Global warming, as a consequence of the dependence on the use of fossil fuels, is one of the major problems that our current society is facing. For this reason, the production of energy by renewable sources has become a great challenge for the scientific community. Thus, researchers have recently focused their efforts on the development of new technologies that guarantee the production of high amounts of energy with low to minimal carbon footprint. Intermittence of renewable energies is a present challenge that requires to develop technologies able to store huge amounts of energy to be easily supplied at periods of low or zero energy production. In this scenario, (photo)electrochemical technologies have become an attractive candidate with the purpose of producing green fuels in order to store energy at the moments of excess of production. Research in the field of (photo) electrocatalysis has mainly been focused on the generation of green H₂, from the oxygen evolution reaction (OER) through the oxidation of H₂O; and the reduction of CO₂ (CO2RR) to obtain green fuels.[1] in these studies, notable advances have been made in recent years, both in terms of understanding the mechanisms of reactivity, as well as the selectivity of the processes activated by sunlight.[2,3]  In both cases, O₂ is obtained as by-product at the anode, with a very low price at the market, thus reducing the economic viability and the interest in this technology.[4] With the aim to store energy at the same time that obtaining compounds with added value, several organic transformations have been pointed out as alternative to the OER^.at the anode and H₂ production or CO2RR at the cathode. [5,6]

One very interesting alternative anodic reaction is based on the oxidation of lignin or compounds from biomass, since these species also allow reducing overpotentials while obtaining valuable products for the chemical industry, with the subsequent biomass revalorization. 5-hydroxymethylfurfural (HMF) is a biomass derived species, which can be easily obtained from C6 natural sugars. The selective oxidation of one of the hydroxyl groups of HMF leads to 2,5-furandicarbox-aldehyde (DFF), which can be used as a monomer in the synthesis of various polymers based in furan; [7,8] while further oxidation of HMF gives rise to 2,5-furandicarboxylic acid (FDCA), a very important component in the pharmaceutical and polymer industry, which can replace the monomers traditionally used in the synthesis of ethylene terephthalate (PET).[9] Besides, the reduction of HMF can lead to 2,5-bi(hydroxymethyl)furan (BHMF), a species that can be used as a precursor for the synthesis of many bio-based polymers, [10] consequently making this process very interesting as alternative to H₂ production or CO2RR in photoelectrochemical processes.

In our project we have performed the transformation of HMF to added value products by means of electrochemistry. The electrochemical oxidation of HMF to 2,5 FDCA has been achieved using easily made and inexpensive NiO-OH electrodes, obtained by Ni electrodeposition on pencil graphite rods (Ni/PGR). We have optimized the reaction conditions to prevent HMF degradation, leading to a nearly complete conversion of HMF into FDCA, with 88% Faradaic Efficiency (FE) and very low degradation in less than 2 hours. The electrochemical reduction of HMF to BHMF has been achieved using Cu foils electrodes decorated with Ag deposition using the galvanic replacement technique. BHMF has been obtained with 89% yield and 87% FE at pH 9 media in less than 3 hours. Impedance Spectroscopy (IS) has been analysed in both electrochemical processes to elucidate the electron transfer mechanism and the adsorption of reactants and intermediate species on the surface of the electrodes.