Gas Diffusion Electrodes For Fuel Cells Research Articles

Stereolithography 3D Printing As a Versatile Tool to Manufacture Porous Electrodes for Redox Flow Batteries

Redox flow batteries show potential for large-scale energy storage; yet their current costs limit widespread implementation. As the porous electrode microstructure defines the available surface area for electrochemical reactions, electrolyte transport, and fluid pressure drop [1], enhancing the electrode performance is a promising strategy to increase power density and thus reduce system costs. Conventional fibrous porous electrodes are repurposed from fuel cell gas diffusion electrodes and have not been tailored to sustain the requirements of liquid-phase electrochemistry. Therefore, new manufacturing techniques offering high control over the electrode microstructure and resulting properties need to be developed [2]. In this context, 3D printing techniques are uniquely suited to manufacture controlled and deterministic architectures, enabling the tuning of electrochemical performance and hydraulic resistance [3].In this work, we employ stereolithography 3D printing using a commercial desktop printer, followed by carbonization, to manufacture model grid structures and investigate their potential as redox flow battery electrodes. We use microscopy, tomography, spectroscopy, fluid dynamics, and electrochemical diagnostics, to evaluate the thermal behavior, manufacturing fidelity, and fluid and mass transport performance of ordered lattice structures in non-aqueous redox flow cells. We find that the flow field geometry (flow through versus interdigitated), printing orientation with respect to the printing platform, and pillar geometry (Figure 1a) influence the flow cell performance. The printing orientation impacts the surface roughness and resin spreading, which determines the electrode morphology, influencing the internal surface area, shrinking direction after carbonization, and thus the charge transfer, mass transfer, and hydraulic resistances. Moreover, mass transfer rates within the electrode are increased by altering the pillar shape to a helical or triangular design, or by using an interdigitated flow field design, providing directions to improve electrolyte mixing through the electrode. Although the commercial electrodes feature a greater internal surface area and therefore a better electrochemical performance, the area-normalized mass transfer coefficients and pressure drop can be reduced by utilizing 3D printed electrodes (Figure 1b-c). Going forward, 3D printing technologies that enable finer features combined with carbonization at elevated temperatures can be used to manufacture multi-scale electrode structures simultaneously providing excellent electrochemical performance and low hydraulic resistance. We anticipate that combining 3D printing with emerging computational topology optimization approaches could enable the bottom-up design of advanced electrode materials for electrochemical devices [4]. Acknowledgments The authors gratefully acknowledge the Dutch Research Council (NWO) through the Talent Research Program Veni (17324) and the 4TU.HTM grant for financial support.

Investigating Mass Transfer Relationships in Stereolithography-Based 3D Printed Electrodes for Redox Flow Batteries

Redox flow batteries are a promising option for large-scale energy storage but remain too costly for widespread deployment. One approach to increase cost competitiveness is to improve the power density and efficiency of the electrochemical cell. Porous electrodes determine the performance of the cell, and their three-dimensional structure determines the electrolyte transport, fluid pressure drop, and the available surface area for electrochemical reactions [1]. Conventional porous electrodes are fibrous mats assembled in coherent structures [2]; however, they are repurposed from fuel cell gas diffusion electrodes and have not been tailored for redox flow batteries. Hence, an effective means to improve the performance of redox flow batteries is by engineering porous electrode structures to sustain the requirements of liquid-phase electrochemistry. To this purpose, there is a need to develop new manufacturing techniques affording a higher degree of control over the electrode microstructure and resulting properties. Additive manufacturing, or 3D printing, can be employed to manufacture customized, controlled, and deterministic architectures, enabling the fine-tuning of the electrical and hydraulic performance of porous electrodes [3].In this study, we manufacture model grid structures using stereolithography 3D printing followed by carbonization (Figure 1a) to tune the physiochemical properties of electrodes to be used in redox flow batteries. We employ a suite of microscopy, tomography, spectroscopy, fluid dynamics, and electrochemical diagnostics to understand the impact of the electrode structure on the mass transport and hydraulic performance of ordered lattice structures in flow cells. Here, the influence of the printing direction, pillar geometry (Figure 1b), and flow field type on mass transport is investigated using an organic electrolyte. We elucidate correlations between the electrode structure and performance metrics, namely surface area, pressure drop, and mass transfer correlations. We find that the printing orientation impacts the electrode performance through a change in electrode morphology caused by resin spreading and surface roughness, affecting the shrinking direction upon carbonization, internal surface area, and thus the charge transfer, mass transfer, and hydraulic resistances. Furthermore, we find that mass transfer rates within the electrode are enhanced by using an interdigitated flow field or by altering the pillar shape to a helical or triangular design, which we hypothesize improves mixing. Compared to commercial carbon-fiber electrodes, the pressure drop is significantly reduced (Figure 1c) as expected due to larger pore sizes (~500 μm for the grids vs. 2-100 μm for the Freudenberg H23 paper electrode and 2-300 μm for the ELAT Cloth electrode). Even though the commercial electrodes feature a superior internal surface area compared to the 3D printed electrodes, their area normalized mass transfer coefficients are lower (Figure 1d). Going forward, the use of printing technologies enabling finer features in combination with carbonization at elevated temperatures can be used to manufacture multiscale electrodes simultaneously providing low hydraulic resistance and excellent electrochemical performance. Additive manufacturing in combination with emerging computational approaches in topology optimization [4] might enable the bottom-up design and manufacturing of advanced electrode materials.

