Sulfated zirconium oxide as electrode and electrolyte additive for direct methanol fuel cell applications

Highlights

• Sulfated zirconium oxide (S-ZrO₂) is used as electrode and electrolyte additive for direct methanol fuel cells (DMFCs).

• Composite electrocatalysts based on Pt/C and S-ZrO₂ display improved methanol tolerance and ORR activity than bare Pt/C.

• S-ZrO₂ as both catalyst and Nafion additive ensures an enhanced membrane/electrode interface stability.

• Superior DMFC performance of the composite membrane electrode assembly in terms of current and power density.

Abstract

Sulfated zirconium oxide (S-ZrO₂) was used as electrode and electrolyte additive for direct methanol fuel cells (DMFCs). Composite Nafion electrolyte membranes and Pt electrocatalysts, both containing S-ZrO₂ at different content, were prepared. The morphology and catalytic activity of prepared catalysts were investigated by scanning electron microscopy, and voltammetric technique. Results indicated that Pt/S-ZrO₂ catalysts showed enhanced efficiency towards oxygen reduction reaction and increased methanol tolerance as compared to bare platinum. Pt/S-ZrO₂-based carbon cloth electrodes were prepared and assembled as cathode in a DMFC, with Nafion/S-ZrO₂ as composite electrolyte membrane. With respect to bare platinum and Nafion, higher values of current and power density were recorded at 110 °C. The use of S-ZrO₂ both as catalyst and electrolyte additive provided enhanced membrane/electrode interface stability, as revealed by EIS spectra recorded during cell operation.

Introduction

Direct Methanol Fuel Cells (DMFCs) are attractive portable power generation devices due to high energy density, low operating temperature, and to the ease of handling, storing and transporting methanol, as compared to hydrogen [1], [2]. However, DMFC market diffusion is still limited by high costs and performance still far from target mainly because of materials constraints [3], [4], [5], [6], [7]. Over the last decades, extensive research efforts have been carried out to replace Nafion and Platinum, which are still the state of the art materials as electrolyte and catalyst, respectively. Despite its high chemical stability, Nafion performance is impaired by methanol crossover and proton conductivity limitations. In fact, at temperatures higher than 90 °C, Nafion undergoes an irreversible swelling process that damages the membrane dimensional stability [8] and introduces interfacial complications when the electrolyte is assembled with electrodes, including high resistance and poor adhesion between membrane and electrodes [9], [10]. To overcome this drawbacks, the fabrication of composite polymer electrolyte membranes has been explored and the use of inorganic fillers as Nafion additives has been found to have a beneficial effect on the water retention properties of the resulting composite membranes [11], [12], [13], [14], [15]. However, the role of polymer/filler interaction on methanol and proton transport through the membrane is far to be completely understood and its consequence on the final DMFC performance is anything but obvious, so that the optimal electrolyte material has yet to emerge.

The use of platinum as both anode and cathode catalyst not only raises the costs of DMFC devices but also limits their performance. On one hand, the self-poison of reaction intermediates on the Pt catalyst results in poor kinetics at the anode. Moreover, the parasitic methanol oxidation at the cathode, which arises as a consequence of methanol crossover through the membrane, brings out a significant cathode depolarization because of the occurrence of a mixed potential and poisoning of Pt active sites [16], [17]. This leads to the need of exploring of new catalyst materials, including noble and non-noble metals [18]. Up to now, numerous Pt-based alloys including Pt–Cr, Pt–Fe, Pt–Co, Pt–Ni and Pt–Au have been investigated as methanol-tolerant cathode catalysts [19], [20], [21], [22], [23] and they have been found to be effective in reducing the active sites for methanol adsorption. Non-noble catalysts such as pyrolyzed transition metal nitrogen-containing complexes [24] transition metal chalcogenides [25], metal oxides/carbides/nitrides/oxynitrides/carbonitrides [26], and organometallic complexes [27], [28] have also been investigated [29] but, although great progress has been achieved, there are still some challenges in the oxygen reduction reaction (ORR) activity and stability. Hence, current hurdles persist and progress is still needed to achieve further materials (both electrolyte and catalyst) improvements and alignment with current targets. In this work, we developed composite Nafion-based electrolyte membranes and composite Pt-based electrocatalysts both containing sulfated zirconium oxide (S-ZrO2) as both electrolyte and catalyst additive.

The use of zirconium oxides as catalysts for ORR reaction has been reported [30] and good methanol tolerance, electrochemical stability in acidic environment, and to some extent also ORR activity, even if still much lower than platinum, have been demonstrated. The use of zirconium oxide and its sulfated form (S-ZrO2) as electrolyte additive has also been explored to prepare composite polymer membranes based on Nafion. Sulfated oxide led to the improvement of Nafion water retention, dimensional stability and proton conductivity, allowing at the same time decreasing of methanol crossover through the membrane [7], [12], [13]. These features motivated us to use S-ZrO2 as additive of both electrolyte (Nafion) and catalyst (carbon-supported platinum) with the aim to i) fabricate a polymer electrolyte with improved proton conductivity with respect to Nafion at the optimal operative conditions of DMFC (T > 90 °C); ii) fabricate a methanol-tolerant cathodic catalyst with high ORR activity; iii) guarantee chemical continuity at electrode/electrolyte interface through the use of the same additive to catalyst and electrolyte. ORR activity and tolerance towards methanol of composite Pt/S-ZrO2 catalysts were assessed by cyclic and linear sweep voltammetry. The final DMFC performance of these materials was also tested and compared with that of reference unfilled Platinum.