Electrocatalysis is key to both sensitive electrochemical sensing and efficient electrochemical energy conversion. Despite high catalytic activity, traditional metal catalysts have poor stability, low selectivity, and high cost. Metal-free, carbon-based materials are emerging as alternatives to metal-based catalysts because of their attractive features including natural abundance, environmental friendliness, high electrical conductivity, and large surface area. Altering surface functionalities and heteroatom doping are effective ways to promote catalytic performance of carbon-based catalysts. The first chapter of this dissertation focuses on developing electrode modification methods for electrochemical sensing of biomolecules. After electrochemical pretreatment, glassy carbon demonstrates impressive figures-of-merit in detecting small, redox-active biomolecules such as DNA bases and neurotransmitters. The results highlight a simplified surface modification procedure for producing efficient and highly selective electrocatalysts. The next four chapters focus on evaluating nitrogen-doped carbon nano-onions (𝑛-CNOs) as electrocatalysts for oxygen reduction and CO2 reduction. 𝑛-CNOs exhibit excellent electrocatalytic performance toward O2 to H2O reduction, which is a pivotal process in fuel cells. 𝑛-CNOs demonstrate excellent resistance against CO poisoning and long-term stability compared to state-of-the-art Pt/C catalysts. In CO2 electrochemical conversion, 𝑛-CNOs demonstrate significant improvement in catalytic performance toward reduction of CO2 to CO with a low overpotential and high selectivity. The outstanding catalytic performance of 𝑛-CNOs originates from the asymmetric charge distribution and creation of catalytic sites during incorporation of nitrogen atoms. High contents of pyridinic and graphitic N are critical for high catalytic performance. This work suggests that carbon-based materials can be outstanding alternatives to traditional metal-based electrocatalysts when their microstructures and surface chemistries are properly tailored.