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Electrochemical energy conversion systems are capable of storing renewable energy in chemical forms which could be subsequently used as fuels or chemical feedstocks. Electrocatalytic reactions are cornerstones of these electrochemical processes and play pivotal roles in determining the overall efficiency of various electrolysis systems. For instance, the oxygen evolution reaction (OER) liberates the electrons and protons for the generation of clean hydrogen (H2) fuel through the hydrogen evolution reaction (HER) in an electrolytic water splitting system integrated with renewable power supplies. In addition, the oxygen reduction reaction (ORR) enables the energy and chemical conversions in fuel cells, metal-air batteries and electro-synthesis of hydrogen peroxide, satisfying our pursuit of a green chemistry. The main obstacle for the large-scale implementation of these energy conversion systems is the sluggish reaction kinetics on both the anodes and the cathodes, including anodic OER, cathodic HER and ORR, which require noble-metal-based electrocatalysts to expedite the reaction at a desirable rate and selectivity (e.g. IrOx and RuO2 for OER, Pt/C for ORR and PtHg for H2O2 production). However, the high cost and poor stability of these noble-metal-containing materials severely hamper their commercial viability. In these regards, the rational design of non-noble-metal electrocatalysts with high activity, good durability, desirable selectivity and low cost is highly sought after. This thesis focuses on the screening, design, synthesis and characterization of first-row transition-metal-based materials as efficient electrocatalysts for various electrolysis systems. To resolve the activity and stability issues haunted by water electrolysis system, two kinds of nanostructured bifunctional HER-OER electrocatalyst are fabricated including (1) ultrathin Fe-N-C nanosheets coordinated Fe-doped CoNi alloy nanoparticles and (2) defective FeCoNi (oxy)hydroxide atomic layers, which aim at an improved catalytic performance and long-term durability. Besides, ORR is the bottleneck of not only fuel cells and metal-air batteries but also other devices targeting at a continuous production of H2O2. To this end, another two novel electrocatalysts are synthesized: (1) epoxy-modified carbon nanotubes with isolated Co atomic sites and (2) thin Cu layer decorated Co nanoparticles encapsulated by a Co-Nx/Cu-Nx co-doped carbon, both achieving a high selectivity in different pathways with excellent activity. New preparation techniques are developed to fabricate these catalysts, and their catalytic properties are evaluated by electrochemical measurements. The structural and chemical properties of catalysts down to atomic scale are also investigated with a variety of physical characterizations (e.g. X-ray adsorption spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, transmission electron microscopy). The correlations established between activity and structural properties give insights into the origin of activity/selectivity within these composites. Last but not the least, the results indicated that, through rational design, transition-metal-based electrocatalysts with high activity, long-term durability and desirable selectivity even superior to the noble metal benchmarks can be achieved with an apparently lower manufacturing cost.