Different approaches to obtain functionalized alumina as additive in polymer electrolyte membranes

Lucia Mazzapioda,Mirko Sgambetterra,Maria Assunta Navarra,Akiko Tsurumaki

doi:10.1007/s10008-021-05025-6

Abstract

A series of sulfated aluminum oxides (S-Al2O3), investigated as an electrolyte additive in Nafion membranes, was synthesized via three different methods: (i) sol–gel sulfation starting from an aluminum alkoxide precursor, (ii) room temperature sulfation of fumed aluminum oxide, and (iii) hydrothermal sulfation of fumed aluminum oxide. Through the characterization of the synthesized S-Al2O3 by means of X-ray diffraction (XRD), thermogravimetric analysis (TGA), and infrared (IR) spectroscopy, a higher sulfation rate was found to be achieved via a hydrothermal sulfation, and the coordination state of sulfate groups was identified as monodentate. By using this hydrothermally synthesized S-Al2O3 as additive, a composite Nafion-based membrane was realized and compared to plain Nafion, by means of thermal analyses and fuel cell tests. Although higher hydration degree was found for the undoped membrane by differential scanning calorimetry (DSC), improved retention of fuel cell performance upon the increase of operation temperature was observed by using the composite electrolyte, confirming the stabilizing effect of the acidic inorganic additive.

Highlights

Polymer electrolyte membrane fuel cells (PEMFCs) are promising electrochemical devices with high energy conversion, high efficiency, and low environmental impact
In order to evaluate the amount of sulfate groups bonded to the aluminum oxide surface, thermogravimetric analysis (TGA) was performed on the four powder samples under air in a temperature range between 25 and 900 °C (Fig. 2(a))
In the case of Hydrothermally sulfated fumed alumina (HSA), the removal of sulfate groups from the sample occurred at the temperature above the calcination temperature, 550 °C, suggesting that the sulfate groups are strongly bonded to the oxide surface

Summary

Introduction

Polymer electrolyte membrane fuel cells (PEMFCs) are promising electrochemical devices with high energy conversion, high efficiency, and low environmental impact. The addition of inorganic particles to the polymer membrane, and in particular of solid acids, is a promising strategy to develop composite electrolytes for high temperature fuel cells due to the ability of increasing the water retention. Conspicuous interest has been devoted to functionalized inorganic materials, including sulfated metal oxides (S-MO2), because of their dual property of high acidity and hygroscopicity which promote the water trapping in Nafion and create additional proton pathways through the membrane. This results in the enhancement of proton conductivity and improvement of the fuel cell performance at the desired conditions described above [13]. The samples were prepared without filtering and drying procedures, respectively, which are named HSA-uf and HSA-ud

Procedure

Evaluation of sulfation ratio using TGA

Conclusions