Proton Conduction in Solids: Bulk and Interfaces
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Introduction Solid protonic conductors stand central in the development of efficient fuel cells that convert hydrogen-rich fuels from renewable or fossil sources to electricity (Figure 1), as well as steam electrolyzers essentially operating as reversed fuel cells and producing hydrogen and oxygen from steam.1 Mixed protonic-electronic conductors furthermore can act as hydrogen permeable membranes for separation of H2 from CO2 and H2S to make sustainable and clean gas- and coal-based power plants (Figure 2).2 The development of stable and robust proton conductors that have sufficient conductivity (i.e., number and mobility of protons) is necessary. Protonic conduction can be accomplished by vehicles such as OH − or H3O+ ions migrating in an aqueous environment. However, that limits the operating temperature to around 100°C, and such materials—including many early and traditional hydrated polymer and inorganic protonic conductors3,4—are not the focus of this article. In protonics,5 the goal is instead conduction by free protons (H+) between stationary host anions. This alleviates the need for molecular water in the structure and allows us to raise the temperature, improve kinetics, and avoid liq-
uid water handling. It will also become increasingly clear that the omnipresent hydrogen in the form of protons is important for other classes of functional oxides and processes—semiconductors, optoelectronics, catalysis, and photocatalysis. Here the protons have roles as charge carriers and defect terminators, but, in the author’s opinion, have not yet been taken sufficiently systematically into account, due to the complexity of the matter and difficulties to detect all charge states of hydrogen. These fields and protonics may fertilize each other in the 21st century quest for better proton conductors and energy conversion processes, in general, and thus will be treated alongside each other in this article.
The Quest for Good, Stable, Solid Proton Conductors True proton conductors utilize a stationary host structure on which protons jump from site to site. High-temperature protonconducting polymers, such as polybenzimidazole, utilize nitrogen as a proton host.6 Protons are added by sulfonation or phosphonation, and the materials can operate at temperatures higher than 100°C, as they do not depend on water as a pro-
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ton mediator, unlike the standard polymer H3O+ conductors, such as Nafion. Essentially, all other proton conductors are oxidic—they rely on oxide ions as proton hosts. The first challenge of proton transport is the strong bond between the proton and host oxide ions. The O–H bond actually holds most of the energy that is gained in a fuel cell or is broken in an electrolyzer. In high-temperature proton conductors, protons migrate by repeatedly breaking and making bonds. The host oxide ion lattice has to have sufficient dynamics to temporarily bring the proton on one host close enough to the neighboring one to allow the proton to jump or tunnel ove
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