Surfactant doped polyaniline coatings for functionalized gas diffusion layers in low temperature fuel cells

Gas diffusion layers (GDLs) are essential for the proper distribution of the reaction gases, the removal of excess water as well as electrical contact in polymer electrolyte fuel cells (PEFCs). The production of state-of-the-art GDLs consists of many steps such as graphitization at high temperatures and hydrophobic treatments with polytetrafluoroethylene (PTFE) which increase the cost. In this study, an electrically conductive and hydrophobic polyaniline (PANI) coating was deposited on carbon paper via dip-coating and electropolymerization to fabricate PTFE-free GDLs. As a proof-of-concept, PANI-coated GDLs were tested as a cathodic GDL in a single cell PEFC and achieved a 42% higher maximum power compared to the reference measurement with a commercial GDL. Furthermore, these PTFE-free GDLs achieved contact angles up to 144° which is in the range of commercial GDLs. The chemical composition of the PANI-coating was investigated via infrared spectroscopy and energy dispersive X-ray spectroscopy (EDX) and the morphology was examined via scanning electron microscopy (SEM). Hence, the proposed method emerges as a possible strategy to simultaneously substitute PTFE and apply a protective and durable coating.</p

Exploring the role of electrode microstructure on the performance of non-aqueous redox flow batteries

Redox flow batteries are an emerging technology for long-duration grid energy storage, but further cost reductions are needed to accelerate adoption. Improving electrode performance within the electrochemical stack offers a pathway to reduced system cost through decreased resistance and increased power density. To date, most research efforts have focused on modifying the surface chemistry of carbon electrodes to enhance reaction kinetics, electrochemically active surface area, and wettability. Less attention has been given to electrode microstructure, which has a significant impact on reactant distribution and pressure drop within the flow cell. Here, drawing from commonly used carbon-based diffusion media (paper, felt, cloth), we systematically investigate the influence of electrode microstructure on electrochemical performance. We employ a range of techniques to characterize the microstructure, pressure drop, and electrochemically active surface area in combination with in-operando diagnostics performed in a single electrolyte flow cell using a kinetically facile redox couple dissolved in a non-aqueous electrolyte. Of the materials tested, the cloth electrode shows the best performance; the highest current density at a set overpotential accompanied by the lowest hydraulic resistance. We hypothesize that the bimodal pore size distribution and periodic, well-defined microstructure of the cloth are key to lowering mass transport resistance

Assessing the Versatility and Robustness of Pore Network Modeling to Simulate Redox Flow Battery Electrode Performance

Porous electrodes are core components that determine the performance of redox flow batteries. Thus, optimizing their microstructure is a powerful approach to reduce system costs. Here we present a pore network modeling framework that is microstructure and chemistry agnostic, iteratively solves transport equations in both half-cells, and utilizes a network-in-series approach to simulate the local transport phenomena within porous electrodes at a low computational cost. In this study, we critically assess the versatility and robustness of pore network models to enable the modeling of different electrode geometries and redox chemistries. To do so, the proposed model was validated with two commonly used carbon fiber-based electrodes (a paper and a cloth), by extracting topologically equivalent networks from X-ray tomograms, and evaluated for two model redox chemistries (an aqueous iron-based and a non-aqueous TEMPO-based electrolyte). We find that the modeling framework successfully captures the experimental performance of the non-aqueous electrolyte but is less accurate for the aqueous electrolyte which was attributed to incomplete wetting of the electrode surface in the conducted experiments. Furthermore, the validation reveals that care must be taken when extracting networks from the tomogram of the woven cloth electrode, which features a multiscale microstructure with threaded fiber bundles. Employing this pore network model, we elucidate structure-performance relationships by leveraging the performance profiles and the simulated local distributions of physical properties and finally, we deploy simulations to identify efficient operation envelopes

Bottom-up design of porous electrodes by combining a genetic algorithm and a pore network model

The microstructure of porous electrodes determines multiple performance-defining properties, such as the available reactive surface area, mass transfer rates, and hydraulic resistance. Thus, optimizing the electrode architecture is a powerful approach to enhance the performance and cost-competitiveness of electrochemical technologies. To expand our current arsenal of electrode materials, we need to build predictive frameworks that can screen a large geometrical design space while being physically representative. Here, we present a novel approach for the optimization of porous electrode microstructures from the bottom-up that couples a genetic algorithm with a previously validated electrochemical pore network model. In this first demonstration, we focus on optimizing redox flow battery electrodes. The genetic algorithm manipulates the pore and throat size distributions of an artificially generated microstructure with fixed pore positions by selecting the best-performing networks, based on the hydraulic and electrochemical performance computed by the model. For the studied VO2+/VO2+ electrolyte, we find an increase in the fitness of 75 % compared to the initial configuration by minimizing the pumping power and maximizing the electrochemical power of the system. The algorithm generates structures with improved fluid distribution through the formation of a bimodal pore size distribution containing preferential longitudinal flow pathways, resulting in a decrease of 73 % for the required pumping power. Furthermore, the optimization yielded an 47 % increase in surface area resulting in an electrochemical performance improvement of 42 %. Our results show the potential of using genetic algorithms combined with pore network models to optimize porous electrode microstructures for a wide range of electrolyte composition and operation conditions.</p

