From Batch to Flow: Paired Electrolysis of H2 and Electrochemical Conversion of Biobased 5-HMF in a Flow Reactor
Authors: Brösgen Linda , Kunkel Robin , Melke Julia 
Published: December 2024

Flow reactors are an attractive process for a variety of applications in the chemical industry. This is due to advantages over batch processes, such as good scalability and constant product quality with precisely controllable reaction conditions. [1] Regarding the industrial transition towards a climate-friendly and CO2-neutral economy, electrolysis and electrosynthesis processes are a focus of current research. These make it possible to use electricity from renewable energies directly and at the same time fulfil several criteria of green chemistry [2]. However, they currently often require the use of platinum group metals (PGM) and need separators that are mostly based on perfluorinated and polyfluorinated alkyl substances (PFAS) [3]. In addition, conventional electrolysis processes used to produce green hydrogen require high overvoltage. These can be traced back to the oxygen evolution reaction (OER). Furthermore, oxygen is not a value-added product. For these reasons, there is increasing interest in coupled systems in which the sluggish OER is replaced by a thermodynamically more efficient reaction that also generates a value-added product. One possibility for this is the electrooxidation of biobased 5-HMF to the platform chemical 2,5-furandicarboxylic acid (FDCA) [4]. A theoretical cell voltage of 0.3 V is required for the FDCAER/HER, whereas the OER/HER requires 1.23 V [5]. [6] The simultaneous production of platform chemicals and green H2 by paired electrolysis is a promising approach to enable sustainable and energy-efficient systems. Currently, this technology is still in the development stage and studies have so far focused on batch cell trials. [4,5,7] However, the use of flow reactors can further reduce overvoltage’s. This is due to factors such as better mass transport, an optimized reactor-volume/electrode ratio and minimized electrode gaps. In addition, more stable process conditions and a simplified scale-up point in favor of transferring to a flow system as early as possible [8,9]. [4–14]
In previous experiments H-cell experiments for electrochemical 5-HMF conversion to FDCA were carried out on nickel base materials [14]. On this foundation, a modular flow-through test cell was developed on a laboratory scale and the feasibility of coupled electrolysis in the flow reactor in recirculating operation was investigated. Particular attention was paid to mild operating conditions (e.g. mild alkaline environment, low operating pressures and low temperatures). For this purpose, polarization curves followed by bulk electrolysis were recorded and the 5-HMF conversion was monitored analytically using UV-VIS. The measurements were carried out on Ni-foam. Subsequently, experiments were carried out to switch from recirculation mode to single-pass operation. For this purpose, Ni-foam and NiOOH foam electrodes were prepared electrochemically and investigated at different flow rates, current densities, and reactant concentrations in the flow cell. The bulk electrolysis was accompanied by analysis (HPLC, UV-VIS) to quantify and classify the derivatives of the electrosynthesis process. These experiments yielded important results about the system, which enabled operational adjustments and a scale-up.
In general, this research is advancing the development of efficient and sustainable energy sources and the synthesis of platform molecules using renewable raw materials in a hydrogen co-electrolysis.

Acknowledgments

The authors thank the European Union's Horizon Europe research and innovation program under grant agreement N° 101070856 ELOBIO (Electrolysis of Biomass) for funding.

References

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