Electrochemical Synthesis with Flow Electrodes: An Innovative Process Concept from Biobased Ressources to Polymers
Authors: Padligur Maria Charlotte , Westerfeld Paul , Linkhorst John , Wessling Matthias 
Published: December 2024

Electrochemical production of chemicals aims to enable the direct use of renewable electricity and sustainable resources such as biomass. A promising biobased product is the polymer furan-2,5-dicarboxylate (PEF). This polymer is synthesized from hemicellulosic biomass via the intermediates 5-hydroxymethylfurfural (HMF) and 2,5-furandicarboxylic acid (FDCA). The synthesis step from HMF to FDCA can take place via an electrochemical route, which is focus of this work.
The main limitation of electrochemical processes is restricting the active area to the electrode surface. We aim to overcome this limitation by suspending catalyst-coated particles in the electrolyte - a so-called slurry electrode or flow electrode. Flow electrodes have already successfully found applications in desalination or energy storage, but we want to show that electrochemical synthesis can also benefit from flow electrodes. This is why, in this work, we show that the electrochemical production of FDCA with flow electrodes is possible. For that we deposit the catalyst NiOOH/Ni(OH)2 on a particle substrate. The coated particles, suspended in 0.1M KOH solution, compose the flow electrode. The flow electrode flows through an electrochemical flow cell, where the catalyst coated particles contact the external current supply at the anodic current collector. This current collectors consists of pure nickel, which does not catalyze the electrochemical oxidation of HMF to FDCA. Our results show that the process concept works, and the synthesis of FDCA happens on the particles within the flow electrode.
We investigated the impact of the particle substrate on the reaction. For that we coated the catalyst on different particle substrates (graphite, glass, and nickel) and used them in a flow electrode. The highest conversion can be achieved with graphite particles, whereas the reaction performs significantly worse on glass particles.
We also explore the reaction mechanisms taking place. The results indicate that the reaction mechanism covers two steps: The catalytic oxidation of HMF to FDCA and the electrochemical catalyst regeneration. These steps in the reaction mechanism do not necessarily have to take place simultaneously. The particles can be pre-charged by applying electrical current without reactant HMF in the solution. When HMF is added after the pre-charging step, the catalytic conversion to HMF increases compared to non-pre-charged particles.
Our findings have significant implications for the practical application of the FDCA production process. Flow electrodes allow the process to operate even with fluctuating substrate concentrations. Without HMF, the particles can be charged, converting the inactive nickel species to the active one. Once HMF becomes available again, the particles' catalytic capacity can convert it to FDCA directly. This dynamic operation of the process enhances its efficiency and makes it more adaptable to real-world conditions, potentially paving the way for its large-scale implementation.