ИСТИНА

Introduction Nowadays the application of fuel cells (FC) as the alternative energy supply is an excellent way to reduce the negative influence on our planet’s ecological situation. However, for a wide FC’s implementation into energetics it is essential to solve a number of problems, one of which deals with finding of an appropriate proton-exchange polymer membrane as a solid electrolyte. The main problem of the currently well-known materials is low membrane proton conductivity under low humidity. This demands additional humidification of the membrane electrode assembly and considerable complication of FC design, which leads to additional expenses. The improvement of transport properties may be achieved by the incorporation of heteropolyacids or theirs acid salts into the membrane matrix, which are capable to keep water at low humidity and to perform high proton conductivity. Experiments The object of this work is concerned with the synthesis and investigation of the transport properties of Nafion-based membranes, doped with acid salts of heteropolyacids M x H 3x PA 12 O 40 , M x H 4x SiA 12 O 40 (M=Rb, Cs; A=W, Mo), including membranes with gradient distribution of a dopant through a thickness of membrane. Hybrid membranes were obtained by casting from polymer solutions, containig dopant’s nanoparticles at different concentrations (from 1.5 to 10 wt%). Hybrid membranes with gradient dopant distribution across the thickness (20% from the total thickness contained dopant) were obtained by layer-by- layer casting from polymer solutions. Solution without dopant was casted, followed by its exsiccation a solution contains dopant nanoparticles was drifted on. Results and Discussion For hybrid membranes with homogeneous dopant distribution it was shown that incorporation of acid salts of heteropolyacids leads to the increase in proton conductivity (Fig.1). The maximum conductivity is provided for samples which contain 3 wt% of dopant. The highest conductivity was achieved for materials containing salts of silicatungstic heteropolyacid. With the decrease of relative humidity, the conductivity of membraneas is decreased, but the difference between conductivity of hybrid membrane (Nafion + 5% Cs x H 4-x SiW 12 O 40 ) more than 2 times higher than that of Nafion membrane and reaches 4.14 mS/cm (Fig. 2). Diffusion permeability of 0.1 М HCl solution is lower in hybrid membranes as compared to the initial one. Taking into account the increase in membrane conductivity, all this proves the increase in selectivity of cation transport in hybrid membranes. Proton conductivity of membranes with gradient dopant distribution in contact with water is not much lower as compared to unmodified membrane. Under low relative humidity hybrid membranes with gradient dopant distribution show higher conductivity as compared to initial one , however, their conductivity is less than that of volume-modified samples. The diffusion permeability measurements were carried out in two directions: when electrolyte diffuse from unmodified (P unmod ) and from modified (P mod ) sides (Table. 1). As in the case of volume-modified membranes, the dopant presence leads to the decrease in diffusion permeability as compared to the initial membrane. Moreover, ion transport asymmetry is observed: diffusion permeability is lower, when solution diffuses from the modified side. Thus, maximum asymmetry coefficient (19%) was obtained for membrane containing 3% Cs x H 3-x PW 12 O 40 in modified layer. Moreover, diffusion permeability coefficient of HCl solutions of different concentrations was measured for this sample. It was estimated that asymmetry decreases with the increase of concentration from 52% for 0.05 M HCl solution to 5% for 0.3 M solution. To sum up, modification of Nafion membrane by acid salts of heteropolyacids results in the increase in proton conductivity as well as in the decrease in diffusion permeability. The asymmetry of ion transport for membranes with gradient dopant distribution is observed. Acknowledgements This work was financially supported by Russian foundation for basic research (project 14- 29-04054_ofi_m).