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If an application requires either more power or more energy, one must buy both more power and more energy. For this reason, it is exceedingly difficult for these types of sys- tems to scale to grid-level storage. In contrast, flow batteries and regenerative fuel cells maintain a modular separation between the power elements of the system (the cell stack) and the energy elements of the system (the reactant and product storage tanks). This per- mits independent scaling of the two. An essential characteristic of any electricity storage technology is its round-trip effi- ciency. Energy is lost both during charging and discharging, making the cell efficiency particularly important, because the efficiency is approximately squared for the round-trip. The benefit of developing a regenerative hydrogen-chlorine fuel cell (as opposed to a re- generative fuel cell of another type, e.g. hydrogen-oxygen) lies in the relatively fast kinet- ics at the chlorine electrode. Fundamentally, this occurs because the chloride oxidation reaction is a one-electron process. Fast kinetics allow for very high round-trip efficiencies, and electrode kinetics arguably present the largest technological barrier to the implemen- tation of any specific fuel cell chemistry. The regenerative hydrogen-chlorine fuel cell (rHCFC) is an electrical energy storage device that facilitates the electrochemical reaction H2(g) + Cl2(g) 2HCl(aq). In dis- charge (galvanic) mode, H2 and Cl2 react to produce electricity and HCl(aq). In charge (electrolytic) mode, electricity is consumed to split HCl(aq) into H2 and Cl2, which is then stored in tanks until the electricity is needed (Figure 1). Compared to most flow batteries, the energy density of the rHCFC can be higher, and the reactants involved are abundant and inexpensive. Also, it is important to note that there is no solid electrode or storage medium involved in a change of state, which can lead to dimensional instability (e.g. dendrite growth) and rapid cycle fatigue. Much of the early research on rHCFCs focused on high-power applications of these cells, often when cost was not of paramount importance. They were studied for use in the MX missile defense program in the early 1980s (2), and rHCFC studies for space power applications were also of interest to NASA (3). In both cases, the high cell potential and corresponding high energy density of the rHCFC were the important cell characteristics motivating this research. During the energy crises of the 1970s, rHCFC storage systems were also studied for general electricity storage applications such as utility-scale grid stor- age and solar balancing (4-6). In the latter reference, power densities in excess of 300 mW were achieved. The studies provided proof of concept and were promising, but, as cm2 energy costs stabilized in the 1980s, interest in energy technologies declined in general, and rHCFC studies, mostly carried out in industry, were discontinued. Because fuel cell materials and systems have undergone vast improvements in the in- tervening years, rHCFCs have potential today as viable grid-scale electrical energy storage systems. Indeed, more recent experimental studies have led to improved cell performance, with power densities now beyond 500 mW (7). This paper presents a simplified rHCFC cm2 model that is intended to provide insight into the relative magnitudes of the cell losses and therefore to guide rHCFC research and development. To our knowledge, there is no extant model in the literature that explores such a large parameter space, particularly in the context of high efficiency operation. Different approaches to modeling this or related types of systems can be found in the literature (4,8,9). Although we do not attempt to 2PDF Image | Regenerative Hydrogen Chlorine Fuel Cell for Grid-Scale
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