(Invited) Rechargeable-Liquid Fuel Cells and Hydrogen Carriers: Recent Progress and Key Challenges
Authors: Perry Mike L. 
Published: February 2020

It has been approximately sixty years since polymer electrolyte fuel cells (PEFCs) were first invented at G.E. [1], and thirty years since Los Alamos National Laboratory (LANL) invented the ionomer impregnated PEFC catalyst layer [2] and demonstrated the modern membrane-electrode assembly (MEA), which is still the heart of the state-of-the-art PEFC [3]. This new MEA architecture developed by LANL initiated a renaissance in PEFC technology, since it became apparent that high performance could be obtained with dramatically reduced catalyst loadings [4]. The result has been tremendous progress in the performance and durability of PEFCs over the past thirty years, which has enabled the growth of deployed PEFC systems, including many types of fuel cell electric vehicles (FCEVs), which has been accelerating over the past few years [5].
However, state-of-the-art FCEVs can be fueled only by hydrogen, which presents significant barriers to market adoption, such as the development of hydrogen-refueling infrastructure, as well as the compression and storage of high pressure hydrogen gas. The need to create a viable H2 infrastructure is the motivation for the U.S. DOE’s H2@Scale initiative, which goes beyond enabling FCEVs, since hydrogen can potentially be used as a clean secondary energy carrier if the economics of the production, distribution, storage, and utilization of hydrogen can be improved [6].
Liquid organic hydrogen carriers (LOHCs) are an alternative to distributing and storing hydrogen; however, the vast majority of the work on LOHC conversions has been done using thermal processes that operate at high temperatures and pressures [7]. Electrochemical hydrogenation and dehydrogenation (EHD) of LOHCs offers the potential for higher round-trip efficiencies, simpler systems, and significantly lower operating temperatures and pressures. Some of the early work, which focused on LOHCs with very high H2 contents, delivered poor results because of complex reaction mechanisms [8-9]. The EHD conversion of more kinetically facile carriers (albeit with lower H2 contents) is deserving of attention, since the energy-density requirements for the distribution and storage of hydrogen are more modest than what is required for transportation applications. Some work has been done with “organic chemical hydrides” (e.g., cyclohexane, isopropanol) [10], and with redox flow battery (RFB) reactants [11], which have demonstrated the concept of rechargeable-liquid fuel cells (RLFCs) using PEFCs. However, the maximum power density of these early cells was poor relative to hydrogen-fueled PEFCs, e.g., 78 mW-cm-2 on neat isopropanol (IPA) and oxygen [10] and 30 mW-cm-2 on 1.6 M V2+/3+ and air [11]. Additionally, two different types of cells were used to charge and discharge these liquids.
UTRC has leveraged our experience in the development of high performance PEFCs and RFB cells [12] to demonstrate RLFCs with improved discharge performance (e.g., 190 and 140 mW cm-2 at 0.6 V with 2 M IPA and oxygen and air, respectively, and 420 mW cm-2 at 0.7 V with 1.5M V2+/3+ and air ). These proof-of-concept cells were also used to demonstrate a novel and efficient recharging method with the identical cell used for both charging and discharging the liquid.
RLFCs are a low maturity technology, with many opportunities for substantial improvements. The key barriers identified by UTRC to further improvements in both the power and energy densities of RLFCs will be presented. These include electrocatalysts designed for the desired EHD reactions, as well as membranes that selectively transport the desired charge carrier while minimizing the crossover of the reactants. PEFC and RFB researchers are well suited to developing the technologies required to make RLFCs and EHD conversion of hydrogen carriers potentially compelling for a variety of applications.

Acknowledgements

The author would like to thank his UTRC colleagues who obtained the original research results that will be included here. A portion of this work was supported by the U.S. DOE as an ARPA-E Open IDEAS project (DE-AR0001002).

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