Atomistic e ngineering of Ag/Pt nanoclusters for remarkably boosted mass electrocatalytic activity

It is of vital importance to boost the intrinsic activity and augment the active sites of expensive and scarce platinum-based catalysts for advancing a variety of electrochemical energy applications. We herein report a mild electrochemical bottom-up approach to deposit ultrafine, but stable, Pt 8 Ag 4 alloy clusters on carbon nanotubes (CNTs) by elaborately designing bimetallic organic cluster precursors with four silver and eight platinum atoms coordinated with µ , σ -bridged ethynylpyridine ligands, i.e., [Ag 4 (C 24 H 16 N 4 Pt) 8 (BF 4 ) 4 ]. The Pt 8 Ag 4 cluster/CNT hybrids present impressively high platinum mass activity that is threefold that of commercial Pt/C toward the hydrogen evolution reaction, as a result of the cooperative contributions from the Ag atoms that enhance the intrinsic activity and the CNT supports that increase the activity sites. The present work affords an attractive avenue for engineering and stabilizing Pt-based nanoclusters at the atomic level and represents a promising strategy for the development of high-efficiency and durable electrocatalysts. alloy cluster with an elaborately designed supramolecular precursor, dedicating to the high density of atomically controlled active sites to promote the associated mass catalytic activity. The present work may open a promising strategy for the controllable synthesis of advanced catalysts for a variety of electrochemical applications.

Keywords: Pt 8 Ag 4 clusters, electrocatalyst, high mass electrocatalytic activity, hydrogen evolution reaction INTRODUCTION Platinum (Pt)-based materials are generally recognized as state-of-the-art catalysts that are indispensable for a variety of electrochemical energy conversion cells. Developing efficient and stable Pt-based catalysts with high intrinsic activity and exposed active sites is crucial for hydrogen and fuel cell technologies, as it is more commercially viable to minimize the loading content of expensive and scarce Pt. On this basis, at least two strategies have been proposed to promote the mass activity of Pt catalysts. One is to develop precisely atomically engineer Pt nanoparticles or sub-nanometer nanoclusters, because the unique features of ultrasmall nanoparticles or nanoclusters, including a high ratio of surface-to-bulk atoms, electronic and geo-metric shell closings and quantum confinement, endow them with a remarkably enhanced catalytic activity that cannot be obtained in bulk samples [1][2][3][4][5][6] . The other strategy is to tailor or alloy Pt with heterometal atoms, which may tune the electronic structure of Pt-based catalysts and thus feasibly promote its intrinsic activity and enhance the utilization efficiency of platinum metal [7][8][9][10][11] . In essence, an ideal electrocatalyst should have rich catalytic active sites, high intrinsic catalytic activity and long-term stability [12][13][14] .
Recently, extensive efforts have been devoted to the controllable synthesis of metallic organic clusters (MOCs) because M n L m -type (M: Metal; n: Number of metal atoms; L: Ligand; m: Number of ligands) nanoclusters tend to present attractive physiochemical properties [15] . Although molecule-like bimetallic alloy nanoclusters represent a new type of catalysts that may bridge homogeneous and heterogeneous catalysis and potentially avoid the disadvantages of conventional pyrolysis alloys, the controllable synthesis of stable alloy nanoclusters remains a grand challenge that is yet to be resolved [16][17][18][19][20] . Moreover, tiny nanoclusters often suffer from poor structural integrity due to their easy aggregation and fusion during the synthesis and treatment processes, leading to a decreased density of active sites and poor catalytic durability and efficiency [19] . It is thus desirable to load these tiny nanoclusters on suitable support but the weak interaction between the metal atoms and supports often leads to metal sintering and deactivation upon catalysis operation. Another disadvantage that must be overcome is the poor conductivity of MOCs, which can also lead to poor electrochemical performance.
