High entropy materials for oxygen electrocatalysis

Oxygen electrocatalysis, including the oxygen evolution and reduction, is the key part in the energy framework with hydrogen as an energy carrier. However, current oxygen electrocatalysts cannot meet the highly efficient conversion between chemical energy (electrolyzer) and electricity (hydrogen fuel cells). Thus, developing high efficient, stable and low-cost oxygen electrocatalyst is the main challenge. High-entropy materials offer a new perspective on developing advanced oxygen electrocatalysts because of synergy effects among multiple components, offering possibilities to break the property limits of traditional materials in the catalysis field. Here, this research, with the research targets of high-entropy materials, aims to develop new oxygen electrocatalysts for oxygen evolution and reduction reaction, and try to solve the issues in oxygen electrocatalysts mentioned above.

Due to four-electron transfer, the oxygen evolution reaction (OER) results in sluggish reaction kinetics, thereby challenging the development of advanced electrolyzer technologies. Ruthenium and iridium-based materials have been recognized as good catalysts in the OER, but their high cost and scarcity impede large-scale applications of electrolyzers.

Thus, given the above consideration, the first part of this thesis focuses on fabricating new low-cost OER catalysts and understanding the relationship between their physical properties and OER activity. More specifically, highentropy metal-organic frameworks (HEMOFs) OER catalysts were synthesized by introducing multi-metal nodes into single metal MOFs. Based on this type of OER catalyst, three main questions are thus addressed, namely i) the influence on electronic structures when introducing multi-metal nodes into MOFs, ii) identifying possible metal catalytic sites in HEMOFs and iii) the main factors affecting the electrocatalytic activity of prepared catalysts. Moreover, the optimized Co-Rich-HEMOFs has an overpotential of 310 mV and a current density of 38 mA cm^-2 at 1.6 V vs. RHE.

The second part is to develop new electrocatalysts for the oxygen reduction reaction (ORR), a bottleneck reaction where chemical energy is converted into electricity in hydrogen fuel cells. In this part, a new facile method combining a solid-state thermal reaction and a carbonization process is employed to synthesize high-entropy alloys (HEAs) encapsulated in hollow carbon tubes, thereby breaking the limitations associated with the wet-chemistry method. The leading research can be divided as follows. First, the synthesis parameters of the catalyst were optimized, including carbonization temperature, as well as the ratio of organic carbon sources and introduced metal elements. Second, ORR activity was measured by electrochemical analytical techniques performed in 0.1 M KOH. The influence of crystal structure, specific surface area (SSA) and degree of graphitization on ORR activity was analyzed. Moreover, the in situ X-ray diffraction method was used to study the HEA formation process, such as the observed formation temperature of HEAs (908 K). Finally, electrochemical tests indicate that the optimized HEAs can maintain a 100% current after 10 hours of operation, revealing good stability in ORR.

The third part focuses on understanding the effects of particle size and metal-rich types on ORR activity for HEA catalysts based on the method developed in the second part. More specifically, by changing the introduced transition metal content (Mn, Fe, Co and Ni), HEA particle sizes can be tailored. The electrochemical results reveal that particle size and metal-rich types affect ORR activity. Specifically, the HEAs with small particle size and Fe-rich type tend to obtain good ORR activity. In addition, the difference in terms of catalytic activity between HEAsand single metal particles by using Fe in the control samples is investigated. The electrochemical result reveals that Fe-Rich-HEAs(1-16) ensures good performance with an E_1/2 of 0.861 V vs. RHE in a 0.1 M KOH solution. These findings offer new references for fabricating new oxygen electrocatalysts and illustrate the potential application of high-entropy materials in catalysis field.