Abstract
Three different porous transport layer (PTL) structures, based on titanium sintered powders, were characterized using X-ray tomographic microscopy to determine key geometric properties such as porosity, pore and particle size distributions as well as effective transport properties. The mass transport through the PTL contributes to the voltage losses in the polymer electrolyte water electrolysis cell. Therefore, influence of the PTL structure on the mass transport overpotential is investigated as function of current densities (≤ 4 A·cm−2), operating pressures (1–100 bar) and temperatures (40–60°C), respectively. A decrease of transport losses was observed with increasing pressure and temperature for all investigated PTLs. At around 100 bar balanced pressure, the transport losses for all PTLs converge to about 40 mV per applied A·cm−2, suggesting that other parts of the cell such as the catalyst layer or their interface contribute to these remaining losses. The performance loss, induced by the different PTL structures, shows a stronger correlation with geometric parameters such as pore and particle size distributions than transport properties like effective diffusivity and permeability. The finest materials with d50 pore and particle diameters of 40–48 and 68 μm, respectively, are performing better than the coarsest material with diameters roughly twice the sizes.
Highlights
Polymer electrolyte water electrolysis (PEWE) is a promising technology to convert electric energy into chemical energy in form of hydrogen to avoid curtailment of the fluctuating renewable electricity sources[1] and reducing the need of fossil fuels.[2]
porous transport layers (PTLs) prepared from titanium sintered spherical powders, with particle diameters in the 50–75 μm range and porosities in the 28–40% range were investigated by Grigoriev et al.[12]
The optimum mean pore diameter was considered a compromise between capillary effects, which become relevant at pore sizes below about 10 μm, and additional parasitic ohmic losses for larger particle diameters
Summary
Polymer electrolyte water electrolysis (PEWE) is a promising technology to convert (surplus) electric energy into chemical energy in form of hydrogen to avoid curtailment of the fluctuating renewable electricity sources[1] and reducing the need of fossil fuels.[2]. Ohmic and kinetic overpotentials, related to the movement of protons in the polymer electrolyte and the finite rates of the electrochemical reactions are reasonably well understood for PEWE.[6,7,8] In most cases, at least at current densities above about 1 A · cm−2, losses in addition to ohmic and kinetic sources are observed and related to mass transport resistances in the porous structures of the catalyst and porous transport layers (PTLs), though their nature and origin in PEWE are not well understood.[6] as long as a sufficient water supply is guaranteed, no transport limitations occur in the form of a turning point in the current/voltage characteristics (i/E-curves), as demonstrated by Lewinski et al.[9] up to 19 A · cm−2. The optimum mean pore diameter was considered a compromise between capillary effects, which become relevant at pore sizes below about 10 μm, and additional parasitic ohmic losses for larger particle diameters
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