Improvement of the Diffusion Coefficient and Concentration of O2 in Nafion® with 2,2’-Bpy

Abstract

The polymer electrolyte fuel cells (PEFCs) are expected as power sources to reduce the consumption of fossil fuel for a solution to global warming. Because oxygen reduction reaction (ORR) at the cathode of PEFCs is sluggish even if Pt/C catalysts are used, new approach is necessary to improve the cathode reaction, thus smooth gas supply to the surface of cathode catalyst may be effective. We reported that the addition of 2,2'-bpy (concentration in film: 10 M) to Nafion® improved oxygen reduction reaction, suggesting that the diffusion coefficient and the concentration of oxygen are improved by the change of morphology. In this study, the effects of 2,2’-bpy on the diffusion coefficient and concentration of O2 in Nafion® film were investigated based on electrochemical measurements, differential scanning calorimetry (DSC) and ab initio calculations. 20 μL of 5 wt% Nafion® solution was cast on a Pt disk electrode (diameter: 6 mm) and dried under N2. The electrode was soaked in a 2,2’-bpy aqueous solution and rotated at 600 rpm to adsorb 2,2’-bipyridine. The concentration of 2,2’-bpy in the Nafion® was estimated using the change of the absorbance at 281.5 nm of the solution. Electrochemical measurements were performed in 0.05 M sulfuric acid solution with the modified working electrode, a Pt wire counter electrode, and an RHE refence electrode with a rotating ring disk electrode system. First, the potential was cycled between 0.05 V to 1.60 V for 20 cycles under N2 atmosphere, and the rotating ring disk electrode measurements and the potential step measurements from 1.60 V to 0.40 V were carried out under 1 atm O2 to estimate the diffusion coefficients and the concentration of O2 in Nafion. The diffusion coefficient of O2 was calculated as follows: time courses of the amount of charge in the potential step measurements were analyzed by the integral form of Cottrell equation: Q(t)=2nFAD1/2Cfπ-1/2 t 1/2 + Q dl(t) +nFAΓ (1) where t [s] is time, Q(t) [C] is the amount of charge at t, n is the number of electrons, F is the Faraday constant, A [cm2] is the electrode geometric area, D [cm2 s-1] and C f [mol cm–3] are the diffusion coefficient and the concentration of O2 in the Nafion®, respectively, Q dl(t) is the capacitive charge, and Γ [mol cm–2] is the surface concentration of an adsorbate reduced by the potential step. The sum of current flow derived from the second and third terms was eliminated by that measured under nitrogen. D f 1/2 C f can be estimated by the slope of Anson plot (a A) as follows: D f 1/2 C f = a Aπ1/2/(2nFA) (2) The time scale used in the analysis was less than 1.5 s when the change of O2 concentration reached the film/electrolyte interface. In the RRDE measurements, the inverse of limit current of the film-coated electrode is given by the following equation: 1/i l =1/(1.554πFAD f 2/3 ω 1/2 v -1/6Cf) + δ/(nFC f D f) (3) where i l [A cm–2] is diffusion limit current, ω [Hz] is the rotating speed of the electrode, and δ is the thickness of Nafion® film on the electrode. D f C f can be estimated using the intercept of i l -1 vs. ω 1/2 plot (b L) as follows: D f C f=δ/(nFb L) (4) D f and C f values were estimated with eqs. 2 and 4. Fig. 1 shows the dependence of the diffusion coefficient and concentration of O2 in Nafion® film on the concentration of 2,2'-bpy. The diffusion coefficient and the concentration of O2 in the Nafion® film increased drastically until 0.5 M and then decreased and became almost constant above 2 M. DSC measurements suggested that the hydrophilic cluster sizes of the Nafion® film increase with increasing concentration of 2, 2 '-bpy, which induces the increase in the parameters. ORR depends on proton concentration in Nafion® as well as the diffusion coefficient and the concentration of O2, thus, the decrease in the parameters was caused by the inhibition of proton conduction via a Grotthuss-type mechanism. Figure 1