 However, a given fuel cell is usually optimized for operating in one mode and may not be built in such a way that it can be operated in reverse.Fuel cells operated backwards may not make very efficient systems unless they are constructed to do so such as in the case of solid oxide electrolyzer cells, high pressure electrolyzers, unitized regenerative fuel cells and regenerative fuel cells.However, current research is being conducted to investigate systems in which a solid oxide cell may be run in either direction efficiently.