Chemical membrane degradation through the Fenton’s reaction is one of the main lifetime‐limiting factors for polymer‐electrolyte fuel cells. In this work, a comprehensive, transient membrane ...degradation model is developed to capture and elucidate the complex in situ degradation mechanism. A redox cycle of iron ions is discovered within the membrane electrolyte assembly, which sustains the FeII concentration and results in the most severe chemical degradation at open circuit voltage. The cycle strength is critically reduced at lower cell voltages, which leads to an exponential decrease in FeII concentration and associated membrane degradation rate. When the cell voltage is held below 0.7 V, a tenfold reduction in cumulative fluoride release is achieved, which suggests that intermediate cell voltage operation would efficiently mitigate chemical membrane degradation and extend the fuel cell lifetime.
Membrane stability controlled by iron ion dynamics: An essential, yet previously missing link in the fundamental understanding of chemical membrane degradation in polymer electrolyte fuel cells is presented. An iron ion redox cycle is discovered within the membrane and catalyst layers during fuel cell operation. This redox cycle is shown to be the underlying mechanism for the cell potential dependency of the chemical degradation of the membrane, which can be mitigated by operation at intermediate cell voltages.
Understanding of degradation mechanisms present in polymer electrolyte fuel cells (PEFCs) is important to continue the integration of this clean energy technology into everyday life. Further ...comprehension of the interaction between various components during fuel cell operation is also critical in this context. In this work, a four-dimensional operando X-ray computed tomography method is developed for combined visualization of all PEFC components as well as transient water distribution residing in the cell, which results as a by-product of the electrochemical reaction. Time resolved, identical-location visualization through degradation stages is uniquely enabled by the non-invasive and non-destructive qualities of this method. By applying an accelerated stress test that targets cathode catalyst layer (CCL) corrosion, novel observations resulting from morphological changes of the CCL such as reduction in the water volume in the adjacent gas diffusion layer, CCL crack formation and propagation, membrane swelling, as well as quantification of local carbon loss is achieved. Additionally, insight into features that contribute to reduced fuel cell performance is enabled by the use of this specialized imaging technique, such as increased membrane undulation causing delamination and separation of the CCL from the microporous layer, which greatly affects liquid water pathways and overall device performance.
Perfluorosulfonic acid ionomer membranes are subjected to simultaneous chemical and mechanical degradation under fuel cell operation. Despite the importance of membrane durability, the understanding ...of its structural degradation and failure modes has been considerably restricted by conventional 2D imaging. In this work, non-invasive micro X-ray computed tomography (XCT) is adopted to visualize the 3D membrane decay at different life stages during combined chemical and mechanical degradation. A detailed survey exhibits damage density of 6 and 10 cracks per mm2 observed at the near-final and final end of life stages respectively. Through-thickness membrane cracks with unbranched I-shaped cracks and Y- and X- shaped cracks with one and two branches respectively are observed. The observed damage development at each life stage is correlated to supplementary diagnostic data including hydrogen leak rate, open circuit voltage, and tensile strength. In particular, large X-shaped cracks formed due to embrittlement from underlying chemical degradation are deemed to have a critical impact on the eventual failure development by facilitating large hydrogen leaks. Overall, the comprehensive 3D perspective enabled by XCT imparts new knowledge pertaining to the degradation process, and could also be extended to other fuel cell failure modes and degradation mechanisms.
Open-cathode fuel cells use air cooling to effectively reduce system cost. However, due to the challenging hygrothermal environment, they generally suffer from low performance compared to ...conventional, liquid-cooled cells. A pre-validated, three-dimensional computational model is used in the present work to determine the effects of different sub-component designs, namely the polymeric membrane, composition of the cathode catalyst layer (CCL), and structure of the cathode microporous layer (CMPL), on the performance of an open-cathode fuel cell. This comprehensive parametric study performed on a total of 90 cases shows the increment in current density to be 7% and 31% by improvising the membrane and CCL design, respectively, at 0.6 V. A steep increase of 87% is also achieved by strategically modifying the CMPL design at 0.4 V operation. An overall increment of 119% and 131% in current density is achieved for the best membrane electrode assembly (MEA) design at 0.6 and 0.4 V, respectively, as compared to the baseline design. These improvements are achieved by collective improvements in kinetics, oxygen mass transport, ohmic resistance, self-heating, and water retention in the ionomer phase. The proposed MEA design could facilitate open-cathode fuel cell stacks with 2× higher power output or 56% lower weight and materials cost for a given power demand.
