Phase Behavior and Superprotonic Conductivity in the System (1–x)CsH₂PO₄ – xH₃PO₄: Discovery of Off-Stoichiometric α-[Cs_1–xH_x]H₂PO₄

Abstract

CsH 2 PO 4 has garnered interest as a protonconducting 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 CsH 2 PO 4 (CDP) and CsH 5 (PO 4 ) 2 , the cubic phase was discovered to form solid solutions of composition [Cs 1−x H x ]H 2 PO 4 , with x extending to at least 2/9. A phase diagram of the composition space (1−x)CsH 2 PO 4 − xH 3 PO 4 , 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 Cs 7 (H 4 PO 4 )-(H 2 PO 4 ) 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 [Cs 1−x H x ]H 2 PO 4 . 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 [Cs 1−x H x ]H 2 PO 4 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.

Superprotonic conductivity in RbH2−3y(PO4)1−y: a phosphate deficient analog to cubic CsH2PO4 in the (1 − x)RbH2PO4 – xRb2HPO4 system

“…Furthermore, the compound Ba 2 KH(PO 4 ) 2 , despite being essentially isostructural to superprotonic Rb 3 H(SeO 4 ) 2 , does not have particularly high conductivity. 19 While these lines of investigation have not proven fruitful, a recent, promising approach to the development of new phosphate based superprotonic electrolytes based on ‘off-stoichiometric’ compositions 20 or ‘heterogeneous doping’ 21 has emerged. Here, one pursues materials in which the cation:polyanion atomic ratio deviates from the canonical values of 1 : 1 as found in the x CsHSO 4 – (1 − x )CsH 2 PO 4 series of compounds 12,22–24 and of 3 : 2 as found in A 3 H(XO 4 ) 2 (A = Cs, Rb, NH 4 , K; X = S, Se) 11 and A 3 H 3 (PO 4 ) 2 20 compounds.…”

“…25 Rather remarkably, the CsH 2 PO 4 and Cs 7 (H 4 PO 4 )(H 2 PO 4 ) 8 compounds exhibit eutectoid behavior, forming a non-stoichiometric cubic superprotonic phase of composition stoichiometry Cs 1− x H 2+ x PO 4 over a wide range in x . 20 In parallel, Gaydamaka et al have pursued an analogous study of the RbH 2 PO 4 – Rb 2 HPO 4 system. 26 These authors have reported that the compound Rb 5 H 7 (PO 4 ) 4 , with a cation : polyanion ratio of 5 : 4, undergoes a superprotonic transition at ∼237 °C at which the conductivity rises from ∼10 −5 to ∼10 −2 S cm −1 , and the activation energy for proton transport becomes ∼0.9 eV.…”

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

“…Solids with mobile ions, such as H + , Li + , Na + , Ag + , F – , and O 2– , play a pivotal role in materials science for a range of applications. − In particular, they are needed to realize all-solid-state lithium and sodium batteries − and to develop oxide fuel cells. ,− The search for proton conductors ,,, to develop proton conducting fuel cells has also reached a rather high level. Classical proton conducting ceramics ,, include oxides such as alkaline earth cerates and zirconate perovskites based on acceptor-doped SrCeO 3 , BaCeO 3 , SrZrO 3 , and BaZrO 3 and other compounds such as CsH 2 PO 4 . ,− Usually, protons are incorporated in, e.g., BaZrO 3 by treatment with water vapor or H 2 gas. Recently, it has been shown that the exact proton concentration in Y-doped BaZrO 3 sensitively affects H + conductivities involving nanoscale percolation pathways with low dimensionality …”

Coexistence of Cationic and Anionic Phosphate Moieties in Solids: Unusual but Not Impossible

“…19 From mixtures of CDP and CsH 5 (PO 4 ) 2 , Wang et al synthesized new proton conducting phases (in particular the compound CPP) with increased proton concentrations compared to CDP. 6,20 In addition to increasing the proton concentration, the partial replacement of Cs ions with H 4 PO 4 + also significantly affects the topology of the hydrogen bond network in CDP. A fluctuating and strong hydrogen bond network is a fundamental prerequisite for efficient Grotthuss-like proton transfer.…”

“…This strategy has already been tested by doping CDP with phosphoric acid, but this direct approach for incorporating excess protons did not significantly change the properties of the superconducting phase of CDP . From mixtures of CDP and CsH 5 (PO 4 ) 2 , Wang et al synthesized new proton conducting phases (in particular the compound CPP) with increased proton concentrations compared to CDP. , In addition to increasing the proton concentration, the partial replacement of Cs ions with H 4 PO 4 + also significantly affects the topology of the hydrogen bond network in CDP. A fluctuating and strong hydrogen bond network is a fundamental prerequisite for efficient Grotthuss-like proton transfer. − Therefore, this type of chemical modification of CDP represents a promising approach for fine-tuning the proton conductivity in this compound.…”