Effect of Micro- and Mesopore Size on Electronic Structure and Oxygen Reduction Kinetics of Platinum
Authors: Nakahara Kota , Ikezawa Atsunori , Okajima Takeyoshi , Arai Hajime 
Published: December 2024

Introduction

Oxygen reduction reaction is an indispensable electrochemical reaction to utilize hydrogen or renewable energy in the form of electrochemical devices, i.e., fuel cells or metal-air batteries. However, the reaction overpotential, or activation and concentration overpotentials, must be reduced to raise its expected role in energy conversion. While porous electrodes contribute to decreasing the activation overpotential owing to their high surface area, the effect of pore structure on the reaction kinetics has not been clarified. It is expected that the electronic structures of electrocatalysts change when the pore size is in the range of a few nanometers. In our recent report, we fabricated platinum model electrodes with a uniform pore size of 1.8 nm and controlled electrode thicknesses.1 The intrinsic ORR kinetics was obtained by using a thin model electrode, and the extract activation overpotential was smaller than that of a planar platinum electrode. Herein, to further elucidate the relationship between the pore structure and the ORR kinetics, we present the effect of pore sizes using the platinum model electrodes with controlled pore sizes of 1.3 nm, 1.8 nm, and 3.0 nm (inset in Figure). Our study indicates that the d-band center is shifted downward with decreasing pore size and that ORR kinetics follows a volcano relationship with a maximum at a pore size of 1.8 nm.

Experimental

Platinum model electrodes were fabricated with the electrodeposition of platinum salt dissolved in a liquid crystalline phase of surfactant.2 The pore diameter was controlled by varying the chain length of the surfactants. The electrode thickness was assumed to be short (~50 nm) enough to mitigate the ORR concentration overpotential.1 Transmission electron microscopy (TEM) was employed to observe the pore structure. The electronic structures of the electrodes were characterized with angle-resolved X-ray photoelectron spectroscopy (ARXPS). The photoelectron takeoff angle was changed to distinguish the local electronic structure of the pore wall from that of the outer surface. Electrochemical measurements were carried out by cyclic voltammetry in N2/O2-saturated 0.1 mol dm–3 KOH solutions with a rotating disk electrode system.

Results and discussion

The model electrodes exhibited characteristic redox peaks of platinum polycrystalline facets in N2-saturated solution. ORR activities were evaluated in the Tafel plot (Figure), where current densities were normalized by electrochemical surface area and corrected for mass transport. It was found that the activation overpotential was the lowest at a pore size of 1.8 nm. Furthermore, the obtained Tafel slope (ca. –60 mV dec–1) was almost the same, suggesting that the rate-determining step is the same and the concentration activation overpotential is small among the electrodes.1 In ARXPS, the binding energy of the Pt 4f7/2 spectrum exhibited a high energy shift with a decrease in the pore size. The result suggests that the d-band center of platinum is shifted downward with decreasing pore size,3 and that the enhanced ORR kinetics at a pore size of 1.8 nm is attributed to the appropriate adsorption energy of the electrode to oxygen-containing species.4 We will also report the results collected in acidic solutions.

References

[1] K. Nakahara et al., ChemElectroChem, 11, e202300654 (2024).

[2] G. S. Attard et al., Science, 278, 838 (1997).

[3] M. Weinert et al., Phys.
Rev. B, 51, 17168 (1995).

[4] V. Stamenkovic et al., Angew. Chem. Int. Ed., 45, 2897 (2006).

Acknowledgements

This work was partially supported by JSPS KAKENHI (JP23KJ0894) and JRP-LEAD with DFG (JPJSJRP20221602).

Figure 1