As the ultimate power source, solar energy is clean, abundant, and free. Direct solar electricity (solar cell or solar thermal) makes up an important portion of the total energy supply of the US today, and it is expected to grow rapidly in the future. The intrinsic intermittency of solar energy brings the challenging problems in energy conversion and storage. By switching the working ion in polymer electrolytes from proton to hydroxide (OH~-), hydroxide exchange membranes (HEMs) have shown the promise to significantly reduce costs, the top commercialization barrier, for a number of electrochemical energy conversion/storage devices including fuel cells, electrolyzers, redox flow batteries, and solar hydrogen generators. For instance, the simple coupling of a fuel cell and an electrolyzer could offer a long-term (months to years) storage solution using H2 as the storage medium; and the redox flow batteries could serve the middle-term (hours to days) storage needs. Different from the conventional proton exchange membranes (PEMs), HEMs have the ability to work with non-precious yet active metal electrocatalysts and themselves are also inexpensive . Equally importantly, the HEMs are completely free from the problems of electrolyte leakage or metal (bi)carbonate formation that the traditional liquid base electrolytes usually bring. The properties of HEMs such as conductivity, solubility and affinity with electrocatalysts directly and significantly impact the performance of electrochemical devices. HEMs are fundamentally controlled by the hydroxide-conducting functional group. For example, the conventional quaternary ammonium hydroxide (QAOH) functional group tends to make their HEMs suffer from low conductivity and poor solubility, limiting the performance of HEM fuel cells.
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