Using a Surface Characterization Analyzer for the Characterization of Carbons

This article details the analysis of full gas adsorption/desorption isotherms of carbon black samples by utilizing a Micromeritics 3Flex micropore instrument. BET surface area together with micro-pore, meso-pore and small macro-pore size distributions can be established from the isotherms, in addition to pore area and pore volume.

Application of Carbon in Industries

Throughout many applications and industries, the use of carbons is common. Carbons, usually as carbon blacks, play a key part in a wide scope of energy production and storage devices such as fuel cells, rechargeable batteries, and super-capacitors. They act as membrane materials, electro-catalysts, catalyst supports, and may be present as pure carbon or doped / impregnated with various precious metals or metal oxides.

The utilization of carbon in the form of graphene, carbon black, activated carbon, carbon nano-tubes, graphite etc. are all presently being utilized or explored. Optimization of electrode porosity is crucial as it affects electrode conductivity, the number and availability of active electrode sites, electrolyte transport within the electrodes, and the intercalation of charge carrier species directly. Through minimizing pore blocking cyclability can also be maximized.

Total pore volume and BET surface area are extremely common measurements which feature in the characterization of cathode and anode materials. Yet, the significance of the porous characteristics of raw materials, intermediates and finished electrodes extends beyond these: the pore size distribution and pore area will provide a more complete understanding of the porous nature of the materials so must also be considered.

Measurement of Nitrogen Adsorption and Desorption Isotherms

Three commercially available carbon black powders which are known for their conductive properties and targeted at battery applications were analyzed. Nitrogen adsorption and desorption isotherms were measured by utilizing a micropore-equipped Micromeritics 3Flex instrument.

Using a Micromeritics VacPrep, samples were first degassed for 6 hours at 300 °C at an ultimate vacuum < 100 mTorr. After this they were transferred to the 3Flex and then continued under vacuum at 300 °C for a further 16 hours. Analyses were performed at 77K, with isothermal jackets being utilized for each sample tube, free space was measured with He at the end of the analysis.

A combination of dosing methods was used to measure adsorption isotherms. The fixed dosing option was employed at 1 cc/g STP up to 0.005 P/Po at 45 s equilibration interval and then 0.5 cc/g STP up to 0.01 P/Po at 30 s equilibration interval. The remainder of the adsorption isotherm to 0.995 P/Po and subsequent desorption measurement to 0.05 P/Po were performed by utilizing the incremental dosing method.

After analysis samples were weighed and this mass applied retrospectively to the analysis files before report generation. The linear isotherm plots are shown in Figure 1, Figure 2 shows the logarithmic isotherm plots. The logarithmic plots are especially useful for ascertaining differences in adsorption within the micro-pore region of the isotherms quickly.

Isotherm Linear Plot

Figure 1. Isotherm Linear Plot

Isotherm Log Plot

Figure 2. Isotherm Log Plot

To choose the most appropriate techniques and models for establishing pore area, volume and size data Micromeritics Microactive software was used. Crucial data for characterizing the porous nature of the materials are detailed in the below table, the samples are labeled A to C in order of increasing BET surface area.

Carbon A Carbon B Carbon C
BET Surface area (m2/g) 228 276 279
t-plot micropore surface area (m2/g) 111 159 22
t-plot external surface area (m2/g) 117 116 257
Single point adsorption volume at 0.985 P/Po (cm3/g) 0.42 0.60 0.58
BJH desorption pore volume pores 2-150 nm diameter (cm3/g) 0.34 0.53 0.54
BJH desorption average pore size (nm) 12.8 20.1 10.0
DFT pore volume in pores ≤ 1 nm width 0.08 0.06 0.03

When analyzing carbon materials, BET surface area is universally reported. In addition, de Boer t-plots have also been constructed which allow the contribution of micropore area to total surface area to be calculated. 2D DFT models were applied to account for variation in the aspect ratio of carbon pore geometries.

Pore size, volume and area data were gathered using a combination of techniques: DFT was applied to the Micropore (< 2 nm diameter) range, Figure 3 shows the pore size distribution.

