ABSTRACT
Fuel Cell EnvironmentThe electrolyte for aqueous studies of PEFC nanomaterials is chosen either to mimic the acidic and adsorption characteristics of the perfluorosulfonic ionomer (PFSI, e.g., Nafion®) used in PEFC electrodes or to accelerate electrocatalyst degradation. The most commonly used electrolytes are aqueous solutions of sulfuric, perchloric, or trifluoromethanesulfonic acid (triflic acid) [215]. The rationales for the use of these three acids are the similarity of the sulfate anion to the sulfonate anion of PFSIs, the non-adsorbing characteristics of the perchlorate anion, and the similarity of the triflate anion to the fluorinated side chain of PFSIs [53, 110, 144, 156, 208]. The most widely used concentrations of these acids are 0.1 to 1.0 M sulfuric acid (typically 0.5 M), 0.1 to 1 M perchloric acid, and 1 to 6 M triflic acid [55, 188, 218]. The pH of a fully humidified PFSI film has been calculated to be approximately –0.05, assuming a water molecule to sulfonic acid group ratio (λ) of 22; therefore Ohma et al. considered 1 M perchloric and triflic acids to be representative of the acidity of a fully humidified PEFC cathode [151, 199]. Spry et al. estimated the local hydronium ion concentration in the water channels of fully humidified Nafion® to be as high as 1.4 M [199]. 4.1.2.1 Electrocatalytic activityThe most common electrocatalysts for PEFCs are nanoparticles of noble metals, supported on high surface area carbon (e.g., Pt/C). Sulfate and bi-sulfate anions are specifically adsorbed on the surfaces of noble metals [20, 79, 218]. Significant specific adsorption has also been observed, using X-ray absorption spectroscopy (XAS), in 6 M triflic acid electrolyte, but not in 1 M triflic acid or 0.1 M
perchloric acid [218]. This specific adsorption can significantly affect the kinetics of fuel cell relevant reactions [219]. For example, numerous studies have shown that the oxygen reduction reaction (ORR) activity of platinum measured in dilute sulfuric acid is lower than that measured in dilute perchloric acid by as much as 80%, as illustrated in Fig. 4.1 [5, 59, 214]. Gasteiger et al. showed that the ORR activity measured in 0.1 M aqueous perchloric acid electrolyte, a non-adsorbing electrolyte, is in quantitative agreement within a factor of two of the ORR activity measured in a MEA [60, 229]. Among the three types of acids, the activation energy for ORR in triflic acid was found to be most similar to that measured at a Pt/Nafion interface [151, 156]. Ohma et al. found that ORR kinetic parameters in 1.0 M triflic acid were most similar to those determined for a PFSI-coated Pt disk as compared to 0.1 M triflic acid or 0.1 M/1.0 M perchloric acid [151]. They attributed the lower ORR kinetic currents for PFSI-coated Pt and Pt in 1.0 M triflic acid to orientation of the anionic group toward the Pt surface and inhibition of O2 adsorption. Relatively few studies have been published on the effect of electrolyte on the anodic hydrogen oxidation reaction, mainly due the minimal contribution of this reaction to the efficiency losses of PEFC. Lamy-Pitara et al. did report that adsorption of (bi)sulfate affects the kinetics of the hydrogen oxidation reaction [108].
