Elsevier

Journal of Power Sources

Volumes 410–411, 15–31 January 2019, Pages 204-212
Journal of Power Sources

Hafnium sulphide-carbon nanotube composite as Pt support and active site-enriched catalyst for high performance methanol and ethanol oxidations in alkaline electrolytes

https://doi.org/10.1016/j.jpowsour.2018.11.021Get rights and content

Highlights

  • Pt@HfSx/CNT is prepared by hydrothermal method.

  • Pt@HfSx/CNT achieves high performance in methanol and ethanol oxidations.

  • CO poisoning effect is reduced by the incorporation of HfSx.

Abstract

The design and development of active and stable electrocatalysts for methanol and ethanol oxidations are needed for the applications of direct alcohol fuel cells. In this study, hafnium sulphide nanoparticle and carbon nanotube-based composites are used as Pt support for anodic reactions of fuel cells. The catalytic performance of the prepared Pt@HfSx/carbon nanotube catalysts for methanol oxidation and ethanol oxidation reactions in alkaline electrolytes are investigated. The electrochemical tests reveal that the Pt@HfSx/carbon nanotube has outstanding catalytic activities for both methanol oxidation and ethanol oxidation reactions in terms of the low overpotential and high current density, which are much better than its counterpart with Vulcan-XC carbon as the Pt support. The results indicate that Pt@HfSx/carbon nanotube has a great potential as electrode catalyst for anodic oxidation in direct alcohol fuel cells.

Introduction

Fuel cells are promising clean energy devices for electric vehicle (EV) transportation and portable electronic devices, which gain a lot of interest due to their low environmental pollution, easy handling, and high energy efficiency [[1], [2], [3], [4], [5], [6], [7], [8]]. Direct alcohol fuel cells (DAFC) use low-molecular-weight alcohols as a fuel are considered better than pure hydrogen fuel because of hassle-free storage and high mass-energy density [2]. DAFC can deliver continuous power compared with rechargeable batteries. In contrast to proton exchange membrane fuel cells (PEMFC), alkaline fuel cells have better current density, low-cost and less corrosive operating condition [9,10]. Also, both the oxidation and reduction kinetics of DAFC will be improved in alkaline electrolyte in contrast to the acidic electrolyte, particularly the adsorbed hydroxyl groups (OH) on the catalyst surface weakens the CO poisoning effect [11]. For DAFC, methanol and ethanol are the most promising fuels used due to their low toxicity, abundance, low permeability across proton exchange membrane and higher energy density (8030 Wh Kg−1 for ethanol and 6100 Wh Kg−1 for methanol). Besides, ethanol has a higher boiling point for safer storage in transportation applications [[12], [13], [14], [15], [16]]. Ethanol can be produced in large quantities by fermentation of sugar-containing raw materials, and thus it has been recognized as a substantial energy source in the future of “green” technology [14,17,18]. Till now, breaking the C–C bond of ethanol in acidic solutions is still difficult. But, the efficiency of ethanol oxidation in alkaline solutions has been demonstrated to be higher than that of the acidic counterpart [19]. For active anodic oxidation, Pt is the well-known efficient electrocatalyst that has been utilized in fuel cells [20]. But the CO poisoning leads to rapid deactivation of Pt catalyst during the electro-oxidation of alcohols. Hence, the design of highly active catalyst is extremely desirable to oxidize the methanol and ethanol fuels efficiently and to lower the harmful intermediates. To that, the inclusion of non-noble metals with Pt can reduce the CO poisoning [3,[20], [21], [22], [23], [24]].

