Pt-M (M = Ni, Co, etc. ) alloy nanoparticles (NPs) have attractive attentions as oxygen reduction reaction (ORR) electrocatalysts used not only in acidic but also in alkaline solution [1]. Although the 3d-transition metal elements M should dissolve into strong acid electrolyte easily, they might remain as (hydro)oxides on the topmost surfaces in alkaline environments [2]. Therefore, ensemble effects of Pt- and M-related surface species of the Pt-M NPs should be considered to develop highly active ORR electrocatalysts in alkaline solution. Also, because such the M elements (Ni and Co) show high oxygen evolution reaction (OER) activity, the Pt-M alloy NPs can be applied to ORR and OER bifunctional catalysts [3] for the metal-air battery. However, effects of the topmost surface structures (atomic arrangements, alloy compositions) of the alloy NPs on ORR and OER properties have been unclear. In this study, we fabricated well-defined Co/Pt(111) bimetallic surface by molecular beam epitaxy and investigated the ORR and OER activity.

Experimental

All the sample fabrication processes were conducted in ultra-high vacuum (UHV; ~10^-8Pa). Pt(111) single-crystal substrate was cleaned by repeated Ar⁺ sputtering and annealing at 1173 K in UHV. Then, n-monolayer(ML)-thick-Co layers (n = 0.13-2.0) were deposited onto the substrates at a room temperature by using an electron-beam evaporator. Hereafter, the Co-deposited samples were denoted as nML-Co/Pt(111).

After the sample transfers from the UHV chamber to an electrochemical system without air exposure, cyclic voltammograms (CVs) were recorded in N₂-purged 0.1 M KOH. Subsequently, linear sweep voltammetry (LSV) was performed in O₂-saturated 0.1 M KOH at a disk rotation rate of 1600 rpm for ORR and OER activity evaluations by using the rotating disk electrode (RDE) method. All the electrochemical measurements have been conducted at a room temperature.

Results

Fig.1 (a) shows CVs of the nML-Co/Pt(111) surfaces recorded in N₂-purged 0.1 M KOH. The clean Pt(111) (dashed line) shows symmetric features derived from hydrogen- and hydroxyl- adsorption-desorption (H_ads&des, OH_ads&des) reactions at 0.05−0.35 V and 0.6−0.9 V, respectively. In contrast, the nML-Co/Pt(111) show sharp redox peaks at around 0.65 V and 0.5 V, respectively, which correspond to oxidation-reduction reaction between Co(OH)₂ and CoOOH [4].

Figs.1 (b) and (c) present the respective LSV curves for ORR and OER of the nML-Co/Pt(111) samples . It can be seen that half wave potentials for ORR (b) shift to higher potentials with increasing Co deposition up to 0.25 ML. However, the diffusion limiting current regions (0.4 V to 0.7 V) become obscure, accompanying the lower-potential shifts for the samples over the 0.5 ML-thick-Co samples. As for OER currents (c), the onset potentials monotonically shift to lower potentials with increasing the Co-thickness, suggesting that the surface Co-related species show much higher OER activity than the clean Pt(111) surface.

Activity enhancement factors for ORR and OER of the nML-Co/Pt(111) samples vs. clean Pt(111) are summarized in Fig. 1 (d). ORR activity enhancements were judged by the j_k values at 0.9 V that estimated using the Koutecky-Levich equation. As for OER, the enhancement factors were compared based on the electrochemical current densities at OER overpotential (η) of 350 mV. The enhancement factors of the ORR peaked at 0.25 ML-Co, where the factor is ca. 1.7 and, above 0.5 ML, the factors become less than 1 (less-active than clean Pt(111)). In contrast, the OER activity increased with increasing the Co-thicknesses up to 2 ML and the enhancement factor of 10 ML-Co sample is lower than that of 1 ML- and 2 ML-Co, suggesting that the surface Co-related-species influenced by the underneath Pt(111) substrate could contribute to the OER enhancements [5]. The results demonstrate that tuning the surface alloy compositions is a key to develop highly-active Pt-based alloy catalysts not only for ORR but also for OER.

Acknowledgments

This study was supported by Nippon Sheet Glass Foundation and Advanced Research and Education Center for Steel (ARECS) in Department of Materials Science, Tohoku University.

References

[1] K. Miyatake, Y. Shimizu, ACS Omega, 2, 2085 (2017).

[2] M. Oezaslan et al., J. Electrochem. Soc., 159, B394 (2012).

[3] S. Hu et al. , Applied Catalysis B: Environmental, 182, 286 (2016).

[4] R. Subbaraman et al. , Nat. Mater., 11, 550 (2012).

[5] B.S. Yeo, A.T. Bell, J. Am. Chem. Soc., 133, 5587 (2011).

Fig.1 (a) Cyclic voltammograms of the nML-Co/Pt(111) surfaces obtained at 50 mV/s in 0.1 M KOH. LSV curves for ORR (a) and OER (b) recorded at 1600 rpm and scan rate of 10 mV/s. (d) The activity enhancement factors for ORR and OER vs. Pt(111), as a function of deposited Co thicknesses.