## 1 Introduction  
The increasing need in clean energy resources is promoting research in the field of proton-conducting materials for low- and intermediate-temperature fuel cells (FCs) [1, 2, 3]. Membrane electrode assemblies (MEAs) play a key role in the overall fuel cell performance, because electrochemical reactions occur [4]. These consist of dense membranes (electrolytes) placed between porous electrodes. In cases where the electrolyte is a proton conductor, protons selectively penetrate the membrane bulk and move through towards the cathode where water and heat elaborate as a result of the reaction [5, 6]. The type and geometry of the membrane should be carefully chosen, not only in accordance with the operation conditions of the fuel cell, but considering other parameters such as working temperature, fuel, moisture presence, catalysts, etc. [7, 8]. The general requirements for electrolyte use as a membrane in electrochemical cells (such as fuel cells), electrochemical reactors and sensors are [9]: high conductivity in the required temperature range; fast proton transport; temperature and phase (structural) stability; absence of side reactions with anode and interconnect materials, i.e., chemical inertness; and high density of the membrane, making it impenetrable for the other components of a gas or fuel mixture. Polymeric membranes are most commonly used for MEA fabrication in fuel cell technology [4; 10; 11]. Their main advantages are low price and easy processability. However, they possess low chemical and thermal stability, as well as high gas bubble adhesion, which can easily result in membrane behavior deterioration [12; 13]. \"Nafion\" family membranes have been known since the 1960s, and because of their superior conductive properties and rather high perm selectivity, they started to be commercially used in several types of fuel cells [14; 15; 16]. However, they possess low conductivity at low water content, and low mechanical strength at high temperatures [14; 17; 18]. As discussed by [PERSON] et al. [17], the problem affecting long-term stability of the FCs is the decrease in conductivity due to decomposition of the polymer membrane. Consequently, these factors have led to searches for other types of inorganic proton conductors, which are less expensive and can overcome the \"Nafion\" family disadvantages. Several inorganic compounds, including fluorite and perovskite-based materials [19; 20], various cerates [21], some acid salts with general composition M\({}_{\text{m}}\)H\({}_{\text{n}}\)(AO\({}_{\text{4}}\))\({}_{\text{(m+n)/2}}\) (M = K, Rb, NH\({}_{4}\), Cs; A = S, Se) and CsH\({}_{2}\)(RO\({}_{4}\)) (R = P, As) [22; 23] have shown rapid proton conduction, up to 10\({}^{-2}\)-10\({}^{-3}\) S/cm. Some ion exchangers such as Sb\({}_{2}\)O\({}_{5}\cdot\)nH\({}_{2}\)O, ZrO\({}_{2}\cdot\)nH\({}_{2}\)O or SnO\({}_{2}\cdot\)nH\({}_{2}\)O are also super proton conductors [24]. In addition, the development of new composite membranes based on inorganic proton conductors combined with an organic matrix [25; 26; 27; 28] has become a trending target for researchers. Despite exhibiting high conductivity, most of the listed compounds undergo phase transitions, resulting in new phases with low conductivity, or lose their chemical stability upon temperature increases. Polyantimonic acid (PAA, Sb\({}_{2}\)O\({}_{5}\cdot\)nH\({}_{2}\)O, where 2 \(<\) n \(<\) 6) is regarded as a potential proton-conducting electrolyte for low- and intermediate-temperature FCs. It is a well-studied crystalline inorganic ion-exchanger [29] which exhibits high proton conductivity up to 10\({}^{-1}\) S/cm and good chemical and thermal stability up to 400 \({}^{\circ}\)C [24; 28; 30]. In addition, PAA is an insoluble, non-toxic compound. Phosphorus-substituted PAA is commercially used for Cs, Rb, Sr and Ba radionuclide fixation in the human body (polysurmin drug) [31]. The formula H(H\({}_{2}\)O)SbO\({}_{3}\cdot\)0.5H\({}_{2}\)O is sometimes used to reflect the non-equivalence of the water molecules in the crystal structure [32].