## 7 Application of Graphene-Enhanced PCMs in Battery Packs  
In this section we give an example of a specific practical application using graphene-enhanced PCMs as energy storage for thermal management in battery packs [29]. The goal in this application was to increase PCM's thermal conductivity without degrading its latent heat storage ability. The battery packs consisted of cylindrical Li-ion batteries connected to a charging-discharging setup that delivered continuous charging-discharging cycles of 16A and 5A, respectively. During the pre-set ten charge-discharge cycles, temperature measurements were logged at assigned time intervals using strategically placed thermocouples and a data acquisition system (DAS). Two thermocouples were attached to the cathode and anode ends of a battery cylinder inside of the battery pack; a third thermocouple was attached to the battery pack shell that was acting as the heat sink; and a fourth thermocouple exposed to the ambient environment. The battery packs themselves were prepared with different media, including air (_i.e._, no PCM), pristine paraffin PCM, and hybrid graphene-PCM with different graphene loading fractions. The paraffin PCM used in these battery packs was melted, mixed with graphene solutions (as applicable), and allowed to cool to RT before testing. All experiments followed the same test and setup protocols. The representative results of the tests of the hybrid graphene-PCM reported by [PERSON] and co-workers [29] are presented in Figure 5.  
The empirical results from four different battery pack design configurations are presented in Figure 5. In the battery pack configuration where no PCM is used heat is dissipated via the ambient air and the metal bottom of the pack. This configuration produces the highest temperature rise at \(\Delta T\)\(\sim\)37 \({}^{\circ}\)C inside the battery to the ambient environment as measured by the thermocouples attached to  
Figure 5: **(a)** Optical image of battery pack with six cylindrical Li-ion batteries. The space among the batteries is filled with paraffin PCM or graphene-enhanced PCM for testing under real-life conditions; **(b)** Diagram of the temperature rise inside the Li-ion battery pack during the first ten charging—discharging cycles for the battery pack without PCM (red), with conventional paraffin PCM (blue), with the hybrid graphene-PCM at 1 wt. % loading (orange) and with the hybrid graphene-PCM at 20 wt. % loading (green). Further reduction in the temperature rise is possible through a more robust design of the outside heat sink. The image in the right panel is reprinted with permission from [29]. Copyright 2014 Elsevier. the anode/cathode ends. Adding conventional paraffin PCM as the medium in the second battery pack configuration decreases the temperature rise to \(\sim\)24 \({}^{\circ}\)C. Further decrease in the temperature rise is realized with graphene-enhanced PCM as the medium in these Li-ion battery packs as indicated in the third battery pack configuration. The lowest temperature rise of \(\sim\)10 \({}^{\circ}\)C was measured during the first charge-discharge cycle which then stabilizes to \(\sim\)16 \({}^{\circ}\)C after the third cycle and beyond. Improvement is observed further at a saturation temperature rise of \(\sim\) 13 \({}^{\circ}\)C in the fourth battery pack configuration with a higher graphene loading fraction of the hybrid graphene-PCM. The behavior observed in the dependence of \(\Delta T\) to the number of cycles, seen in Figure 5, reflects the physical cooling mechanism characteristic of conventional PCMs and hybrid graphene-PCMs. The conventional PCM medium absorbs and stores most of the heat, while conducting a small amount to the outer battery pack shell.