Development of platinum-group-metal-free catalysts for oxygen electrocatalysis in energy conversion and storage devices

One of the greatest challenges of this century is to advance the clean energy transition by deploying efficient and cost-effective energy production and storage devices. Water electrolyzers and H2-O2 fuel cells are crucial to achieving H2 production from renewable sources and using H2 to generate electricity. These technologies emerge as beneficial solutions to reduce our dependence on fossil fuels and contribute to the decarbonization of various industries and energy sectors. Another critical aspect of the growth of sustainable technologies is the intermittency problem associated with renewable energy sources. Longterm energy storage solutions like metal-air batteries can address this issue. Water electrolyzers (WEs), H2-O2 fuel cells (FCs), and metal-air batteries (MABs) have reached a level of maturity but still depend heavily on critical raw materials (CRM) in their key components. Those devices share the oxygen electrode, which substantially affects their performance and durability due to the intrinsically slow kinetics for oxygen evolution/reduction and poor durability under harsh operating environments. The use of platinum-group-metal (PGM)-based electrodes, although catalytically active toward oxygen electrocatalysis, is not sustainable. Therefore, the rational design of advanced electrocatalysts for oxygen electrocatalysis is critical for advancing the development of WE, FC, and MAB systems with low cost, high efficiency, and extended durability. This thesis aims to develop innovative platinum-group-metal (PGM)-free materials for accelerating the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) for reducing costs and improving sustainability while guaranteeing high performance as compared to the current technologies based on PGM electrodes. After a brief introduction to the political and economic scenario related to the development of WE, FCs, and MABs, Chapter 1 illustrates the catalytic mechanism for ORR and OER, reviewing the most recent advances in the design and development of advanced electrocatalysts with a particular emphasis on applications in anion-exchange membrane fuel cells (AEMFCs), anion-exchange membrane water electrolyzers (AEMWEs), and zinc-air batteries (ZABs). 6 Chapter 2 reports the development of ORR catalysts for application in AEMFC, based on a carbon matrix with a high surface area decorated with iron and nitrogen (Fe-N-C). We optimized a soft-templating approach based on a phenol melamine resin for obtaining catalytic precursors with hierarchical porosity. The high electroactive surface area and NxFe sites allow for high catalytic performance, although the Fe percentage is lower or as high as 1.3 wt.%. We found that the presence of Fe plays a key role in the graphitization process of the carbonaceous matrix, favoring its higher stability and conductivity under AEMFC operation. Chapter 3 focused on the design of OER electrocatalysts derived from Fe/Ni-based Prussian blue analogs. The activity and durability of oxygen evolution were enhanced by functionalization with phosphorus (P). Once assembled in oxygen electrodes in AEMWE devices, this electrocatalyst class allowed for high electrochemical performance. The results obtained in Chapters 2 and 3 were used to design a strategy for the synthetization of bifunctional catalysts for ORR/OER based on iron and nickel (Fe/Ni-NC). Chapter 4 illustrates that tailoring the synthesis conditions, particularly pyrolysis conditions and the Fe: Ni ratio, allowed the obtaining of a highly graphitic carbon matrix with N inclusions decorated with Fe/Ni functionalities highly active towards OER and ORR. Given the excellent bifunctional oxygen reduction and evolution activity, the electrocatalysts were integrated as air cathodes into a rechargeable zinc-air battery, showing enhanced long-term stability and power density retention compared to commercial PGM-based cathodes. Overall, the results highlighted that the PGM-free Fe-based catalysts designed and optimized in this work are promising and cost-effective candidates to replace PGM-based oxygen electrodes in AEMFC, AEMWE, and ZAB devices.