Abstract
Molybdenum carbide (Mo2C)-based electrocatalysts were prepared using two different carbon supports, commercial carbon nanotubes (CNTs) and synthesised carbon xerogel (CXG), to be studied from the point of view of both capacitive and electrocatalytic properties. Cation type (K+ or Na+) in the alkaline electrolyte solution did not affect the rate of formation of the electrical double layer at a low scan rate of 10 mV s−1. Conversely, the different mobility of these cations through the electrolyte was found to be crucial for the rate of double-layer formation at higher scan rates. Molybdenum carbide supported on carbon xerogel (Mo2C/CXG) showed ca. 3 times higher double-layer capacity amounting to 75 mF cm−2 compared to molybdenum carbide supported on carbon nanotubes (Mo2C/CNT) with a value of 23 mF cm−2 due to having more than double the surface area size. The electrocatalytic properties of carbon-supported molybdenum carbides for the oxygen reduction reaction in alkaline media were evaluated using linear scan voltammetry with a rotating disk electrode. The studied materials demonstrated good electrocatalytic performance with Mo2C/CXG delivering higher current densities at more positive onset and half-wave potential. The number of electrons exchanged during oxygen reduction reaction (ORR) was calculated to be 3, suggesting a combination of four- and two-electron mechanism.
Highlights
Demand for sustainable and efficient energy sources to complement and eventually substitute existing fossil fuel-based ones has arisen with the increasing energy demand and environmental pollution
X-ray diffraction analysis (XRD) peaks were assigned to the orthorhombic structure of α-Mo2C (ICSD card #1326) and crystallite sizes of 22.3 and 28.6 nm were evaluated for Mo2C/carbon nanotubes (CNTs) and Mo2C/carbon xerogel (CXG), respectively
It should be mentioned that a previous study of oxygen reduction reaction (ORR) at different phases of molybdenum carbide supported on carbon in acidic media have shown that their activity towards ORR is strongly influenced by the carbides’ structure so that α-Mo2C/C shows higher activity compared to δ-MoC/C, most likely due to the stronger affinity of the former for O2 adsorption [42]
Summary
Demand for sustainable and efficient energy sources to complement and eventually substitute existing fossil fuel-based ones has arisen with the increasing energy demand and environmental pollution. Electrochemical energy conversion devices such as fuel cells (FCs), batteries, and supercapacitors, are believed to be the most feasible alternatives among different energy technologies considered. The oxygen reduction reaction (ORR) has received much attention due to its application in fuel cells and metal-air batteries [1,2,3,4]. Sluggish ORR kinetics is one of the main limiting factors in the energy conversion efficiency of fuel cells and metal-air batteries, since ORR requires high overpotential and has a complex mechanism involving numerous steps [5,6,7,8]. ORR in alkaline media such as in alkaline fuel cells (AFCs) has faster kinetics, enabling the use of non-platinum (Pt) electrocatalysts in these cells [9]. Pt-based electrocatalysts are currently the most common electrocatalysts in AFCs and FCs in general, holding back their large-scale production and application
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