The so-called hydrogen economy is high on the agenda for chemists, materials scientists, and others. In this scenario, hydrogen becomes our primary energy storage material—generated using sustainable means such as solar and wind power—and we dispense with polluting and unsustainable fossil fuels. Platinum-based electrocatalysts are the most promising option for the oxygen reduction reaction (ORR), which generally has a large overpotential and slow kinetics, but is crucial for the hydrogen economy.
Avoiding Noble Metals
There are problems with relying on platinum, of course. This noble metal is rare, costly, and of greater concern, sourced almost entirely from a single country (South Africa). Problems such as unevenly distributed mineral and energy resources already lead to international diplomatic tensions. In the future, such issues could become increasingly pressing as resources become more scarce and national borders are tightened. Thus, there is an urgency in the search for sufficiently effective alternative catalysts—ones that replace precious metals with abundant materials such as iron.
Soon Gu Kwon, Yung-Eun Sung, Taeghwan Hyeon, Institute for Basic Science (IBS), Seoul, and Seoul National University, Republic of Korea, and colleagues have examined how a material comprised of iron, nickel, and carbon can be endowed with a multimodal porous structure to make it an effective catalyst for the ORR that entirely precludes the need for a noble-metal component.
Doping the Hydrogen Economy
The team created a highly active Fe-C-N electrocatalyst. The developed synthetic procedures allowed them to prepare three types of nitrogen-doped carbon model catalysts, which could then be compared systematically to study the effects of their precise porous structures.
The researchers found that different macro- and mesoporous structures in their nitrogen-doped carbon materials loaded with iron contribute to different stages of the reaction kinetics. It is the hierarchical nature of the porous structures that has the biggest impact on activity. Additionally, however, the specific iron loading also affects the efficacy of the electrocatalysts. Their optimized catalyst structure has one of the best performances in the ORR and excellent long-term stability, the team reports.
Porous All the Way
The research reconciles our understanding of porosity, the distribution and interconnectedness of macro-, meso- and micropores in a way that has not been clarified before. Moreover, it has given the team a more general set of criteria for understanding what affects catalytic behavior and offers guiding principles for the design of a new generation of electrocatalysts.
The researchers explain, “The mesoporous structure is a channel for the reaction medium contributing to electrolyte wetting of the surface area.” The mesopores, thus, expose a much larger part of the physical surface area that would otherwise remain unreachable by the reactants, and thus, electrochemically inactive. The macropores then facilitate kinetic accessibility of the active sites during the reaction.
“We believe that our results motivate the rational design of porosity engineering for the development of high-efficiency electrochemical energy devices,” the team concludes. Such non-precious catalysts would help sidestep the geopolitical limitations of rare, noble metals such as platinum.
- Design Principle of Fe-N-C Electrocatalysts: How to Optimize Multimodal Porous Structures?,
Soo Hong Lee, Jiheon Kim, Dong Young Chung, Ji Mun Yoo, Hyeon Seok Lee, Min Jeong Kim, Bongjin Simon Mun, Soon Gu Kwon, Yung-Eun Sung, Taeghwan Hyeon,
J. Am. Chem. Soc. 2019.