Barbara Mecheri

Due to the intermittency of solar and wind energy generation, efficient energy storage solutions are essential to ensure a global transition to renewable energy sources. Bioelectrochemical Power-to-Hydrogen systems are a promising storage pathway, yet their development is limited by high costs and low productivity compared to conventional hydrogen production. Novel, sustainable, and cost-effective materials, such as carbon-based electrodes, can help to overcome these challenges. This study evaluates five cathodes for hydrogen and methane production in microbial electrolysis cells (MECs) operated at 600 and 800 mV: stainless steel mesh (SSM), two custom-made biochars derived from olive mill waste (OMW-1, OMW-2), and two commercial carbon-based materials (Carbon Black and Black Pearls). OMW-1 achieved a H₂ yield of 257 ± 62 mL L^–1 d^–1 at 800 mV, showing the potential of noncommercial biochar. CB and SSM performed better, reaching 493 ± 57 and 496 ± 9 mL L^–1 d^–1 H₂, respectively. Cyclic voltammetry and next-generation sequencing revealed that hydrogen-oxidizing bacteria colonization negatively impacted H₂ yields. At 600 mV, increased CH₄ production was observed for OMW-2, BP, and CB. Energetically, OMW-2 (3.0 ± 0.2 kWh L^–1 d^–1) performed comparably to CB and BP (both 3.3 kWh L^–1 d^–1), outperforming SSM at both voltages. These findings support the viability of carbon-based cathodes as sustainable alternatives to metal-based ones with the potential to reduce electrode costs while maintaining or improving energy productivity.

Redox flow batteries (RFBs) are a promising technology as a grid-level energy storage system and have attracted a growing amount of attention. In these devices, electrochemical storage is carried out through the reduction and oxidation of chemical species. The peculiarity of RFBs is that active species are in solutions, with the reaction occurring at the solid–liquid interface. In the present work, we propose the use of nickel tetra-(4-sulfonatophenyl)­porphyrin (NiTPPS) as an innovative bipolar redox-active molecule (BRM) for aqueous organic redox flow batteries (AORFBs). Thanks to its distinctive redox properties, this single yet complex molecule serves as both an anolyte and a catholyte. This symmetry allows RFBs to use identical components, offering simplified storage and reduced crossover benefits. To increase NiTPPS stability in aqueous solution, we explored the ionic liquid (IL) 1-butylpyridinium tetrafluoroborate (BupyBF₄) as a supporting electrolyte for AORFBs (Huang et al., 2019). Using an IL was also advantageous in broadening the water electrochemical potential window. Actually, by DFT calculations and aggregation studies, carried out by UV–vis spectroscopy, it was observed that the insertion of the metal atom enhances the chemical and electrochemical stability of the porphyrin macrocycle. In addition, the use of BupyBF₄ as a supporting electrolyte improved the resolution of redox processes, avoiding problems associated with water electrolysis and demetalation of the electroactive species.