Modern precious metal catalysts are regarded to be expensive and inefficient compared to carbon-based electrocatalysts. Heteroatom doping can create highly active catalytic centers, but it also result in poorer electronic conductivity, impeding electrocatalysis.
A Janus MOF heterostructure composed of ZIF-8 crystals and boron-containing MOF nanosheets (B-MOF) was constructed through a “molecular clipping and re-suturing” process. The pyrolysis of ZIF-8/B-MOF yielded Janus carbon structures consisting of nitrogen-doped carbon block and boron, nitrogen co-doped carbon nanosheets. Image Credit: ©Science China Press
To overcome this limitation, a group from the South China University of Technology created a Janus carbon electrocatalyst with various levels of heteroatom doping between the two sides, which might resolve the conflict between intrinsic activity and electronic conductivity and improve performance in electrocatalytic hydrazine oxidation reactions.
Electrocatalysis is the process by which electrical energy is converted into chemical energy. Electrocatalytic processes must run smoothly due to the design of electrocatalysts with extremely active centers and effective electron conduction. Carbon materials are a significant class of electrocatalysts. The trade-off between intrinsic activity and electronic conductivity is the key impediment to improving the performance of carbon materials.
Prof. Yingwei Li’s team at the South China University of Technology has now overcome this issue by producing a carbon-based catalyst with a Janus structure. A conductive nitrogen-doped carbon block (NC) and catalytically active boron-, nitrogen co-doped carbon nanosheets (BNC) make up the Janus carbon electrocatalyst.
The design of Janus carbon nanomaterials is not an easy task. Carbon materials are usually prepared by the carbonization of carbon-containing precursors. However, conventional precursors lack the designability to synthesize carbon materials with tunable structures and compositions.
Yingwei Li, Professor, South China University of Technology
Li continues, "Our group has been engaged in the development of efficient catalysts based on metal-organic frameworks (MOFs), a class of materials with high designability, tunable compositions, and ordered atomic distributions. The interesting properties of MOFs motivated us to design a Janus MOF as the precursor for Janus carbon nanomaterials."
For the Janus MOF, the researchers devised a “molecular clipping and re-suturing” method. The boric acid solution in methanol was used to heat ZIF-8 crystals. After progressively etching ZIF-8 with boric acid to liberate metal ions and ligands, nucleation and growth of B-MOF on etched ZIF-8 were performed. After that, ZIF-8/B-MOF was used as precursors for the synthesis of Janus NC/BNC.
Compared to the BNC side, the NC side had a lower doping level and, consequently, a higher electronic conductivity. The BNC side, on the other hand, has catalytically active BO3 sites with increased intrinsic activity.
The combination of NC and BNC could ensure the hybrid’s high electronic conductivity and induce greater charge delocalization of active sites on the BNC side, resulting in increased catalytic activity. NC/BNC demonstrated considerably higher activity in the electrocatalytic hydrazine oxidation process than single counterparts and simple physical mixtures.
Given the large family of MOFs, the group believes the suggested MOF-templated technique can be broadened to synthesize a variety of Janus carbon materials with configurable compositions and structures. This, hopefully, will add to the toolbox of tailorable chemistry and nanotechnology for prospective applications such as interfacial stabilizers, drug delivery, and phase-transfer catalysis.
Journal Reference:
Ding, J., et al. (2022) A Janus heteroatom-doped carbon electrocatalyst for hydrazine oxidation. National Science Review. doi.org/10.1093/nsr/nwac231.