Synthesis of Nafion/CeO₂ hybrid for chemically durable proton exchange membrane of fuel cell

Abstract

Nafion/CeO₂ hybrid composites prepared through a self-assemble route are proposed for chemically durable electrolyte membrane of fuel cell. Self-assembly of Nafion and CeO₂ nanoparticles between the positively charged CeO₂ and the negatively charged SO₃⁻ end groups of Nafion ionomers gives the composite structure a good compatibility on the interface. As a result, the Nafion/CeO₂ hybrid proton exchange membranes present conductivities slightly lower than that of the pristine Nafion membrane at 100 RH% condition, whereas much higher than the later when humidification is below 75 RH%. The Nafion/CeO₂ hybrid membranes also display excellent radical scavenge ability under hydroxyl radical corrosion condition. With Fenton reagent of 30 wt% H₂O₂ solution containing 20 ppm FeCl₂ at 85 °C, the fluoride emission rates for the self-assembled Nafion/CeO₂ membranes with ceria content of 1, 3, 5, 10 wt% are 43.05, 8.67, 6.01, 4.47 mg h⁻¹, respectively, much lower than that for pristine Nafion membrane (55.78 mg h⁻¹) and Nafion/CeO₂ membrane prepared from the conventional sol–gel method (11.64 mg h⁻¹). Another important aspect of the self-assembled Nafion/CeO₂ membrane contributed to the durability is the low humidity-induced stress and dimensional stability. The humidity-induced stress of the self-assembled Nafion/CeO₂ membranes with ceria content of 1–10 wt% was 1.22–1.93 MPa, in comparison to 2.25 MPa of pristine Nafion membrane. Thus, the proton exchange membranes prepared from self-assembled Nafion/CeO₂ present high durability under the fuel cell operating conditions. In the open circuit voltage accelerated test under in situ accelerating RH cyclic test, the OCV reduction rate of self-assembled Nafion/CeO₂ membrane with ceria content of 5 wt% is 1.13×10⁻⁴ mV/s, much lower than 11.7×10⁻⁴ mV/s of the pristine Nafion membrane and 5.78×10⁻⁴ mV/s of the Nafion/CeO₂ hybrid membrane prepared from the conventional sol–gel method.

Introduction

Polymer electrolyte membrane (PEM) fuel cell has been extensively studied as a clean and efficient power source for electric vehicles and residential power sources. The degradation of proton exchange membrane continues to present challenge for fuel cells to meet the required lifetime. This is especially the case when the fuel cell is operated in transportation applications with extreme or cyclic changes in load [1], [2]. The integrity of the PEM is one of the most crucial factors affecting the lifetime of fuel cells because it functions both as an electrolyte and a separator of the reactant gases. Failure of the former decreases the performance, while failure of the latter aggravates the gas crossover through the electrolyte membrane and destroys the cell [3], [4] Perfluorosulfonic acid (PFSA) membranes, such as Nafion, are deemed as the state-of-art PEM material because of its excellent chemical, mechanical, and thermal stability, as well as its relatively high proton conductivity when fully hydrated [5], [6]. However, chemical destruction and conductivity degradation of the Nafion membrane are frequently observed in fuels cells that have been operated for a long term [7], [8], [9].

Researchers believe that chemical degradation of the PEM typically occurs via a two-step process including the formation of reactive oxygen species such as the hydroxyl radical OH, and reaction of reactive oxygen species with the PEM [10], [11], [12], [13]. Radicals can originate from electrochemical and chemical reactions on fuel cell electrode with the presence of transition metal cations which can split hydrogen peroxide produced from a two-electron oxygen reduction [14], [15], [16], or direct reaction of H₂/O₂ on the surface of the Pt catalyst because of oxygen crossover from the cathode at low currents and hydrogen crossover from the anode at high currents [17], [18], [19], [20]. In these cases, unstable terminal –COOH end groups of the perfluorinated sulfonic acid ionomers (PFSA) originated from weak polymer end groups and/or the side chain cleavage are oxidized to carbon dioxide, reforming the linked CF₂ units to another terminal carboxyl group and resulting in unzipped degradation [21], [22], [23].

In the past several years, much effort has been geared toward the development of alternative, chemically stable PEMs. Cross-linked polyelectrolyte [24], grafted copolymer electrolyte [25], and multilayer electrolytes with aromatic polymers [26] fall into this category. Although each material has its own advantages, most are restricted by the imbalance of physical stability and proton conductivity of the electrolyte membrane due to the inherently low conductivity of the hydrocarbon electrolyte. Another approach is to introduce an inorganic abnormal valence catalyst such as cerium oxide [27], [28], [29], manganese oxide [30], heteropoly acids [31], or zirconia [32] into the proton exchange membrane to decompose the hydroperoxyl radicals during operation.

Nafion metal oxide hybrid membrane may be produced either by physical mixing of oxide powder with Nafion solution [33], [34], [35], or by the sol–gel reactions [36]. In the case of in situ sol–gel derived Nafion/silicate composite membranes, membranes were soaked in a bath containing TEOS, allowing the hydrolyzed TEOS molecules to permeate and diffuse into the organic phase. The sol–gel reaction is confined mainly to polar clusters at short reaction times, but upon further reaction time, the inorganic oxide phase begins to percolate and becomes knit together through the ionomer matrix. This is indicated by the observation of the agglomeration of inorganic fillers and the formation of cracks on the surface of the composite membrane and the formation of inorganic oxide-rich surface on the Nafion/inorganic composite membranes. The formation of inorganic filler-rich surface is detrimental to the performance and durability of PEM fuel cells.

Stronger interaction along the interface between the host matrix and the filler particles results in a greater modification of the original properties of the membrane [37], [38], and inorganic oxides functioned by acidic groups would improve the proton connectivity of the hybrid membranes [39]. Enhancement of the interfacial compatibility between the inorganic nanoparticles and the proton conducting polymer should also be critical for the radical scavenge ability and durability of the composite membranes. It was demonstrated recently that highly dispersed silica [40], [41] or zirconia [42], [43] nanoparticles with size of several nanometers can be prepared by combining the self-assembly route and Nafion/inorganic oxide hybrids. Thus, if cerium dioxide radical scavenges can be synthesized and stabilized by Nafion, it is anticipated that an intimate interface between the Nafion-radical scavenges particles and the Nafion matrix could be achieved, compatibilized by Nafion ionomers bridging between the radical scavenges nanoparticles and the Nafion polymeric matrix. In this study, we report a novel Nafion/CeO₂ composite membrane prepared from the Nafion-stabilized CeO₂ nanoparticles by the self-assembly technique. Our choice of CeO₂ as a target regenerative free radical scavenger for PEMs arises from multiple roles played owing to its unique acid–base and redox properties. CeO₂ nanoparticles have been shown to be excellent free radical scavengers in biological systems [44], [45], [46]. Additionally, they have been shown to be regenerative in nature, especially in acidic media. The preparation procedure and properties of the composite membrane of the Nafion/CeO₂ membranes were studied in detail. The results show that Nafion/CeO₂ hybrid membranes possess excellent anti-oxidation abilities, good interfacial properties, uniformly dispersed CeO₂ nanoparticles, high mechanical properties and high durability, in comparison with the conventional Nafion/CeO₂ composite and pristine Nafion membranes.