Lung-inspired, fractal flow-fields hold great potential in improving the performance of polymer electrolyte membrane fuel cells (PEMFCs) by providing uniform gas distribution across the electrodes ...and ensuring minimum entropy production in the whole system. However, the inherent susceptibility of the fractal flow-fields to flooding renders their use inadequate at high humidity conditions. In-depth understanding of water management in lung-inspired flow-fields is indispensable for the implementation of alternative outlet channel geometries or engineered water removal strategies to alleviate flooding. Here, liquid water formation and transport across the lung-inspired and serpentine flow-field based PEMFCs are evaluated using neutron radiography. The results reveal a propensity to flooding in the interdigitated outlet channels of the fractal flow-field with N = 4 generations as a result of slow gas velocity and narrow channel dimensions, which leads to significant performance deterioration. Neutron images also elucidate the importance of ensuring a well-defined internal channel structure of the fractal flow-fields to prevent backflow of liquid water via wicking and capillary pressure build-up arising from the narrow inlet gas channels and hydrophobic gas diffusion layer.
•Neutron radiographs are presented for the lung-inspired and serpentine flow-fields.•A well-defined channel structure of the fractal flow-field is indispensable.•Water removal strategies required to alleviate flooding in the fractal flow-field.
An efficient approach to improve the catalytic activity of titanosilicates is introduced. The Doehlert matrix (DM) statistical model was utilized to probe the synthetic parameters of mesoporous ...titanosilicate microspheres (MTSM), in order to increase their catalytic activity with a minimal number of experiments. Synthesis optimization was carried out by varying two parameters simultaneously: homogenizing temperature and surfactant weight. Thirteen different MTSM samples were synthesized in two sequential ‘matrices’ according to Doehlert conditions and were used to catalyse the epoxidation of cyclohexene with
tert
-butyl hydroperoxide. The samples (and the corresponding synthesis conditions) with superior catalytic activity in terms of product yield and selectivity were identified. In addition, this approach revealed the limiting values of each synthesis parameter, beyond which the material becomes catalytically ineffective. This study demonstrates that the DM approach can be broadly used as a powerful and time-efficient tool for investigating the optimal synthesis conditions of heterogeneous catalysts.
Inefficient usage of expensive platinum catalyst has plagued the design of PEM fuel cells and contributed to the limited production and use of fuel cell systems. Here, it is shown that hierarchical ...optimization can increase platinum utilization 30-fold over existing catalyst layer designs while maintaining power densities over 0.35W/cm2. The cathode catalyst layer microstructure is optimized with respect to platinum utilization (measured as kilowatts of electricity produced per gram of platinum). A one-dimensional agglomerate model that accounts for liquid water saturation is used in this study. The cathode catalyst layer microstructure is optimized by manipulating the platinum loading (mPt), platinum-to-carbon ratio (Pt|C), and catalyst layer void fraction (εVcl). The resulting catalyst layer microstructure features ultra-low platinum loadings of roughly 0.01mg/cm2.
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•Layer wise PCB based fractal lung-inspired cathode flow field.•Cost effective and light weight method of fractal flow field development.•Enhanced performance over serpentine flow field at a range of ...operating conditions.•Uniform and sustained operation of fractal flow field with better water management.
Fractal cathode flow-fields, inspired by the flow mechanism of air inside lungs, can provide homogeneous, scalable and uniform distribution of reactants to polymer electrolyte fuel cell (PEFC) electrodes. However, the complex 3D flow-fields demonstrated previously face manufacturing challenges, such as requiring selective laser sintering, an additive manufacturing method that is expensive to scale up. Here, a lung-inspired cathode flow-field is introduced and fabricated using low-cost, lightweight printed circuit boards (PCB). The uniformity and alignment between individual PCB layers producing the fractal hierarchy of flow channels have been characterised using X-ray computed tomography (X-ray CT). The performance of the fractal flow-field exceeds that of conventional single-serpentine flow-fields and is particularly beneficial when operating on air with a low relative humidity. The lung-inspired design is shown to lead to a more stable operation than the single-serpentine design, as a result of uniform distribution of reactants.
•Design, development and testing of two different configurations of fractal PEMFCs.•Evaluation of the hydration state of fractal PEMFCs via acoustic emission method.•Performance diagnosis of fractal ...PEMFCs using acoustic emission analysis.•Correlation of acoustic emission analysis with conventional electrochemical studies.
Techniques for evaluating water management are critical to diagnose the performance of polymer electrolyte membrane fuel cells (PEMFCs). Acoustic emission as a function of polarisation (AEfP) has been recently introduced as a non-invasive, non-destructive method to analyse the water generation and removal inside a PEMFC during polarisation. AEfP was shown to provide unique insight into water management within a conventional PEMFC and correlating it to cell performance. Here, AEfP is used to characterise the performance of fractal PEMFCs by evaluating the hydration conditions inside them. This is achieved by probing the water dynamics inside two different fractal flow-field based PEMFCs, namely 1-way and 2-way fractal PEMFCs, and measuring the corresponding acoustic activity generated from them. AEfP is performed on the fractal PEMFCs under relatively humid (70% RH) and fully humidified (100% RH) reactant relative humidity (RH) conditions. Flooding in the 2-way fractal PEMFC, as opposed to the 1-way fractal PEMFC, is demonstrated under different operating conditions by the relatively higher acoustic activity it generates. Corroborating evidence of flooding in the 2-way fractal flow-field under different conditions is provided by its polarisation curves, impedance tests and galvanostatic (current hold) measurements.
