3D Noble Metal-Based Porous Materials Self-Assembled By Atomically Modified Building Blocks As Electrocatalysts Boosting Small Molecule Electro-Oxidation or Reduction
Authors: Shi Qiurong , Zhu Chengzhou , Du Dan , Lin Yuehe 
Published: February 2020

Three-dimensional (3D) noble metal-based electrocatalysts are currently being intensively studied due to their unique hybrid porous architectures that could effectively enlarge their active sites, favor the mass and electron transfer rate, avoid the corrosion issues of carbon support and so on. Modifying building blocks is also of great importance for constructing the self-assembled 3D porous electrocatalysts with significantly improved electrochemical performances. However, the synthesis strategies for tailoring the structures and morphologies from atomic scale are still challenging and the mass activity (MA) and stability are still far from sufficient toward small molecule (i.e., ethanol, oxygen) electro-oxidation or reduction. Here, we rationally designed a series of 3D architectures by finely tailoring the morphologies, compositions, and surface atomic configurations etc. of building blocks for boosting their electrocatalytic performances.

Core-shell structures are very popular because of their ligands effects, ensemble effects and electronic effects, which are related to charge transfer and structure constraints that could favor desorption of oxygen-containing intermediates from the Pt active sites. Therefore, we firstly synthesized Au@Pt3Pd ternary metallic aerogels via a one-pot gelation strategy employing the dendritic core-shell structured Au@Pt3Pd nanoparticles (NPs) with size of about 24.5 nm as the building block. The as-obtained Au@Pt3Pd metallic hydrogels exhibited an improved MA of 0.812 A/mgPt+Pd and enhanced stability of showing 11.8 mV of half-potential shift after ADT test compared to commercial Pt/C toward oxygen reduction reaction (ORR). Both macro- and micro-structures contributed to the enhanced catalytic performances, like the high porosity for fast mass diffusion, cross-linked nanowires for rapid electron transfer, dendritic surface for providing more active sites, core-shell structure for offering synergistic effect and so on.

Compared to disordered alloyed structures, intermetallic nanocrystals (IM-NCs) possesses specific atomic structures with predictable electronic and geometric effect, high selectivity achieved through reducing the homoatomic coordination number of active sites, and stable chemical structures realized by the more negative enthalpy for generation of intermetallics compared to that of alloys. In order to improve the MA, selectivity and durability of the electrocatalysts toward ORR, for the first time, we successfully synthesized 3D IM-Pd3Pb nanowire networks (NNs) using ethylene glycol as the solvent and PVP as the surfactant at 170 ℃ for 1 h. The MA of IM-Pd3Pb NNs increased to be 1.06 A/mg and showed a half-potential shift of only 8 mV after 10000 potential cycles test. Besides, IM-Pd3Pb NNs displayed negligible current changes with addition of methanol while commercial Pt/C showed significant oxidation peak of methanol, indicating a high selectivity of oxygen as the cathode electrocatalysts. What’s more, IM-Pd3Pb NNs also shows a MA of 3.2 A/mg, which was two times higher than commercial Pd black toward ethanol oxidation.

Well established is the knowledge that catalytic activities leapt via downsizing the NPs to nanoclusters or single atoms, which is attributed to the remarkably increased active sites along with the altered surface defects, atomic and electronic structure. The as-resulted geometric and electronic/synergistic effects are all beneficial for boosting catalytic activities in electrochemical reactions. As for the ethanol oxidation, it had been revealed that Pd ensembles rather than isolated Pd atoms that serve as highly activated sites toward ethanol oxidation. Based on this principle, we successfully anchored Pd ensembles on the surface of the core-shell structured Au2Cu metallic hydrogels (denoted as Au2Cu@Pd) via galvanic displacement reaction under ice bath and achieved a MA of 22.1 A/mg toward ethanol oxidation, which is 15.8 times higher than Pd black. Also it suffers only 10 % degradation after 300 potential cycle test. The MA and long-term durability of Au2Cu@Pd exhibited remarkable improvement due to the modified surface composition and atomic configuration, core-shell structured NNs, 3D porous structures and so on.

All in all, the high porosity and interconnected architecture of 3D porous materials are significant for electrocatalysts because they provide free paths and open channels for electron and mass transfer. However, the modification of the building blocks by optimizing the compositions and structures, tailoring the surface atomic configurations also hold a key for boosting the electrocatalytic performances due to the as-resulted electronic and geometric effect from the atomic scale.