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Subnanoscale Pt clusters constructed by domain-limited strat

Time:2020/01/06 丨 source:未知 丨 visit count:

Subnanoscale Pt clusters constructed by domain-limited strategy for efficient HER

[Research Background]

Due to its zero-pollution nature, hydrogen energy is considered as a green energy source that can substitute for fossil fuels. At present, hydrogen is produced mainly through natural gas reforming, which inevitably leads to an increase in carbon dioxide emissions. In order to achieve carbon-neutral economy, hydrogen production by electrolysis of water driven by renewable energy is a feasible method. Although Pt-based and its intermetallic alloys are still the most active HER catalysts, their high price and poor stability limit their large-scale application. Therefore, enhancing the quality activity and durability of Pt-based electrocatalyst is the key to reduce the cost of electrochemical hydrogen production. Large-size metal nanoparticles (NPs) with low atom utilization are usually synthesized by conventional methods such as wet chemical method and thermal reduction. Compared with metal nanoparticles, metal clusters show excellent surface structure and abundant active sites, which can improve the catalytic activity. However, the separation and aggregation of clusters in the synthesis process lead to a serious degradation of the final performance, which limits their practical application. Therefore, it is of great value to design and synthesize nanostructured Pt cluster catalysts to promote the electro-catalysis of HER.

[ Introduction ]

Recently, a team led by Professor Xiong Wen Lou of Nanyang Technological University reported a method for precisely controlling subnanoscale Pt clusters. High efficient Pt-based electrocatalysts can be constructed by confining the Pt-based clusters to the hollow carbon spheres. The HER electrocatalyst (Pt5 / HMCS) with ultra-fine Pt clusters will maximize the use of Pt Atoms. At the same time, the Pt Nanoclusters in the Nanopore Exhibit Higher Stability and less aggregation tendency during the heat treatment process. When used as an electrocatalyst for HER, significant improvements were achieved in both activity and durability. Compared with commercial Pt / C catalysts, the mass activity of Pt5 / HMCS in alkaline solution was increased by one order of magnitude. The paper, titled Confining Sub-Nanometer Clusters in Hollow mesophilic Carbon Spheres for Boosting Hydrogen Evolution Activity, was published in the international journal Advanced Materials.
[ Highlights ]

Firstly, the SiO2 / RF composite was synthesized by the hard template method, and the hollow porous HMCS carbon spheres were obtained by selective etching of SiO2 after carbonization at high temperature. The HMCS carbon spheres were then mixed with Pt5(GS)10 cluster solution and stirred continuously. Pt5(GS)10 cluster adsorbed on the surface of the porous carbon and formed Pt5(GS)10 / HMCS structure due to the intermolecular interaction. Further, the Pt Nanoclusters with Zero Valence were selectively formed and the final Pt5 / HMCS electrocatalyst was formed under the conditions of hydrogen gas flow and appropriate calcination.

Figure 1 Pt5 / HMCS synthesis schematic.
The Mass Spectra in Fig. 2 show that the mass distribution m/z of Pt5(GS)10 clusters is 4038, which is in good agreement with the theoretical value 4038. The Pt5(GS)10 clusters have a hollow and porous carbon sphere structure similar to that of HMCS. High resolution transmission electron microscope (tem) images confirm that Pt5(GS)10 clusters are uniformly loaded in porous carbon spheres, where dark spots smaller than 1nm correspond to monodispersed Pt5(GS)10 clusters, which indicates that monodispersed Pt5(GS)10 / HMCS have been successfully synthesized.

Fig. 2 Pt5(GS)10 cluster and Pt5(GS)10/HMCS Characterization
Haadf-stem in Fig. 3 shows a single and discrete Pt Nanocluster on the porous carbon sphere HMCS. The size distribution of the Pt Nanocluster is 0.77 nm, the element mapping diagram reveals the uniform distribution of the Pt species within the Hollow Porous Carbon Shell. The Pt Clusters on HMCS after ligand removal at different temperatures were further characterized by energy scattering x-ray spectroscopy. It can be seen that the content of s decreases significantly after heat treatment, and the molar ratio of Pt: S increases from 0.5 in original Pt5(GS)10 clusters to 5 after annealing at 650 °C. XPS shows that the binding energy of Pt 4 f7//2 moves toward lower binding energy with the increase of annealing temperature, and the new peak of binding energy is Pt12 cluster with 71.6 eV, which shows that increasing annealing temperature will eliminate thiol ligand at the same time, and reduce Pt2 + to metal Pt0. By further increasing the temperature to 550 °C, about 3/4 of Pt2 + can be converted into metal Pt0 in the Pt5 / HMCS sample, thus providing enough active sites for hydrogen evolution.

Figure 3 Pt5 / HMCS Characterization
Fig. 4 Electrocatalytic performance of HER was tested in 0.5 M H2SO4 and 1 M KOH with Pt Wire and Ag / AgCl as the pair electrode and reference electrode, the Properties of commercial Pt/C catalysts (5 and 20wt% Pt / C-5% and 20% Pt / C-20%) were compared. Compared with commercial Pt/C catalysts, Pt5 / HMCS samples show higher catalytic activity under the same Pt loading, and the overpotential is only 20.7 mV acidity and 46.2 mV basicity at 10 mA cm-2 Hydrogen evolution current. At the same time, the durability test showed that after 3000 CV tests, the commercial Pt/C Catalyst had obvious performance degradation, while the performance degradation of Pt5/HMCS was very weak, and the test in alkaline environment had almost no degradation, showing better durability.

Fig. 4  Polarization curve, load and durability test in 0.5 M H2SO4 and 1 M KOH environment.
[ Conclusion ]

In conclusion, we have successfully developed an effective domain-limiting strategy to form Pt5 / HMCS by confining Pt Nanoclusters in hollow mesoporous carbon spheres, which can be used as HER electrocatalysts with high activity and excellent durability. In principle, the ultra-small nanoclusters maximize the use of a single Pt Atom, while providing a unique surface and electronic structure, as compared to conventional Pt nanoparticles, the Pt clusters with high activity can be effectively stabilized by immobilizing the Pt clusters in the mesoporous channels of carbon carriers. At relatively low Pt loading (5.08 wt%) , the electrocatalytic activity of Pt5 / HMCS is much higher than that of commercial Pt / C catalysts with high Pt loading (20 wt%) . In addition, Pt5/ HMCS provides higher Pt quality activity in alkaline electrolyte, the quality activity of Pt5/ HMCS will be 12 times that of commercial Pt / C. This work can be used as a reference for the design and control of active metal catalytic centers and the synthesis of high-performance catalysts.
Xian-Kai Wan, Hao Bin Wu, Bu Yuan Guan, Deyan Luan, and Wen (David) Lou*, Confining Sub-Nanometer Pt Clusters in Hollow Mesoporous Carbon Spheres for Boosting Hydrogen Evolution Activity, Adv. Mater., 2019, DOI: 10.1002/adma.201901349

The source from Energist

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