•Acoustic emission technique to probe water formation and removal inside fuel cells.•Acoustic emission polarisation (AEP) in correlation with performance polarisation.•Identify the presence of water ...in flow channels based on cell polarisation level.•AEP validated by conventional electrochemical and current hold measurements.
Understanding water management is a crucial aspect in the development of improved polymer electrolyte fuel cells (PEFCs). Separating the performance degradation due to dehydration, water flooding and reactant starvation in PEFCs is a major challenge. In this study, acoustic emission (AE) analysis, a non-invasive and non-destructive diagnostic tool, is utilised to probe water formation and removal inside an operating fuel cell. In the acoustic emission as a function of polarisation (AEfP) method, AE activity from the PEFC is measured in terms of cumulative absolute AE energy (CAEE) hits during operation at discrete points on the polarisation curve. AEfP can identify the presence of liquid water in flow channels and correlate its formation and removal with the level of cell polarisation, and consequent internal temperature. Correlation between acoustic activity and water generation, supply and removal is achieved by varying current (polarisation), cathode air feed relative humidity (RH) and cell temperature, respectively. Features such as initial membrane hydration, liquid water formation, ‘flushing’ and the transition from ‘wet-channel’ to ‘dry-channel’ operation are identified using AE analysis, thereby providing a powerful and easy to implement diagnostic for PEFCs.
Some of the new liquid water management systems in polymer electrolyte membrane (PEM) fuel cells hold great potential in providing flood-free performance and internal humidification. However, current ...water management systems entail major setbacks, which either inhibit implementation into state-of-the-art architectures, such as stamped metal flow-fields, or restrict their application to certain channel configurations. Here, a novel water management strategy is presented that uses capillary arrays to control liquid water in PEMFCs. These capillaries are laser-drilled into the land of the flow-fields and allow direct removal (wicking) or supply of water (evaporation), depending on the local demand across the electrode. For a 6.25 cm2 active area parallel flow-field, a ∼92% improvement in maximum power density from capillary integration was demonstrated. The proposed mechanism serves as a simple and effective means of achieving robust and reliable fuel cell operation, without incurring additional parasitic losses due to the high pressure drop associated with conventional serpentine flow-fields.
•New capillary-based water management strategy is presented for PEM fuel cells.•The capillaries allow redistribution of liquid water across the electrode.•Parallel flow-field modified with capillaries outperforms serpentine flow-field.
•Simulations of a lung-inspired polymer electrolyte membrane fuel cell are presented.•The optimal number of generations corresponds to transition from flow to diffusion.•The effect of GDL thickness ...on the lung-inspired PEMFC performance is investigated.•The potential for 80% increase in PEMFC volumetric power density is demonstrated.
A finite-element model of a polymer electrolyte membrane fuel cell (PEMFC) with fractal branching, lung-inspired flow-field is presented. The effect of the number of branching generations N on the thickness of the gas diffusion layer (GDL) and fuel cell performance is determined. Introduction of a fractal flow-field to homogenize reactant concentration at the flow-field | GDL interface allows for the use of thinner GDLs. The model is coupled with an optimized cathode catalyst layer microstructure with respect to platinum utilization and power density, revealing that the 2020 DoE target of ~8 kW/gPt is met at N = 4 generations, and a platinum utilization of ~36 kW/gPt is achieved at N = 6 generations. In terms of the overall fuel cell stack architecture, our results indicate that either the platinum loading or the number of cells in the stack can be reduced by ~75%, the latter option of which, when combined with a 100 µm GDL, can lead to >80% increase in the volumetric power density of the fuel cell stack.
Metal foam flow-fields (MFFs) exhibit immense potential for enhancing the performance of polymer electrolyte fuel cells (PEFCs) owing to their advantageous pore connectivity and abundant gas ...pathways. Nevertheless, challenges remain with the conventional MFF concerning reactant homogeneity and water management. To address these concerns, this study incorporates a fractal manifold into the MFF design. By employing operando neutron imaging, device-level testing, and electrochemical impedance spectroscopy (EIS), a comprehensive understanding of mass transfer and water management characteristics across the fractal manifold MFF is obtained. This novel design delivers better cell performance and lower mass transport resistance compared to the conventional MFF under all experimental conditions investigated. Notably, neutron imaging reveals that the fractal manifold MFF consistently exhibits a reduced liquid water content and more uniformly distributed liquid water compared to the conventional MFF. These superior characteristics of the design contribute to a substantial ∼15% increase in maximum power density compared to the conventional MFF-based PEFC. The results indicate the potential for further performance improvement by optimizing manifold parameters.
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•First use of a fractal manifold to improve mass transport and water management in the PEFC using metal foam flow fields.•Operando neutron imaging confirms the water distribution in the PEFC is more even using fractal manifold design.•Electrochemical performance is improved by around 15% with this approach.
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