Peat Derived Carbon – Perfect Fit as an Electrode Material for High Power Supercapacitors and High Capacity Sodium-Ion Batteries
Authors: Paalo Maarja , Adamson Anu , Härmas Meelis , Lust Enn , Janes Alar 
Published: December 2024

In this work, we show how peat, abundant and cheap biomass, can be successfully used as a precursor to synthesize peat-derived carbons (PDCs), applicable as electrode materials both for supercapacitors (SC) and sodium-ion batteries (SIB).

For supercapacitors, high surface area micro- and mesoporous carbon material is needed. For that, ZnCl2 activation method was used. From the comparative data characterizing the material with and without the ZnCl2 activation, it can be seen that mesopores have been formed in noticeable quantities after the ZnCl2 activation step (Fig. 1a), and the S
BET value is about 1270 m2 g─1 for the ZnCl2 activated PDC. Also, the Raman spectra of these two materials (with and without the activation step) show that real graphitization took place during the peat-derived carbon activation process. The peat-derived carbon material (activated with ZnCl2) was tested as an electrode material in a supercapacitor cell, and an EMImBF4 ionic liquid was used as an electrolyte. The electrochemical testing data shows that the material exhibits high capacitance value, approximately 120 F g─1. The constant power test shows that the material is applicable for stationary electricity storage in wind farms and local PV electricity generating fields. Experimental Ragone plots (Fig. 1b) show that very high energy densities (E = 50 Wh kg─1) at moderate power densities (P = 10 kW kg─1) have been measured [1].

A two-step pyrolysis method combined with base-acid solution treatment steps was used to synthesize carbon-rich material for battery anode material with low surface area. Hard carbons were obtained by pre-pyrolysing the peat, treating it with base and acid solutions, and finally, post-pyrolysing the material. A broad range of pre-pyrolysis (300 °C – 800 °C at 100°C intervals) and post-pyrolysis temperatures (1000 °C – 1500 °C at 100°C intervals) were used to optimize the best graphitization degree and level of purity to allow for effective sodium storage. The combination of lowering the pre-pyrolysis temperature and increasing the post-pyrolysis temperature has an enormous effect on the structural properties of the hard carbons while maintaining some of the peat's cellular structure. Using the multiple synthesis steps enables to change the ordering of the carbon structure and impact the surface area – carbon materials with surface areas as low as 6 m2 g─1 were prepared. Peat contains many impurities (mostly Ca and Fe hydroxides), which lower the capacity significantly. However, the base and acid treatment steps are highly effective in removing these impurities from materials that have been previously pyrolyzed at lower temperatures (300–500 °C), enhancing noticeably the half-cell capacity. The base-acid treatment steps of the hard carbons pre-pyrolysed at 450 °C elevate the capacity to 328 mA h g─1. The best results have been obtained for the peat material pre-pyrolysed at 450 °C, treated with KOH and HCl and post-pyrolysed at 1400 °C (PDC-450-1400 A). This material demonstrated great sodium storage capability, achieving a reversible capacity of 330 mA h g─1 with a plateau region of 245 mA h g─1 at 50 mA g─1 current density (350 mA h g─1 with a plateau of 250 mA h g─1 at 25 mA g─1) (Fig. 1c and 1d) and an 80% initial coulombic efficiency. Based on the analysis of the electrochemical results of the half-cells completed, it has been established that peat can successfully be used as a precursor for obtaining the high capacity hard carbon electrode materials for sodium-ion batteries [2].

Acknowledgements

This work was partially supported by the Estonian Ministry of Education and Research (TK210), project „Increasing the knowledge intensity of Ida-Viru entrepreneurship“ co-funded by the European Union (ÕÜF1), Personal Research Grant PRG676, and R&D project EAG228.

References

[1] M. Paalo, M. Härmas, T. Romann, A. Jänes, and E. Lust, Electrochem. Com.
153, 107543, 2023.

[2] A. Adamson, R. Väli, M. Paalo, J. Aruväli, M. Koppel, R. Palm, E. Härk, J. Nerut, T. Romann, E. Lust, and A. Jänes, RSC Adv.
10, 20145, 2020.

Figure 1