Non-Noble Metal Oxides/Hydroxides on Carbon Substrates for Effective Oxygen Electrocatalysis

Developing cost-effective and durable electrocatalysts for the sluggish oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is at the heart of advancing energy conversion and storage technologies, such as rechargeable metal–air batteries. In this thesis, several strategies were investigated for this purpose, with a focus on non-noble transition metal derivatives (Mn, Co, Ni, Fe oxides/hydroxides) and functional carbon substrates (oxidized carbon nanotubes and defective graphene). The enhancement in electrochemical performance was realized by rational design of the hybrid structure. Three series of hybrids were synthesized and analyzed: (1) Manganese cobalt oxide/nitrogen-doped multiwalled carbon nanotubes hybrids were rationally integrated by fine control of surface chemistry and synthesis conditions, including tuning of functional groups at surfaces, the congruent growth of nanocrystals with controllable phases and particle sizes, and ensuring strong coupling across catalyst–support interfaces. The hybrid structure exhibits tunable and durable catalytic activities for both ORR and OER, with a lowest overall potential difference of 0.93 V. The long-term electrochemical activities are also sustained by rational design of hybrid structures from the nanoscale. (2) Defect-rich graphene was realized by a two-step treatment (thermal reduction and annealing) to enhance the effectiveness of ORR and OER. The dominant mechanism for the enhancement is the increased density of active sites, which can be controlled by the annealing temperature in relation to the O/C ratio, surface area and pore structure. This defective graphene substrate can reduce the amount of manganese cobalt oxide needed to achieve comparable performance against the commercial standard Pt/C, proving an effective strategy of developing cost-effective oxygen electrocatalysts. (3) Nickel-iron layered double hydroxide on defective graphene was developed for highly efficient oxygen evolution electrocatalysis. The hybrids with annealed graphene as the substrate exhibit more efficient oxygen evolution than the other graphene-based materials studied earlier and in this work, in terms of high current response, low overpotential and Tafel slope. The main reason is due to the extensive defects, high electrical conductivity and hierarchical pore size distribution. The morphology, phase and electronic state of the nickel-iron hydroxides were further tuned by the atomic ratio of Ni and Fe and the synthesis conditions, leading to a much reduced low overpotential of 285 mV and 418 mV to achieve 10 mA cm−2 and 100 mA cm−2, respectively, which is among the best oxygen evolution electrocatalysts. The thesis also reviewed the concurrent progress of this subject area, outlined the perspective of this emerging field and proposed further work