In this regard, we herein report the first synthesis of Pt 8 Ag 4 bimetallic organic clusters (Pt 8 Ag 4 MOCs) through the reaction between a Pt-based metalloligand and Ag + , which can be evolved into alloyed Pt 8 Ag 4 clusters through in-situ electrochemical decoration on the surface of carbon nanotubes (CNTs). This approach greatly improves the conduction of the catalyst and also can increase its stability during operation by increasing the interaction between the metal atoms and supports. Finally, the formation of the Pt 8 Ag 4 cluster/CNT hybrids results in an enhanced mass electrocatalytic activity toward the hydrogen evolution reaction (HER) with mg-1 Pt at an overpotential of 70 mV. reduction method. For comparison, Pt/CNTs were also prepared for comparison using Pt metalloligands as precursors with a similar process to the Pt 8 Ag 4 clusters/CNTs. The X-ray single-crystal structure of the Pt 8 Ag 4 -MOC precursor is shown in Figure 1A, which reveals a twolayered sandwich structure with a rhombic shape defined by four Pt 2 Ag units, with diagonal lengths of 1.12 and 2.28 nm and a thickness of 0.39 nm (defined by the corresponding Pt to Pt distances). Each layer consists of four (bpy)Pt(≡ py) 2 metalloligands and four Ag ions, which are all sandwiched between two (bpy)Pt(≡ py) 2 ligands through multiple µ-coordination to the ethynyl groups and PtAgPt metallophilic interactions (with Pt-Ag distances of 2.86-3.42 Å). Interligand Ag-py coordination along with inter-layer π-π stacking interactions between the (bpy)Pt planes finally connect together with the unique discrete supramolecular architecture [ Figure Figure 1E-F] exhibit the HRTEM images of the Pt 8 Ag 4 cluster/CNT hybrids, from which one can observe that the Pt 8 Ag 4 clusters, like Pt 8 Ag 4 MOCs, are uniformly dispersed on CNTs with a size range of 1.5-3.5 nm, close to that of the Pt 8 Ag 4 MOCs This indicates that the robust structure of the Pt 8 Ag 4 MOCs can effectively prevent metal clusters from aggregation during the electrochemical reduction process. From the TEM images [ Figure 1G], one can observe that the geometry of the Pt 8 Ag 4 clusters is close to that of the rhombic crystal molecules and diffraction fringes with a d-spacing of 0.230 nm [inset of Figure 1G], corresponding to the (111) crystal plane of the face-centered cubic structure [21][22][23] . It is found that the d-spacing of the diffraction fringes in Pt 8 Ag 4 are located in between the dspacing of the crystal planes of Pt (0.226 nm) and Ag (0.235 nm) [24,25] .

RESULTS AND DISCUSSION
Based on the XRD pattern of the Pt 8 Ag 4 clusters/CNTs, Pt/CNTs and Pt 8 Ag 4 MOCs/CNTs [ Supplementary  Figure 2], one can clearly observe two broad peaks at ~26° and ~44° for all samples, corresponding to the characteristic crystal planes of CNTs [26] . There are no diffraction peaks corresponding to metallic Pt or Ag, likely due to the low Pt and/or Ag contents and their ultrasmall nanoclusters in the hybrids. Figure 1H-J show the elemental mapping images of Ag and Pt for the Pt 8 Ag 4 clusters/CNTs, suggesting that Pt and Ag are well overlapped in each cluster and are uniformly distributed on the CNTs, as further verified by the high-angle annular dark-field (HAADF) image. The inductively coupled plasma mass spectrometry measurements [Supplementary Table 2] reveal that the atomic ratio of Pt (3.86 wt.%) and Ag (0.896 wt.%) is close to 2:1, consistent with the atomic ratio in the MOCs. Figure 2A and B show the HAADF-STEM images of the Pt 8 Ag 4 -clusters/CNTs, in which a number of nanoclusters are uniformly decorated on the CNT surface. The structure of the Pt 8 Ag 4 clusters/CNTs was further studied by X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), which are sensitive to electronic and local atomic structures. Figure 2C   between zero and 4+ [10] .
