pH-dependent protic ionic liquid tuning effect on oxygen reduction activity of a molecular iron catalyst and its electrochemical interfacial origin

doi:10.1016/S1872-2067(25)64882-0

Abstract

Abstract:

Protic ionic liquid (IL) modification has been demonstrated to be a promising approach for improving the oxygen reduction reaction (ORR) activity and electrochemical stability of catalysts. However, its fundamental mechanism remains largely elusive and controversial, and the possible roles of electrocatalytic interface microenvironment has been ignored so far. Herein, taking the well-structured iron phthalocyanine (FePc) as a model catalyst, it is found that the ORR activity evolution behavior induced by the protic IL modification exhibits a striking pH-dependence, that is, ORR is promoted in acid while slightly inhibited in alkaline. Integrating the electrokinetic analyses, ab initio molecular dynamics simulation and in situ surface-enhanced infrared absorption spectroscopy, we show that the discrepancy in activity evolution arises from the regulation of IL modification on the vastly dissimilar electrochemical interfacial structures under acid and alkaline ORR conditions. Such mechanistic picture can be further supported by the fact that the protic IL with a lower pK_a renders a higher acid ORR activity. This study provides a unique interfacial perspective for understanding the IL modifiers-modulated ORR performance and highlights opportunities for developing cost-effective and high-efficiency proton exchange membrane fuel cells through the functional modulation of electrocatalytic interfaces.

Key words: Oxygen reduction reaction, Protic ionic liquid modification, Electrocatalytic interface, Proton-coupled electron transfer, pH-dependence

Yana Men, Yuzhou Jiao, Yanxing Zheng, Xiaoyan Wang, Shengli Chen, Peng Li. pH-dependent protic ionic liquid tuning effect on oxygen reduction activity of a molecular iron catalyst and its electrochemical interfacial origin[J]. Chinese Journal of Catalysis, 2026, 80: 258-269.

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URL: https://www.cjcatal.com/EN/10.1016/S1872-2067(25)64882-0

https://www.cjcatal.com/EN/Y2026/V80/I1/258

Figures/Tables 5

Fig. 1. Characterizations of the IL modification. (a) XPS surveys of the pristine FePc/C and FePc/C-[MTBD][NTf2]-0.0025 catalysts. TEM (b) and HAADF-STEM (c) images of the FePc/C-[MTBD][NTf2]-0.0025 sample with the corresponding elemental mapping (d-h).

Fig. 2. IL modification induced pH-dependent ORR activity evolution. (a) LSV curves of FePc/C-[MTBD][NTf2]-α in O2-saturated 0.1 mol/L HClO4, with a scan rate of 5 mV/s and a rotation speed of 1600 rpm. (b) Correlation between the E1/2 of FePc/C-[MTBD][NTf2]-α catalysts and the IL content α in 0.1 mol/L HClO4. (c) Kinetic current density jk calculated at different potentials in 0.1 mol/L HClO4. (d) LSV curves of FePc/C-[MTBD][NTf2]-α in O2-saturated 0.1 mol/L KOH. (e) Correlation between the E1/2 and the IL content α in 0.1 mol/L KOH. (f) Kinetic current density jk calculated at different potentials in 0.1 mol/L KOH.

Fig. 3. Discrepancies between the interfacial microenvironment under acid and alkaline ORR conditions. (a) Representative snapshots of the FePc/water interface at the PZFC condition. The Fe, N, C, O and H are colored with brown, blue, gray, red and white, respectively. The brown dashed lines represent the hydrogen bonds. (b) Pourbaix diagram showing the pH dependence of the ORR reaction potential (0.9 V vs. RHE is used here), the PZFC of FePc/C electrode and the PZTC of FePc/C-*O electrode. Given the persistent presence of oxygenated intermediates during ORR process, the *O species is chosen as an example here. (c,d) Representative snapshots of the interfacial structures at ORR potentials on the *O adsorbed FePc/C electrode for alkaline and acid systems. The arched shadows represent the double-layer microenvironments around reactive centers. The *O and F- anion are colored with green and cyan, respectively. (e,f) Close-ups of the interfacial water molecules closest to the *O intermediates at alkaline and acid interfaces.

Fig. 4. Interfacial origin of the pH-dependent tuning effect of IL modification on ORR activity. (a) Schematic diagram of the protic IL regulating the interfacial double-layer microenvironments for acid and alkaline ORR systems. (b,c) ORR LSV curves of the FePc/C-[DEMA][NTf2]-α catalysts tested in O2-saturated 0.1 mol/L HClO4 and 0.1 mol/L KOH. (d) Comparison of the E1/2 of FePc/C-[DEMA][NTf2]-α and FePc/C-[MTBD][NTf2]-α catalysts in acid electrolyte. (e,f) Simulated acid and alkaline ORR interfaces on FePc/C electrodes with IL cation [MTBD]+. (g) Comparisons of the radial distribution functions g(r) between *O and the interfacial H atoms of water and [MTBD]+ (denoted as Hinterface) before and after introducing IL component at interfaces.

Fig. 5. In situ spectroscopic validation. (a?d) In-situ SEIRAS spectra recorded from 0.8 to 0.4 V vs. RHE for the pristine FePc/C and FePc/C-[MTBD][NTf2] (α = 0.0025) catalysts in Ar-saturated 0.1 mol/L HClO4 and KOH solutions. IR background was taken at 1.0 V in the corresponding solution. (e,f) Changes of the O?H stretching wavenumber of interfacial water with the electrode potential in HClO4 and KOH solutions.

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