The development of solid-state proton conductors with high proton conductivity at low temperatures is crucial for the implementation of hydrogen-based technologies for portable and automotive applications. Here, we report on the discovery of a new crystalline metal acid triphosphate, ZrH5(PO4)3 (ZP3), which exhibits record-high proton conductivity of 0.5 - 3.1 x 10-2 S cm-1 in the range 25 - 110 °C in anhydrous conditions. This is the highest anhydrous proton conductivity ever reported in a crystalline solid proton conductor in the range 25 - 110 °C. Superprotonic conductivity in ZP3 is enabled by extended defective frustrated hydrogen bond chains, where the protons are dynamically disordered over two oxygen centers. The high proton conductivity and stability in anhydrous conditions make ZP3 an excellent candidate for innovative applications in fuel cells without the need for complex water management systems, and in other energy technologies requiring fast proton transfer.

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/anie.202218421

Anhydrous Superprotonic Conductivity in the Zirconium Acid Triphosphate ZrH5(PO4)3.

The development of ceramic proton conductors is currently attracting great attention, as they might be useful to construct new energy storage systems. Li6La3ZrTaO12 (LLZTO) is known for its rapid Li+ diffusivity as has been directly revealed by 7Li NMR measurements. Exchanging parts of the highly mobile Li+ ions by protons through treatment of a single crystal in water or glacial acetic acid yields a mixed proton–lithium ionic conductor. Here, H+ proton diffusivity and Li+ diffusivity have separately been studied with element-specific 1H and 7Li NMR spectroscopy. While long-range 7Li diffusion is noticeably slowed in Li–H exchanged LLZTO, we directly observe rather high H+ diffusivity, which is, however, significantly slower than Li+ dynamics. With the help of spin–lattice relaxation measurements we were able to measure local (and long-range) energy barriers (0.20(1) eV vs 0.45(3) eV) as well as the self-diffusion coefficient DH of H+ dynamics (1.2 × 10–15 m2 s–1 at 125 °C). These encouraging results are assumed to open new directories in designing ceramics offering fast transport pathways for protons. 

https://pubs.acs.org/doi/pdf/10.1021/acs.jpcc.3c02330

Solids with Two Mobile Ions: Proton H+ Self-Diffusion in Li–H Exchanged Garnet-Type Li6La3ZrTaO7 as Seen by Solid-State 1H NMR Relaxation

Alternative processing routes on CsH2PO4 proton conductors: Cold sintering and ball-milling routes

The development of solid-state proton conductors with high proton conductivity at low temperatures is crucial for the implementation of hydrogen-based technologies for portable and automotive applications. Here, we report on the discovery of a new crystalline metal acid triphosphate, ZrH5(PO4)3 (ZP3), which exhibits record-high proton conductivity of 0.5–3.1×10−2 S cm−1 in the range 25–110 °C in anhydrous conditions. This is the highest anhydrous proton conductivity ever reported in a crystalline solid proton conductor in the range 25–110 °C. Superprotonic conductivity in ZP3 is enabled by extended defective frustrated hydrogen bond chains, where the protons are dynamically disordered over two oxygen centers. The high proton conductivity and stability in anhydrous conditions make ZP3 an excellent candidate for innovative applications in fuel cells without the need for complex water management systems, and in other energy technologies requiring fast proton transfer. 

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/ange.202218421

Anhydrous Superprotonic Conductivity in the Zirconium Acid Triphosphate ZrH 5 (PO 4 ) 3 **

High temperature phase transformations and Superprotonic conductivity in Cs2(HSeO4)(H2PO4)

CsH2PO4 has garnered interest as a proton-conducting electrolyte due to its exceptional conductivity at intermediate temperatures (228–300 °C) at which it adopts a cubic structure with a high degree of disorder. Here, through a study of mixtures of CsH2PO4 (CDP) and CsH5(PO4)2, the cubic phase was discovered to form solid solutions of composition [Cs1–xHx]H2PO4, with x extending to at least 2/9. A phase diagram of the composition space (1–x)CsH2PO4 – xH3PO4, 0 ≤ x ≤ 2/9 was developed through thermal analysis, high-temperature in situ X-ray diffraction experiments, and variable-temperature NMR spectroscopy. At temperatures above about 90 °C, monoclinic, stoichiometric CDP exists in equilibrium with Cs7(H4PO4)(H2PO4)8. These two phases displayed eutectoid behavior, with a eutectoid reaction temperature and composition of 155 °C and x = 0.18, respectively, to form cubic [Cs1–xHx]H2PO4. The structural studies revealed, rather remarkably, that the cubic phase accommodates vacancies on the cation site that are charge-balanced by excess protons, where the latter are chemically associated with phosphate groups. The conductivities of cubic phases of various compositions, measured by impedance spectroscopy, are comparable to that of CDP. The excellent proton conductivities of off-stoichiometric, cubic [Cs1–xHx]H2PO4 at temperatures well below the superprotonic transition of stoichiometric CDP present the opportunity to extend the low-temperature operating limit of CDP-based devices. More generally, the off-stoichiometric phase behavior demonstrated here introduces a new approach for the modification of superprotonic solid acid compounds. 

Phase Behavior and Superprotonic Conductivity in the System (1–x)CsH2PO4 – xH3PO4: Discovery of Off-Stoichiometric α-[Cs1–xHx]H2PO4

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Phase Behavior and Superprotonic Conductivity in the System (1–x)CsH2PO4 – xH3PO4: Discovery of Off-Stoichiometric α-[Cs1–xHx]H2PO4