This study suggests a novel electrochemical reactor design for fuel cells and water electrolysis. Reactions in the fuel cells and electrolyzers occur at a solid (electrode: electrons)/liquid (electrolyte: ions)/gas (cathodic and anodic reactants: gases) three-phase boundary, which leads to a low power density and low efficiency of hydrogen production, respectively. In addition, heat removal is an important consideration, especially in a large scale fuel cell or electrolyzer, because heat is generated at the three-phase boundary due to the high overpotential. Improvements in mass (ions and gases) transfer and heat transfer characteristics are necessary to enhance the performance of these chemical devices. In our previous research, a new type of electrochemical reactor called Fuel Cell/Battery (FCB) system was proposed. In the FCB system, the fuel cell/water electrolysis reactions at the solid/liquid/gas three-phase boundary are split into reactions at solid/gas and solid/liquid two-phase boundaries, thereby leading to large reaction areas. This reaction mechanism is applied to the present electrochemical reactor design, which can improve both hydrogen and power generation efficiencies. Furthermore, this paper demonstrates the electrochemical reactor designs with novel electrode structures such as fibrous electrode, wave shaped electrode, and three dimension electrode.