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

In the power-to-gas process, hydrogen, produced by water electrolysis, is used as storage for excess, fluctuating renewable electric power. Reconversion of hydrogen back to electricity with the maximum possible efficiency is one pre-requisite to render hydrogen storage technically and economically viable. Pure oxygen is a byproduct in the electrolysis of water. The use of pure oxygen as the oxidant in a polymer electrolyte fuel cell (PEFC) is a possible way of increasing the conversion efficiency of hydrogen to power, by reducing the fuel cell's cathodic kinetic overvoltage, which is the most important energy loss process in low temperature PEFCs. As we demonstrate in this work, when using pure oxygen, either high efficiencies at current densities around 1 A cm À2 are obtained or a very high power density operation (up to 1.6 W cm À2 at cell voltages above 0.62 V) can be reached, giving the technology a broad window of operation and application. The fuel cell stack durability is assessed in accelerated longterm tests of up to 2700 h. The potential of the technology is demonstrated with the realization of a complete 25 kW prototype system delivering a peak efficiency of 69% LHV (57% HHV).

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