Advancing the Science of Solid Acid Superprotonic Conductors

Superprotonic solid acid electrolytes, materials with chemical and physical properties intermediate between conventional acids (e.g., H₃PO₄) and conventional salts (e.g., Cs₃PO₄), have emerged as attractive candidates for fuel cell and other electrochemical applications. Key characteristics of these materials are tetrahedral oxyanion groups linked by hydrogen bonds, and a polymorphic structural transition to a disordered state at moderate temperatures. Rapid oxyanion reorientation and dynamic disorder of the hydrogen bond network facilitate high proton conductivity in the high temperature phase. Materials exhibiting a superprotonic transition include CsHSO₄, CsH(PO₃H), Cs₃H(SeO₄)₂, CsH₂PO₄, and Cs₂(HSO₄)(H₂PO₄). Of these, CsH₂PO₄ (or cesium dihydrogen phosphate, CDP) has garnered particular attention due to its unique stability in both hydrogen and oxygen environments, rending it applicable in a range of electrochemical devices. A critical limitation of CDP is its need for high steam partial-pressures at the temperatures at which the superprotonic phase occurs ( ≥ 228 °C). In the absence of sufficient humidification, the material decomposes via loss of bulk H₂O to form CsH₂P₂O₇ and/or CsPO₃. Here we describe efforts to stabilize the superprotonic phase to lower temperatures via chemical modifications, specifically, by replacing Cs with Rb and K, with the goal of decreasing the humidification requirement [1]. Surprisingly, both substituents result in a decrease in proton conductivity in the superprotonic cubic phase, which is accompanied by a decrease in activation energy for proton transport. Thus, the decrease in conductivity in the Cs_1-xM_xH₂PO₄ materials (M = Rb, K) with increasing x is a consequence of a strong decrease in the pre-exponential term. The reason for this behavior remains unknown, with simple arguments based on jump distances unable to rationalize the result. Attempts to replace Cs with the NH₄ polycation produced unexpected results and the eventual discovery of (i) proton substituted CDP, i.e., Cs_1-xH_2+xPO₄ in the cubic, superprotonic structure, and (ii) the ordered compound Cs₇(H₄PO₄)(H₂PO₄)₈, (or hepta-cesium tretra-hydroxyphosphonium octadihydrogenphosphate , shortened to CPP). In the former, Cs vacancies are charge-balanced by the presence of additional protons at the phosphate groups [2]. In the latter, Cs cations are replaced with phosphate groups with effective positive charge (H₄PO₄⁺), and the ordered arrangement of these species results in a 4 × 4 × 4 supercell relative to the CsCl structure type of conventional, superprotonic CDP [3]. The compound CPP forms at a relatively low temperature of ~ 90C and has a conductivity of ~ 10^-3 S/cm at 150 °C. Though this conductivity is ~ 20× lower than that of CDP at 250 °C, the humidification requirements are moderate at 150 °C, laying the groundwork to higher efficiency in electrochemical system operation. At compositions intermediate between those of stoichiometric CDP and of the new compound CPP, the system displays eutectoid behavior. The eutectoid temperature is 155 °C, above which Cs-deficient CDP occurs as a phase that is effectively a solid solution between stoichiometric CDP and phosphoric acid. The observation of Cs vacancies in cubic CDP opens the door towards new strategies for stabilizing superprotonic materials towards lower temperatures [4].

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2. L. S. Wang, S. V. Patel, E. Truong, Y.-Y. Hu, and S. M. Haile, Chem. Mater. 34 1809-1820 (2022).
3. L. S. Wang, S. Patel, S. Sanghvi, Y.-Y. Hu, and S. M. Haile, J. Amer. Chem. Soc. 142 19992-20001 (2020).
4. G. Xiong, L. S. Wang, and S. M. Haile, Materials Horizons 10, 5555-5563 (2023).