## 4 Results and Discussion
### Stabilization of Graphite
#### 4.1.2 Effect of pH Value on Zeta Potential  
The zeta potential was measured for the concentrations at which a plateau was reached in the viscosity curves, corresponding to 20 m% for PVP and 40 m% for TX-100 (see Figures 2 and 3). Figure 4 shows the influence of the pH value on the zeta potential. The standard deviation was taken from three samples on the basis of three single measurements. The pH value was adjusted by a dosing needle with an internal diameter of 0.61 mm. The measured values do not display regular intervals because the addition of such small quantitites is subject to a certain degree of inaccuracy. Figure 3: Viscosity as a function of TX-100 content for (**a**) an aqueous dispersion containing 10 m% graphite nanoparticles (see Table A3) and (**b**) a TX-100 solution without nanoparticles (see Table A4). The pH value dependence was primarily investigated around the neutral or in the slightly basic range, as previous works had shown that anodic electrode dispersions with a pH value in this range positively influenced the final cell performance [44]. In the acidic range (pH = 4.2), the zeta potential is \(-\)22 mV for the PVP dispersion and \(-\)16 mV for the TX-100 dispersion. A strong decrease in the zeta potential can be observed with increasing pH for both dispersion types. The electrostatic repulsion forces among the particles increase successively, which results in higher stability of the dispersions [38]. For the PVP dispersion, a minimum zeta potential of approximately \(-\)45 mV is attained at a pH value of 9. This was found to correlate well with previous studies. [PERSON] et al. [30] stated that the most suitable pH value with regard to stability is approximately 9, while [PERSON] et al. [26] specified for this a pH value of 9.5. The zeta potential curve of the TX-100 dispersion reaches its minimum at a pH value of 10.75. This is at \(-\)34 mV. The minimum zeta potential indicates that the most stable dispersion is obtained at this pH level. A further increase in the pH value leads to an increase in the zeta potential. This is due to the increased ion concentration in the aqueous medium, according to [PERSON] et al. [26]. This has a compressive effect on the diffuse double layer, leading to a reduced zeta potential. Varying statements regarding the zeta potential and its dependence on stability can be found in the literature. [PERSON] et al. [39] and [PERSON] [1] determined that dispersions with a zeta potential of less than \(-\)30 mV and more than +30 mV are stable. According to this definition, the graphite dispersion with PVP is stable within a pH value range between 8 and 11. The graphite dispersion in the case of TX-100 only appears to be stable for pH values that are between 10.7 and 11. However, [PERSON] et al. [33] stated that even zeta potential values of less than \(-\)15 mV or more than 15 mV can indicate sufficient stabilization. According to this limitation, the graphite dispersions with PVP and TX-100 are stable throughout the entire investigated pH value range of 4 to 11. In this case, no pH value adjustment is required, since the natural pH value of the investigated dispersion is around 4. It is worth mentioning that all of the dispersions with TX-100 displayed isolated agglomerates, characterizing a not-stable state. This makes it clear that a strict classification of dispersions into stable and non-stable according to the given zeta potential limits is not applicable for all dispersion types. It allows for an evaluation of dispersions with regard to electrostatic stabilization, but not to steric stabilization [1]. However, it was observed that reproducible values can only be obtained for electrostatic stabilization if no strong sedimentation occurs. It also appeared that adjusting the pH value in the basic range of dispersions containing TX-100 required the addition of more NaOH than for dispersions with PVP. To gain a deeper understanding of the influencing factors, the correlations between pH value and both dispersant and particle content were investigated.