Heterogeneous Ir3Sn–CeO2/C as alternative Pt-free electrocatalysts for ethanol oxidation in acidic media
Graphical abstract
Introduction
Over the last decade, the global energy crisis and environmental issues stimulate much attention on new renewable energy storage and conversion. Particularly, direct alcohol fuel cells (DAFCs) have triggered increased interest, mainly due to their potential applications in electric vehicles and portable electronic apparatus [1], [2], [3]. However, its practical feasibility is still restricted by the high cost and low stability of Pt-based catalysts [4], [5]. For reducing the Pt usage and driving down the price of fuel cells, it is urgent to develop alternative Pt-free catalyst with high catalytic performance [6], [7], [8]. Various Pd and Pd-based catalysts have been recognized as promising substitutes to Pt in alkaline media for anode reaction [7], [9], [10]. Although fuel cells in acidic media have numerous advantages, such as good proton conductivity and mechanical stability [11], the development of high-efficiency Pt-free electrocatalysts for ethanol oxidation reaction (EOR) in acidic media still remains a huge challenge.
One effective strategy to design advanced catalysts is the use of multicomponent systems, which contains more than two different nanoscale components [12]. The heterogeneous catalysts exhibit the intrinsic property of constitutive components as well as the synergistic interaction between them [13]. With this interaction, heterogeneous catalysts exhibit novel physical and chemical characters, which could lead to the development of highly active Pt-free electrocatalysts for fuel cells. As one of the most stable materials in acidic conditions, Ir shows good performance in some catalytic reactions due to its low cost, high resistance to COads, excellent oxygen evolution and good oxygen reduction [14], [15], [16]. However, only few studies have been reported about the Ir-based catalysts as anode electrocatalysts for EOR. In 2007, Lei et al. investigated the Ir3Sn/C catalyst towards EOR in acidic media for the first time, strongly demonstrating the significant potential of Ir metal as an alternative Pt-free electrocatalyst in anode reaction. Then, in the pursuit of more efficient Ir-based catalysts for DAFCs, a highly active Ir/Ir–Sn/SnO2 heterogeneous catalyst was found for EOR by Du's group [15]. It indicated that both the IrSn alloy and SnO2 on the surface could play an important role in catalytic performance. This research is very encouraging to design novel Ir-based heterogeneous catalysts by combining the alloy and oxide. Questions, most of the metal oxides are easy to dissolve in extreme conditions, and then, would affect the catalytic activity and stability [17].
Recently, CeO2 has been received remarkable attention as a promoter to improve the catalytic performance, due to its anticorrosion ability in acidic media, oxygen vacancy defects and high oxygen-storage capacity [18], [19]. These features make the CeO2-metal hybrid structure particularly useful for some reactions. In the last years, several literature have been demonstrated that CeO2 can increase the dispersion of active metal and obtain synergistic effect between noble metal nanoparticles and CeO2 [20], [21], [22], thus are promising to achieve a high catalytic performance. In this respect, it is supposed that combining CeO2 and Ir-based alloy into a heterogeneous structure should possess high electrochemical performance for EOR. Herein, a novel Ir3Sn–CeO2/C heterogeneous catalyst as alternative Pt-free electrocatalyst is designed, characterized and tested for EOR. The electrochemical results prove that, relative to commercial Pt/C, as-prepared Ir3Sn/C and Ir/C catalysts, Ir3Sn–CeO2/C heterogeneous catalyst exhibits high EOR activity and good stability in acidic conditions. In addition, the effects of concentrations and scan rates on Ir-based catalyst are performed for the first time.
Section snippets
Preparation of CeO2/C precursor
CeO2/C was prepared using a traditional precipitation procedure. According to the pre-calculated amount, Ce(NO3)3·6H2O aqueous solution was mixed with Vulcan XC-72R carbon black. Then, the dilute NH3·H2O solution was added dropwise to the obtained solution under steady stirring, until the value of pH reached 10. Next, the mixture was stirred for 3 h. After that, the suspension was centrifuged several times and washed with ultrapure water. Then, the sample was dried in a vacuum oven (for 6 h at
Physical characterization
XRD is employed to obtain structural information of the CeO2/C, Ir3Sn–CeO2/C, Ir3Sn/C, Ir/C and Pt/C (Fig. 1). For CeO2/C precursor, diffraction peaks at 28.6°, 33.1°, 47.5°, 56.5°,76.7°, 79.4°and 88.6° are ascribed to the (111), (200), (220), (311), (331), (420) and (422) characteristic crystalline planes of the fluorite structure CeO2. This indicates the successful preparation of CeO2/C by the precipitation method [10]. For Pt/C catalyst, the broad peak at 25° corresponds to the carbon (200),
Conclusions
In conclusion, a novel Ir3Sn–CeO2/C heterogeneous catalyst was prepared, characterized and tested as alternative Pt-free electrocatalyst for EOR in acidic media. Electrochemical measurements prove that the heterogeneous Ir3Sn–CeO2/C catalyst exhibits high EOR activity and good stability in comparison with the Pt/C, Ir3Sn/C and Ir/C catalysts. This high performance can be explained by the strong synergistic effect among Ir, Sn and CeO2. Besides, the kinetics of Ir3Sn–CeO2/C catalyst towards EOR
Acknowledgement
We thank to the National Natural Science Foundation of China (21561019), the program of Changjiang Scholars and Innovative Research Team in University (IRT_15R56) and the Innovative Research Team of Gansu Province (1606RJIA324).
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