Kraft black Liquor: from a Paper Industry’s by-product towards Energy Storage

Kraft black liquor is the main by-product of the paper industry. It is produced by the eponymous kraft process, which separates the cellulose needed to make paper from the other wood constituents, hemicellulose and lignin. This process is distinguished from others by the use of an alkaline soda solution (NaOH). Here, the black liquor is merely a waste product, the main product being the paper obtained from the white liquor. In so doing, paper mills seek to valorize this black liquor... as much as possible. For the time being, it can only be burned to provide energy for the mill (pressurized water vapor turns electro-generating turbines), and a few low-value-added molecules of interest can be obtained, such as surface-active molecules (soaps) and turpentine.

However, the presence of lignin and hemicellulose fragments in kraft black liquor makes it particularly interesting as a biosourced carbon source, and opens the way to other, much more eco-responsible means of recovery. Certain carbon sources derived from renewable materials, such as coconuts, are of growing academic interest, in part to compensate for the increasing scarcity of fossil fuels. Here, we present a summary of our work to transform a cheap and abundant waste product from the paper industry into a realistic alternative to petro-based carbon sources for the generation of new materials (carbons) dedicated to the energy transition, whether for hydrogen storage or as electrode materials for supercapacitors. Black liquor-derived carbons and their hybridizations offer clear advantages in these topical "energy transition" fields, where the valorization of a co-product of the paper industry is approached in an eco-responsible manner.