Lignin-based batteries

Challenging to produce suitable lignin for flow batteries

Researchers have tested methods for depolymerizing the macromolecule lignin in the search for suitable molecules for redox flow batteries that can be used to balance fluctuations in renewable electricity generation.

There are strong environmental and climate-related reasons to increase the share of renewable electricity in the power grid. However, one challenge is that renewable electricity generation is typically more variable than traditional baseload power. One way to smooth out these fluctuations is through the use of flow batteries. When electricity generation exceeds demand, flow batteries can store electrons in an electrolyte solution, which can then be kept in tanks and used when needed. This provides greater flexibility and enables longer-duration energy storage compared with conventional batteries.

The aim of this project was to develop new natural, water-based organic redox electrolytes derived from lignin for use in redox flow batteries (RFBs). RFB technology has the potential to significantly reduce the cost of stationary energy storage compared with alternatives such as lithium-ion batteries or vanadium-based flow batteries.

Lignin has long been regarded as an attractive starting material for the synthesis of chemicals and fuels. However, it has proven difficult to break this macromolecule down into smaller components that can be readily used for producing fuels and chemicals because of its highly stable structure.

In this project, researchers employed a novel method based on lignin depolymerization. The method, developed at Lund University, enables the oxidative depolymerization of lignin into small molecular fragments with short residence times using heterogeneous catalysts. In this way, molecules with practical applications can be produced efficiently. The resulting solutions were subsequently electrochemically characterized in flow batteries at the KTH Royal Institute of Technology in Stockholm.

The project initially focused on optimizing the oxidative depolymerization process to generate suitable lignin-derived fragments, both directly after depolymerization and following further modification. For the batteries, key performance parameters such as cell resistance, capacity retention, and coulombic efficiency were evaluated, and a techno-economic assessment was carried out.

Improving the properties of lignin proved more challenging than anticipated. Regardless of the type of lignin used, it was difficult to identify appropriate chemical substitution methods to make the molecules more water-soluble, increase their charge-carrying capacity, and enable operation at different redox potentials. Further research aimed at improving the electrochemical reversibility of lignin-derived compounds and exploring other lignin types for RFB applications therefore represents a logical next step.

To facilitate eventual commercial-scale implementation, continued efforts will be required to further improve the performance of these redox electrolyte solutions.

More from the project

The following articles have been produced:

• Lignin-Based Electrolytes for Aqueous Redox Flow Batteries (2024)
• Recent strides toward transforming lignin into plastics and aqueous electrolytes for flow batteries (2024)
• Oxidative Depolymerization of Kraft Lignin to Aromatics Over Bimetallic V–Cu/ZrO2 Catalysts (2023)
• On the Oxidative Valorization of Lignin to High‐Value Chemicals: A Critical Review of Opportunities and Challenges (2022)