RESUMO
The photoelectrocatalytic reduction of CO2 (CO2RR) onto bismuth oxyhalides (BiOX, X = Cl, Br, I) was studied through physicochemical and photoelectrochemical measurements. The successful synthesis of the BiOX compounds was carried out through a solvothermal methodology and confirmed by XRD measurements. The morphology was analyzed by SEM; meanwhile, area and pore size were determined through BET area measurements. BiOI and BiOCl present a lower particle size (3.15 and 2.71 µm, respectively); however, the sponge-like morphology presented by BiOI results in an increase in the BET area, which can enhance the catalytic activity of this semiconductor. In addition, DRS measurements allowed us to determine bandgap values of 1.9, 2.4, and 3.6 eV for BiOI, BiOBr, and BiOCl, respectively. Such results predict better visible light harvesting for BiOI. Photoelectrochemical measurements indicated that BiOX shows p-type semiconductor behavior, being the holes the majority charge carriers, making BiOI the most active material to carry out photoelectrocatalytic CO2RR. In the second stage, three different composites, BiOI-Pd, BiOI-Cu, and BiOI-PdCu, (BiOI-M; M = Pd, Cu, PdCu), were fabricated to study the influence of active metal nanoparticles (NP's) in the BiOI CO2RR activity. XRD measurements confirmed the interaction between BiOI and the metallic NP's, the three composites overpassed by 20% the BET area of pristine BiOI. Photoelectrochemical measurements indicate that all BiOI-metal composites are suitable materials to perform CO2 reduction in neutral media efficiently; however, the BiOI-PdCu composites surpassed the faradaic current of BiOI-Pd and BiOI-Cu at 0.85 V vs. RHE (3.15, 2.06 and 2.15 mA cm-2, respectively). BiOI-PdCu presented photoactivity to carry out the CO2 reduction evolving formic acid and acetic acid as the main products under visible-light irradiation.
RESUMO
Bioelectrochemistry has gained importance in recent years for some of its applications on waste valorization, such as wastewater treatment and carbon dioxide conversion, among others. The aim of this review is to provide an updated overview of the applications of bioelectrochemical systems (BESs) for waste valorization in the industry, identifying current limitations and future perspectives of this technology. BESs are classified according to biorefinery concepts into three different categories: (i) waste to power, (ii) waste to fuel and (iii) waste to chemicals. The main issues related to the scalability of bioelectrochemical systems are discussed, such as electrode construction, the addition of redox mediators and the design parameters of the cells. Among the existing BESs, microbial fuel cells (MFCs) and microbial electrolysis cells (MECs) stand out as the more advanced technologies in terms of implementation and R&D investment. However, there has been little transfer of such achievements to enzymatic electrochemical systems. It is necessary that enzymatic systems learn from the knowledge reached with MFC and MEC to accelerate their development to achieve competitiveness in the short term.