How pH Affects the Oxygen Reduction Reactivity of Fe–N–C Materials

Abstract

While Fe–N–C materials exhibit great potential for catalyzing the oxygen reduction reaction (ORR), their activity origin, especially the significant activity difference in acidic and alkaline media, remains a long-standing conundrum hindering the development of such catalysts. Here, we show an unanticipated pH-dependent regulation mechanism in Fe–N–C materials via first-principles microkinetic computations that explicitly consider the pH, solvation, and electrode potential effects. We find that, under typical operating potentials, the well-established FeN4 centers of Fe −N–C catalysts, regardless of the pyridinic and pyrrolic-type N-coordination environments, are not adsorbate-free but covered by an intrinsic intermediate *OH at pH = 1 and *O at pH = 13, resulting in FeN4–OH and FeN4–O centers formed in situ. We evaluate the pH- and potential-dependent kinetics and thermodynamics of the real active Fe centers of Fe–N–C catalysts against experimental measurements. We demonstrate that the activity difference of Fe–N–C catalysts is attributed to the *O coordination-induced optimization of the electronic structure and intermediate adsorption over the *OH case. Our work provides the mechanistic insight into the pH effects and paves the way toward a more effective catalyst design.

Optimizing reaction intermediate adsorption by engineering the coordination structure of single-atom Fe–N5–C electrocatalysts for efficient oxygen reduction

“…To date, metal-nitrogen-doped carbon (M-N x -C) single-atom electrocatalysts have emerged as promising materials in the ORR to replace the precious-metal-based catalysts because of their fascinating characteristics (good activity, low cost, maximum atomic utilization, tunable electronic structures, etc.). [9][10][11][12][13] However, many challenges still remain to improve the electrocatalytic performance of the single-atom catalysts.…”

“…To date, metal–nitrogen-doped carbon (M–N x –C) single-atom electrocatalysts have emerged as promising materials in the ORR to replace the precious-metal-based catalysts because of their fascinating characteristics (good activity, low cost, maximum atomic utilization, tunable electronic structures, etc .). 9–13 However, many challenges still remain to improve the electrocatalytic performance of the single-atom catalysts. Therefore, many research studies have been devoted to overcome these challenges, such as trying to break through the limitation of dimension on the catalytic activity, improving the density of the single atomic active sites, and preventing the aggregation of metal atoms during the calcination process of the catalyst precursor, and remarkable results have been achieved.…”

Exploring the catalytic activity of graphene-based TM-NxC4-xsingle atom catalysts for the oxygen reduction reactionviadensity functional theory calculation

“…20 So far numerous TM-N-C (TM = Fe, Co, Ni, etc.) electrocatalysts have been experimentally synthesized in the past decade, [21][22][23][24][25] but a precise control of the coordination environment on the transition metal site in TM-N-C is still difficult due to the amorphous nature of the graphene matrix. Thus, porous defects, topological defects, substitution of coordination atoms and various coordination structures of the TM center may exist, which give a chance to computational chemists to dig out how these factors affect the catalytic performance of TM-N-C. Up to now the effect of N/C coordination on the ORR activity and mechanism has been extensively reported via simulation.…”

Reaction mechanism and kinetics of oxygen reduction reaction on the iron–nickel dual atom catalyst

“…[17][18][19][20][21][22][23][24] The theoretical studies for ORR have mainly reported the Gibbs free energy (DG) of the main reaction intermediates such as H*, OH*, O*, OOH*, and OO* at zero net charge with the goal of predicting the limiting potential (4) and overpotential (h = 1.23 − 4). [25][26][27] In most calculations, the dependence of the Gibbs free energy of the intermediates is assumed to be linear in the applied potential (U) as:…”

“…17–24 The theoretical studies for ORR have mainly reported the Gibbs free energy (Δ G ) of the main reaction intermediates such as H*, OH*, O*, OOH*, and OO* at zero net charge with the goal of predicting the limiting potential ( φ ) and overpotential ( η = 1.23 − φ ). 25–27 In most calculations, the dependence of the Gibbs free energy of the intermediates is assumed to be linear in the applied potential ( U ) as:Δ G OH* ( U ) = Δ G OH* | at U =0 V − eU Δ G O* ( U ) = Δ G O* | at U =0 V − 2 eU Δ G OOH* ( U ) = Δ G OOH* | at U =0 V − 3 eU where e is the electron charge. 28–30 However, linear dependence is assumed and has been shown to be incorrect in some studies.…”