No abstract

Polymer electrolyte membrane water electrolysis cells were studied using in-operando neutron radiography, revealing insights into the gas-water distribution inside the cells. Cells were operated at current densities up to 2 A/cm² and for water flow rates ranging from 0.5 ml/(min cm²) up to 5 ml/(min cm²). The averaged gas amount in the flow channels was quantified, revealing that the ratio of gas to water inside the channels decreases for an increasing water flow rate on the anode side. This examination also demonstrates that neutron radiography is very suitable to study gas/water two-phase flow phenomena in running electrolysis cells.

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