Engineering Redox Flow Battery Electrodes with Spatially Varying Porosity Using Non-Solvent-Induced Phase Separation

Redox flow batteries (RFBs) are a promising electrochemical platform for efficiently and reliably delivering electricity to the grid. Within the RFB, porous carbonaceous electrodes facilitate electrochemical reactions and distribute the flowing electrolyte. Tailoring electrode microstructure and surface area can improve RFB performance, lowering costs. Electrodes with spatially varying porosity may increase electrode utilization and provide surface area in reaction-limited zones; however, the efficacy of such designs remains an open area of research. Herein, a non-solvent-induced phase-separation (NIPS) technique that enables the reproducible synthesis of macrovoid-free electrodes with well-defined across-thickness porosity gradients is described. The monotonically varying porosity profile is quantified and the physical properties and surface chemistries of porosity-gradient electrodes are compared with macrovoid-containing electrode, also synthesized by NIPS. Then, the electrochemical and fluid dynamic performance of the porosity-gradient electrodes is evaluated, exploring the effect of changing the direction of the porosity gradient and benchmarking against the macrovoid-containing electrode. Lastly, the performance is examined in a vanadium RFB, finding that the porosity-gradient electrode outperforms the macrovoid electrode, is independent of gradient direction, and performs favorably compared to advanced electrodes in the contemporary literature. It is anticipated that the approach motivates further exploration of microstructurally tailored electrodes in electrochemical systems.</p

Framework for additive manufacturing of porous Inconel 718 for electrochemical applications

Porous electrodes were developed using laser powder bed fusion of Inconel 718 lattice structures and electrodeposition of a porous nickel catalytic layer. Laser energy densities of 83-333 J/m were used to fabricate 500 $\mu$m thick electrodes made of body centered cubic unit cells of 200-500 $\mu$m and strut thicknesses of 100-200 um. Unit cells of 500 $\mu$m and strut thickness of 200 $\mu$m were identified as optimum. Despite small changes in feature sizes by the energy input, the porosity of more than 50 percent and pore size of 100 $\mu$m did not change. Nickel electrodeposition created a network of submicrometer pores. The electrodes' electrochemical efficiency was assessed by analysing hydrogen/oxygen evolution reaction (HER/OER) in a three-electrode setup. For HER, a much larger maximum current density of -372 mA/cm$^2$ at a less negative potential of -0.4 V vs RHE (potential against reversible hydrogen electrode) was produced in the nickel-coated samples, as compared to -240 mA/cm$^2$ at -0.6 V in the bare one, indicating superior performance of the coated sample. For OER, however, both bare and nickel-coated electrodes similarly showed a maximum current density of 350 mA/cm$^2$ at 1.8 V vs RHE due to performance trade-offs arising from sample composition and structure

Framework for additive manufacturing of porous Inconel 718 for electrochemical applications

Porous electrodes were developed using laser powder bed fusion of Inconel 718 lattice structures and electrodeposition of a porous nickel catalytic layer. Laser energy densities of 83-333 J/m were used to fabricate 500 $\mu$m thick electrodes made of body centered cubic unit cells of 200-500 $\mu$m and strut thicknesses of 100-200 um. Unit cells of 500 $\mu$m and strut thickness of 200 $\mu$m were identified as optimum. Despite small changes in feature sizes by the energy input, the porosity of more than 50 percent and pore size of 100 $\mu$m did not change. Nickel electrodeposition created a network of submicrometer pores. The electrodes' electrochemical efficiency was assessed by analysing hydrogen/oxygen evolution reaction (HER/OER) in a three-electrode setup. For HER, a much larger maximum current density of -372 mA/cm$^2$ at a less negative potential of -0.4 V vs RHE (potential against reversible hydrogen electrode) was produced in the nickel-coated samples, as compared to -240 mA/cm$^2$ at -0.6 V in the bare one, indicating superior performance of the coated sample. For OER, however, both bare and nickel-coated electrodes similarly showed a maximum current density of 350 mA/cm$^2$ at 1.8 V vs RHE due to performance trade-offs arising from sample composition and structure