(Invited) Direct Ammonia Fuel Cell Enabled By Precious-Metal-Free Cathode
Authors: Zhao Yun , Setzler Brian P. , Wang Junhua , Yan Yushan 
Published: February 2020

For several decades, hydrogen fuel cell vehicles (FCVs) have been considered best candidates for achieving high conversion efficiency, long range and zero-tailpipe-emissions, particularly so for the passenger vehicle sector. However, hydrogen FCVs suffer from some tall challenges in fuel distribution and fuel tank fill-up, associated with the physical state of the fuel‒a highly compressed gas. Gas pressure of 700 bars is needed to achieve >5 wt% H2 storage in the two, gas-filled Mirai tanks, required to match the effective energy content in a single, tank-full of gasoline.
An alternative approach to hydrogen distribution infrastructure in transportation is using hydrogen-rich fuels. Instead of highly compressed hydrogen gas, a hydrogen-rich fuel (e.g. ammonia, hydrazine, methanol, ethanol, etc.) which is in liquid form under ambient, or close to ambient conditions, is to be transported and refilled. This could significantly simplify and lower the cost of the fuel distribution infrastructure. In terms of CO2 emission, hydrocarbon fuels such as methanol, ethanol, etc. are excluded because they does contain carbon, therefore, will release CO2 when used as fuel in a fuel cell. In the quest for sustainable fuels with high energy density, interest has similarly been turned to nitrogen-containing fuels, such as ammonia and hydrazine. As the starting point for nearly all nitrogen-containing fuels, ammonia has a natural cost advantage over said alternatives.1
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2 In 1960s, the earliest type of direct ammonia fuel cells (DAFCs) were reported by Wynveen based on alkaline fuel cells using a KOH electrolyte with a typical operating temperature of 50‒200 oC.3 Recently, as the development of polymeric HEMs for fuel cell application, DAFCs-HEM have been tested and shown more attractive. DAFCs-HEM sound attractive but they also have some significate drawbacks: first, the overall performance of DAFCs-HEM are rather low (≤11 mW cm−2) due to the lack of high-performance HEMs specially operating at ≥ 80 oC and the relatively sluggish AOR even on noble metals;4
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5 second, the crossover of ammonia through the polymeric membranes is unavoidable, decreasing the OCV and efficiency. The oxidation of diffused ammonia at cathode may generate toxic NO because Pt not only serve oxygen reduction reaction (ORR) but also proceeds AOR.6

In the present work, the precious-metal-free ORR catalysts (Acta 4020) were implemented in DAFCs to relieve the ammonia crossover issue for the first time. Acta 4020 presents a better ORR activity than Pt in presence of ammonia in KOH solution and significantly higher DAFC-HEM performance is obtained switching Pt cathode for Acta 4020 cathode. DAFC-HEM with Acta 4020 cathode test run on 3M NH3 in 3M KOH solution shows a peak power density of 135 mW cm−2.

R. Lan and S. Tao, Frontiers in Energy Research,
2 (2014).

D. J. Little, I. I. I. M. R. Smith, and T. W. Hamann, Energy & Environmental Science,
8 (9), 2775-2781 (2015).

E. J. Cairns, E. L. Simons, and Tevebaug.Ad, Nature,
217 (5130), 780-781 (1968).

T. L. Lomocso and E. A. Baranova, Electrochimica Acta,
56 (24), 8551-8558 (2011).

L. A. Diaz, A. Valenzuela-Muniz, M. Muthuvel, and G. G. Botte, Electrochimica Acta,
89 413-421 (2013).

T. Matsui, S. Suzuki, Y. Katayama, K. Yamauchi, T. Okanishi, H. Muroyama, and K. Eguchi, Langmuir,
31 (42), 11717-11723 (2015).