Estimating charge-transport properties of fuel-cell and electrolyzer catalyst layers via electrochemical impedance spectroscopy

Capacitance Determination for the Evaluation of Electrochemically Active Surface Area in a Catalyst Layer of NiFe-Layered Double Hydroxides for Anion Exchange Membrane Water Electrolyser

The determination of the electrochemically active surface area (ECSA) of a catalyst layer (CL) of a non-precious metal catalyst is of fundamental importance in optimizing the design of a durable CL for anion exchange membrane (AEM) water electrolysis, but has yet to be developed. Traditional double layer capacitance (Cdl), measured by cyclic voltammetry (CV), is not suitable for the estimation of the ECSA due to the nonconductive nature of Ni-based oxides and hydroxides in the non-Faradaic region. This paper analyses the applicability of electrochemical impedance spectroscopy (EIS) compared to CV in determining capacitances for the estimation of the ECSA of AEM-based CLs in an aqueous KOH electrolyte solution. A porous electrode transmission line (TML) model was employed to obtain the capacitance-voltage dependence from 1.0 V to 1.5 V at 20 mV intervals, covering both non-Faradic and Faradic regions. This allows for the identification of the contribution of a NiFe-layered double hydroxide (LDH) catalyst and supports in a CL, to capacitances in both non-Faradic and Faradic regions. A nearly constant double layer capacitance (Qdl) observed in the non-Faradic region represents the interfaces between catalyst supports and electrolytes. The capacitance determined in the Faradic region by EIS experiences a peak capacitance (QF), which represents the maximum achievable ECSA in an AEMCL during reactions. The EIS method was additionally validated in durability testing. An approximate 30% loss of QF was noted while Qdl remained unchanged following an eight-week test at 1 A/cm2 constant current density, implying that QF, determined by EIS, is sensitive to and therefore suitable for assessing the loss of ECSA. This universal method can provide a reasonable estimate of catalyst utilization and enable the monitoring of catalyst degradation in CLs, in particular in liquid alkaline electrolyte water electrolysis systems.

Good Practices and Limitations of the Hydrogen Pump Technique for Catalyst Layer Protonic Conductivity Estimation

The hydrogen pump technique has been shown to be an effective method to measure the effective protonic conductivity of intermediate layers (ILs) that mimic the catalyst layers used in proton exchange membrane fuel cells and electrolyzers. It has been hypothesized, however, that the technique is limited to testing ILs that are inactive during the hydrogen reaction as proton transport through the ionomer in the layer can be bypassed by transferring the charge to the electronic phase via the reaction. This work uses numerical modeling, supported by experimental testing, to investigate the impact of IL hydrogen reaction activity, thickness, and electronic conductivity on the prediction of the IL protonic conductivity. A transient, 2-D, through-the-channel model is developed and implemented using the finite element method to predict the performance of hydrogen pump cells and perform electrochemical impedance spectroscopy. It is shown both numerically and experimentally that for iridium black and for platinum-/carbon-based ILs, the protonic phase is almost entirely bypassed, reducing the overall cell resistance and making the determination of the true conductivity difficult. The model can be used to provide an estimate of the resistance of the active layers, which is not possible using only experiments. In addition, the interfacial contact resistance between the membrane and the catalyst layers is determined using the high-frequency resistance, and the alternating current method for the hydrogen pump is studied to determine the accuracy of the method. Finally, further insights are provided through a breakdown of the resistances of each phase, as well as the potential profiles, in an active IL, and through parametric studies on the impact of varying the IL activity, thickness, and electronic conductivity.

