Development Of Non-precious Metal-based Metalloporphyrin Frameworks As Bio-inspired Oxygen Reduction Electrocatalysts

With rising concerns about energy crisis, tremendous efforts have been devoted to the research of direct methanol fuel cell(DMFC) and microbial fuel cell(MFC). Yet, the cathodic electrocatalyst for oxygen reduction reaction(ORR) in DMFC and MFC is still the bottleneck limiting their practical applications. To date, Pt-based catalysts are still the most widely used ORR electrocatalysts. However, the scarcity, high cost, and vulnerability to poison have strongly hindered the industrialization of Pt-based catalysts. Therefore, it remains a critical question and is of great significance to develop a non-precious metal catalyst with high efficiency and low cost to replace Pt-based catalysts for the application of DMFC and MFC.Natural coenzymes and cofactors with important biological functions have provided valuable inspiration to the development of ORR catalysts. These coenzymes and cofactors often contain macrocyclic metals(e.g. iron or cobalt porphyrins) and play critical roles in oxygen transportation and redox chemistry in many organisms. Inspired by nature, a variety of macrocycle-chelated non-precious metal complexes have been explored for their ORR catalytic activity. To this end, Fe and Co are the most commonly investigated non-precious metals and macrocycles are often based on porphyrin, phthalocyanine and their derivatives. However, these discrete complexes are prone to dissociation in the medium, causing a stability problem for long-term FC or MFC operation. In order to improve the stability of macrocyclic non-precious metal complex in solution, covalent organic frameworks(COFs) embedded with multiple metal centers have been created by using discrete complexes as building blocks. COFs are a class of relatively ordered covalent organic polymers with high porosity and stability. Hence, COF-based catalytic materials with embedded metals have become a hot topic in recent research of ORR catalysts.Herein, 2D and 3D COFs embedded with multiple macrocycle-chelated non-precious metal complexes, inspired by the catalytic centers of natural coenzymes and cofactors, have been synthesized and investigated to address a key question in the research of air-cathode DMFC and MFC. A series of physical/chemical characterization and electrochemical measurements have been conducted on the obtained materials and some of the materials have also been applied in actual MFCs. The specific works and key conclusions are:1. A conjugated microporous metalloporphyrin framework(Co PEF) has been synthesized through alkyne metathesis polymerization and mixed with carbon black, a low-cost conductivity enhancer, to obtain the composite electrocatalyst of Co PEF/C. Co PEF/C shows excellent ORR catalytic activity in both acidic and alkaline media. Compared to discrete metalloporphyrin monomers(M1), the conjugated framework exhibits considerably enhanced ORR catalytic activity in terms of onset potential and current density under otherwise identical conditions, which is likely due to the high porosity and conjugated nature of the framework, leading to better exposure of active sites and efficient electron and mass transport. Importantly, the Co PEF/C catalyst is able to achieve the full reduction of O2 via a 4-electron pathway. The catalyst also shows superior durability and enhanced resistance to methanol-poisoning compared to the composite containing the monomer M1, further suggesting the potential of such a polymer catalyst in DMFC applications.2. A highly porous metalloporphyrin polymer(Co POP) which can act as an efficient electrocatalyst for ORR has been developed. The as-synthesized Co POP shows remarkable durability in both acidic and alkaline media without any loss of ORR current density over the period of 50000 s. After pyrolysis, although the materials lose some stability and porosity, they show considerably enhanced ORR catalytic activity. Pyrolysis of Co POP yields nitrogen-doped carbon materials with encapsulated Co NPs, as evidenced by HR-TEM images. It has been found that the optimal pyrolysis temperature is 800 °C, which produces the material(Co POP-800) with the highest catalytic activity comparable to or even surpassing the commercial Pt/C. In addition, Co POP-800/C catalyzes the direct 4e reduction of O2 to H2 O with superior durability and methanol-tolerance under both acidic and alkaline conditions.3. A composite of Fe-Nx/C has been achieved from a novel and highly porous metalloporphyrin polymer and applied as the cathodic catalyst in single-chamber MFCs. Fe-Nx/C has displayed excellent ORR activity in p H-neutral medium, which is probably due to a synergistic process by Fe-based nanoparticles and N-doped graphitic carbon layer. Moreover, MFC using Fe-Nx/C catalyst demonstrates superior performance than that with Pt/C, in terms of cell voltage, maximum power density and Coulombic efficiency, suggesting that Fe-Nx/C is more tolerant and durable than Pt/C in a system with bacteria metabolism and thus holds better potential for practical MFC applications.