Revisiting the electrochemical oxidation of ammonia on carbon-supported metal nanoparticle catalysts

Abstract

The ammonia electro-oxidation reaction (AOR) has been studied due to its promising applications in ammonia electrolysis, wastewater remediation, direct ammonia fuel cells, and sensors. However, it is difficult to compare and analyze the reported electrocatalytic activity of AOR reliably, likely due to the variation in catalyst synthesis, electrode composition, electrode morphology, and testing protocol. In this paper, the electro-oxidation of ammonia on different carbon-supported precious metal nanoparticle catalysts was revisited. The effect of experimental conditions, electrochemical test parameters, electrocatalytic activity, thermodynamics, and possible deactivation mechanism of the catalysts were investigated. Pt/C catalyst possesses the highest electrocatalytic activity, while Ir/C and Rh/C show lower overpotential. The onset potential of the AOR is related to the hydrogen binding energy of the catalyst. N ads is one major cause of deactivation accompanied with the formation of surface O/OH ads at high potentials. The coulombic efficiency of N ads formation on Pt is about 1% initially and gradually decreases with reaction time. Increase in ammonia concentration leads to increase in current density, while increase in hydroxyl ions concentration can enhance the current density and reduce the overpotential simultaneously. The slopes of AOR onset potential and hydrogen adsorption/desorption potential of Pt/C as a function of pH follow Nernst equation. In contrast, potentials measured at different current densities exhibit non-Nernstian behavior, suggesting a critical role of the local pH change.

Electro-Oxidation of Ammonia over Copper Oxide Impregnated γ-Al2O3 Nanocatalysts

“…The anodic peak current increases with CuO contents in CuO/γ-Al2O3 nanomaterials; however, 16%-CuO/γ-Al2O3 shows the best performance towards AEO compared to other serial materials (Figure 6c), bare CuO and γ-Al2O3 (Figure 6d). The onset potential for AEO is found to be about −0.35 V vs. Ag/AgCl (i.e., −0.2 V vs. NHE) which is comparable to the Pt electrode [5,[63][64][65]]. Here, i p is the peak current, n is the number of electrons transferred, A is the electrochemical active surface area (cm 2 ), D is the diffusion coefficient (0.76 × 10 −5 cm 2 •s −1 at 25 • C), [61] υ is the scan rate (V•s −1 ) and C is the concentration of the analyte.…”

“…ECSA of catalysts are compared in Figure 6b 6d). The onset potential for AEO is found to be about −0.35 V vs. Ag/AgCl (i.e., −0.2 V vs. NHE) which is comparable to the Pt electrode [5,[63][64][65].…”

“…The D NH3 using all the catalysts can be estimated from the slope of the Randles-Sevcik (i p vs. υ 1/2 ) plot at a constant scan rate (100 mV•s −1 ) at 25 • C [73]. Resultantly, 16%-CuO/γ-Al 2 O 3 gives the highest value of the diffusion coefficient as compared to the other materials Ni/Pt [65] and Ni/Ni(OH) 2 [18], as enlisted in Table 3. Usually, the diffusion coefficient for AEO in KOH electrolyte (10 −9 ) is much lower than that of water (2.4 × 10 −5 ) [66] due to the presence of hydroxyl (OH − ) ions in the electrolyte [20].…”

A High-Performance 75 W Direct Ammonia Fuel Cells Stack

“…Such drop in ammonia crossover is likely the consequence of HEM degradation. Another possible reason is AOR catalysts deactivation caused by the formation of strongly chemisorbed ammonia oxidation intermediates during the stability test 32,[50][51][52] .…”

“…This stack was operated first for 3.5 h before shutting it down for the night, and the voltage degradation is seen to slightly recover on stack restart on the next day. The beneficial effect of open-cicuit conditions on recovery of the DAFC anode performance has been explained by reductive desorption of strongly bonded AOR intermediates at the PtIr catalyst 50 .…”