More Progress for Automotive-Oriented Direct Ammonia Fuel Cells
Byon January 03, 2019
Shimshon Gottesfeld’s paper The Direct Ammonia Fuel Cell and a Common Pattern of Electrocatalytic Processes leads with a big number: “A record power density of 450 mW/cm2 has been demonstrated for a direct ammonia fuel cell [DAFC] using an alkaline membrane electrolyte.” We know it’s big because it’s 80% higher than the 250 mW/cm2 that Gottesfeld’s team had achieved in the fall of 2017 and that Gottesfeld, Adjunct Professor of Chemical Engineering at the University of Delaware, reported at the November 2017 NH3 Energy+ Topical Conference.
Further contextualization comes from two other numbers. The first is order-of-magnitude 10 mW/cm2. This was the state of the art before the UDel team started their DAFC program, according to Gottesfeld’s paper, published on December 7, 2018 in the Journal of the Electrochemical Society (JES). (“Recent reports in the literature on the performance of such cells of lab scale dimensions have not been too encouraging, exhibiting peak power levels on the order of 1–10 mW/cm2.”) The second number is order-of-magnitude 1,000 mW/cm2. This is the power density of commercially produced polymer electrolyte fuel cells (PEFCs) that run on hydrogen. In his 2017 NH3 Energy+ talk, Gottesfeld said that “as a first-order approximation,” DAFCs will need to achieve a peak power density of at least 50%, relative to direct hydrogen fuel cells, to “secure competitive performance in [fuel cell electric vehicles] FCEV[s].” So a power density of 450 mW/cm2 is, beyond all doubt, a big number: both a long way from the starting point of 10 and awfully close to the critical benchmark of 500.
It should be noted that the record power density was achieved at an operating temperature of 100 °C. This meets a condition that had been stipulated by the UDel team at an early stage of their work. As described in a December 2017 Ammonia Energy post, the team was seeking “a sweet spot on the fuel cell’s operating temperature gradient. Low temperatures dampen the rate of catalytic generation of hydroxide (OH-) ions from water and decomposition of ammonia into nitrogen and hydrogen. High temperatures promote degradation of the electrolyte membrane. The team initially chose 100 °C as the likeliest compromise between these two considerations. The make-or-break focus of their work will be finding a catalyst that can produce sufficient chemical reactivity and a membrane that will prove sufficiently durable at that temperature.”
The work described in Gottesfeld’s paper is part of the Direct Ammonia Fuel Cells for Transport Applications project that the UDel team (which also includes Brookhaven National Laboratory, Rensselaer Polytechnic Institute, advanced materials start-up Xergy, and fuel cell R&D company PO-CellTech) is carrying out with funding from the U.S. Department of Energy Advanced Research Projects Agency’s Renewable Energy to Fuels through Utilization of Energy-Dense Liquids (REFUEL) Program. As described in a December 2016 Ammonia Energy post on REFUEL-funded projects, “the heart of the Delaware concept is a membrane electrolyte that is similar to that used in [polymer electrolyte fuel cells (PEFCs)], except that it conducts hydroxide (OH-) ions instead of protons. [PEFC] fuel cells operate at close to ambient temperatures which makes them the technology of choice for fuel-cell vehicles.”
The search for a highly effective catalyst system was subject to another constraint: the cost of the catalyst materials themselves, which can easily reach prohibitive levels given that the traditional choices come from the platinum group metals (PGMs). In his JES paper, Gottesfeld cites the “ultimate cost target of $30/kW for the fuel cell stack in a passenger vehicle” and the “major contribution” to stack cost from precious metals. Gottesfeld estimates that the cost target can be met with a PGM loading of 0.1 grams of platinum per kW of fuel cell power capacity. (He observes that the 10 grams of PGMs that this implies for a 100 kW fuel cell is “a similar mass of Pt [to what] is being used in catalytic converters of ICE powered vehicles which are to be replaced by the FCEV.”) Gottesfeld characterizes as “good” the “prospects for further lowering to 0.1 gPt/kW in the next 2–3 years” based on extending the advances that have brought the loading down to 0.2 gPt/kW over the last several years.
The bulk of Gottesfeld’s paper lays out the theory and practice of anode and cathode catalysis that will enable the PGM loading target to be achieved. The discussion is dense. A key conclusion is that “a mechanistic pattern common to three multi-step electrocatalytic processes, the AOR [ammonia oxidation reaction], ORR [oxygen reduction reaction], and HOR [hydrogen oxidation reaction], involves a Pre-Step preceding the RDS [rate determining step] and defining the onset potential [voltage].”
This statement and the evidence supporting it will be comprehended mostly easily by specialists. But the paper gives even non-specialists the grounds to ask provocative questions. For example, how will fuel cell vehicle producers respond if the UDel team and other groups active in the field ultimately succeed in developing a direct ammonia PEFC whose cost and performance rival those of hydrogen devices? How much of their proprietary PEFC technology could be adapted to an alternative chemistry? And will hydrogen infrastructure developers embrace the apparent technical and economic advantages and become ammonia energy infrastructure developers? In short, is disruption looming in the wings?