Elsevier

Applied Surface Science

Volume 392, 15 January 2017, Pages 27-35
Applied Surface Science

Carboxyl functionalized carbon fibers with preserved tensile strength and electrochemical performance used as anodes of structural lithium-ion batteries

https://doi.org/10.1016/j.apsusc.2016.09.017Get rights and content

Highlights

  • Carboxyl functionalized CF is acquired by simple chemical oxidation method.

  • These CF have preserved the tensile strength, better electrochemical properties.

  • The presence of H3PO4 prevented the turbostratic carbon from over-oxidization.

  • There CF can be used as anodes of multifunctional structural battery.

  • The preservation and improvement is result from the hindered over-oxidization.

Abstract

Carboxyl functionalized carbon fibers with preserved tensile strength and electrochemical properties were acquired through a simple chemical oxidation method, and the proposed underlying mechanism was verified. The surface of carboxyl functionalizing carbon fibers is necessary in acquiring functional groups on the surface of carbon fibers to further improve the thermal, electrical or mechanical properties of the fibers. Functionalization should preserve the tensile strength and electrochemical properties of carbon fibers, because the anodes of structural batteries need to have high strength and electrochemical properties. Functionalized with mixed H2SO4/HNO3 considerably reduced the tensile strength of carbon fibers. By contrast, the appearance of H3PO4 preserved the tensile strength of functionalized carbon fibers, reduced the dispersion level of tensile strength values, and effectively increased the concentration of functional acid groups on the surface of carbon fibers. The presence of phosphoric acid hindered the over-oxidation of turbostratic carbon, and consequently preserved the tensile strength of carbon fibers. The increased proportion of turbostratic carbon on the surface of carbon fibers concurrently enhanced the electrochemical properties of carbon fibers.

Introduction

The use of structural batteries could effectively decrease the whole mass of devices by acting as integrated load-carrying parts (e.g., car roof) [1], [2], [3]. By combining electrochemical and mechanical properties in the same material, a structural battery could function both as a structural element and as energy storage, having the possibility to become a fully integrated part of a device. In addition, the power and energy density could be increased on a system level. Carbon fiber (CF) is a promising lightweight material used as electrode material in structural power batteries because of its high specific tensile properties and carbonaceous microstructure, which enables reversible lithium-intercalation reactions [4], [5], [6]. However, CF used in lithium-ion batteries display poor capacity, only about 100 mAh g−1 at moderate lithiation rates [7]. Moreover, surfaces are chemically inert and manifest weak interfacial interactions [8]. Therefore CF cannot be easily used to form composites with high-capacity materials (e.g. silicon [9], ZnCo2O4 [10] and WO3 [11]) to effectively improve electrical properties [12], [13], and there is poor interfacial strength between CF and solid polymer electrolytes [14], [15], [16]. Therefore the surface carboxyl functionalization of pristine carbon fibers is necessary.

There are a number of existing functionalization methods are studied to address different application requirements of functionalizing carbon fibers, but there are few is aimed at carbon fibers as structure electrode material. Different with the other application requirements, carbon fibers used as structure electrode material have a high demands on both tensile strength and electrochemical properties. Hence the appropriate functionalization method is necessary and urgently needed. In this study, chemical oxidation method is improved to address the high demands for carbon fibers as structure electrode materials because of the following reasons. First, compared with other functional groups, carboxyl groups are proved more suitable for adhesion with nanoparticles [12], the addition of surface polar functional groups effectively increases surface wettability of fibers, and the chemical oxidation using strong oxidizing acids is easier to obtain more carboxyl groups than the others [17]. Second, chemical oxidation method is simpler and cost less, and has less potential to damage electrochemical properties. Third, although surface functional group grafting procedures could preserve tensile strength of CF [18], [19], grafting reactions using solid organic matters as secondary materials have great potential to damage electrode electrical properties. Besides, grafting reactions using carbon nanotube (CNT) [20], carbon aerogel or graphene as a secondary material suffer high cost and low productive efficiency on the synthesis of CNT or graphene and grafting processes.

However, chemical oxidation is usually accompanied by the cleavage of Csingle bondC bond, and creates surface defects, which are undesirable for fiber tensile strength, and cannot be ignored. The negative effect of chemical oxidation method should be decreased. Otherwise, the application of carbon fibers would be influenced. A previous study indicated that the use of phosphoric acid as secondary acid can protect the vicinal diols by forming a cyclic structure, thereby preventing or retarding the over-oxidation of the CNT and nanoribbon [21]. Therefore, the presence of phosphoric acid may be used to prevent Csingle bondC from breaking. Meanwhile the tensile strength of carbon fibers is determined by the turbostratic structure. Hence, we hypothesize that the addition of phosphoric acid could preserve the strength of carbon fibers.

In this study, we developed a carboxyl functionalization method with mixed phosphoric/sulfuric/nitric acids (H3PO4/H2SO4/HNO3) for carbon fiber surface. The effectiveness of this proposed method on increasing the concentration of surface acid functional groups, tensile strength and electrochemical properties of carbon fibers was assessed. The proposed functionalization mechanism was also verified by several tests. The PAN-based T800 carbon fibers were viewed as a highly promising structural electrode material and have a promising prospect in further researching during the process of developing a functional structural battery in the previous study [22]. Hence PAN-based T800 carbon fibers are utilized in this study.

Section snippets

Experimental

PAN-based carbon fibers were coated particularly with epoxy or polyurethane resin. PAN-based T800 carbon fibers were desized through solvent extraction using a superfluous volume of acetone for 72 h at room temperature to avoid the influence of surface resin on tensile strength and electrochemical properties especially the reversible capacity. The desized carbon fiber (DCF), was considered as the unmodified carbon fiber. The carbon fibers were then filtered and dried at 60 °C for 12 h to remove

Tensile properties and concentration of surface acid groups

The measured data were integrated through a two-parameter Weibull model [25]. This model is based on the weakest link theory, and it is widely used to analysis and evaluate fiber tensile strength. The failure of carbon fiber can be attributed to the presence of random flaws. In this model, the failure probability of each single-filament carbon fiber P was given by Eq. (1):P=in1

Where n is the data point number and i is the rank of the ith data point ranked from smallest to largest. The failure

Conclusion

Carbon fibers were functionalized with mixed H3PO4/H2SO4/HNO3. This kind of functionalized carbon fibers had preserved tensile strength and improved electrochemical properties. The functionalization degree of carbon fibers was not lower than that of carbon fibers functionalized with existing functionalization method. The PFCF fibers attained good reversibility, lower charge-transfer impedance and higher stable capacity than DCF in the charging and discharging process. Moreover, the proposed

Author contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes

The authors declare no competing financial interest.

References (28)

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