Platinum-trimer decorated cobalt-palladium core-shell nanocatalyst with promising performance for oxygen reduction reaction

Advanced electrocatalysts with low platinum content, high activity and durability for the oxygen reduction reaction can benefit the widespread commercial use of fuel cell technology. Here, we report a platinum-trimer decorated cobalt-palladium core-shell nanocatalyst with a low platinum loading of only 2.4 wt% for the use in alkaline fuel cell cathodes. This ternary catalyst shows a mass activity that is enhanced by a factor of 30.6 relative to a commercial platinum catalyst, which is attributed to the unique charge localization induced by platinum-trimer decoration. The high stability of the decorated trimers endows the catalyst with an outstanding durability, maintaining decent electrocatalytic activity with no degradation for more than 322,000 potential cycles in alkaline electrolyte. These findings are expected to be useful for surface engineering and design of advanced fuel cell catalysts with atomic-scale platinum decoration.


Supplementary Note 1|Details about the CNT supports used for synthesis.
In this study, multi-wall CNTs were purchased from Cnano Technology Ltd.
(product number: MWCNT-FT9400). The diameter of these CNTs is about 20-40 nm and the length is 10-20 μm. For a better dispersion of the catalysts, the CNTs were treated in aqueous solution of 4.0M sulphuric acid at 80 oC for 4h. FTIR spectra and HRTEM images of the CNT w/ and w/o acid treatments are compared in Fig. S9 presents the FTIR spectra of the CNTs with and without the acid treatment.
According to our TEM observation, the size of Co@Pd-Pt nanoparticles on the initial CNTs is within a range of 6.0-8.0 nm. In contrast, the average size of Co@Pd-Pt nanoparticles on treated CNTs is about 4.12 nm. Adsorption of Co2+ ions tend to form at the defective and ligand sites on an acid-treated CNT surface, thus resulting in high densities of Co nuclei for the subsequent heterogeneous crystal growth of Pd crystal. Therefore, the particle size of Co@Pd-Pt nanoparticles on acid-treated CNTs is smaller.

Supplementary Note 2|Synthesis of Co@Pd-Pt/CNT catalysts
In this study, proper controls of sequences and time in all reaction steps enable the growth of Pt3 decorated Co@Pd core-shell structure. For clarifying the rationales, reaction pathways for all steps are introduced as follow: Step 1: Chemisorption of Co 2+ in CNT surface for 4 hours. In this step Co 2+ ions will be chelated or chemisorbed by ligands in CNT surface. Shown by FTIR analysis (Fig. S9), high contents of ligand (COO -, C=O, and O-H) might assist chelation and distribution of Co 2+ in CNT surface.
Step 2: Reduction of Co 2+ @CNT by interacting with exceed NaBH 4 (NaBH 4 / Co 2+ = 4.0 mole ratio) to form solution A. Solution A contains suspension of Co/CoO x @CNT powder and exceed NaBH 4 molecules.
Step 3: After 10 seconds, Pd 2+ ions are added in solution A. In this step, Pd 2+ ion in solution and adsorbed in liquid phase will be reduced by NaBH 4 into metallic Pd clusters. Pd 2+ ions adsorbed in Co/CoO x @CNT surface will be reduced into Pd cluster.
Those Pd clusters formed in liquid phase will rapidly adsorb in heterogeneous interfaces (CNT or Co/CoO x @CNT) or agglomerate into homoatomic crystals.
Considering to kinetics of crystal growth, heterogeneous crystal growth is naturally preferred due to its relatively lower energy barrier as compared to that of homogeneous ones. After reaction, sample of Co@Pd/CNT was placed in room temperature for 10 minutes in order to consume all reduction agents.
Step 4: Formation of Pt clusters on Co@Pd surface by adsorption of Pt 4+ ions followed by their reduction by NaBH 4 . With a short adsorption time (10s), galvanic replacement between Pt 4+ ion and atom in solid phases (Pd, Co, CoO x ) is suppressed.
After addition of reduction agent, agglomeration between Pt atoms into nanoclusters of nanoparticles are inhibited due to the strong bonding of Pt atom to adsorbate in which homoatomic clusters between Pt atoms are suppressed due to a presence of high defect density in surface.

Supplementary Note 3|Calculation of ORR mass activity
The mass activity (MA) is calculated by following equation where J k is the kinetic current density (mA/cm 2 ) and area is the geometric area of working electrode. The mass activity of the catalyst is estimated via the calculation of J k and normalization to the catalyst loading (CL) of the disk electrode. In our study, CL is ~0.073 mg (comprising 0.0042 mg of Pt, 0.046 mg of Pd, and 0.0256 mg of Co) and J k is 12.5 mA cm -2 for Co@Pd-Pt/CNT catalyst in the working electrode.
In the manuscript, MA Pt is calculated by dividing total J k to Pt loading because the MA contribution from Co@Pd structure is very limited, according to the electrochemical results of the control samples Co/CNT and Co@Pd/CNT catalysts (Table S5). It can be found that the Co/CNT catalyst exhibit no ORR activity at 0.85 V vs. RHE. Moreover, J k of the control sample Co@Pd/CNT is 5.15 mA/cm 2 , and the corresponding MA contribution from Pd is very limited, only 72.2 mA/mg Pd . Thus, considering the similar Pd content in the Co@Pd/CNT and Co@Pd-Pt/CNT samples, it is reasonable to conclude that main ORR activity of the Co@Pd-Pt/CNT catalyst definitely comes from the decorated Pt 3 species.

Supplementary Note 4|CV analysis of Co@Pd-Pt and control samples.
The discussion is based on the comparison of CV curves of Co@Pd-Pt/CNT and various control samples (Pt/CNT, Pd/CNT, Co@Pt/CNT) and DFT calculation outputs.