Non-precious metal catalysts (NPMCs) hold great promise in energy conversion applications, thanks to their exceptional performance in the oxygen reduction reaction (ORR) within fuel cells. Recent research efforts have focused on creating transition metal-containing carbon-based conductive and porous catalyst materials for ORR applications, where active sites are crucial for oxygen electrocatalysis.
A team of scientists has engineered a composite catalyst using micro-mesoporous metal-organic framework (MOF) and carbon nanotubes (CNTs), demonstrating impressive ORR electrocatalytic activity in alkaline solutions with minimal catalyst usage. The incorporation of MNx sites, achieved by introducing transition metals into the MOF- and CNT-derived carbon structure, played a pivotal role in enhancing the catalysts’ electrocatalytic performance.
Their groundbreaking work was documented in the Industrial Chemistry & Materials journal on September 6, 2023.
In the battle against global warming, the research and development of renewable energy generation have garnered significant attention. Anion exchange membrane fuel cells (AEMFCs), which employ O2 or air at the cathode to generate electricity, have become a focal point due to their relatively mild operating conditions. However, the slow kinetics of the cathodic oxygen reduction reaction (ORR) and the limited durability of Pt-based catalysts have hindered their practical application.
Over the past decade, substantial efforts have been devoted to creating highly active transition metal-nitrogen-carbon (M-N-C) catalysts for ORR. Although various transition metal-based electrocatalysts have been explored as cost-effective alternatives in alkaline media, identifying a single-metal catalyst to replace Pt-group metals for ORR remains challenging. Bimetallic electrocatalysts, with their altered electronic properties, are expected to enhance electrocatalytic activity.
Metal-organic frameworks (MOFs) offer valuable advantages for catalyst tuning and structure optimization, often providing highly dispersed active sites, improved charge transfer networks, and hierarchical porosity for efficient reactant mass-transfer. The research team succeeded in fabricating MOF- and CNT-based carbon composites with iron and cobalt coordination to nitrogen in the doped carbon material, resulting in a three-dimensional micro-mesoporous structure used for ORR electrocatalysis.
They achieved this by subjecting ordered zinc-based zeolitic-imidazolate framework (ZIF) to high-temperature pyrolysis, creating the micro/mesoporous carbon composite with CNTs. The composite catalysts exhibited remarkable electrocatalytic ORR activity in alkaline media (0.1 M KOH), surpassing the performance of Pt/C catalysts with a half-wave potential of 0.85 V vs. RHE. When tested in AEMFC, the prepared bimetallic catalysts achieved a maximum power density of 171 mW cm-2 using an Aemion+ 15 μm anion exchange membrane (AEM).
The research team envisions further optimization of various M-N-C type cathode catalysts to achieve even higher power outputs from AEMFCs. They also anticipate potential applications in zinc-air batteries, proton exchange membrane fuel cells, and beyond. Professor Kaido Tammeveski, from the University of Tartu, shared their aspirations for future research endeavors.
The team comprised Rohit Kumar, Marek Mooste, Zubair Ahmed, Srinu Akula, Ivar Zekker, Margus Marandi, Maike Käärik, Jaan Leis, Arvo Kikas, Alexey Treshchalov, Markus Otsus, Jaan Aruväli, Vambola Kisand, Aile Tamm, and Kaido Tammeveski from the University of Tartu, Estonia.