Study of Desorption Process of Nitrogen Poisoning Species Generated during Ammonia Oxidation on Pt/C
Authors: Harada Yuya , Shiroishi Hidenobu , Saito Morihiro 
Published: February 2020

A hydrogen society are one of the solution to alleviate the current global warming. However, since the energy density of hydrogen is low in the gas state, hydrogen must be transported under high pressure or liquid state, which causes energy loss. Ammonia is a promising hydrogen carrier because of its low production cost and ease in liquefaction and transport. A low temperature direct ammonia type fuel cell (DAmFC) is a device for directly extracting energy from ammonia. However, even Pt and Ir do not have enough catalytic activity for ammonia oxidation at low temperatures, which is disturbing practical application. The reason is that an inert surface poisoning by strongly adsorbed nitrogen (Nad) takes place competitively with a reaction generating nitrogen molecules. We investigated the desorption process of Nad on an Ir disk electrode and multiwalled carbon nanotube supported Ir nanoparticles (Ir/MWCNT) by normal pulse voltammetry (NPV). The purpose of this study is to analyze the desorption process of Nad generated during ammonia oxidation on Pt/C.

To prepare catalyst modified electrode, 2 mg of 20 % Pt/XC-72 (E-TEK) was dispersed in 0.1 wt%Nafion-2-PrOH solution, and 10 μL of the dispersion was cast on a glassy carbon disk electrode (6 mm) and dried by hot wind. A three-electrode cell configuration was adopted using the modified electrode as the working electrode, a Pt wire as the counter electrode and RHE as the reference electrode. Electrochemical measurements were performed in 0.1 M NH3– 0.1 M KOH aqueous solution in nitrogen atmosphere. All potentials shown in this abstract are quoted with respect to the RHE.

To analyze the desorption process of Nad, a waveform used in the NPV was shown in Fig. 1. First, Nad was generated by applying a potential step to 0.6 V, where ammonia oxidation occurs, for 180 s to generate Nad. After that, Nad was attempted to be desorbed by applying potential step to E
recover [V] (0.4 V – 0.1 V) for a certain time (t
recover, 1 s – 180 s). Finally, ammonia oxidation current with non-Faradaic current was obtained by reapplying potential step to 0.6 V. The ammonia oxidation current was determined by subtracting the current value considering the change in solution resistance and charging capacity in 0.1 M KOH. Assuming that desorption process consists of two independent pseudo first-order reactions, the oxidation current is expressed by the following equation:

i
(0.6V,0.5s)= I
max
Γ [p{1-exp(-k1
t
recover) }+ (1-p){1-exp(-k
2
t
recover)}] (1)

where I
max[μAcm-2
ECSA] is the maximum ammonia oxidation current normalized by electrochemically active surface area (ECSA), Γ is the relative coverage of the ammonia, p is the rate of the fast reaction, and k
1 [s-1] and k
2 [s-1] are the pseudo-first order reaction rate constants, and each parameter was determined by the nonlinear least square method.

Fig. 1 shows the t
recover dependence of the ammonia oxidation current after applying various E
recover. The current reached almost a plateau after 60 s of applying E
recover, suggesting that the Nad dissociation reaction reached equilibrium at the potentials. The current increased with decreasing E
recover. Nad dissociation was promoted at the hydrogen adsorption/desorption potential region. Thus, it is predicted that hydrogen atoms adsorbed on the Pt atoms react with Nad, followed by desorption of N
x
H
y
.

Figure 1