(588b) The Bipolar Stack

Industrial and transportation applications of electrochemical systems invariably require high voltages. Given that the potential of individual cells is about one to four volts, hundreds of cells may need to be connected electrically in series. Where possible, building voltage is commonly done with a bipolar configuration. Despite the industrial importance of this design, its features are typically unfamiliar to novices in electrochemical engineering; and even well-seasoned academic researchers have misunderstandings and relatively poor intuition about its operation. This tutorial will explore the bipolar configuration, providing practical information and identifying common misconceptions.

We begin with a more detailed look at the need for developing high voltages, which leads to many cells being connected in series. Next, the options for configuring cells to build voltage are discussed. The primary methods use either bipolar or monopolar cells, and the advantages and disadvantages of each are considered. Why are fuel cells and redox flow batteries almost exclusively bipolar designs whereas lithium-ion batteries always use the monopolar approach? What are the key characteristics that make the bipolar configuration viable? At the same time, the chlor-alkali industry uses both configurations. Why? The critical role of current density on economics is examined.

A few topics that are particularly relevant to the bipolar stack configuration are detailed. The first is the current flow through the stack and the central role of the bipolar plate. Current collection for bipolar and monopolar designs is described. Shunt currents occur when cells are connected in series and there is an ionic path between cells. These currents are explained with simple analytic expressions that allow a more intuitive grasp of the phenomena. In addition to their impact in reducing efficiency, the factors that lead to corrosion of cell components is elucidated. This corrosion risk is far more insidious, but rarely acknowledged outside of industry. A number of mechanical challenges with the bipolar plate assembly are identified.

Lastly, bipolar plates are often a key driving of the cost of electrochemical systems. Often the design is maligned for its high cost, yet, rarely are the alternatives considered on a fair basis.