The design and development of new functional materials for energy conversion and storage devices is emerging as one of the potential research areas. Fuel cells have been considered as a promising energy conversion device for automobiles and portable electronic devices. The operating principle of fuel cell involves cathodic reduction of oxygen. The performance of the fuel cell largely depends on the activity of cathode catalyst. Traditionally Pt-based catalysts have been used for the oxygen reduction reaction (ORR). However, the high cost, lack of durability and catalyst poisoning during fuel cell operation are some of the serious concerns of the traditional catalysts. Non-precious metals, metal oxides and metal-free catalysts have been proposed s alternative catalysts.¹ Recently, the transition metal nitride-based catalysts are emerging for various applications due to their interesting physicochemical properties.² These metal nitrides are very promising for electrocatalytic oxidation and reduction reactions. Although several methods have been developed for the synthesis of metal nitrides including Cu₃N, the electrocatalytic performance has not been well explored. We are interested in the development of new functional materials for ORR.^3,4 In an attempt to explore the catalytic activity of metal nitrides, we have synthesized nanosized Cu₃N and evaluated the ORR activity.

The electrocatalytically active Cu₃N was synthesized in one-pot by solvothermal approach (200 ⁰C) using hexamethylenetetramine as a nitrogen source. The X-ray diffraction pattern of as-synthesized Cu₃N nanocrystals confirms the formation of cubic anti-ReO₃ structure. The high resolution N1s X-ray photoelectron spectrum shows the presence of characteristic Cu₃N peak at 397.4 eV. Transmission electron microscopy (TEM) measurement shows that the Cu₃N nanoparticles have quasi-spherical shape with an average diameter of 80 nm. The electrocatalytic activity towards ORR was investigated by hydrodynamic voltammetry in alkaline pH. The mass specific activity was calculated to be 27.58 mA/mg which is significantly higher than the other Cu₃N-based catalysts available in the literature.⁵ The number of electrons (n) involved in ORR was 3.4 and the peroxide yield (%H₂O₂) was 34% implying the ORR follows mixed electron transfer pathway. In order to promote the 4-electron pathway for the reduction of O₂ to H₂O, the Cu₃N nanoparticles were integrated with reduced graphene oxide (rGO). Interestingly, well-defined voltammogram for ORR was obtained with the integrated hybrid catalyst (rGO-Cu₃N). A 35% increase in the limiting current density and a 90 mV positive shift in the onset potential with respect to the as-synthesized Cu₃N were observed, highlighting the excellent activity. We observed only 14% H₂O₂ suggesting that the hybrid catalyst promotes 4-electron pathway. The mass normalized activity was 96.45 mA/mg which is ~3.5 times higher than the as-synthesized Cu₃N. The kinetics of ORR was further evaluated by the mass-transport corrected Tafel analysis. The Tafel slope in the low overpotential region on as-synthesized Cu₃N and rGO-Cu₃N was calculated to be 106 and 56 mV/decade, suggesting the oxygen reduction kinetics on rGO-Cu₃N is similar to that of Pt. The pronounced activity of rGO-Cu₃N over as-synthesized Cu₃N can be attributed to the synergistic effect of conductive rGO sheets and semiconductor Cu₃N (band gap 1.68 eV).

Reference:

1. Raj, C. R.; Samanta, A.; Noh, S. H.; Mondal, S.; Okajima, T.; Ohsaka, T. J. Mater. Chem. A 2016, 4, 11156-11178.
2. Wang, D.; Li, Y. Chem. Commun. 2011, 47, 3604−3606.
3. Bag, S.; Roy, K.; Gopinath, C. S.; Raj, C. R. ACS Appl. Mater. Interfaces 2014, 6, 2692–2699.
4. Bag, S.; Mondal, B.; Das, A. K.; Raj, C. R. Electrochim. Acta 2015, 163, 16–23.
5. Wu, H.; Chen, W. J. Am. Chem. Soc. 2011, 133, 15236–15239.