BJH, which can be observed in Figure 4, was applied to mesopore (2-50 nm diameter) and small macropore (50 – 150 nm diameter) ranges. The BET surface area of carbons B and C are almost identical, whilst that of carbon A is 17% less. Yet, the extent of micro-porosity, the pore size and volume distributions, and so the source of surface area are hugely different.

The t-plot data are particularly useful when considering surface area: Carbon C has the lowest contribution of micropore area to overall surface area, around 8% of total surface area being found within micro-pores. On the other hand, carbons A and B have a more equal contribution of micro-pore area to total surface area, 49% and 58% respectively.

Investigation of Micropores Using DFT Technique

Pore size is crucial to a number of applications of carbons as it often directly affects performance and, can be considered through the distribution of pore volume and pore sizes, in addition to pore area. Using the DFT technique micro-pores can be investigated, and this shows the pore size distributions to be similar.

Yet, significantly, carbon A has the highest micropore volume with the majority of micro-pores found in particularly small pores (< 0.7 nm width). Carbon C on the other hand has the lowest micropore volume with pores present within two size ranges: < 0.7 nm and > 0.7 nm width with similar volumes in each. Carbon B can be considered as being intermediate with respect to both micropore volume and size.

DFT dV/dlog(W) Pore Volume vs. Pore Width

Figure 3. DFT dV/dlog(W) Pore Volume vs. Pore Width

The extent of macro-porosity and meso-porosity should also be considered, particularly since these pores are often present as ‘transport pores’, giving access to any micro-pores within a porous network. BJH pore models are especially useful in this respect.

Though the pore volume for carbon A is considerably less than for carbons B and C, all three carbons have appreciable porosity in the range 2 – 150 nm diameter. Whilst the average pore size present in carbon B is double that of carbon C, the volume of pores is very similar for carbons B and C.

BJH Desorption dV/dlog(D) Pore Volume

Figure 4. BJH Desorption dV/dlog(D) Pore Volume

There are some very significant differences in pore sizes and volumes, despite the similarity in BET surface area, which can have profound effects on the performance of the carbons. The separation of micro-pore and meso / macro-pore data is crucial.

The higher micro-porosity of Carbons A and B may supply shortened diffusion pathways through the material and so, quicker electron transfer with better conductivity, plus an abundance of active sites for electrochemical processes.

Conversely, as with Carbon B, higher micro-pore volume located within particularly small pores may make the material more prone to pore clogging and blocking during use, and so decrease lifetime. The relative absence of micro-porosity within Carbon C shows that this material would be least prone to pore clogging and blocking.

Often, meso and macro-porosity supply essential pathways to micro-pores: Therefore, Carbon B could be expected to show better transport of charge carrier species, for example Li+, to and from active sites, as long as the smallest micro-pores do not themselves limit diffusion.

Carbon C could be expected to exhibit good charge carrier transport properties due to appreciable meso-porosity but may have a smaller concentration of active sites because of the relative lack of micro-porosity.

It is probable that selection of the best carbon will depend on the precise application. For example, Carbon A may be more suited to Li-Ion battery cathodes where a balance of micro and meso-porosity is needed with control over micro-pore size. Where doping with, for example, a metallic species is required, it would also be a good choice.

Carbon C appears to be especially suited to anode fabrication throughout a range of device types, where high surface area will aid quick charge carrier transfer. Carbon B may be applicable to either cathode or anode fabrication but effects of the particularly small micro-pores would need more extensive investigation of system performance.

Conclusion

Knowledge of the key differences in porosity is crucial to the understanding of the selection, application and performance of carbon materials. This can only be realized when the entire isotherm is gathered and considered through the application of a variety of pore models.

This article was based on an application paper written by Dr. Darren Lapham, Operations Manager at MCA Services, a UK contract analysis laboratory and consultancy for physical materials characterization since 2002. Darren specializes in techniques of gas adsorption, mercury porosimetry and chemisorption. Darren obtained his Ph.D in Physical Chemistry from the University of Essex in 2000 and has undertaken post-doctoral research at Greenwich University and the University of Cambridge, working on semi-conducting and solid state gas sensors and rechargeable battery technology.

This information has been sourced, reviewed and adapted from materials provided by Micromeritics Instrument Corporation.

For more information on this source, please visit Micromeritics Instrument Corporation.

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