Abstract
Ammonia electrooxidation reaction (AOR) plays a pivotal role in H2 production, direct ammonia fuel cells, electrosynthesis, electrochemical sensors as well as understanding of nitrogen cycle.1-2 AOR kinetics is slow requiring electrocatalysts among which Pt and Ir are the two best choices.2 AOR is also very sensitive to structure of electrocatalyst as (100) facets exhibit maximum activity.3-4 Among alloys, PtIr has shown the best performance but there are inconsistent reports on the amount of Ir required for enhancing the activity.5 Designing more durable electrocatalysts for AOR has also been a bottleneck. In this context, Ni could be a possible solution according to a report based on theoretical calculations.6
In this study, PtIr (several ratios), PtNi and PtIrNi cubic electrocatalysts with (100) facets were prepared by wet chemistry methods with CO as selective capping agent. Structure, morphology and composition of the obtained electrocatalysts were examined by TEM, EDX, XRD and ICP-AES. Quantitative estimation of (100) surface sites were carried out by GeO2 irreversible adsorption and analysis of H desorption peaks in KOH solution. Currents shown in Figure 1 were normalized by electrochemical surface areas obtained via the Cu UPD method. LSV and chronoamperometry methods were used to elucidate the activity and stability of electrocatalysts, respectively.
It was found that only a trace amount of Ir could increase the activity of Pt cubes by more than twice. As illustrated in Figure 1, PtIr (85:1) cubes exhibited specific activity of 1.25 mA cm-2 compared to Pt cubes with 0.58 mA cm-2. Increasing the amount of Ir further decreased the activity. PtNi cubes performed better than Pt cubes with respect to stability although its specific activity of 0.51 mA cm-2 was slightly less than Pt cubes. Addition of trace amount of both Ir and Ni to Pt cubes, i.e. ternary PtIrNi cubes demonstrated the best performance in terms of stability. These PtIrNi cubes had specific activity of 0.72 mA cm-2 which was 41% more than Pt cubes. Additionally, PtIr cubes showed the least value of onset potential which was 0.429 V vs RHE as can be seen in Figure 1.
This work adds new insights showing that a minute amount of Ir could improve the AOR of Pt significantly. Furthermore, Ni plays a crucial role in improving stability for AOR.
Figure 1. LSVs of electrocatalysts in 0.1M KOH + 0.1M NH3 aqueous solution at 50 mV s-1
References
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Figure 1