The development of energy conversion application and the utilization of renewable energy carriers have been popular in research area in order to meet the growing energy demand. Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are two of dominant the energy conversion reactions, as they both associate with four-electron transfer steps and essentially need a large potential to conquer the kinetic limit. Recently, to develop the energy conversion devices for large-scale applications, seeking for low-cost and abundant materials as electrocatalysts is required.

Transition metal-based nanomaterials, including nickel (Ni), iron (Fe), and cobalt (Co) components that are earth-abundant and not expensive, have been found to present encouraging activities as oxygen-related electrocatalysts. With the efforts for years, the performance of some Pt-free ORR catalysts (such as Fe and Co compounds) has been comparable to that of benchmark Pt/C with satisfactory oxygen binding energetics. However, a relatively large overpotential (above 300 mV) is still required by most of non-noble metal-based catalysts for OER to reach the current density of 10 mA·cm^-2. Furthermore, there is still room for improvement in terms of the long-term stability of OER and ORR catalysts considering their industrial application. Hence, to explore highly active and robust transition metal-based electrocatalysts with rational material engineering is still in urgent demand.

Generally, the performance of electrocatalysts is affected by the intrinsic activity and number of active sites. Specifically, improving the electrical conductivity facilitates the charge transfer during electrocatalytic process and enhances the intrinsic activity which can be solved by constructing conductive structure. Besides, morphology modulating strategy mostly contributes to the solution for number of active sites problems. Crystal structure engineering and morphology modulation are likely to concurrently improve the stability of electrocatalysts.

To this end, this thesis aims to obtain high-performance oxygen electrocatalysts with superior stability. First, I focus on the development of Fe-Co bimetallic electrocatalysts, which showcases great promise for both ORR and OER and the application in Zinc-air batteries (ZABs). A series of FeCo alloys grown on carbon matrix with different particle sizes were characterized. The size modulation of FeCo nanoparticles could provide more active sites without changing their chemical properties. FeCo alloy nanoparticles can be uniformly distributed to carbon template with abundant nanotubes (FeCo/NC-3). FeCo/NC-3 demonstrates superior electrocatalytic activity towards ORR by a half-wave potential of 0.88 V and robust stability after 10000 cycles. After assembling the liguid ZAB with FeCo/NC-3 catalysts, the open-circuit potential (Voc) is evaluated to be 1.40 V, which is close to that of ZAB with commercial Pt/C+IrO₂. This finding not only demonstrates the appropriate polymer precursors for pyrolyzed transition metal-carbon materials, but also offers prospects for achieving high electrocatalytic performance of ORR, OER by modulating particle sizes with altering precursor components.

Then, in the second work, an innovative layered double hydroxides (LDHs)-assisted strategy is developed to synthesize two-dimensional metal organic framework nanosheets (2D MOF NSs) as efficient OER catalysts. This strategy preserves the active components toward OER by coordinating organic linkers with metal elements in LDHs. Additionally, the characterizations have exhibited that introducing carboxyl organic ligands to coordinate with metal centers can promote the proton transfer in the electrochemical OH_ads → O_ads process since the organic functionalities in MOF structure can act as proton transfer mediators. The as-prepared freestanding NiFe-2D MOF NSs deposited on glassy carbon electrode show a 260 mV overpotential (@10 mA·cm^-2). The overpotential can be significantly reduced to 221 mV when this approach is processed on NiFe LDH grown on nickel foam. Furthermore, this LDH-assisted transformed strategy is also applicable to NiCo, NiMn, and NiV based bimetallic systems, confirming the universality of this strategy. This excelllent OER activity and the universal applicability supports the capability of raional design of multimetallic MOF systems for water oxidation electrocatalysts. This strategy offers a general route to 2D MOFs synthesis in order to achieve MOF electrocatalysts that can be used directly in electrocatalytic devices.

In the last part of this thesis, we focused on the exploration of electroactive and stable MOFs by structure engineering due to the flexible tunability of coordination environment. Here, a molecularly engineered MOF system (e.g., Ni(DMBD)-MOF) which features a 2D coordination network incorporating with mercaptan-metal links was designed and the new structure was synthesized. This new molecularly engineered route enables Ni-S to be integreted into coordination links to build MOF structure. The crystal structure of Ni(DMBD)-MOF is solved from microcrystals by continuous rotation electron diffraction (cRED) technique. DFT computational results indicate that Ni(DMBD)-MOF features a metallic electronic structure as a result of the Ni-S coordination, highlighting the effective design of the thiol ligand (DMBD) for enhancing electroconductivity. In addition, the DFT calculations and several characterizations indicate that (DMBD)-MOF affords better performance in the electrocatalytic OER than non-thiol (e.g., 1,4-benzene dicarboxylic acid) analog (BDC)-MOF, as it poses fewer energy barriers during the rate-limiting *O intermediate formation step. To further enhance the OER performance, Fe species are introduced to form the NiFe(DMBD)-MOF catalysts, which requires an overpotential of 280 mV (@100 mA·cm^-2). A water-splitting cell with NiFe(DMBD)-MOF/NF as anode and Pt/C/NF as cathode exhibits a voltage of 1.50 V @10 mA·cm^-2 in alkaline electrolyte, outperforming most OER electrocatalysts. This work not only opens a new MOF platform for efficient and stable OER catalysis, but also demonstrates an effective strategy for leveraging molecular design and crystal engineering for achieving well-defined crystalline electrocatalysts.