Proton Conduction in a Quaternary Organic Salt: Its Phase Behavior and Related Spectroscopic Studies

One of the key challenges of fuel cell technology is to find solid electrolytes which are cheap and environmentally friendly with high proton (H⁺) conductivities. In this context, designing new materials based on organic cocrystals/salts appears very promising to expand the scope of H⁺ ion conductors. In our approach, we have synthesized a quaternary organic salt consisting of gallic acid, isoniazid, sulfate (SO₄^2–) ion, and water by a slow evaporation method which exhibits high proton conductivity of 0.2 mS cm^–1 at 293 K to serve as a model system. The proton conductivity value observed in our system is comparable and in some cases better than recently published coordination polymers, metal organic frameworks, and covalent organic frameworks. The system crystallizes as monoclinic with space group <i>P</i>2₁/<i>c</i> (<i>Z</i>′ = 3; <i>Z</i> = 12), which depicts a layered structure with extensive O–H···O and N–H···O hydrogen bonding networks. Further, it exhibits interesting order–disorder phase transitions at both high and low temperatures. Calculation of the activation energy (∼0.39 eV) from conductivity plots for the system reveals the mechanism of proton conduction to be Grotthuss type. Thus, our novel design strategy of preparing an organic salt for proton conduction applications opens up a pathway to generate easy synthesis of cheap and environmentally friendly materials