sources, such as wind and solar, are intermittent in nature, and must be combined with large-scale energy storage to provide stable and reliable power. Hydrogen-based reversible fuel cells (RFCs), which combine the functionality of a polymer electrolyte membrane (PEM), fuel cell (FC), and water electrolyzer (WE), could play a crucial role in providing this energy storage. During periods of excess renewable power generation, RFCs can electrolyze water to generate hydrogen. During periods of excess demand and insufficient renewable power generation, the stored hydrogen can be utilized in FC mode to generate power. Combination of FC and WE functionality into a single electrochemical device, that is, a unitized reversible fuel cell (URFC), could provide smaller, simpler, and less expensive systems. [1] However, the conflicting needs of FC and WE operation, particularly with regard to water management, have constrained progress on development of robust URFC technology. Herein, we report a new approach to URFC design based on hydrochannel porous transport layers (PTLs), which enables effective water management in both URFC operation modes. This hydrochannel PTL enables substantial improvements in performance and round-trip efficiency (RTE).A schematic representation of the URFC operation and the membrane electrode assembly (MEA) configuration is shown in Figure 1. In WE mode, water is supplied to the oxygen electrode (anode), where the water is split into oxygen and protons (H + ) through use of an oxygen evolution reaction (OER) catalyst. The resulting H + are transported through the PEM to the hydrogen electrode (cathode), which utilizes a hydrogen evolution reaction (HER) catalyst to combine H + with electrons to generate hydrogen. In FC operation mode, the hydrogen is supplied to the hydrogen electrode (anode) for the hydrogen oxidation reaction (HOR), and oxygen (air) is supplied to the oxygen electrode (cathode) for the oxygen reduction reaction (ORR). In the present work, the URFC is operated in the constant gas electrode mode, where OER and ORR (or HOR and HER) occur on the same electrode to avoid fuel and oxidant mixing. [2] For constant gas electrode mode operation, as shown in Figure 1, the hydrogen electrode catalyst consists of a supported platinum catalyst, which serves as both HOR and HER catalyst. The oxygen electrode catalyst consists of a combination This work presents a novel porous transport layer (PTL), the hydrochannel PTL, that enables improved water management and record high round trip efficiency in unitized reversible fuel cells (URFCs). URFCs require rapid transport of O 2 and H 2 O to provide high performance in both fuel cell and water electrolyzer operation, but cell design is complicated by conflicting water management requirements: electrolyzers perform best with high liquid water saturation, whereas fuel cells perform best when liquid water saturation is as low as possible while still maintaining effective ionomer hydration. The hydrochannel PTL circumvents this obstacle by providing ...
Demand for high purity hydrogen production using renewable energy sources is growing to meet the clean energy demands. Polymer electrolyte membrane water electrolyzer (PEMWE) is one of the viable options for H2 production, but its high capital cost and operational expenditures increase the cost of H2. Improving the interface between the catalyst layer (CL) and the porous transport layer (PTL) is critical to increasing the efficiency of PEMWEs and thereby lowering the cost of H2. Increased contact between the CL and PTL improves catalyst utilization, and the optimal structure of the PTL reduces the mass transport issues related to O2 bubble removal.(1-3) Improved understanding of the PTL microstructure is necessary to improve the performance and efficiency of the PEMWE. This work presents a systematic study to elucidate the effect of PTL properties (morphology, thickness, and porosity) and their impact on PEMWE performance under different operating conditions. Polarization curves with different anode PTL (felt, sinter and pore graded hierarchical PTL) are presented in Figure 1a. The separation of mass transport resistance and the contract resistance for the different PTLs will be elucidated to show the impact of water management and interfacial contact. Mass transport in an operating electrolyzer is also studied by estimating the water content using neutron imaging. Figure 1b shows water thickness across a membrane electrode assembly (MEA) with a pore graded hierarchical PTL in anode at different current densities. Water content across the MEA with different PTL is also studied. The cells with these PTLs were evaluated in operando using micro x-ray computed tomography (CT) and x-ray radiography. The x-ray techniques revealed oxygen distribution within the PTLs on the pore-scale at varied current densities, complementing neutron imaging water thickness studies and providing micro-scale insight into transport. Acknowledgment This research is supported by the U.S. Department of Energy (DOE) Hydrogen and Fuel Cell Technologies Office, through the H2NEW consortium. References J. K. Lee, C. Lee, K. F. Fahy, B. Zhao, J. M. LaManna, E. Baltic, D. L. Jacobson, D. S. Hussey and A. Bazylak, Cell Reports Physical Science, 1, 100147 (2020). T. Schuler, J. M. Ciccone, B. Krentscher, F. Marone, C. Peter, T. J. Schmidt and F. N. Büchi, Advanced Energy Materials, 10, 1903216 (2020). P. Lettenmeier, S. Kolb, F. Burggraf, A. S. Gago and K. A. Friedrich, Journal of Power Sources, 311, 153 (2016). Figure 1
Unitized reversible fuel cells (URFCs) are a promising grid scale energy storage solution that can leverage intermittent renewable energy sources. URFCs operate in both fuel cell (FC) mode during discharge and water electrolyzer (WE) mode during recharge. Inherent disparity in the requirement of the system components of these two operational modes introduces several technical challenges that must be overcome to make URFCs competitive with other energy storage technologies. The wettability requirements of the porous transport layer (PTL) in the oxygen electrode are different for the two operation modes: hydrophilic PTLs provide optimal H2O transport in WE mode, while hydrophobic PTLs provide faster O2 transport in FC mode. PTL design needs to be optimized for both modes in order to resolve this conflicting requirement. We have previously shown that amphiphilic titanium (Ti) felt improved performance in both modes of operations. In this work, a corrosion-resistant micro-porous layer (MPL) is incorporated to facilitate further improvement in the transport of oxygen and water in both operational modes. The MPL is fabricated from corrosion-resistant powders (titanium nitride [TiN], antimony doped tin oxide [ATO], and niobium doped titanium dioxide and polytetrafluoroethylene (PTFE) dispersion. We present experimental results for cell performance and durability through accelerated stress testing of PTLs with amphiphilic MPLs. Acknowledgment: Financial support from the US Department of Energy through the Office of Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cells Technology Office is gratefully acknowledged.
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