(Digital Presentation) A Study on the Poly(Amidoamine) Dendrimer Encapsulated Platinum Nanoparticles for Hydrogen Evolution Reaction in Proton Exchange Membrane Water Electrolyzer

Abstract

Hydrogen is a non-polluting energy carrier being considered for use in many sectors, including transportation, chemical industry, as well as the energy storage. The main motivation is to reduce utilizing fossil fuels and thus to stop global warming. Hence, speeding up research and development efforts in hydrogen production is critical to ensuring a sufficient share of hydrogen to be used in the envisaged economy sectors in the coming decades. Proton exchange membrane water electrolyzis (PEM WE) is the perspective technology to supply, in combination with the renewable energy sources, so called green hydrogen connected with a zero carbon dioxide emissions. This technology is characterized by high process intensity, good faradaic efficiency, quick response time and the possibility of high-pressure operation [1]. Important aspect, however, represents improvement of its economic competitiveness to the alternative hydrogen production routes. Important aspect represents an effective electrocatalysis of the desired electrode reactions.Number of catalysts are being studied for the hydrogen evolution reaction (HER). However, to the date, Pt-based nanoparticular catalysts are still considered to be a state of the art, robust option for use as a cathode catalyst in PEM WE. But the high cost and limitted stability are still obstacle to use Pt based catalysts in PEM WE effectivelly [1]. Hence, the effort has to be made to improve the stability of Pt nanoparticles and thus to retain its catalytic activity even under reduced loads. Poly(amidoamine) dendrimer (PAMAM) encapsulation of the nanoparticles represents a promising approach to solve this issue. PAMAM works as a template, protecting encapsulated Pt nanoparticles without passivating their surface and preventing them from agglomeration. This approach has been validated by using dendrimer encapsulated Pt nanparticles (Dend-Pt) as an oxygen reduction reaction catalyst in the fuel cells [2]. The PAMAM are monodispersed macromolecules, having a three-dimensional structure. Typically, there are three types of PAMAM, namely cationic (-NH2), anionic (-COONa) and neutral (-OH). Thus, the charge of the surface groups determines the dendrimer type. Based on the size and molecular weight, each type of PAMAM dendrimers has been classified into different generation for example, G1 and G10 are denoted as 1st and 10th generations, respectively [2].The goal of this work is to reduce Pt load required for efficient and stable HER in a PEM WE. PAMAM encapsulated Dend-Pt incorporated into multilayer graphene oxide (MLG) are used for this purpose.At first, the most suitable dendrimer template for Pt encapsulation needs to be identified and an optimum loading improving its HER efficiency in PEM WE determined. Our previous communication has proved the excellent HER activity of the carbon supported Dend-Pt, revealing an important role of the various carbon supports in enhancing conductivity, stability and catalytic activity of the Dend-Pt for HER [3]. Thus, Dend-Pt were synthesized encapsulated within the different types of dendrimer templates with varying surface end groups (-NH2, -COONa and –OH). Different generations of each type of dendrimer were used. The synthesis method consists in a simple chemical reduction in an aqueous solvent at the room temperature. The encapsulated amount of Pt was fixed for the each dendrimer under study. Commercially available MLG was used as a carbon support. The Dend-Pt was incorporated into MLG through chemical covalent linkage using (1-ethyl-3-(3-dimethylaminopropyl) carbodiimide) (EDC) and N-hydroxyl sulfoxuccinimide (NHS) as the cross-linking agents. The amount of the cross-linking agents (EDC and NHS) used was optimised as a the mass ratio of Dend-Pt and MLG with respect to the HER activity of the resulting catalyst. The formation of the zero-valent Dend-Pt was confirmed using UV-Vis absorption and XPS spectroscopy. The structure of Dend-Pt and MLG/Dend-Pt composites were determined using TEM. The loading of the Dend-Pt on MLG surface was determined using the ICP-OES spectrometry. The electrochemical HER catalysis by different catalysts was studied by means of voltammetric methods and electrochemical impedance spectroscopy in an environment of the 0.5 M H2SO4 electrolyte solution. As a proof of concept, the PEM water electrolysis in single-cell setup was perfomed using either a commercial catalyst (40% Pt/C) or Dend-Pt as a cathode catalyst. Iridium dioxide was used as an anode catalyst.AcknowledgementThe authors gratefully acknowledge financial support from “The Grant Agency of the Czech Republic” (GAČR) under project no. 20-21105Y.Reference:[1] M. Prokop, M. Drakselova, K. Bouzek, Curr Opin Electrochem, 20 (2020) 20-27.[2] B. Devadas, A.P. Periasamy, K. Bouzek, Coord. Chem. Rev, 444 (2021) 214062.[3] B. Devadas, T. Imae, Electrochem. Communications, 72 (2016) 135-139. Figure 1