This article presents a new approach for environmentally benign, low‐cost batteries intended for single‐use applications. The proposed battery is designed and fabricated using exclusively organic ...materials such as cellulose, carbon, and wax and features an integrated quinone‐based redox chemistry to generate electricity within a compact form factor. This primary capillary flow battery is activated by the addition of a liquid sample and has shown continuous operation up to 100 min with an output voltage that can be conveniently scaled to match the voltage needs of portable electronic devices (1.5–3.0 V). Once depleted, the battery can be disposed of without the need for any recycling facility, as its components are nontoxic and shown to be biotically degradable in a standardized test. The practical utility of the battery is demonstrated by direct substitution of a lithium ion coin cell in a diagnostic application.
A microbattery entirely based on nontoxic, organic, abundant, and inexpensive raw materials and fabricated with cost‐efficient scalable manufacturing processes is presented. The nature of the device allows it to be discarded without the need for any specific recycling facilities after use, because it biotically degrades to simple compounds, such as CO2, CH4, H2O, and N2, as a result of the action of micro‐organisms.
This work demonstrates the feasibility of measuring electrochemical reaction rates on common flow-through porous electrodes by traditional Tafel analysis. A customized microfluidic channel electrode ...was designed and demonstrated by measuring the intrinsic kinetics of the V2+/V3+ and VO2+/VO2+ redox reactions in carbon paper electrodes under forced electrolyte flow. The exchange current density of the V2+/V3+ reaction was found to be nearly two orders of magnitude slower than the VO2+/VO2+ reaction, indicating that this may be the limiting reaction in vanadium redox flow batteries. The forced convection in this technique is found to generate reproducible exchange current densities which are consistently higher than for conventional electrochemical methods due to improved mass transport.
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•A customized microfluidic cell is devised to study flow-through porous electrodes.•Tafel analysis is shown for the first time to be feasible on porous carbon paper.•The kinetics of VO2+/VO2+ and V2+/V3+ reactions are directly evaluated and compared.•The V2+/V3+ reaction is likely the limiting reaction in vanadium redox batteries.•This technique has potential for understanding mass transport in porous electrodes.
Creep as a time-dependent mechanical damage acting either independently or in conjunction with other degradation mechanisms is known to reduce the membrane durability of polymer electrolyte fuel ...cells (PEFCs). Due to the important ionomer coupling of membrane and catalyst layers in PEFCs, the present work evaluates membrane creep when constrained within a catalyst coated membrane (CCM). Three key factors dominating creep life in commonly used perfluorosulfonic acid (PFSA) ionomer membranes, including creep stress, temperature, and relative humidity, were investigated by applying ex-situ creep loading and unloading experiments under controlled temperature and humidity conditions. The creep strain and recovery of the CCM were found to be highly dependent on the environmental conditions and applied stress levels, where the temperature effect on creep strain was the most significant. Repetitive creep – recovery cycles revealed that significant creep damage can accumulate in the material over time. This accumulated creep damage was found to be independent of the loading frequency while both peak strain and permanent deformation increased with the stress duration. Based on the present findings, it is recommended to reduce the operating temperature and ensure adequate membrane hydration in order to mitigate harmful creep effects in PEFCs.
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•Creep properties of catalyst coated membranes were compared to pure membranes.•CCM creep was strongly influenced by the applied stress and hygrothermal conditions.•Catalyst layer microcracks were formed at the yield point of the CCM.•Accumulation of creep damage was observed during successive creep-recovery steps.
