Carbon Nanocomposite Catalysts for Sustainable Electrochemical Energy Conversion

摘要

Oxygen electrocatalysis, namely the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER), governs the performance of numerous electrochemical energy systems such as polymer electrolyte membrane fuel cells (PEMFCs), water electrolyzers, reversible fuel cells, and metal--air batteries. However, the sluggish kinetics of these two reactions and their dependency on expensive noble metal catalysts (e.g., Pt or Ir) limit the sustainable commercialization of these highly innovative and in-demand technologies. Development of highly active and stable bifunctional ORR and OER catalysts from earth-abundant elements is a grand challenge in electrochemical energy conversion.;The major contributions of this dissertation are: (1) establishment of a structure property relationship of Fe-N-C based electrocatalyst and subsequent optimization to achieve high ORR activity in acid and alkaline media. (2) Development of a review to deepen the knowledge of physicochemical properties of perovskite oxide catalysts relevant to their bifunctional ORR and OER activity. (3) Development of transition metal derived carbon nanocomposite catalysts with high ORR and OER activity and exceptional electrochemical stability successively meeting the milestones set by the US Department of Energy.;In this dissertation, first, we have extensively focused on understanding ORR on a widely studied Fe-N-C based catalyst in both acid and alkaline media. Structures and morphologies of Fe-N-C catalysts are crucial for overall catalyst performance for the ORR. Unfortunately, the relevant understanding is still lacking for their rational design. By employing multiple nitrogen/carbon precursors, including polyaniline (PANI), dicyandiamide (DCDA), and melamine (MLMN), catalyst morphology and structure were optimized in terms of the maximum catalyst activity and stability. Going beyond previously-studied single precursors, a synergistic effect was explored by using multiple precursors during the synthesis. In particular, the best performing Fe-N-C catalyst derived from PANI and DCDA is superior to individual PANI and DCDA-derived ones. Multiple key factors associated with density of active sites are elucidated including the optimal pore size distribution, highest electrochemically active surface area, presence of dominant amorphous carbon, and thick graphitic carbon layers with more pyridinic nitrogen doped at edge sites.;Bifunctional perovskite oxides have emerged as a new class of highly efficient non-precious metal catalysts (NPMC) for oxygen electrocatalysis in alkaline media. We have discussed the state-of-the-art understanding of the physiochemical properties of perovskites with regard to their OER/ORR activity in alkaline media and review the associated reaction mechanisms on the oxides surface and the related activity descriptors developed in the recent literature. Some strategies are also summarized relating to the role of surface redox chemistry and oxygen deficiency in perovskite oxides in order to further improve their performance for the ORR/OER.;The major hurdle facing perovskite oxides in their poor electrical conductivity and low surface areas, which have limited their ORR and OER activity to a great extent. In contrast, carbon catalysts have many obvious advantages including low cost, high electrical conductivity, high surface area, easy surface functionalization, and processibility. The most important contribution of this dissertation is the development of nitrogen doped large sized graphene tubes (>500 nm) decorated with FeCoNi alloy particles as electrocatalysts with excellent ORR and OER activity and electrochemical durability over a wide potential window (0 to 1.9 V) in alkaline media. Moreover, the electrochemical durability of the catalyst was further improved by introducing Mn and optimizing its molar ratio relative to Fe, Co and Ni. In addition to FeCoNiMn metal alloys/oxides, the carbon composites comprise a substantial carbon tube forest growing on a thick and dense graphitic substrate. The dense carbon substrate with high degree of graphitization results from Mn doping, while active nitrogen-doped carbon tubes stems from FeCoNi. Catalyst structures and performance are greatly dependent on the Mn content. Various accelerated stress tests (AST) and life tests verifies the encouraging ORR/OER stability of the nanocarbon composite catalyst with optimal Mn doping. Extensive characterization before and after AST elucidates the stability enhancement mechanism, which is attributed to (i) unique hybrid carbon nanostructures with enhanced resistance to oxidation and (ii) in-situ formation of beta-MnO2 and FeCoNi-based oxides capable of preventing carbon corrosion and promoting activity. Note, the improvement in stability due to Mn doping is accompanied by slight activity loss.