Development of Carbon-supported Nonprecious Metal Electrocatalysts

Abstract

In this dissertation, we report three promising carbon-supported nonprecious metal catalysts for the efficient electrocatalysis of oxygen molecules, which are key reactions in the wide range of renewable energy conversion and storage devices. First, we revealed that a rationally designed carbon nanotube–graphene (CNT–GR) hybrid support stabilizes selectively the small, nitrogen-rich phase of iron nitride nanoparticles and incorporates effectively nitrogen species into the carbon lattices. This catalyst–support synergistic effect leads to superior oxygen reduction reaction (ORR) activity and durability against carbon corrosion and metal dissolution, compared to the independent use of CNTs and graphene as supports as well as Pt/C catalysts in alkaline media. This hybrid support is also applicable to cobalt nitride catalysts with similar promotional effects. Therefore, our work explicitly reveals critical new roles of the CNT–GR hybrid material as efficient support for developing strongly coupled and highly dispersed catalyst/support composites that could open up new avenues for use in a wide range of electrochemical and catalytic applications. Second, we presented a powerful method of stabilizing single-atom catalytic sites onto a highly reduced carbon host using a few-minutes-long ‘selective microwave annealing’ (SMA) procedure.5 We used the carbon nanotube (CNT) support as the main microwave susceptor to fabricate a highly reduced carbon support that can host atomic Fe–N4 sites. In the selective microwave-annealed catalyst (MA-Fe-N/CNT), abundant single Fe sites were uniformly dispersed in the form of Fe–N4, and a few layers of well-crystallized, graphene-like carbon structures coated the CNT surface. In contrast, severe agglomeration of Fe metal and thick, amorphous carbon layers were observed in the thermally annealed catalyst (TA-Fe-N/CNT). Thus, the MA-Fe-N/CNT catalyst shows unprecedented oxygen reduction reaction (ORR) activity and pH-universal durability superior to those of thermally annealed Fe–N4/CNT and expensive Pt/C catalysts. Furthermore, an aqueous Na–air battery with our Fe–N4/CNT catalyst operates as effectively as the device with the Pt/C catalyst. The method provides a new concept for the design of various strongly coupled and highly dispersed carbon-supported catalysts, which could open up new avenues for use in a wide range of electrochemical and catalytic applications. Third, we reported a catalyst, containing nitrogen–coordinated single-iron-sites specifically on the highly crystallized, basal-plane edges. Our synthesis method involved few-minutes-long microwave irradiation, anchoring of metal precursors, and generation of cut-off bonds. We used carbon nanotube–graphene (CNT–GR) hybrid as the main microwave susceptor and as a frame material, which is an excellent 3D structured carbon network, to fabricate a carbon-embedded nitrogen-coordinated atomic-iron-sites (Fe-N/S-CNT-GR). The edge-hosted single-iron-sites driven by the generation of concentrated basal-plane edges in the Fe-N/S-CNT-GR exhibited comparable ORR activity with superior stability regardless of pH, and comparable OER activity and durability in alkaline condition as well. Finally, we verified that the Fe-N/S-CNT-GR catalyst became a highly efficient charging and discharging air-cathode operating as effectively as a Pt/C-loaded cathode in a Zn-air battery. Overall, our findings contribute to the development of renewable energy technologies in such ways that include:

i) Demonstration of the diversified use of carbon nanomaterials. ii) Development of the alternative high-temperature annealing method, SMA. iii) Investigation of the origin of catalytic activity and stability iv) Verification of the practical applicability of newly developed catalysts.

We believe that harmonizing the metal-based catalysts with rationally designed carbon materials is significant for the design of a new generation of more robust and well-dispersed carbon-supported catalysts. In this aspect, the CNT–GR hybrid and microwave-derived synthetic strategy are projected to open up new avenues for use in a wide range of practical research field.

