Carbonaceous Materials Investigated by Small-Angle X-ray and Neutron Scattering

Eneli Härk,Matthias Ballauff

doi:10.3390/c6040082

Abstract

Carbonaceous nanomaterials have become important materials with widespread applications in battery systems and supercapacitors. The application of these materials requires precise knowledge of their nanostructure. In particular, the porosity of the materials together with the shape of the pores and the total internal surface must be known accurately. Small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS) present the methods of choice for this purpose. Here we review our recent investigations using SAXS and SANS. We first describe the theoretical basis of the analysis of carbonaceous material by small-angle scattering. The evaluation of the small-angle data relies on the powerful concept of the chord length distribution (CLD) which we explain in detail. As an example of such an evaluation, we use recent analysis by SAXS of carbide-derived carbons. Moreover, we present our SAXS analysis on commercially produced activated carbons (ACN, RP-20) and provide a comparison with small-angle neutron scattering data. This comparison demonstrates the wealth of additional information that would not be obtained by the application of either method alone. SANS allows us to change the contrast, and we summarize the main results using different contrast matching agents. The pores of the carbon nanomaterials can be filled gradually by deuterated p-xylene, which leads to a precise analysis of the pore size distribution. The X-ray scattering length density of carbon can be matched by the scattering length density of sulfur, which allows us to see the gradual filling of the nanopores by sulfur in a melt-impregnation procedure. This process is important for the application of carbonaceous materials as cathodes in lithium/sulfur batteries. All studies summarized in this review underscore the great power and precision with which carbon nanomaterials can be analyzed by SAXS and SANS.

Highlights

Electrodes of supercapacitors [1,2,3,4,5,6,7,8,9] and batteries [10,11,12,13,14,15,16,17] are often made of partially ordered carbon powders
The evaluation of the small-angle data relies on the powerful concept of the chord length distribution (CLD) which we explain in detail
Quantitative small-angle scattering experiments provide a powerful tool for the determination of structural parameters and the nanoscopic morphology of carbon materials used in material engineering for energy conversion or for supercapacitors and batteries

Summary

Introduction

Electrodes of supercapacitors [1,2,3,4,5,6,7,8,9] and batteries [10,11,12,13,14,15,16,17] are often made of partially ordered carbon powders. The performance of the supercapacitors is predominantly determined by the properties of the carbon [27,28,29] and structural features, e.g., the specific surface area, pore size, or pore size distribution have been discussed as determining parameters [2,6,22,30,31,32,33]. Most studies of carbon materials used for energy applications have relied on differential pore size distributions obtained from gas sorption isotherms [6,23,24,32,34,35,36,37,38].

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