JOURNAL OF MATERIALS SCIENCE 27 (1992) 6357-6364 Proton conducting polymer electrolyte: II poly ethylene oxide + N H41 system K. K. MAURYA, NEELAM SRIVASTAVA, S. A. HASHMI, S. CHANDRA* Department of Physics, Banaras Hindu University, Varanasi 221 005, INDIA A new proton conducting polymer electrolyte PEO + NH41 system has been investigated. The solution-cast films of different stochiometric ratios have been prepared and characterized. Proton transport has been established using various experimental studies, namely optical microscopy, X-ray diffraction, differential thermal analysis, infrared, coulometry transient ionic current and electrical conductivity measurements at different temperatures and humidity. The maximum conductivity of the complexed material has been found to be ,-~10-~ Scm -1. Both H § ion and I- anion movements are involved with respective transference numbers and mobilities as tH+ =0.74, t~- =0.09, Ix,+ =4.97 x 10 -a cm 2 V -1 s -1 and Ix~- =7.65 x 10-7 cm2V-1 s-1 1. Introduction The ion-conducting polymer electrolytes have attrac- ted wide interest after the pioneering work by the groups of Wright [1, 2] and Armand [3, 4] in the 1970s. The ionically conducting polymer electrolytes are mostly of alkali ions such as Li + and Na + (for reviews see [5-8]). Recently it has been realized that developing proton (H § ion conductors have vast technological applications, particularly in fuel cells. Some, but very few, proton conducting polymer elec- trolytes have been reported so far, e.g. polyethylene oxide (PEO):NH4SCN and NH4SO3CF 3 [9], PEO and poly acrylic acid (PAA):NH4HSO 4 [10], PEO:NH4C10 4 [11, 12], PEO:H3PO 4 [13], poly vinyl alcohol (PVA):H3PO 4 [14, 15] poly ethylene imine (PEI):HzSO 4 and H3PO 4 [16], HC1 [17], CH3COOH [18]. In general, it is difficult to establish the proton conduction unambiguously. In the present paper, we report a new proton conducting polymer electrolyte obtained by complexing PEO polymer with NH4I. A wide variety of experiments (X-ray diffraction, differential thermal analysis, infrared, complex impedance analysis for conductivity, cyversus 1/T, cy versus relative humidity, transference number, mobility) have been carried to establish proton trans- port and to characterize the material. It has been observed from the above studies that the PEO + NH4I system is predominantly an H § ion conductor together with a small contribution to the total conductivity due to the I ~ motion. 2. Experimental procedure Films ( ~ 200-400 pm) of PEO (mol. wt 600 000) com- plexed with NH4I were prepared of different compositions, i.e. NH~-/EO ratios varying from *Author to whom all correspondenceshould be addressed. 0022-2461 9 1992 Chapman & Hall ~0.016~).138. The stoichiometric ratio of different compositions was solution cast followed by slow evap- oration. Distilled methanol was used as solvent. The films were dried rigorously in a high vacuum to eliminate all traces of methanol (confirmed by infrared studies). Optical micrographs of the films of different com 2 positions were taken using a Leitz optical microscope (LABORLUX "D'). The X-ray diffraction (XRD) studies were carried out using Philips X-ray diffrac- tometer (PW 1710). The infrared (IR) spectral study of the films of puTe PEO and complexed PEO of different compositions was carried out using a Perkin-Elmer IR spectrophotometer (Model 883). Differential ther- mal analysis (DTA) was carried out using Linseis instruments (Type 2045). Samples of ~ 60 mg of each composition were kept in a platinum crucible and heated in a static air atmosphere at a heating rate of 5~ C min -1" The total ionic transference number was computed using the polarization method. In this method, the d.c. current was monitored as a function of time, on application of a fixed d.c. voltage across the cell A1/PEO + NH41 electrolyte/A1. The transference number, rio,, was calculated from the initial current, ii, and final residual current, ie, after polarizing the electrolyte, using the formula tion ~-- ( i i - ie)/ii (1) However this method does not give information re- garding the cationic/anionic contribution to the total conductivity. To circumvent this problem, a coulo- metric investigation based on Faraday's law of elec- trolysis was carried out using a specially designed double-arm coulometer or electrolysis cell. Details of this method is given elsewhere [19, 20]. The gas 6357