5.2. Future Perspective Unlike early-stage carbon-supported nonprecious metal catalysts, where various types of nonprecious metals, chalcogenides, oxides as well as single-atomic-sites were explored, atomically dispersed M–N–C catalysts have recently received intensive attention because of their maximal metal atom utilization and high catalytic activity.6 However, the precise design of M–N–C catalysts remains limited by unpredictable structural changes and severe agglomeration of metal ions during synthesis. Furthermore, the limited durability of M–N–C catalysts also remains a grand challenge. Therefore, a new synthetic strategy, advanced in situ characterization tools, and in-depth mechanistic studies are needed to improve catalytic activity and to address stability problems of current M–N–C catalysts. Controlling the structure of precursors such as a metal-organic framework (MOF), a covalent organic polymer (COP), and other polymer hydrogel-based carbon structures can explosively immobilize a large density of atomically dispersed M–Nx site on the robust carbon structures. Another approach for the advanced M–N–C catalysts is to develop a novel heat treatment method, which can selectively yield single-iron-sites instead of large-sized particles. From this aspect, pyrolysis-free synthetic approaches, silica-protective-layer coating methods, and microwave-derived synthetic approaches have been reported to suggest a new platform for designing enhanced M–N–C catalysts. Translating their high half-cell activity and stability into high-performance single-cell devices remains another grand challenge due to the lack of accurate control of a triple-phase boundary and the complexity of the cell system. To overcome these obstacles, the origin of promoted catalytic activity and stability should be firstly investigated by cutting-edge characterization tools to understand the reaction and degradation mechanisms. In addition to the catalyst development, extensive engineering of single-cell devices, including the balance between hydrophilicity and hydrophobicity, optimized pressure and temperature, and membrane electrode assembly (MEA) fabrication should be developed together. In the foreseeable future, electrocatalyst-based energy technologies enable the vision, fossil-free pathway for sustainable energy systems. We believe that this is accelerated by governments, international organizations, energy companies, investors, and citizens based on the stated policy scenario (STEP). | In response to a rising global population, increasing energy demands, and impending climate change, major concerns have been on the rise over the security of our energy future. Therefore, a blossoming interest in the development of renewable energy technologies has recently been witnessed for a sustainable energy future. For this vision, one prospective goal is to develop advanced electrocatalysts with the proper cost, efficiency, and selectivity. Oxygen evolution and reduction reactions (OER and ORR) are two key electrochemical processes taking place in a wide range of renewable energy conversion and storage devices such as water electrolyzers, fuel cells, and metal-air batteries. However, the overall efficiency of these devices has been substantially constrained by the sluggish kinetics in the oxygen electrocatalysis. In this dissertation, we deal with cost-effective and high-performance nonprecious metal electrocatalysts for electro-catalytic reduction or evolution of molecular oxygen. In Chapter 1, we briefly describe why renewable energy technologies would be needed and how electrocatalysts could play key roles in our energy future. In Chapter 2, we present a composite of metal nitrides and N-doped carbon nanomaterials, which is effectively capable of forming high-density M–N–C (M=Fe and/or Co) active sites into carbonaceous materials for enhanced ORR activities. Herein, we revealed the beneficial roles of a rationally designed carbon nanotube–graphene (CNT–GR) hybrid nanomaterial as an excellent carbon support through various comparative analyses with control supports, including solitary CNTs and GR sheets. This hybrid carbon support can provide abundant anchoring and defect sites, which can effectively confine metals as well as nonmetals, to selectively yield small-sized particles with N-rich FeN phase and allow a high concentration of nitrogen atoms in the carbon lattices. Furthermore, it can possess a 3D porous structure with a large surface area, thereby facilitating the much more favorable oxygen diffusion. Finally, we demonstrate that this CNT–GR hybrid support is also generally effective for cobalt nitride catalysts. In Chapter 3, we propose an alternative high-temperature synthetic strategy termed ‘selective microwave annealing’ (SMA), which can stabilize abundant single-atom catalytic sites onto a highly reduced form of carbon host with only a few minutes of microwave irradiation. In this method, carbon serves as an excellent microwave absorber (susceptor) as well as a support for the catalyst. Upon microwave irradiation, the carbon support selectively absorbs the microwave radiation, keeping the catalyst precursor temperature low, but effectively reducing itself; heat is then funneled from the hot carbon to the catalyst precursors, stabilizing the catalyst precursor specifically on the carbon support with a strong catalyst–carbon interaction. The mechanism of SMA is distinguished from that of usual thermal annealing in a furnace, wherein heat transfer along all directions heats the sample non-selectively, leading to agglomeration. This SMA strategy was applied to incorporate carbon-hosting Fe-porphyrin catalysts on a CNT support to produce MA-Fe-N/CNT catalysts with abundant single atomic, nonplanar Fe–N4 sites that were embedded in few-layer, well-crystallized carbon structures. In contrast, many agglomerated Fe particles were observed on thick amorphous carbon layers in thermally annealed TA-Fe-N/CNT. In Chapter 4, we present carbon-embedded nitrogen-coordinated atomic-iron sites on the sulfur-doped CNT–GR hybrid (Fe-N/S-CNT–GR), which is bifunctional toward the conversions between oxygen and water (OER and ORR). Our synthetic route involves the few-minutes-long microwave irradiation, the anchoring of metal precursors, and the formation of cut-off bonds. In addition to the unique heat transfer driven by microwave irradiation, the anchoring strategy promotes the formation of a single generic type of FeNxCy moieties instead of agglomerated Fe-based particulates. By extension, the cut-off bonds effectively tailor the high density of plane edges/steps, particularly comprising FeNxCy moieties. The unique edge-hosted single-iron-sites in the Fe-N/S-CNT-GR show unprecedented ORR activity and pH-universal durability while demonstrating comparable OER activity and durability in alkaline conditions. Density functional theory (DFT) calculations also demonstrated that such edge-hosted structures observed in the Fe-N/S-CNT–GR showed a relatively high ORR and OER activity. Furthermore, the density of state (DOS) analysis combined with crystal orbital Hamilton population (COHP) suggested that the incorporation of sulfur atoms induced the increase of bonding strength between Fe-dz2 and O-pz orbitals, which should be beneficial to the ORR and OER pathways. Finally, we verified that the Fe-N/S-CNT–GR catalyst became a highly efficient charging and discharging air-cathode that effectively operated as a Pt/C–Ir/C-loaded cathode in an aqueous Zn–air battery.