Investigation of Pt-Based Alloy Catalysts for Direct Ammonia Fuel Cells
Authors: Li Zhe-Fei , Beverage Kevin , Wang Yuxuan , Velraj Samgopiraj , Botte Gerardine G 
Published: February 2020

Introduction

Due to the urgent energy and environmental issues, the use of renewable energy to replace conventional fossil fuels is highly demanded. In this regard, fuel cells have attracted tremendous attention owing to its high power density, high efficiency and cleanness. However, conventional H2/O2 fuel cells suffer from several critical problems, including sluggish reaction kinetics, high cost, transportation and storage issues of hydrogen, safety. Instead of hydrogen, many other different fuels have been intensively investigated, i.e. methanol, ethanol, formic acid, ammonia, etc.1-4

Ammonia is considered to be very promising, because of its low cost, high energy density, and relatively easy handling. The standard potential of ammonia oxidation is -0.77 V vs. SHE, only 0.06 V less negative than the value of -0.83 V for hydrogen oxidation in alkaline media. The electrochemical oxidation of ammonia have been extensively studied during the past several dcades.5-6 Pt catalysts have shown the most promising catalytic activity, better than any other metal alone. However, the activity and durability of Pt catalysts is still unsatisfactory. The catalytic activity of Pt can be tuned by alloying with other metals. Therefore, much work have been carried out to synthesize Pt-based alloys (i.e. Ir, Rh, Pd) to improve the activity of ammonia oxidation. Particularly, PtIr and PtRh alloys showed most promising activity, possibly due to the improved adsorption of amine species and inhibition of poisonous Nads adsorption. Although some previous reports on PtIr and PtRh alloys have been published, there is some disagreement on the optimal content of Ir and Rh in the catalysts. In addition, most works have been done by half-cell tests rather than in real fuel cells. Therefore, there is still a demand of systematic study on ammonia oxidation of PtIr and PtRh catalysts.

Results

In this work, PtIr and PtRh nanoparticles supported on Vulcan XC-72 with different compositions (Pt:M ratio from 20:1 to 1:1) were prepared under identical condition. The catalysts possessed small particles size of ~2-3 nm and uniform distribution. The electrochemical performance was evaluated by cyclic voltammetry scan in 1M KOH and 1M NH3, and the representative results are shown in Figure 1. It can be seen that both PtIr and PtRh catalysts exhibited better performance than pure Pt catalyst. As compared to Pt catalyst, the onset potential of PtIr and PtRh catalysts significantly shifted to lower one, indicating improved ammonia oxidation activity and possibly less poisonous Nads adsorption. Detailed synthesis and characterization will be provided later.
The fuel cell tests were conducted to further evaluate the anode catalyst performance and shown in Figure 2. The fuel cell tests were carried out using a membraneless configuration, where a porous polypropylene membrane saturated with 1M KOH + 1M NH3 was used as the separator and were sandwiched by cathode and anode electrodes. The cathode fuel is 50 mL/min CO2-free air, and the anode fuel is 1M KOH + 1M NH3. The cell temperature was kept at 50 °C. The polarization curves were recorded by linear sweep from OCV to 0.05 V at a scan rate of 5 mV/s. It can be seen from Figure 2 that PtIr/C anode can deliver much higher current density, particularly at high overpotential. However, the current density is still very low, when compared to H2/O2 PEMFC. The sluggish ammonia oxidation reaction and Nads poisoning is probably one of major causes. It should be noted that the reaction is also significantly inhibited by the poor mass transport within the anode, due to the formation of N2bubbles. Further optimization of anode catalyst composition and electrode structure is ongoing and the results will be presented later.

Figure 1. CV curves of Pt/C, PtIr/C, PtRh/C, and Ir/C in 1M KOH + 1M NH3at a scan rate of 20 mV/s.

Figure 2. Fuel cell performance of Pt/C and PtIr/C in 1M KOH + 1M NH3at 50 °C.

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Figure 1