promising substances. [2] However, HMF can also be electrochemically oxidized to increase its value. Scheme 1 illustrates the individual oxidation products, from HMF to 2,5-diformylfuran (DFF) or 5-hydroxymethyl-2-furancarboxylic acid (HMFCA) to 5-formyl-2-furancarboxylic acid (FFCA) and 2,5-furandicarboxylic acid (FDCA). Of particular economic interest is the end product FDCA, which is used as a monomer for the production of polyethylene furanoate (PEF). [3] PEF is entirely obtainable from renewable raw materials and also possesses thermal, mechanical and barrier properties superior to the currently used petroleum-based polyethylene terephthalate (PET). [4] As an alternative to a thermo-catalytic oxidation of HMF using oxygen, electrocatalysis bears the potential to combine biomass as renewable carbon resource directly with renewable electrical energy. In addition, very benign reaction conditions are possible avoiding the need for oxygen pressure and elevated temperature. [2b] One of the first literature-known electrocatalytic oxidation of HMF to FDCA was carried out in 1991 in a two-chamber electrolysis cell. Skowronski et al. obtained a FDCA yield of 71% by using NiO/Ni(OH) 2 as electrocatalyst in a very basic aqueous solution (1 m NaOH). [5] Despite the promising yield, scientific work on the HMF oxidation did not continue for over two decades. In recent years, however, the topic of the electrochemical conversion of HMF has gained considerable importance and found increasing attention. In 2012, Strasser's group investigated the oxidation of HMF on Pt electrodes and discovered the strong influence of the pH value on the reaction yield. [6] Nowadays, in particular noble-metalfree [7] and stable [8] electrocatalysts attract increasingly attention driven by the need for efficient, selective and robust catalyst systems. The herein performed upgrading of HMF over structured nickel oxide reveals the great dependence of electrocatalysts for oxidation on material morphology and nanostructure. The reaction was conducted in a 3D-printed electrolysis cell with two separated half-cells. The simple synthesis of NiO in the CMK-1 structure led to a significant current density, selectivity and yield increase in comparison with unmodified amorphous NiO.