The performance of lithium (Li) ion batteries and fuel cells are limited by unwanted or inecient electrochemical reactions at the electrode surfaces. This thesis studies such reactions with simulations and density functional theory (DFT) calculations, to understand the reactions at the atomic scale. It is important because the limitations of Li ion batteries and fuel cell inhibits the widespread use of electric vehicles, and fundamentally understanding these limitations can help to design future solutions for powering electric vehicles. The first part of the thesis considers reactions in batteries. The specic energy and the stability of Li ion batteries are limited by electrolyte oxidation at the positive electrode. The oxidation reaction of the electrolyte solvent, ethylene carbonate, is studied on surfaces of Li metal oxide electrodes. The Li content and transition metal are varied systematically, and this approach can help to understand the underlying principles of the oxidation reaction. The investigation nds that surface oxygen atoms are the active sites, and that the thermodynamics and the kinetics of the oxidation reaction generally can be described by the hydrogen adsorption energy for all the surfaces considered. Electrolyte decomposition also occurs by reduction reactions at the negative electrode, but here it results in an important layer called the solid electrolyte interphase (SEI), whose composition aects many important parameters of Li ion batteries. A study is presented on the formation of LiF, which is an SEI component, by the reduction HF impurities in the electrolyte. The reaction mechanism is studied on single crystal surfaces with explicit inclusion of the electrolyte, using state-of-the-art methods. The results are compared to experiments and it is found that HF can only be dissociated through interaction with Li ions. The second part of this thesis deals with understanding and improving the catalysis of the oxygen reduction reaction (ORR), which uses costly materials and limits the performance of fuel cells. A numerical model is developed to predict the ORR current at high overpotentials. It is based on the hypothesis that electrolyte ions blocks the active surface sites, which limits the ORR rates. The model is fitted with and compared to experimental results, that for the rst time have obtained the ORR current at high overpotentials without mass transport limitations. A potential region of low coverage of protons and anions determines the potential of maximum current. The last study deals with molecular catalysts containing two dierent active metals sites. These catalysts have the potential to direct the ORR in a dierent reaction mechanism, and thereby exceed the catalytic activity of conventional catalysts, using low-cost materials. The results leads to design principles for such bimetallic molecular catalysts and good candidates are identied.