Fuel Cells and Flow Batteries: A Comparative Process and Design Analysis

Abstract

The energy industry needs an increase of efficiency of energy conversion and inexpensive energy storage. Electrochemical flow processes, such as fuel cells, flow batteries and electrolysers, could become key technologies in our energy future. The increasing amount of fluctuating renewable electric power generation needs the installation of electric storage. Hydrogen economy might be one solution involving fuel cells and electrolyzer, but classic secondary batteries as batch processes have size and cost limitations. The development of flow batteries changed the process of classic design of electrochemical batch process for storing electricity to a flow process like a fuel cell. For further development and a future research strategy it is important to understand their similarities and synergies to fuel cells and electrolyzer. A process and design analysis allows identifying similarities and differences between fuel cells and flow batteries. Electrolyzer also can be discussed. A thermodynamic consideration shows that the reaction coordinate (fuel utilization or state of charge (SoC)) of fuel cells and flow batteries is a function of space. SoC of secondary batteries is a function of time because they are batch processes. Flow processes generally allow high energy storage by its reaction products that can be stored independently from their stack design. A comparison of the major design criteria, as kind of process, BoP needs, ionic transport in membranes, substance transport, operation conditions, transformation of electrodes, reaction progress, comparison open system and closed system etc. shows a number of similarities between fuel cells and flow batteries. Only the energy storage by ionic substances in flow batteries differs clearly from energy storage by reaction enthalpy of electrical neutral substances in fuel cells. The storage of sufficient electric work in a certain volume or mass still remains a problem. This may be the use of high pressure with the necessary pressure vessels or heavy metallic hydrates for storing hydrogen or the use of strongly diluting supporting electrolytes for flow batteries. For that reason it is important to identify the quality of energy storage in an early state of design. A new method allows a very early and simple evaluation of the future technical feasibility of electrochemical processes as fuel cells or flow batteries for electricity storage.