Funding and Acknowledgment: We thank the analytical services from the chemistry department of the Hamburg university, especially the central elemental analysis service around Dr. Dirk Eifler, the NMR-service around Dr. Thomas Hackl, Ute Gralla for measuring numerous Raman samples and Thomas Marx from the research group of Prof. Dr. Peter Burger for lending the equipment and assisting with electrochemical measurements. Part of this work was supported by the Cluster of Excellence Fuel Science Center (EXC 2186, ID: 390919832) funded by the Excellence Initiative by the German federal and state governments to promote science and research at German universities as well as by German Research Foundations in the frame of NFDI4CAT (ID 441926934). We also acknowledge funding by the Federal Ministry of Food and Agriculture granted by the Agency for Renewable Resources (FNR, 2220NR101X). S. D. Mürtz thanks Cusanuswerk e.V. for funding.

The dataset covers the research data of the publication in ChemCatChem. (doi.org/10.1002/cctc.202301632) with the title "Transition-metal substituted polyoxometalates as soluble redox mediators in electrocatalytic biomass conversion". The provided data comprehend the measured current density, CVs, HPLC data, NMR data, UV VIS data as well as a python script for plotting.