Supercapacitors from Waste: Converting Pulp and Paper Mill Waste to Nitrogen Doped Supercapacitors

Abstract

The conversion of waste into energy storage materials has become an important topic in the field of sustainability. The challenge being that new materials from waste should perform better than those that are currently available, be cost effective, environmental friendly and scalable. The was streams from pulp and paper mills make an ideal candidate for upgrading, as they contain a wide range of biomaterials that have the potential to be converted into carbon materials for supercapacitors. The challenge with these waste streams is that they are wet and contain a range of inorganic materials. To overcome this, hydrothermal carbonization was utilized to convert the waste streams into carbonaceous materials. Hydrothermal carbonization is a wet, low temperature, thermochemical carbonization technique that excels in converting wet biomass into carbon products that can be easily activated an converted into carbon electrodes for supercapacitors. Additionally, the carbon materials can be doped with nitrogen, phosphorus, boron and sulfur by simply adding the dopant to the raw material prior to carbonization. Doped carbon materials have displayed improved performance and stability over their non-doped counterparts, as the dopants provide additional charge storage sites, increasing capacitance. In this work we have examined the potential to upgrade black liquor, an inexpensive by product of the pulping process, via hydrothermal carbonization for supercapacitors. These materials have been doped with a range of readily available nitrogen sources (NH4Cl, melamine, urea) to increase the stability and capacitance of the electrode. Additionally, a range of activation agents (KOH, K2CO3, KHCO3 and melamine) were investigated to improve the capacitance and stability. The electrodes were tested in aqueous electrolytes, 4M KOH and 0.5M Na2SO4, with stable voltage windows of 1.1V and 1.8V, respectively. To examine the capacitance and cycle stability, cyclic voltametry and galvanostatic charge-discharge measurements were employed, finding that most materials had a capacitance <190 Fg-1 at 25 mV s-1. The electrochemical variations between the materials were further examined by Step Potential Electrochemical Spectroscopy, which allowed the decoupling of double layer and pseudocapacitance contributions. These were combined with in-depth X-ray Photoelecton Spectroscopy of the surface to draw correlations between pseudocapacitane from the carbon-oxygen/carbon-nitrogen functional groups and capacitance contributions from the double layer.