Greenhouse effect associated to human activity is one of the greatest threads to our society according to the intergovernmental panel on climate change from united nations. Present compromises on CO₂ emission reductions are insufficient to avoid an increase in global temperature 1.5ºC above preindustrial records, as European Union is demanding, and there is a growing probability of crossing the 2ºC limit that is marked as non-return limit for climate change.¹ These facts add more pressure on finding effective and sustainable ways both to transform fossil fuel economy into a green-powered one and to extract the excess greenhouse gasses from the atmosphere.    

While green hydrogen has been considered the best choice for the first objective, several approaches are being considered to fix carbon from CO₂ into valuable products that pays off the effort of the whole process. One of the processes to achieve this objective is using waste to produce products of interest for the industry. As an example, it can be mention the use of 5-hydroxymethylfurfural (HMF), which is a derived species from biomass sugars (either free or obtained from lignocelluloses) that can be either electrochemically oxidised to 2,5-furandicarboxylic acid (FDCA), or reduced to 2,5-bi(hydroxymethyl)furan (BHMF), both components of great interests in the pharmaceutical and polymer industry.

In order to develop more efficient systems and it is key to develop appropriate characterization methods to identify the mechanism that control and limit the performance of our materials and devices. modulated techniques such as Impedance Spectroscopy (IS), Intensity-Modulated Photocurrent Spectroscopy (IMPS) and Intensity-Modulated Photovoltage Spectroscopy (IMVS) are well-established techniques to characterize optoelectronic devices in operation conditions. Here first we will present some advances in the use of IS for the understanding of the origin of the processes occurring in Ni-oxide electrodes during the oxidation of HMF, what allowed to separate redox processes in the Ni from reaction mechanisms of HMF and how they are related with the onset of the different reactions occurring in the electrode.

Then I will show how the IS, IMPS and IMVS may be combined to provide key information that helps to select and fit accurate electrical models that describe the physico-chemical response of optoelectrochemical devices. Several examples of the extraordinary potential of the combination of these three techniques will be shown including the analysis of a silicon sensor and BiVO4 photoelectrodes used for water splitting. Through this analysis the main limitations for the operation of these devices can be identified and, in the particular case of BiVO4 photoelectrodes, the ambipolar response of this electrode for the different illumination side will be unveiled.

[1] https://www.ipcc.ch/