(16c) Metal Phenolic Network Modified Core-Shell Nanoparticles As Electrocatalysts for Alkaline Hydrogen Evolution Reaction

Hydrogen evolution reaction is a crucial reaction for renewable energy storage. Among all different types of electrocatalysts for hydrogen evolution reaction, nanoparticle electrocatalysts are gaining more attention. Nanoparticle electrocatalysts are advantageous for their high surface area and tunability. Both noble and non-noble metal-based nanoparticle electrocatalysts have been developed, such as Pt, Ru, Ag, Mo, Co, Ni, and so on. Although nanoparticle electrocatalysts show high catalytic efficiency and relatively low cost, they still suffer from lacking stability and high conductivity. A way to improve both stability and conductivity is by forming core-shell nanostructures mainly based on carbon. It provides a protecting layer for the catalysts and also acts as a conductive layer. Most of the core-shell structures are derived from metal organic frameworks (MOFs) or adding ethanol with calcination. These core-shell structures have been proved to improve the catalytic ability of existing nanoparticle electrocatalysts. However, multiple synthesis steps and high temperature required are not ideal for reducing the use of chemicals and energy.

Here, we explored a new type of coating, metal phenolic network (MPN), which is a 2D porous framework formed by the chelation of metal ions and polyphenol groups. Herein, we developed a simple one-step coating method to form these MPN core-shell nanoparticles with iron ion and tannic acid. Tannic acid is a polyphenol that mainly came from plant extracts. The coating method is simply mixing both reactants, iron ion and tannic acid, and following with self-assembling of MPN coating by tuning the solution pH. The shell has shown to be formed uniformly on nanoparticle electrocatalysts, confirmed by different techniques such as transmission electron microscopy (TEM), energy-dispersive X-ray (EDX) mapping, and X-ray diffraction (XRD). With electrochemical impedance spectroscopy (EIS), this MPN coating layer provided low resistance and high hydrophilicity, which are both important factors for hydrogen evolution reaction. The coating improved the catalytic behavior of nanoparticle electrocatalysts by lowering the overpotential under certain current based on linear sweep voltammetry (LSV). Long term stability was also tested using chronopotentiometry (CP). Our modifying method with MPN coatings is demonstrating a new way to optimize existing electrocatalysts with low cost and simple synthesis by taking the advantage of self-assembling property of MPNs.