Distribution of relaxation times analysis of electrochemical hydrogen pump impedance spectra

Polybenzimidazole-based electrochemical hydrogen pumps (EHPs) allow hydrogen separation from gas mixtures at low cell overpotential. An operating temperature of up to 180 °C provides robustness towards catalyst poisoning by common impurities in steam reformate, like CO or sulfur compounds. Electrochemical impedance spectroscopy (EIS) coupled with the distribution of relaxation times (DRT) analysis is performed on single-cell EHPs supplied by H$_2$ contaminated with N$_2$, CO$_2$, and CO to investigate and quantify the underlying physicochemical processes. By systematically varying the operating parameters, five different processes were identified in the DRT spectrum: the proton transport in the electrode, the hydrogen evolution reaction (HER), the hydrogen oxidation reaction (HOR), the mass transport (MT) in the anode gas diffusion electrode, and the movement of phosphoric acid anions from the cathode to the anode at high current densities. At high contaminant concentrations, the HOR and the MT resistances increase. The HOR inhibition is dominant for CO, while for N$_2$ and CO$_2$, the MT resistance increase is more pronounced. At 180 °C cell temperature, the performance with 50% CO$_2$ in the gas feed was worse than with 1% CO, highlighting the possibility of operating an EHP with a CO-contaminated gas feed at elevated operating temperature

Investigating the V(IV)/V(V) electrode reaction in a vanadium redox flow battery – A distribution of relaxation times analysis

Due to the worldwide increasing energy demand and the urgency to act due to climate change, new energy storage technologies are required to balance the intermittent power supply of renewable energy sources. While the vanadium redox flow battery (VRFB) must still overcome lifetime and efficiency challenges, the technology is a promising candidate for large-scale energy storage. Thus, conducting experiments in a setup that closely mimics the operating conditions is vital for gaining new insights into the reactions and transport processes in a VRFB. We developed a novel 3D printed flow cell to study the individual half cell reactions of a VRFB under precisely controlled operating conditions. Using electrochemical impedance spectroscopy combined with the distribution of relaxation times analysis, we could identify the processes occurring in the half cell with the V(IV)/V(V) redox reaction by varying experimental parameters. We assigned peaks in different frequency ranges to the electrochemical reaction, the transport processes through the porous electrode structure, and the ion transport. This information is essential in the search for optimized operating conditions to improve the VRFB efficiency

Distribution of Relaxation Times Analysis of High-Temperature PEM Fuel Cell Impedance Spectra

In this study, Distribution of Relaxation Times (DRT) was successfully demonstrated in the analysis of the impedance spectra of High-Temperature Polymer Electrolyte Membrane Fuel Cells (HT-PEMFC) doped with phosphoric acid. Electrochemical impedance spectroscopy (EIS) was performed and the quality of the recorded spectra was verified by Kramers-Kronig relations. DRT was then applied to the measured spectra and polarization losses were separated on the basis of their typical time constants. The main features of the distribution function were assigned to the cell’s polarization processes by selecting appropriate experimental conditions. DRT can be used to identify individual internal HT-PEMFC fuel cell phenomena without any a-priori knowledge about the physics of the system. This method has the potential to further improve EIS spectra interpretation with either equivalent circuits or physical models

Synchrotron X-Ray radiography of vanadium redox flow batteries – Time and spatial resolved electrolyte flow in porous carbon electrodes

A porous carbon electrode fully saturated with electrolyte is one crucial aspect of vanadium redox flow battery efficiency. It determines the electrochemically active surface area, provides more active sites for the reaction during operation, and prevents local degradation due to inhomogeneities in electrolyte distribution. We investigate the electrolyte invasion and distribution at open-circuit potential in heat-treated carbon felt electrodes at varying compression ratios and flow field configurations, using synchrotron X-ray radiography. The quantitative analysis yields time-resolved saturation values of the injection and resolves local changes in saturation to detect areas of lower electrolyte accessibility. Compression ratios of 50% and above lead to a high electrode utilization with more than 97% saturation over the felt thickness. In contrast, carbon felts at 25% and 17% compression only reach 49% and 15% saturation near the flow fields. However, increasing the flow velocity after the injection causes the boundary area next to the flow field to fill even at low compressions. This area is especially critical for the electrode utilization since it is invaded after the bulk. Depending on the compression level, it does not reach full saturation

FIB‐SEM and ToF‐SIMS Analysis of High‐Temperature PEM Fuel Cell Electrodes

The phosphoric acid (PA) distribution in the electrodes is a crucial factor for the performance of high-temperature polymer electrolyte fuel cells (HT-PEM FCs). Therefore, understanding and optimizing the electrolyte distribution is vital to maximizing power output and achieving low degradation. Although challenging, tracking the PA in nanometer-sized pores is essential because most active sites in the commonly used carbon black-supported catalysts are located in pores below 1 µm. For this study, a cell is operated at 200 mA cm−2 for 5 days. After this break-in period, the cathode is separated from the membrane electrode assembly and subsequently investigated by cryogenic focused ion beam scanning electron microscopy (cryo FIB-SEM) coupled with energy-dispersive X-ray spectroscopy (EDX) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). PA is located on the surface and in the bulk of the cathode catalyst layer. In addition, the PA distribution can be successfully linked to the gas diffusion electrode morphology and the binder distribution. The PA preferably invades nanometer-sized pores and is uniformly distributed in the catalyst layer

Carbon felt electrodes for redox flow battery: Impact of compression on transport properties

In a flow battery setup, carbon felt materials are compressed to obtain higher performance from the battery. In this work, a commercially available carbon felt material, commonly used as electrodes in Vanadium Redox Flow Battery setups was evaluated for the transport properties (diffusivity, permeability, pressure drop required for maintaining flow, among others) while under seven set levels of compression, using an image analysis coupled with pore network modeling approach. X-ray computed tomography has been used to obtain the microstructure of a commercially available electrode under compressed conditions. An open-source pore network modeling tool, OpenPNM has been used to investigate the transport properties of the porous felt material at each of the set compression levels. The results from the modeling are compared against experimentally obtained electrolyte transport patterns visualized using synchrotron X-ray radiography. The electrical resistance of the carbon felt electrode was measured experimentally using a four-probe method. The compression resulted in a 58% reduction in permeability, and a 25% reduction in single-phase diffusion. This combination of ex-situ characterization of the electrical and fluid transport through the electrodes provides valuable data for modeling flow battery systems, and validating hypothesis from in situ testing

Revealing the Multifaceted Impacts of Electrode Modifications for Vanadium Redox Flow Battery Electrodes

Carbon electrodes are one of the key components of vanadium redox flow batteries (VRFBs), and their wetting behavior, electrochemical performance, and tendency to side reactions are crucial for cell efficiency. Herein, we demonstrate three different types of electrode modifications: poly(o-toluidine) (POT), Vulcan XC 72R, and an iron-doped carbon–nitrogen base material (Fe–N–C + carbon nanotube (CNT)). By combining synchrotron X-ray imaging with traditional characterization approaches, we give thorough insights into changes caused by each modification in terms of the electrochemical performance in both half-cell reactions, wettability and permeability, and tendency toward the hydrogen evolution side reaction. The limiting performance of POT and Vulcan XC 72R could mainly be ascribed to hindered electrolyte transport through the electrode. Fe–N–C + CNT displayed promising potential in the positive half-cell with improved electrochemical performance and wetting behavior but catalyzed the hydrogen evolution side reaction in the negative half-cell