10. These planar, circular membrane electrode assemblies (MEAs) are packaged within the stack architecture shown in the figure. MEAs are bonded within a ceramic frame, and the frame is packaged within ferritic-steel interconnects. Numerous multi-cell stack prototypes have been demonstrated. A three-cell stack based on BaCe0.2Zr0.6Y0.2O3-d (BCZY26) electrolyte shows near-theoretical open-circuit voltage and promising power density under methane fuel (183 mW / cm2 at 500 ºC). In this presentation, we will review our continuing efforts to improve the performance and durability of protonic-ceramic fuel cell stacks. Figure 1

Advances in Solid Oxide Fuel Cells: Review of Progress through Three Decades of the International Symposia on Solid Oxide Fuel Cells

Solid oxide fuel cell (SOFC) technology has made significant progress in the past 30 years and is currently in the early commercialization stage. The progress of the technology can be seen clearly through review of the papers and presentations at the biennial series of International Symposium on Solid Oxide Fuel Cells initiated in 1989. This paper details the growth and evolution of the Symposium series as the technology progressed and highlights key advances in materials development, understanding of electrode reactions and chemical interactions, and design, performance, durability and operation of SOFC single cells, multi-cell stacks and power systems as documented in the Symposium publications.

Cell and System Design of Lithium Air Batteries for Electric Vehicles

Many studies have been carried out on the lithium-air batteries due to their high theoretical specific energy. The results have been published in the range of scientific papers: electrolytes, catalysts, cathode materials and structures, lithium anode and their fundamental reaction mechanisms for the last ten years. Recently a few researchers have published their analysis for the system level considerations for lithium-air batteries to reach long-term automotive applications.Even though there are still many scientific challenges such as poor cyclability and low energy efficiency, we focused on a cell and system design of lithium-air batteries for electric vehicles to investigate their practical achievable specific energy and energy density. For this purpose, a cell size of 270 x 100 mm2 was fabricated and tested their electrochemical performance. We also designed a novel multi-cell stack structure including bipolar plates for the optimal air distribution. A lithium-air 13Ah cell stack was made and tested their electrochemical performance. The details will be discussed in the PRiME 2016.

Modelling and simulation of thermal behaviour of vanadium redox flow battery

This paper extends previous thermal models of the vanadium redox flow battery to predict temperature profiles within multi-cell stacks. This involves modelling the thermal characteristics of the stack as a whole to modelling each individual cell. The study investigates the thermal behaviour for two different scenarios: during standby periods when the pumps are turned off, and in a residential power arbitrage scenario for two types of membranes. It was found that the temperature gradient across the cells is most significant during the standby case, with the simulation results showing completely different thermal behaviours between the two systems.

Protonic Ceramic Fuel Cell Stacks

In this report, we present our work on development of fuel cell stacks based on proton-conducting ceramic materials. Such protonic-ceramics have demonstrated great promise for addressing the technological challenges now facing the solid-oxide fuel cell community. Protonic ceramic fuel cells (PCFCs) based on BaCe0.7Zr0.1Y0.1Yb0.1O3-δ have demonstrated high power density at low operating temperatures, achieving 450 mW / cm2 at 500 ºC under hydrogen fuel [1, 2]. PCFCs may also be more amenable to internal reforming of hydrocarbon fuels (CH4 + H2O => 3 H2 + CO). Proton conduction removes H2from the anode stream, thereby shifting the internal-reforming equilibrium chemistry towards the products, promoting higher methane conversion. While these materials show great promise, no multi-cell PCFC stacks have been fabricated to date. In this presentation, we review our progress in extending previous single “button-cell” experiments to small fuel-cell stacks. The stack design is shown in Figure 1; PCFC membrane-electrode assemblies (MEAs) are bonded within a ceramic frame, and the frame is packaged within ferritic-steel interconnects. Fabrication of MEAs involves solid-state reactive sintering (SSRS), where the protonic-ceramic fuel cell material is formed from simple parent oxides during co-sintering of the anode-electrolyte assembly. This single-step fabrication greatly reduces MEA-manufacturing costs and improves yields. Multi-cell stack prototypes have been demonstrated. A three-cell stack based on BaCe0.2Zr0.6Y0.2O3-δ electrolyte shows open-circuit voltage that approaches the theoretical Nernst potential (Figure 1), and demonstrates excellent stability over 500 hours of continuous operation. The stack also yields reasonable power density, reaching 250 mW/cm2at 650 ºC. Moving forward, higher performance is expected as we improve fabrication and packaging methods. Citations: [1] C. Duan, J. Tong, M. Shang, S. Nikodemski, M. Sanders, S. Ricote, A. Almansoori, R.P, O’Hayre, “Readily processed protonic ceramic fuel cells with high performance at low temperatures,” Science 349: 6254 (2015) 1321-1326. [2] J. Kim, S. Sengodan, G. Kwon, D. Ding, J. Shin, M. Liu, G. Kim, “Triple-conducting layered perovskites as cathode materials for proton-conducting solid oxide fuel cells,” ChemSusChem 7(2014) 2811–2815. Figure 1

Channel Dimensional Error Effect of Stamped Bipolar Plates on the Characteristics of Gas Diffusion Layer Contact Pressure for Proton Exchange Membrane Fuel Cell Stacks

Thin metallic bipolar plates (BPPs) fabricated by stamping technology are regarded as promising alternatives to traditional graphite BPPs in proton exchange membrane (PEM) fuel cell. However, during the stamping process, dimensional error in terms of the variation in channel height is inevitable, which results in performance loss for PEM fuel cell stack. The objective of this study is to investigate the effect of dimensional error on gas diffusion layer (GDL) pressure characteristics in the multicell stacks. At first, parameterized finite element (FE) model of metallic BPP/GDL assembly is established, and the height of channels is considered as varying parameters of linear distribution according to measurements of actual BPPs. Evaluation methods of GDL contact pressure are developed by considering the pressure distribution in the in-plane and through-plane directions. Then, simulation of the assembly process for a series of multicell stacks is performed to explore the relation between dimensional error and contact pressure based on the evaluation methods. Influences of channel number, cell number, and clamping force on the constitutive relation are discussed. At last, experiments are conducted and pressure sensitive films are used to obtain the actual GDL contact pressure. The numerical results show the same trend as experimental results. This study illustrates that contact pressure of each cell layer is in severely uneven distribution for the in-plane direction, and pressure change is unavoidable for the through-plane direction in the multicell stack, especially for the first several cells close to the endplate. The methodology developed is beneficial to the understanding of the dimensional error effect, and it can also be applied to guide the assembling of PEM fuel cell stack.