The Pt EXAFS spectra of the Pt 8 Ag 4 clusters/CNTs show a very weak peak at ~2.7 Å [ Figure 2D, Supplementary Figure 3B], which is similar to the Pt-Pt shell peak in Pt foil (2.65 Å) and also close to the Pt-Ag shell peak in the PtAg alloy (2.64 Å). Furthermore, the apparent first shell peak at ~1.56 Å was ascribed to the Pt-C/O interaction, revealing that Pt in the Pt 8 Ag 4 clusters/CNTs was formed as PtAg alloy nanoclusters. X-ray photoelectron spectroscopy (XPS) was also carried out to investigate the elemental composition and analyze the chemical state of the respective elements [ Supplementary Figure 4]. Figure 2E shows the highresolution XPS spectrum of Pt 4f, where the Pt 8 Ag 4 MOCs/CNTs are quite similar to the form of Pt 4+ . However, both the metallic and oxidation states (Pt 2+ and Pt 4+ ) of Pt are present in the Pt 8 Ag 4 clusters/CNTs. Although metallic Pt is the major phase in Pt 8 Ag 4 -cluster /CNTs, the major phases in Pt 8 Ag 4 MOCs/CNTs and Pt/CNTs are the oxidation states (Pt 2+ and Pt 4+ ) of Pt. The high-resolution XPS spectrum of Ag 3d is displayed in Figure 2F, which reveals that the Ag in the Pt 8 Ag 4 clusters/CNTs is metallic Ag, different from that (Ag + ) in Pt 8 Ag 4 MOCs/CNTs [27,28] . By combining the results of the elemental maps, EXAFS spectra and XPS data, it can be found that the PtAg alloy bonded with the MOC or CNT substrates after electrochemical reduction [29] .
The electrocatalytic properties of the Pt 8 Ag 4 clusters/CNTs for the HER were evaluated using linear sweep voltammetry (LSV) at room temperature in 0.5 M H 2 SO 4 . A set of electrodes, including PtMLs/CNTs, Pt/CNTs, Pt 8 Ag 4 MOCs/CNTs, Pt 8 Ag 4 MOCs, Pt 8 Ag 4 clusters and the commercial Pt/C, were prepared as references for comparison. Figure 3A and Supplementary Figure 5 show the LSV curves of the set of electrodes. The overpotential at 10 mA cm -2 (η 10 ) for the Pt 8 Ag 4 clusters/CNTs is 18 mV, which is much lower than those of PtMLs/CNTs (408 mV), Pt/CNTs (117 mV), Pt 8   Furthermore, the LSV curves are normalized by net Pt mass [ Figure 3B]. The mass activity at -70 mV for the Pt 8 Ag 4 clusters/CNTs reaches as high as 9.98 A mg Pt -1 , which is 50 times higher than that of the Pt/CNTs (0.32 A mg Pt -1 ) and three times higher than that of the commercial Pt/C (2.94 A mg Pt -1 ). These results signify that the alloyed Pt 8 Ag 4 cluster contributes to the significantly enhanced mass activity for the HER.
Cycling within a non-Faradaic region at various scan rates allow us to calculate the double-layer capacitance (C dl ), which is 4.60, 1.47 and 3.04 mF cm -2 for Pt 8 Ag 4 clusters/CNTs, Pt/CNTs and Pt/C, respectively [ Supplementary Figure 6, Supplementary Table 4]. The electrochemical surface area (ECSA) [ Figure 3D]evaluated based on C dl was ~131.43 cm 2 for the Pt 8 Ag 4 clusters/CNTs, which is three times larger than that (38.57 cm 2 ) for the Pt/CNTs, suggesting that atomistically Ag-engineered Pt also leads to an increase in activity sites. The current densities of different electrodes are thus normalized to the ECSA, which can provide information to evaluate the intrinsic activity. The Pt 8 Ag 4 clusters/CNTs exhibit a current density of 35.0 μA cm -2 at -70 mV, at least 2.5 times higher than those of both Pt/CNTs (8.2 μA cm -2 ) and Pt/C (12.6 µA cm -2 ), demonstrating that the Pt 8 Ag 4 clusters hold a much higher intrinsic catalytic activity for the HER. The specific current densities for the other electrodes, i.e., Pt 8 Ag 4 MOCs/CNTs, Pt 8 Ag 4 MOCs and PtMLs/CNTs, are remarkably lower than for their electrochemically reduced electrode counterparts [ Supplementary Figure 5], further signifying that the electrochemical reduction treatment can significantly boost the associated intrinsic activity. It is noteworthy that there is only a slight difference in the specific current density between the Pt 8 Ag 4 clusters/CNTs and Pt 8 Ag 4 clusters, implying that the CNT support in the hybrids significantly contributes to the increased number of active sites, other than the intrinsic activity.
