Theoretical studies can provide fundamental understanding into the microscopic domain of atoms and molecules during chemical reactions. Our recent efforts to understand important electrochemical conversions show that transition-metal phosphides (TMPs) are promising catalysts for the hydrogen evolution reaction (HER) and CO2 reduction reaction (CO2RR). A broad range of experimental observations on HER activity can be explained by investigating the (nonstoichiometric, voltage- and pH-dependent) surface reconstructions of six different nickel phosphides. With pH and potential change, Ni2P offers selective CO2RR toward C3 and C4 products. In addition, we study how surface coating techniques can effectively tune mediocre catalysts into systems with high efficiency toward CO2RR. For example, molecularly functionalized TMPs convert CO2 to C3 products with a high efficiency. Furthermore, our ab initio thermodynamic calculations on lithium-air batteries demonstrated that Mo3P delivers an ultralow overpotential (η < 0.1 V) among different electrode candidates. The discharging LiO2 monomer product was found favorable to be transported away from the electrode surface with the assistance of high “donor number” electrolytes, leading to a solution model of the discharge products growth mechanism and a toroidal morphology of the ultimate discharge products. Therefore, the deactivation and passivation of the electrode due to the gradual covering of the surface by discharge products (Li2O2) can be avoided. The two metrics of electrolytes (donor number and HOMO-LUMO gap) are found to be inversely linearly correlated, demonstrating that the long-term activity and sustainable low overpotentials of electrodes strongly depend on the electron donor ability of aprotic solvents in the electrolytes. Overall, we highlight the importance of understanding surface, solution, and interfacial chemistries from a fundamental first-principles perspective. The garnered insights can be useful for a wide range of technologies, including fuel cells, CO2 utilization, and Li batteries.