Deactivation Mechanism of Pyridinic Nitrogen-Doped Carbon for ORR in Acidic Media

Nitrogen doped (N-doped) carbons are attracting attention as Pt-free catalysts for oxygen reduction reaction (ORR) in fuel cells. N-doped carbon catalysts exhibit high activity comparable to that of platinum in basic media whereas the activity is significantly reduced in acidic media. To solve the deactivation mechanism based on kinetics, we have investigated the reaction pathway of N-doped carbon catalysts. The active site of pyridinic nitrogen (pyri-N) is protonated to be pyridinium (pyri-NH+) in acidic media due to acid-base equilibrium of pyri-N + H+ ⇄ pyriNH+ (1) Following to Eq. (1), we recently exhibited that pyriNH+ + e- + O2⇄ O2a + pyri-NH(2) proceeds using model catalysts with homogenous active sites. In the step, the electrochemical reduction from pyri-NH+ to pyri-NH and thermal O2 adsorption simultaneously occurs wherein the adsorption of O2 promotes the electron transfer into π* orbital of pyri-NH. We proposed that the hydration of pyri-NH+ lowers the redox potential of Eq. (2) decreasing the ORR activity in acidic media. In the present study, we further considered the acid-base equilibrium of pyri-N in Eq. (1) to comprehensively answer why the ORR activity of N-doped carbon catalyst is high in alkaline media and is decreased in acidic media. To investigate the relationship between acid-base equilibrium and the activity, the electrochemical measurements were performed in electrolyte solutions of various pH. Figure 1 shows the onset potential (voltage at -100 μA/cm2) as a function of pH measure with a model catalyst of 1,10-phenanthroline/(carbon black). In the range from pH 5 to 13, the onset potential decreased with a slope of ~59 mV/pH, while at below pH 5, the onset potentials were almost constant. This indicates that the reaction mechanism changes due to the shift of the acid-base equilibrium. In low pH, the reactions determining the onset potential is assumed to be Eq. (2). On the other hand, at high pH, the reaction should start from pyri-N but not from pyri-NH+. We have figured out that pyri-N + H2O +e- + O2⇄ O2a + pyri-NH + OH- (3) takes place in alkaline madia. Figure 2 shows the N1s peak of the model catalyst applied voltage at pH 13 in N2-(left) and O2-(right)saturated electrolyte. A peak at 400.5 eV appeared at higher applied voltage in O2-saturated electrolyte. This means that pyri-N (399.2 eV) is reduced at a higher potential in the presence of O2 to form pyri-NH. In other words, oxygen adsorption and reduction of pyri-N simultaneously occurred in the presence of O2. The redox potentials of Eq. (2) and (3) explain the pH dependences of the onset potentials. The Nernst equations of Eq. (2) and Eq. (3) show pH-independent redox potential vs. RHE and pH-dependent redox potential vs. RHE with the slope of 59 mV/pH, respectively. This also means that the activity depends on the acid-base equilibrium of pyri-N. We also calculated the redox potentials based on DFT as shown in Figure 3. The additional important point is that the redox potentials of Eq. (2) is strongly influenced by the hydration whereas the redox potentials of Eq. (3) is not influenced by the hydration because the stability of pyri-N is not changed by the hydration. Therefore, the ORR activity in alkaline media is high because the redox potential of Eq. (2) vs. RHE is higher for the higher pH and is not influenced by the hydration. On the other hand, the activity in acidic media is lowered by the hydration. This is that the suppression of the hydration by the introduction of the hydrophobicity at the vicinity of the active site can significantly promote the activity in acidic media. Figure 1