Tuning of Nafion® by HKUST-1 as coordination network to enhance proton conductivity for fuel cell applications

Abstract

Metal-organic frameworks can be intentionally coordinated to achieve improved proton conductivity because they have highly ordered structures and modular nature that serve as a scaffold to anchor acidic groups and develop efficient proton transfer pathways for fuel cell application. Using the concept of a coordination network, the conductivity of Nafion® was tuned by the incorporation of HKUST-1. It has CuII–paddle wheel type nodes and 1,3,5-benzenetricarboxylate struts, feature accessible sites that provides an improved protonic channel depending on the water content. In spite of the fact that HKUST-1 is neutral, coordinated water molecules are contributed adequately acidic by CuII to supply protons to enhance proton conductivity. Water molecules play a vital part in transfer of proton as conducting media and serve as triggers to change proton conductivity through reforming hydrogen bonding networks by water adsorption/desorption process. Increased ion exchange capacity and proton conductivity with lower water uptake of the H3PO4-doped material, and improved thermal stability (as confirmed by thermogravimetric analysis) were achieved. The structure of HKUST-1 was confirmed via field emission scanning electron microscopy and X-ray diffraction, while the porosity and adsorption desorption capacity were characterized by porosity analysis. The H3PO4-doped HKUST-1/Nafion® composite membrane is demonstrated to be a promising material based on its proton conductivity. HKUST-1 has an average particle diameter of around 15–20 µm. The proton conductivity, IEC values, and the thermal stability of the 2.5 wt% HKUST-1/Nafion® composite membrane suggest that HKUST-1 may be a promising candidate as a proton-conductive material in the polymer electrolyte fuel cell membrane due to its reasonable proton passageway, favorable surface area, lower water uptake with the higher IEC, and proton conductivity of the H3PO4-doped material and facile synthesis.