Role of the microporous layer in the redistribution of phosphoric acid in high temperature PEM fuel cell gas diffusion electrodes

In this work, pore network modelling was used to investigate the role of the microporous layer (MPL) in the redistribution of phosphoric acid (PA) in a high temperature polymer electrolyte membrane fuel cell (HT-PEMFC) gas diffusion electrode (GDE). A HT-PEMFC GDE composed of a catalyst layer (sprayed in-house), MPL, and fibrous substrate was assembled, and its three-dimensional (3D) geometry was imaged using synchrotron X-ray microcomputed tomography. The spatial distribution of pore spaces and their connections were identified based on the 3D geometry, and an equivalent pore network of the GDE was obtained for simulating the PA transport with an invasion percolation algorithm. The predicted mass redistribution of PA in the GDE was found to be in excellent agreement with experimental values reported in the literature. The presence of the MPL was found to encourage the redistribution of PA within the CL and inhibited PA leaching toward the channel. To the authors’ best knowledge, this study is the first numerical simulation of PA transport in a HT-PEMFC GDE.

Aging Studies of Pt/Glassy Carbon Model Electrocatalysts

A model Pt catalyst nanoarray on planar glassy carbon (GC) was synthesized using a diblock copolymer synthesis route and subjected to a potential cycle aging procedure. Preliminary experimental data show initial and degradation behavior similar to that of proton exchange membrane fuel cell gas diffusion electrodes. Commonalities include similar cyclic voltammetry and CO stripping voltammetry features, Pt active surface area temporal changes, and Pt particle size effect on CO oxidation peak potential. Furthermore, atomic force microscopy provides evidence for both Pt dissolution and Pt nanoparticle migration/coalescence as aging mechanisms. Experimental data obtained with Pt model catalysts also revealed that Pt dissolution may be the rate-determining step under certain operating conditions. Thus, the beneficial use of Pt/GC model electrocatalysts as surrogates for commercial gas diffusion electrodes is validated.

Properties of Gas Diffusion Electrodes Containing Sulfonated Poly(ether ether ketone)

The use of sulfonated poly(ether ether ketone) as the proton-conducting medium within the electrocatalytic layer of fuel cell gas diffusion electrodes (GDEs) is described. An electrochemically active surface area determined by cyclic voltammetry and electrochemical impedance spectroscopy was studied as a function of ion exchange capacity. Maximum activity is related to the concentration of sulfonate groups within the catalyst layer. Despite their relatively high electrochemical surface area and catalyst utilization, fuel cell performance is rather poor compared to Nafion-based GDEs. Fuel cell performance is greatly enhanced when polytetrafluoroethylene (PTFE) is incorporated into the catalyst layer. The addition of PTFE results in GDEs with larger pore diameters that are less prone to flooding. The incorporation of an ionomer of similar chemical structure to the proton-conducting membrane employed is shown to reduce interfacial resistance. Electrode structures of this type may prove useful for investigating other novel proton-conducting materials.

Characterization of Ionic Conductivity Profiles within Proton Exchange Membrane Fuel Cell Gas Diffusion Electrodes by Impedance Spectroscopy

Characterization of Ionic Conductivity Profiles within Proton Exchange Membrane Fuel Cell Gas Diffusion Electrodes by Impedance Spectroscopy

Measurements of proton conductivity in the active layer of PEM fuel cell gas diffusion electrodes

This paper reports further studies to understand and optimize the Membrane and Electrode Assembly (MEA) structure in Polymer Electrolyte Membrane Fuel Cells (PEMFCs). The effective proton conductivity in the active catalyst layer was measured as a function of its composition, which consisted of platinum catalyst on carbon support (E-Tek) and Nafion® polymer electrolyte (DuPont de Nemours). The conductivity was calculated from the resistance added to a standard MEA by the addition of an inactive composite layer in the electrolyte path between the anode and cathode. The specific conductivity of the active layer was found to be proportional to the volume fraction of Nafion® in the composite mixture, following the relationship κ eff H + ≈0.078 ε Nafion+0.004 S cm −1. Modeling studies showed that this ionic conductivity limits the utilized active layer thickness to 20–25 μm.

Impedance studies of porous electrodes

Various approaches to the application of impedance methods to the study of porous electrodes are reviewed with reference to two examples. These are: (i) double-layer charging in porous structures; (ii) fuel cell gas-diffusion electrodes. Simple pore models are described first, and their extension to cases with charge-transfer and mass transport is then discussed. The thin film and distributed agglomerate models for gas electrodes are also described, and, finally, a numerical approach to the solution of the alternating curent and potential distribution in an arbitrary geometry by solution of Laplace's equation is mentioned. The relationship between pore models and macroscopic approaches is outlined.