Tafel plots were constructed to study the reaction kinetics and the HER mechanism [ Figure 3E]. The slope of 25.9 mV dec -1 for the Pt 8 Ag 4 clusters/CNTs suggests that the Tafel step (H ads + H ads →H 2 ) is the ratedetermining step [30][31][32] . The Tafel slopes of the set of catalysts decrease in the following order: Pt 8 Figure 3F] on the Pt 8 Ag 4 cluster/CNT electrodes was conducted by applying an overpotential of 33 mV (vs. RHE), which shows the capability of running for over 50 h with a gradual increase in current density. In contrast, the Pt/C electrode at an overpotential of 75 mV displayed a rather rapid drop in current density within 10 h.
We also investigated the HER catalytic performance of the Pt 8 Table 8 and 9], suggesting a more pronounced ORR electrocatalytic activity for the Pt 8 Ag 4 clusters/CNTs than the Pt/CNTs and the commercial Pt/C. Density functional theory (DFT) calculations were employed to acquire atomic insights into the HER activity of the Pt 8 Ag 4 /CNT catalysts. The model structure of the Pt 8 Ag 4 clusters/CNTs is shown in Figure 4A and the reference Pt/CNTs are shown in Supplementary Figure 12. The value of ∆E H* (Gibbs energy of absorption of H*) is introduced as an effective activity descriptor for the HER activity and a catalyst with ∆E H* = 0 can be considered as an outstanding candidate for the HER. Therefore, the adsorption free energy of H (∆E H* ) at different sites (Supplementary Figure 13 and 14) on the Pt 8 Ag 4 clusters/CNTs and Pt/CNTs was calculated to evaluate their HER activity. The results suggest that the Pt site of the Pt 8 Ag 4 /CNTs has excellent activity for the HER. As shown in Figure 4B, the ∆E H* at dominant active sites in the Pt 8 Ag 4 clusters/CNTs is close to the ideal value of the HER, which proves that the introduction of Ag atoms to Pt 8 Ag 4 /CNTs contribute to the improved HER activity. In contrast, the Pt 8 Ag 4 /CNTs also have superior performance for the ORR. The value of ∆E O* (Gibbs energy of adsorption of O*) is used to assess the ORR activity of catalysts based on the theoretical volcano relationship [33] . The positions of oxygen atoms adsorbed on the cluster were found by DFT calculations, as shown in Supplementary Figure15. As shown in Figure 4C, the ORR activity of the Pt 8 Ag 4 clusters/CNTs is higher than that of the Pt/CNTs based on the theoretical volcano model, which is also consistent with the experimental results. Therefore, the introduction of Ag to the Pt 8 Ag 4 -clusters/CNTs leads to high activity toward both ORR and HER catalysis.

CONCLUSION
In summary, we here presented a new CNT-supported alloyed nanocluster hybrid catalyst derived from a unique bimetallic organic cluster, which shows remarkably boosted catalytic activity with an ultralow Pt content loading. This study provides a new synthetic method for a bimetallic alloy cluster with an elaborately designed supramolecular precursor, dedicating to the high density of atomically controlled active sites to promote the associated mass catalytic activity. The present work may open a promising strategy for the controllable synthesis of advanced catalysts for a variety of electrochemical applications.