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 d 50 pore and particle diameters of 40-48 and 68 μm, respectively, are performing better than the coarsest material with diameters roughly twice the sizes. 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 thus reducing the need of fossil fuels. 2 Depending on the use of hydrogen, the gas has to be provided at pressure levels up to 1000 bar, i.e. for the use in mobility applications.3 Commercial electrolyzers are operated at pressures in the order of 30 bar (hydrogen and optionally oxygen) 4 to reduce downstream compression and drying efforts. 5For energy applications, high efficiency of the electrolysis conversion process is a key property. As in any electrochemical cell, also with PEWE, losses/overpotentials incur when a current is passed. 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 Nevertheless, 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 . In polymer electrolyte fuel cells (PEFC), a technology similar to PEWE, the effect of mass transport in the micro-porous structures of the el...

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Inside a PEFC, the distribution of reactions during the cell operation is generally inhomogeneous, which may lead to reducing the cell performance and/or enhancing the degradation of an MEA. For the commercialization of PEFCs, elucidating the distributions of physical and chemical parameters is important, such as temperature, oxygen partial pressure (p(O2)), water vapor pressure, CO2 concentration, etc. We have so far visualized the distributions of p(O2)1- 4) and liquid water3 ,4) during the power generation and CO2 concentration5) during the degradation.   We have constructed a new system, which enables to visualize the distributions of p(O2) and current density. A cathode plate of a cell is made of a transparent acrylic resin, and the current collectors are segmented to nine both at the cathode and the anode (Fig.1). The electrochemical potentials of the current collectors were controlled to be the same by the external circuit.  The ribs are partially transparent, and p(O2) can be visualized even under the ribs at those locations. As an oxygen sensor, a luminescent dye compound, PtTFPP ([tetrakis(pentafluorophenyl)porphyrinato] platinum]),1) was used, which absorbs 390-nm blue light and emits 650-nm red light, and the emission intensity lowers as p(O2) increases. This dye film was coated on the GDL. During the visualization, a 407-nm laser light was diffused, spread and distributed uniformly onto the transparent cathode plate. The emission from the dye on the GDL surface was filtered (> 600 nm), and the images were captured with a CCD camera (500 x 500 pixel, 1 pixel à 0.12 mm). Meanwhile, the current density at each current collector was recorded. The MEA was prepared with a Pt/CB catalyst (TEC10E50E), a Nafion® membrane (NRE211) and a GDL (SIGRACET® 25BCH).   Fig.2 shows the distributions of p(O2) and the current density at 80 oC and 80% RH, at the oxygen utilization (Uo2) of 10 (a), 40 (b), and 80% (c). As Uo2 increased, oxygen is clearly seen to be consumed. p(O2) under ribs was seen lower than that under flow channels. At Uo2 = 10%, the current densities near the outlet were higher than that near the inlet due to the higher proton conductivity inside the membrane by the generated water existing more near the outlet. At Uo2 = 40%, the current densities became relatively uniform because of the rather uniform water distribution in the membrane. At Uo2 = 80%, the current densities near the inlet became higher than that near the outlet. The change at Uo2 = 80% can be explained by the lower oxygen diffusion to the catalyst layer and the excess formation of liquid water near the outlet. In order to understand the difference of p(O2) under flow channels and ribs, p(O2) is plotted in Fig. 3 at Uo2 = 10, 40, and 80% along the white line drawn on the cell image on the left. Large modulation is seen at any Uo2; p(O2) at the middle of the channels was the highest, and lower under the ribs. Near the inlet, the modulation was larger at higher Uo2, probably because of the increased water vapor pressure under the ribs during the power generation at a fixed air flow rate. Near the outlet, the modulation became lower than that near the inlet. This is probably because of the mixture of gasses across the flow channel and the GDL as the gas moved from the inlet to the outlet. These results indicate that the oxygen diffusion might be lowered by the increased generated water at higher Uo2 with lower p(O2). This low diffusion of oxygen could be a reason for the inhomogeneous distribution of reactions, especially under the ribs.   This technique is expected to be used for the improvement of MEAs, cell designing, and for the optimization of operating conditions. Acknowledgement This research was performed under the HiPer-FC project supported by NEDO, Japan. References 1)  J. Inukai et al., Angew. Chem. Int. Ed., 47, 2792 (2008). 2)  Y. Ishigami et al., J. Power Sources, 269, 556 (2014). 3)  K. Takada et al., J. Power Sources, 196, 2635 (2011). 4)  K. Nagase et al., J. Power Sources, 273, :873 (2014) 5)  Y. Ishigami et al., Electrochem . Solid-State Lett ., 15, B51 (2012).  Figure 1