This review is devoted to membrane electrolysis, in particular utilizing ion-selective membranes, as an important part of both existing and emerging industrial electrochemical processes. It aims to ...provide fundamental information on the history and development, current status and future perspectives of membrane electrolysis. An overview of the history of electromembrane processes is given with the focus on brine electrolysis since it is the predominant electrochemical industrial technology utilizing ion-selective membranes. This is followed by a summary of the wide range of hydrogen-based energy conversion processes with different degrees of maturity, i.e. water electrolysis and fuel cells, which promise to become the next generation of major electromembrane processes. The overview of the state-of-the-art is rounded off by a number of smaller-scale processes utilizing ionically conducting solid electrolytes and ion-selective membranes that are already commercially available. The article concludes by considering potential future developments in this exciting field of electrochemistry.
•FIB-SEM tomography on catalyst layer (CL) bonded with polybenzimidazole is feasible.•Polybenzimidazole-bonded CL has homogenous Pt distribution but cracked structure.•In polybenzimidazole-bonded CL ...Pt penetrates into the microporous layer.•Heat treatment of CL improves its electric conductivity but decreases permeability.
The main topic of this study is the microstructure of catalyst layers (CLs) of a high-temperature gas diffusion electrode in a PEM fuel cell, bonded by polybenzimidazole. Using focused ion beam scanning electron microscopy in both the secondary and backscattered electron modes, Pt distribution in the CLs was studied. It was found that the choice of binder and preparation history had an important impact on the CL 3D microstructures. Polybenzimidazole-bonded CLs exhibited smooth surface, low porosity and homogeneous distribution of Pt. On the other hand, penetration of Pt into the microporous layer of the electrode was observed. Good thermal and mechanical stability of the CL made it possible to carry out FIB-SEM tomography and to reproduce its 3D microstructure by alternating use of ion milling and imaging of emerging cross-sectional planes. The 3D microstructure obtained served as an input for mathematical models allowing an evaluation of the effective macroscopic transport properties of the CL. This represents important information for reliable mathematical modelling and optimisation of the high-temperature PEM fuel cell.
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•Setups for electrical conductivity transport properties of catalyst layer implemented.•Numerical analysis of FIB-SEM tomography images corresponds to the experiments.•Electrical conductivity of ...catalyst layer improved by heat treatment.•Transfer properties of catalyst layer less affected by heat treatment.
In this study, the experimental approach for determination of high-temperature PEM fuel cell, PBI-bonded catalyst layer conductivity and transfer properties was developed and optimised. Electrical conductivity was determined by chronopotentiometry combined with mathematical model, using uncompressed catalyst layer on a conductive glass support. Effective transport properties were determined using gas-diffusion electrode samples fixed in the Wicke-Kallenbach cell. Experimental results were compared with quantities calculated from FIB-SEM tomography. Very good agreement between the results was reached. Theoretical data evaluated on the base of tomography may thus be considered as validated. The results confirm the heat treatment of the catalyst layer to lead to its reorganisation to less porous and better-defined structure with improved electrical conductivity. Changes in effective transport properties, however, were not pronounced. This is due to the nature of gas-diffusion electrode samples. Determined electrical conductivities and transfer properties of catalyst layer underlined the importance of gas-diffusion electrode preparation procedure. This represents an important input for further studies, focused on optimisation of membrane-electrode assemblies for HT PEM FC.
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This paper investigates the effect of dwell time and heat treatment on the modified friction stir clinched (MFSC) joint of AA2024-T3/AA6061-T6 Al alloys. The precipitation-hardening heat treatment ...method involves the combination of solution heat treatment (at 520 °C for 1 h) and aging (at 165 °C for 18 h) processes. The microstructure, failure load, hardness, and fracture behavior of the as-welded and heat-treated MFSC joints were investigated. TEM images show that re-precipitation of strengthening Al2CuMg and MgZn2 phases, dislocation density, and tangles are more pronounced in the heat-treated MFSC joint. A rise in dwell time increases the average grain sizes (1.39–6.65 μm), tensile-shear strength (101–133 MPa), and cross tension strength (59–88 MPa) of the MFSC welded 2024-T3/6061-T6 joints due to an upsurge in the in-process exposure time-induced heat input and inter-material flow. An increase in dwell time beyond 15 s is undesirable. It induces the formation of nugget cracks and micro-voids in the joints and an impaired joint failure load consequently ensues. Heat treatment processing further causes grain coarsening (2.48–9.15 μm) and improves the hardness (at the weld center), tensile-shear (146 MPa), and cross tension (102 MPa) failure strengths of the MFSC joints due to the re-precipitated strengthening phases.
Degradation of the Pt catalyst is one of the most serious problems related to the high-temperature fuel cell with a proton-exchange membrane (HT PEM FC). Despite many publications on this topic, no ...clear relationship between Pt nanoparticle growth and HT PEM FC operating conditions has been formulated yet. The goal of the presented study is to determine the dependence of Pt nanoparticle growth on the operational voltage of the HT PEM FC by means of experimental single-cell testing with subsequent post mortem analysis of the catalyst layers on the two electrodes. Well-defined, long-term tests performed at constant voltage in combination with several post mortem instrumental methods enabled the prediction of the development of Pt nanoparticle distribution. At higher voltages, Pt nanoparticle growth was less pronounced than at low voltages. This was likely caused by the change in the rate-limiting step in Pt nanoparticle growth.
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•The FSEW-B process was used for the first time for welding of AA5083 to Cu using pure Zn as interlayer.•The shoulder geometry noticeably affects the materials flow and weld strength.•The fracture ...load of joints is determined by the filling of pre-threaded hole and the width of brazed zone.
