# Novel Highly Dispersed Additive for Proton-Conducting Composites
## 4 Conclusions  
In this work, the synthesis and the thermal decomposition of zinc hydroxostannate were studied in detail. ZnSn(OH)\({}_{6}\) with the cubic crystals of 150-250 nm in size was synthesized by hydrolytic precipitation from the solutions of ZnCl\({}_{2}\) and Na\({}_{2}\)SnO\({}_{3}\). Zinc stannate with a high specific surface area (85 m\({}^{2}\)/g) was formed through the thermolysis of ZnSn(OH)\({}_{6}\) in the temperature range of 300-520 \({}^{\circ}\)C. A weakly crystallized ZnSnO\({}_{3}\) phase with a particle size of 15-20 nm was formed at 540-600 \({}^{\circ}\)C, while at an annealing temperature of 700 \({}^{\circ}\)C, a mixture of nanocrystalline SnO\({}_{2}\) with the cassiterite structure (20-40 nm) and Zn\({}_{2}\)SnO\({}_{4}\) (less than 60-70 nm) with the spinel structure was formed. As a result of the thermolysis study, optimal conditions for the formation of highly-dispersed zinc stannate were found, and the obtained materials were further used for the creation of the composite solid electrolytes.  
The proton conductivity of the composite systems with the compositions of (1-x)CsH\({}_{2}\)PO\({}_{4}\)-\(x\)ZnSnO\({}_{3}\) (\(x\) = 0.2-0.8) was studied. The CsH\({}_{2}\)PO\({}_{4}\) crystal structure remained in the composite systems, but dispersion and partial salt amorphization were observed due to the interface interaction with the ZnSSnO\({}_{3}\) matrix. As a result, the conductivity in the low-temperature region increased up to 2.5 orders of magnitude, went through a smooth maximum at \(x\) = 0.2, and then decreased due to the percolation \"conductor-isolator\" effect. The samples with the highest proton conductivity at \(x\) = 0.2 demonstrated stable high conductivity values (8 \(\times\) 10\({}^{-3}\) S/cm) under long-term isothermal exposure (150 min at 230 \({}^{\circ}\)C) at lower partial pressures of water vapor. This will allow this material  
Figure 9: Dependence of conductivity of (1–\(x\))CsH\({}_{2}\)PO\({}_{4}\)–\(x\)ZnSnO\({}_{3}\) (\(x\) = 0.2) system on the time of long-term storage at 230 \({}^{\circ}\)C (water vapor content was 20 mol%).  
to be used in electrochemical applications. Thus, it has been demonstrated that X-ray amorphous zinc stannate ZnSnO\({}_{3}\) is a suitable heterogeneous additive in other composite solid electrolytes. The composite solid electrolytes CsH\({}_{2}\)PO\({}_{4}\)-ZnSnO\({}_{3}\) may be used as solid electrolyte membrane materials in intermediate temperature electrochemical resistors for hydrogen production. Such an approach might be competitive compared with alternative hydrogen production techniques [66; 67; 68; 69; 70].  
A.I.A., A.G.B. and A.M.: investigation, methodology, data curation, and writing--original draft. A.V.L., N.U., V.V.G. and A.S.A.-Q.: investigation, data curation, and methodology: conceptualization, paper revision, and English correction; V.P.: writing--review and editing. I.B.: methodology and investigation. All authors have read and agreed to the published version of the manuscript.  
The work was supported by the Russian Federation Ministry of Science and Higher Education (state research target, project no. FSUN-2023-0008).  
Not applicable.  
The data is available from corresponding author upon a reasonable request.  
The authors declare no conflict of interest.