PGM-Free Electrocatalytic Layer Characterization by Electrochemical Impedance Spectroscopy of an Anion Exchange Membrane Water Electrolyzer with Nafion Ionomer as the Bonding Agent

Low-cost anion exchange membrane (AEM) water electrolysis is a promising technology for producing “green” high-purity hydrogen using platinum group metal (PGM)-free catalysts. The performance of AEM electrolysis depends on the overall overvoltage, e.g., voltage losses coming from different processes in the water electrolyzer including hydrogen and oxygen evolution, non-faradaic charge transfer resistance, mass transfer limitations, and others. Due to the different relaxation times of these processes, it is possible to unravel them in the frequency domain by electrochemical impedance spectroscopy. This study relates to solving and quantifying contributions to the total polarization resistance of the AEM water electrolyzer, including ohmic and charge transfer resistances in the kinetically controlled mode. The high-frequency contribution is proposed to have non-faradaic nature, and its conceivable nature and mechanism are discussed. The characteristic frequencies of unraveled contributions are provided to be used as benchmark data for commercially available membranes and electrodes.

Exploring the Impacts of Conditioning on Proton Exchange Membrane Electrolyzers by In Situ Visualization and Electrochemistry Characterization

For a proton exchange membrane electrolyzer cell (PEMEC), conditioning is an essential process to enhance its performance, reproducibility, and economic efficiency. To get more insights into conditioning, a PEMEC with Ir-coated gas diffusion electrode (IrGDE) was investigated by electrochemistry and in situ visualization characterization techniques. The changes of polarization curves, electrochemical impedance spectra (EIS), and bubble dynamics before and after conditioning are analyzed. The polarization curves show that the cell efficiency increased by 9.15% at 0.4 A/cm², and the EIS and Tafel slope results indicate that both the ohmic and activation overpotential losses decrease after conditioning. The visualization of bubble formation unveils that the number of bubble sites increased greatly from 14 to 29 per pore after conditioning, at the same voltage of 1.6 V. Under the same current density of 0.2 A/cm²; the average bubble detachment size decreased obviously from 35 to 25 μm. The electrochemistry and visualization characterization results jointly unveiled the increase of reaction sites and the surface oxidation on the IrGDE during conditioning, which provides more insights into the conditioning and benefits for the future GDE design and optimization.

Insight into the performance discrepancy of GAC and CAC as air-cathode materials in constructed wetland-microbial fuel cell system

Constructed wetland-microbial fuel cell (CW-MFC) has exhibited the performance discrepancy between using granular activated carbon (GAC) and columnar activated carbon (CAC) as air-cathode materials. No doubt, this is linked with electrochemical performance and decontaminants characteristics in the CW-MFC system. To provide insight into this performance discrepancy, three CW-MFCs were designed with different carbon-material to construct varied shapes of air-cathodes. The results showed that the ring-shaped cathode filled with GAC yielded a highest voltage of 458 mV with maximum power density of 13.71 mW m^-2 and >90% COD removal in the CW-MFC system. The electrochemical characteristics and the electron transport system activity (ETSA) are the driven force to bring the GAC a better electron transportation and oxygen reduction reaction (ORR). This will help elucidating underlying mechanisms of different activated carbon for air-cathode and thus promote its large application.

Effects of Deep Cryogenic Treatment on the Microstructure and Properties of Rolled Cu Foil

The development of fifth-generation (5G) communication and wearable electronics generates higher requirements for the mechanical properties of copper foil. Higher mechanical properties and lower resistance are required for flexible copper-clad laminate and high-frequency and high-speed Cu foil. Deep cryogenic treatment (DCT), as a post-treatment method, has many advantages, such as low cost and ease of operation. However, less attention has been paid to the impact of DCT on rolled Cu foil. In this study, the effects of DCT on the microstructure and mechanical properties of rolled Cu foil were investigated. The results show that as the treatment time increased, the tensile strength and hardness first increased and then decreased, reaching a peak value of 394.06 MPa and 1.47 GPa at 12 h. The mechanical property improvement of rolled Cu foil was due to the grain refinement and the increase of dislocation density. The dislocation density of rolled Cu foil after a DCT time of 12 h was determined to have a peak value of 4.3798 × 10¹⁵ m^-2. The dislocation density increased by 19% and the grain size decreased by 12% after 12 h DCT.