In the first paper of this series the bulk and surface structural properties of Ti-fiber based porous transport layers (PTL) were characterized and described. In this second part the correlation of structure to the performance in polymer electrolyte water electrolysis cells is analyzed by determination of the three main overpotentials of ohmic, kinetics and mass transport losses. The strongest correlation between the PTL bulk transport properties and cell performance is obtained for heat transport. The current density dependent temperature gradients show good agreement with ex situ determined heat conductivity from part one. However, surface properties of the PTL materials, have a stronger influence on cell performance than the bulk properties. Catalyst layer utilization and ohmic interfacial resistances correlate with the interfacial contact areas reported in part one and performance increases with increasing contact area. This is due to a local mass transport resistance decreasing with increasing catalyst layer utilization.
In the first paper of this series, the bulk, surface and transport properties of porous transport layer materials (PTL) for polymer electrolyte water electrolysis cells (PEWE) are characterized. A systematic PTL matrix of Ti fiber materials with 3 fiber diameters and 2 nominal porosities as well as a state of the art sintered powder material are investigated to get a better understanding of the governing parameters in electrolysis cells. X-ray tomographic microscopy analysis of ex situ PTL structures and post operando membrane electrode assemblies is performed. On the tomographic structures, bulk (porosity, pore/solid size distributions, fiber orientation), mass transport (diffusivity, permeability, conductivities) and surface parameters (roughness, membrane deformation, PTL surface area and interfacial contact area) are determined. The second paper of this series will correlate the structural parameters with in-depth electrochemical analysis. The new insights into the effect of PTL properties on PEWE performance allow to isolate governing key parameters. From know-how obtained on the fiber based materials fundamental design guidelines for optimization of PTL structures are deduced.
renewable power, from wind or solar by medium-to-long-term storage using hydrogen as the energy vector, [1,2] enabling effective decarbonisation of the energy sector. [3] PEWE converts electric power by electrochemical water splitting into storable chemical energy. [4][5][6][7] Hydrogen can be reconverted to power or used in other sectors, [8,9] such as fuel cell based mobility [10] and chemical industries. [11] To make hydrogen production with polymer electrolyte water electrolysis a technoeconomically relevant contender, capital and operational cost need to be reduced substantially. [12][13][14][15] Operational cost, which becomes the main cost driver for operation schemes with high duty ratio (≥6000 h p.a.), is governed by power prices and the hydrogen production efficiency of the PEWE plant, which in turn strongly depends on the electrochemical performance and efficiency of the PEWE cell technology employed. [16] The electrochemical efficiency is governed by the underlying electrochemical loss mechanisms of kinetic, ohmic, and mass transport losses, whereas capital cost is driven by limited power density and high noble metal catalyst loadings.Recent studies revealed that the structure of the interface between the anodic porous transport layer (PTL) and the catalyst layer (CL) is a crucial factor limiting cell efficiency. [17][18][19][20][21] Today's PEWE technology relies on porous, Ti based, transport layer materials in the form of sintered materials [17,18,[22][23][24][25] with original applications in filtration [26] or medical tissue growing. [27][28][29] The lack of PTL materials with suitable surface characteristics tailored for this application inhibits the further improvement of cell efficiency and development of PEWE technology.Kinetic losses are governed by the sluggish kinetics of the oxygen evolution reaction (OER). The related overpotential depends on intrinsic catalyst parameters, such as specific exchange current density, activation energy, and number of active catalyst sites. [30] It has been established with model experiments such as optical imaging of the PTL/CL interface [19,20,31] and correlation of in-depth electrochemical analysis with PTL surface structure [17,18] that the catalyst is only partially utilized and CL domains not directly contacted by the porous Timaterials showed no gas evolution hence no electrochemical activity. Schuler et al. [17,18] have quantified the effect, showing
Significant mass-transport resistances in polymer-electrolyte-fuel-cell catalyst layers (CLs) impose a lower limit on Pt-loading levels, hindering widespread fuel-cell commercialization. The origin of this resistance remains unclear. Minimization of CL mass-transport resistance is imperative to achieve better CL design and performance. In this paper, an operando method based on H 2 limiting current is used to characterize and quantify CL resistance in traditional porous Pt/carbon-based electrodes. CL sub-resistances are isolated using continuum multiscale modeling and experiments, investigating the effects of reactant molecular weight, pressure, and ionomer to carbon weight ratio. The results expose CL resistance including both interfacial and transport components, although the majority of the CL resistance is ascribed to a local resistance close to the Pt reaction sites, which includes interfacial resistance and local transport resistance. Variations in temperature, humidity, and primary particle loading (Pt:C ratio) highlight the impact of operating conditions and CL design parameters on CL sub-resistances. The observed trends guide optimization of CL design to achieve novel low-loaded fuel-cell electrodes.
The reaction order of water and the electro-kinetic parameters are determined for the oxygen evolution reaction on iridium oxide.
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