# Novel Highly Dispersed Additive for Proton-Conducting Composites
## 1 Introduction  
Materials based on the oxide systems Zn-Sn-O possess numerous functional properties and are thus of growing interest to researchers. For instance, zinc stannate has recently been utilized as anode material for sodium-ion and lithium-ion batteries [1; 2; 3]. Additionally, several works [4; 5; 6] have highlighted the potential for stannates as materials for supercapacitors and solar cell electrodes. Because of its chemical reactivity, excellent electronic properties, and perovskite structure, ZnSnO\({}_{3}\) is widely used for gas and organic compound sensors [7; 8; 9; 10]. Zinc stannates also have numerous applications in electronics, catalysis, and photocatalysis [21; 22; 23; 24; 25; 26; 27]. In the Zn-Sn-O system, zinc metastannate ZnSnO\({}_{3}\) and zinc orthostannate Zn\({}_{2}\)SnO\({}_{4}\) are present. Zinc metastannate ZnSnO\({}_{3}\) has an orthorhombic elementary lattice with a perovskite-like crystal structure, while zinc orthostannate Zn\({}_{2}\)SnO\({}_{4}\) has a cubic lattice with a spinel-type structure. Various methods are used to obtain these compounds, including low-temperature ion exchange, sol-gel technology, and coprecipitation followed by thermal treatment [4; 28; 29; 30; 31; 32]. Among them, the sol-gel method is the most commonly used approachdue to its ease of control, low-temperature, and high efficiency for obtaining various homogeneous nanostructures. However, for practical applications, coprecipitation is typically used as it is a more readily available and easy-to-implement method. Previously, it was demonstrated that hydroxostannates MSn(OH)\({}_{6}\) of alkali-earth metals MSnO\({}_{3}\) and M\({}_{2}\)SnO\({}_{4}\) (M = Mg, Ca, Sr, Ba) can be obtained by thermal decomposition [33; 34; 35; 36]. Notably, the stannates obtained by thermolysis possess a relatively high specific surface area (10-100 m\({}^{2}\)/g) and can be utilized as functional additives to obtain composite solid electrolytes [34]. According to the literature data [5; 37; 38; 39], zinc stannates are mostly synthesized through the thermal decomposition of the hydroxostannate ZnSn(OH)\({}_{6}\). This process results in the formation of highly dispersed products, which can be utilized to obtain the composite materials. However, zinc stannates are characterized by higher chemical resistance to acids compared with alkaline earth metal stannates. This makes them a promising heterogeneous additive for creating composite proton solid electrolytes with acidic alkali metal salts, such as CsH\({}_{2}\)PO\({}_{4}\), which have a high conductivity up to 6 \(\times\) 10\({}^{-2}\) S/cm at 230 \({}^{\circ}\)C [40; 41; 42]. Cesium dihydrogen phosphate undergoes a superprotonic phase transition at 230 \({}^{\circ}\)C, which results in a significant increase in its conductivity to 6 \(\times\) 10\({}^{-2}\) S/cm. At low temperatures (20-230 \({}^{\circ}\)C), the conductivity dependence has an Arrhenius form with an activation energy of 0.9 eV, while at high temperatures (230-260 \({}^{\circ}\)C), the superionic conductivity activation energy is 0.42 eV. An abrupt increase in conductivity during the superionic phase transition is observed, from 10\({}^{-7}\) S/cm to 6 \(\times\) 10\({}^{-2}\) S/cm (at T \(>\) 230 \({}^{\circ}\)C). To suppress the dehydration and increase the stability range of the high-temperature phase, the formation of an increased partial pressure of water vapor is necessary [43]. CsH\({}_{2}\)PO\({}_{4}\) is a promising proton membrane for medium-temperature fuel cells [44; 45]. Currently, investigations are focused on creating highly conductive composite and polymer electrolytes with modified salts in a larger temperature range, as well as searching for electrochemically active electrode compositions for medium temperature fuel cells based on cesium dihydrogen phosphate [46; 47; 48; 49; 50; 51; 52; 53; 54; 55; 56; 57; 58].