Generally, the specific activity of the catalysts strongly depends on their size, distribution, and the support. Although, the catalyst nanoparticles with small size and uniform distribution attain large surface-to-volume ratio tends to high electrocatalytic activity [25]. In recent years, various metal sulphides have been widely investigated due to their potential applications in energy storage and conversion devices, photocatalytic and electrocatalytic reactions. Particularly, metal sulphides such as nickel sulphide (NiS), cobalt sulphide nanoneedles (CS-NN) and silver sulphide (Ag2S) have been utilized as an electrocatalyst for methanol oxidation reactions [21,26,27]. Whereas, the reports of metal sulphides for ethanol oxidation are very limited. Parallelly, the distribution of catalyst nanoparticles like Pt on different architectures greatly modulate the energy density and conversion efficiency [2,28]. Carbon materials have been studied widely as electrocatalyst supports because of their good conductivity, chemical stability and low cost. The strong bonding of CO on the Pt surface can be weakened by carbon support [22]. The weak interaction between the carbon support and Pt nanoparticles resulting in a poor stability that leads to fast and continuous degradation of the carbon support and eventually causes the migration and aggregation of Pt nanoparticles [9]. Though carbon black has been widely used due to its high surface area, which generally enhances particle dispersion, the presence of micropores limits its application as a catalyst because the metallic particles get trapped in the micropores and become inaccessible, as observed in the case of methanol molecules [25,28,29]. On the other hand, carbon nanotubes (CNT) and carbon nanofibers are very promising carbon-based support materials for catalysts due to their unique structure, good electrochemical stability, high electronic and thermal conductivities. Though, carbon nanotubes have been applied as alternative supports for metal catalysts and also numerous earlier investigations have demonstrated that Pt-based electrocatalysts supported by CNTs would show enhanced electrocatalytic activity for the methanol and ethanol oxidation at the anode of fuel cells [22,25,28]. Previously, the oxidation characteristics of hafnium disulphide (HfS2) for electronic device applications were reported [30], which brought more interest on hafnium sulphide (HfSx) based catalysts for methanol and ethanol oxidations. By considering the superiority of metal sulphide, we select the hafnium sulphide as support for Pt and investigated the catalytic performance. To enhance the conductivity of the hafnium sulphide-based support, the CNT and carbon black were adopted.

This study reports the preparation, characterization and electrochemical measurements of Pt@HfSx/CNT, Pt@HfSx/C (Vulcan-XC carbon), Pt/CNT and Pt/C catalysts for methanol oxidation and ethanol oxidation reactions in alkaline solutions. The hafnium-based catalyst is first ever utilized for methanol and ethanol oxidations. Electron microscopic images affirm the structures of Pt@HfSx/CNT and Pt@HfSx/C catalysts. Cyclic voltammograms showcased the electrochemical activities of Pt@HfSx/CNT, Pt@HfSx/C, Pt/CNT and Pt/C catalysts towards MOR and EOR and the mass-specific activities of Pt on different carbon support are also identified.

Section snippets

Catalyst synthesis

Initially, CNT was treated with 2M HCl for 1 h stirring. To prepare the catalysts, 300 mg of urea (CO(NH)2) was dissolved in ultra-pure water (55 ml) and then 50 mg of CNT or Vulcan-XC carbon black was added under continuous stirring. After fine dispersion, 1 g of hafnium tetrachloride (HfCl4) powder was gradually added to the stirring solution. Ten minutes later, 600 mg of sodium sulphide (Na2S) crystals were added to the mixture solution. After ultrasonication for 1 h, the mixture solution

Results and discussion

The X-ray diffraction patterns of Pt@HfSx/CNT, Pt@HfSx/C, Pt/CNT and Pt/C catalysts are shown in Fig. 1a. For Pt@HfSx/CNT and Pt@HfSx/C, the peaks at 28.32° (100), 31.67° (101), 50.28° (110) and 55.8° (103) correspond to the HfS2 phase of JCPDS 28-0444 [33]. Likewise, the peaks at 31.67° (012), 34.3° (200), and 46° (211) belongs to HfS3 phase (JCPDS 16-0039). These peaks ensure the sulphides formation of the prepared catalysts. Among the diffraction patterns, Pt@HfSx/CNT have sharp intense

Conclusion

The catalytic activity of Pt@HfSx/CNT and Pt@HfSx/C in the oxidation of methanol and ethanol were investigated. Results showed that the Pt@HfSx/CNT catalyst delivered the highest peak currents for the oxidation of both MOR and EOR. For both methanol and ethanol oxidation reactions, the Pt@HfSx/CNT also achieved the lowest Tafel slope. The mass activity of Pt of Pt@HfSx/CNT is predominant among all the prepared catalysts for both MOR and EOR. The high catalytic activity of Pt@HfSx/CNT should be

Acknowledgments

Financial support for this work was provided by the China Scholarship Council (No. 2015DFH586) for International Doctoral Students in China, the National Natural Science Foundation of China (No. 51302022 and No. 21707102), the Shandong Natural Science Foundation (ZR2018MB036 and ZR2017QB009), the Science Development Project of Shandong Provincial (2017GGX40115), the Project of Shandong Province Higher Educational Science and Technology Program (J17KA094, J13LD08), and the Scientific Research

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