Friction spot extrusion welding-brazing of pure copper to 5083 aluminum alloy sheet using pure Zn as interlayer has been elucidated. The effect of shoulder geometry on the microstructural characterizations and mechanical behavior of the joints was explored. Results indicated that changing the geometry of the shoulder from cylindrical to triangular not only enlarged the brazed zone from 14.98 μm to 19.65 μm, but also the tensile/shear strength had a rising trend and improved from 3268 N to 4398 N, which can be attributed to the larger brazed zone and complete filling of the pre-threaded hole in the joint produced by the triangular shoulder.
In this work, a combined approach, utilising both experimental cyclic voltammetry and mathematical modelling, was adopted in order to determine the kinetics of Pt oxidation to PtO and chemical ...dissolution of PtO to Pt2+ in concentrated H3PO4 at elevated temperatures. Experimental cyclic voltammograms were corrected for chemical dissolution of PtO based on a charge balance calculation and the corresponding kinetic constant was evaluated. The corrected voltammograms were compared with the results of a 0-dimensional dynamic model of electrochemical Pt oxidation and the kinetic parameters of this reaction were evaluated. Finally, the two surface reactions were integrated into a mathematical model. This model is able to simulate cyclic voltammograms of Pt in concentrated H3PO4 electrolyte within the temperature range of 120–160 °C. It was shown that Pt electrochemical oxidation is the dominating reaction on the Pt surface, based on total recorded charge. However, the chemical dissolution of PtO is by no means negligible, since as much as 10% of the PtO monolayer formed can chemically dissolve under specific conditions.
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This paper presents the experimentally studied degradation of a gas-diffusion electrode under a potentiostatic regime. The experimental conditions corresponded to the operation of a high-temperature ...fuel cell with a proton-exchange membrane, e.g. in 99.6 wt% H3PO4, at a temperature of 160 °C. A one-dimensional mathematical model of the degradation of a gas-diffusion electrode was validated using experimental data and utilised for determination of kinetics data of the electrochemical dissolution of Pt. The mathematical model predicted a general mechanism of Pt degradation during electrode polarisation, comprising the electrochemical oxidation of the surface of smaller nanoparticles to PtO, followed by the chemical dissolution of PtO to Pt2+(sol) and electrochemical reduction of the formed Pt2+(sol) on the bare Pt surface of larger nanoparticles. The intensity of degradation varied with the electrode polarisation potential. At potentials close to 0.7 V vs. dynamic hydrogen electrode (DHE), only small nanoparticles were dissolved, while at potentials close to 1 V vs. DHE, Pt dissolution took place on a wider range of nanoparticle sizes, resulting in a higher concentration of Pt2+(sol) on the electrode and, consequently, in a higher rate of nanoparticle growth. The mathematical model presented can be used, with modifications, to make an approximate estimate of the extent of degradation and Pt nanoparticle size distribution in a gas-diffusion cathode, depending on the polarisation potential within the range of 0.7–1 V vs. DHE.
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Polymer electrolyte membrane (PEM) water electrolysis is an attractive way of producing carbon-free hydrogen. One of the drawbacks of this method is the need for precious metal-based ...electrocatalysts. This calls for a highly efficient utilization of the precious metal, which can be obtained by dispersing the precious metal compound onto a catalyst support. Electrocatalysts with 50, 70 and 90 wt.% of IrO2 on a TaC support were tested in a laboratory PEM water electrolyser and compared with pure IrO2. The temperature was set at 90, 110, 120 and 130 °C respectively and the cell voltage was varied between 1.4 and 1.8 V. The load characteristics and electrochemical impedance spectra were obtained and compared for a range of electrocatalysts. The highest current densities and the lowest charge transfer and cell resistances were found for the 70 wt.% IrO2 electrocatalyst. By contrast, the pure IrO2 electrocatalyst showed the lowest current densities and the highest charge transfer and cell resistance. For example, the relative difference in current densities between the 70 wt.% IrO2 and the pure IrO2 electrocatalyst attained 36% at 130 °C and 1.7 V. All of the supported electrocatalysts showed a higher efficiency of utilization of the precious metal than the pure IrO2.
•As the optimum electrocatalyst composition, 70 wt.% IrO2 on TaC was identified.•PEMWE test expanded and refined findings from cyclic and linear voltammetry.•Conductivity of support may be unimportant when concentration of IrO2 is ≥50 wt.%.•Benefits of utilisation of TaC as an IrO2 support for PEM water electrolysis proven.•IrO2 concentration higher than 50 wt.% secures ample conductivity of the electrode.
•The AA5083AZ31 Mg joint of MFSCB was explored using Zn as interlayer for the first time.•The keyhole defect is completely removed after MFSCB.•No deleterious and brittle phase such as Al3Mg2 or ...Al12Mg17 was formed in MFSBed Al/Zn/Mg joint.•The fracture load of welds is determined by the filling of keyhole and the phases formed at the interface.
AA5083-H112 Al alloy and AZ31 Mg alloy with pure Zn as an interlayer was joined through a Modified friction stir clinching brazing (MFSCB) process. Results revealed that no deleterious phase such as Al3Mg2 was present in MFSBed Al/Mg joint. The fracture load of the joint under tensile/shear and cross-tension loading reached 3809 N and 2543 N, respectively. By the addition of Zn as an interlayer, the brazing zone was formed at the edges of the Al/Mg joint, which was the main reason for the betterment of the fracture load. Furthermore, plug fracture occurred under tensile/shear and cross-tension loading.
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