Abstract Efficient and durable catalysts for the oxygen evolution reaction (OER) are of great importance for energy storage and conversion devices. However, an objective evaluation and fair comparison of different catalysts remain challenging due to the different catalyst loadings and substrates for OER measurements. In this work, we investigated NiFe layer double hydroxide and commercial Ni/NiO catalysts with different loadings and substrates of glassy carbon (GC), porous nickel foam (NF), and carbon paper (CP). The activity, cycling stability, and potentiostatic stability of the catalysts are compared with respect to the loading and substrate. Catalyst loading exhibits a volcano trend with OER activity, while it has little impact on stability. The 3D substrates NF and CP significantly improved the OER activity of the catalysts compared to GC, especially at higher loadings. The consistent degradation trend of the catalysts confirms the validity of using NF or CP as substrates for the stability test.
Abstract In‐plane synchrotron radiography with a resolution of a few micrometers was applied to study transport processes within a Proton Exchange Membrane (PEM) electrolyzer cell. The degradation process of the catalyst layer, gas production with bubble formation at the catalyst layer and in the porous transport layer (PTL) was analyzed. From this, a new cell design was developed that allows for high X‐ray transmittances at the membrane plane in the in‐plane viewing direction. During the measurement, a bubble growth and movement was observed. Furthermore, a detachment of catalytic material from the catalyst layer was detected. Afterwards, a post mortem EDX analysis was conducted to determine the position of the catalyst particles. Despite Iridium being initially used as the anode catalyst and platinum as the cathode catalyst, the EDX measurement revealed Pt and Ir particles on both electrodes following cell operation.
In this paper reactions having strong influence on the lifetime of PEMFCs are described. These lifetime limiting reactions are related to the catalyst (increase of the particle size, catalyst dissolution, oxidation of the carbon catalyst support, catalyst deactivation by contaminants), to the membrane (degradation, loss of membrane humidification, increase of membrane resistance), and corrosion of structural components (e.g. metallic bipolar plates). Examples for such reactions are shown and rate determinining parameter are discussed.
Polymer Electrolyte Membrane Water Electrolysis (PEMWE) is increasingly attracting interest from academia and industry as it offers an option for storing renewable energy from sources such as wind or solar [1]. Further, electrolysis is considered one of the few available pathways to achieve a 100% renewable electricity supply [2]. In PEMWE, water is typically supplied via flow channels and distributed across the catalyst layer through a porous transport layer (PTL). Gas evolution occurs at the catalyst layer, and the gas is transported back through the PTL and discharged into the flow channels where it is then transported out of the cell as a two-phase mixture with the feed water. This evolution of oxygen and hydrogen in the surrounding water leads to distinct two-phase flow phenomena, which are investigated in this study in-operando inside operating PEMWE cells using synchrotron X-ray radiography as well as neutron radiography. In the past, both methods have allowed to visualize processes inside operating fuel cells, elucidating water management issues [3], the formation of liquid water, its accumulation rate and transport in polymer electrolyte membrane fuel cells (PEFC) [4], as well as the carbon dioxide evolution in liquid fed direct methanol fuel cells (DMFC) [5]. PEM water electrolysis systems are currently scaled to the megawatt range, necessitating an increase of the active area of an individual cell. This scale-up goes along with challenges concerning media distribution on large cell areas. Information about the oxygen saturation and distribution inside the PTL at different points of operation but also at different locations a large area PTL is therefore of special interest. In this work, operating PEMWE cells are examined using synchrotron X-ray radiography at BESSY II and using neutron radiography at BER II (both Helmholtz-Zentrum Berlin). In both setups, a synchrotron X-ray beam or a neutron beam is directed at the running cell, and the attenuation dependence of the beam on the elements inside the cell allows visualizing the processes occurring inside the cells. Both methods are to a certain degree complimentary – the synchrotron radiography setup allows inspecting small areas of some mm² with a spatial resolution of a few µm, while the neutron radiography allows inspecting areas of several cm² with a spatial resolution of lower than 100µm. The especially strong neutron attenuation in hydrogen offers the possibility to visualize the gas/water distribution on large areas, while the comparably low neutron attenuation in metals allows using a typical cell setup with only minor adjustments. The PEMWE cells examined using neutron radiography contained different PTLs, which were compared in terms of the gas/water distribution under different operating conditions. On the anode side, an additional injection of oxygen from the bottom of the cell was used to simulate the effects occurring in a scaled cell exhibiting a larger area. This setup allowed examining the dynamics in the transport and quantifying the distribution along the cell area. In the PEMWE cells examined using synchrotron radiography, oscillations in the gas bubble discharge from the PTL into the flow channel were investigated in terms of their frequency and the gas discharge volumes. Furthermore, the implications of this on the gas transport within the PTL are discussed, and the number of gas bubble discharge sites is correlated with the current density. Thereby it is demonstrated that both synchrotron X-ray and neutron radiography represent valuable tools to study transport processes and to gain insights into the gas-water distribution inside running PEMWE cells. These results pose important implications for the design of cell components with facilitated gas removal, for the modeling of two-phase flow in PEMWE cells and for the modeling of mass transport losses in PEMWE. References [1] M. Carmo et al., Int. Journal of Hydrogen Energy, 2013, 38, 4901 - 4934 [2] G. Pleßmann et al., Energy Procedia, 2014, 46, 22-31 [3] R. Satija et al., Journal of Power Sources, 2004, 129, 238 - 245 [4] I. Manke et al., Applied Physics Letters, 2007, 90, 174105 [5] C. Hartnig et al., Electrochemistry Communications, 2009, 11, 1559 - 1562 [6] M.A. Hoeh et al., Electrochemistry Communications, 2015, 55, 55 - 59 Fig. 1 a) Neutron radiograph of a full cell with an area of 17.6 cm², showing a meander-shaped flow channel on the anode side with water flow from bottom to top b) Synchrotron radiograph of an individual channel with gas bubbles visible inside the channel Figure 1
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