We propose new membraneless microfluidic fuel cell architectures employing graphite rod electrodes. Commonly employed as mechanical pencil refills, graphite rods are inexpensive and serve effectively ...as both electrode and current collector for combined all-vanadium fuel/oxidant systems. In contrast to film-deposited electrodes, the geometry and mechanical properties of graphite rods enable unique three-dimensional microfluidic fuel cell architectures. Planar microfluidic fuel cells employing graphite rod electrodes are presented here first. The planar geometry is typical of microfluidic fuel cells presented to date, and permits fuel cell performance comparisons and the evaluation of graphite rods as electrodes. The planar cells produce a peak power density of 35
mW
cm
−2 at 0.8
V using 2
M vanadium solutions, and provide steady operation at flow rates spanning four orders of magnitude. Numerical simulations and empirical scaling laws are developed to provide insight into the measured performance and graphite rods as fuel cell electrodes.
We also present the first three-dimensional microfluidic fuel cell architecture with multiple electrodes. The proposed fuel cell architecture, consisting of a hexagonal array of graphite rods, enables scale-up/integration of microfluidic fuel cell technology as well as power conditioning flexibility beyond that of the traditional fuel cell stack. When provided the same flow rate as the planar cell, the array cell generated an order of magnitude higher power output. The array architecture also enabled unprecedented levels of single pass fuel utilization, up to 78%.
Open-cathode polymer electrolyte membrane fuel cells (PEMFCs) utilize a unique air-cooled system design to eliminate the humidifiers, air compressor, and liquid cooling loop of conventional, ...liquid-cooled PEMFC systems, thereby greatly reducing system cost. However, the open-cathode PEMFC performance is restricted by poor humidification, high membrane and charge transfer resistances, and overheating due to inefficient thermal and water management. This work aims to strategically modify the membrane electrode assembly (MEA) design to overcome these issues and achieve high open-cathode PEMFC performance that approaches that of liquid-cooled systems. The use of thinner membrane along with short side chain ionomer is found to elevate the cell performance due to increased water retention at the cathode catalyst layer (CCL) and decreased ohmic losses. Thinner gas diffusion layers with high porosity enable additional cell performance increment by improving oxygen availability at the CCL. An overall current density rise of 88% at 0.6 V and 53% at 0.4 V is achieved by the strategically designed MEA for open-cathode cells. The enhanced power density enabled by the custom MEA can both reduce the stack cost and expand the power range of open-cathode PEMFCs, thus expanding their potential use for low-cost fuel cell system applications.
Highlights
Five strategically designed MEAs are made and tested in an open-cathode fuel cell.
Thin membrane with short side chain ionomer enhances water retention.
Thin, high-porosity GDL improves oxygen transport to the cathode.
Strategic MEA design can offer high power density for open-cathode fuel cells.
Catalyst coated membranes (CCMs) in polymer electrolyte fuel cells are subjected to mechanical stresses in the form of fatigue and creep that deteriorate the durability and lifetime of the cells. The ...present article aims to determine the effect of in-situ hygrothermal fatigue on the microstructure and mechanical properties of the CCM. The fatigue process is systematically explored by the application of two custom-developed accelerated mechanical stress test (AMST) experiments with periodic extraction of partially degraded CCMs. Cross sectional and top surface scanning electron microscope (SEM) images of the end-of-test CCMs reveal the formation of mechanically induced cracks and delamination due to cyclic tensile and compressive fatigue stress. Tensile and expansion tests are conducted at different stages of degradation to evaluate the evolution in the mechanical and hygrothermal properties of the CCM. The tensile test results indicate gradual reductions in final strain, ultimate tensile strength, and fracture toughness with increasing number of fatigue cycles. The decay in tensile properties is attributed to the microstructural damage and micro-cracks formed during the AMST. Moreover, it is shown that the hygrothermal expansion of the CCM is more sensitive to conditioning than mechanical degradation.
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