Influence of Operating Conditions and Material Properties on the Mass Transport Losses of Polymer Electrolyte Water Electrolysis

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 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...

Communication—Contribution of Catalyst Layer Proton Transport Resistance to Voltage Loss in Polymer Electrolyte Water Electrolyzers

“…Data describing the cells with different PTLs will be designated with T5/T10/T20. The previous study from Suermann et al 5 Operating conditions -electrolysis regime.-CCMs were conditioned for 12 hours prior to measurements by cycling the current density between 1 and 2 A cm −2 in 5 min intervals at 60…”

“…12,13 The water reaches the CL a through the voids on the PTL surface and relies on lateral diffusion through the ionomer binder to reach the active sites under the PTL solid particles (Figure 4). The T20 PTL surface consists of larger particles and pores, 5 in larger diffusion distances for the water in the CL a ionomer, and thus higher η mt x−rest . The different η mt x−rest for the T5 and T10 might stem from the more constrained T5 structure, with a higher fraction of small, potentially bottlenecking pores.…”

“…The different η mt x−rest for the T5 and T10 might stem from the more constrained T5 structure, with a higher fraction of small, potentially bottlenecking pores. 5 The compaction of the CL a under the PTL particle leads to higher degree of ionomer binder confinement and inhomogeneous wetting of the CL a active sites, resulting in increased R H + C La .…”

“…This overpotential is often unaccounted for in PEWE and has so far been lumped together with general 'mass transport' losses. [5][6][7] Recently, Bernt and Gasteiger 12 calculated the effective proton transport resistance in the CL a from the electrode sheet resistance for proton conduction to be in a range from 6 to 171 m cm 2 , depending on CL a ionomer content. The study presented here is the first to report experimentally determined R H + C La values and its dependence on the PTL a /CL a interface.…”

Electrochemical Hydrogen Compression: Efficient Pressurization Concept Derived from an Energetic Evaluation

“…Commercial catalyst coated membranes (CCMs) with Ir-and Pt-based catalysts (Nafion 117CS and Nafion 212, Greenerity E400, Greenerity, DE) are sandwiched between two 1.0 mm thick porous transport layers (PTLs) with a porosity of 35 ± 2% made from titanium sintered powder (SIKA T10, GKN Sinter Metal Filters, DE). 19 Gas and water is distributed from and to the PTLs by flow-fields with five parallel channels (2 × 1 mm, width x depth) made from gold-coated stainless steel, to eliminate contact resistances and corrosion issues.…”