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Mechanically interlocking redox-active anthraquinone onto single-walled carbon nanotubes (AQ-MINT) gives a new and advanced example of a noncovalent architecture for an electrochemical platform. Electrochemical studies of AQ-MINT as an electrode reveal enhanced electrochemical stability in both aqueous and organic solvents compared to physisorbed AQ-based electrodes. While maintaining the electrochemical properties of the parent anthraquinone molecules, we observe a stable oxygen reduction reaction to hydrogen peroxide (H2O2). Using such AQ-MINT electrodes, 7 and 2 μmol of H2O2 are produced over 8 h under basic and neutral conditions, while the control system of SWCNTs produces 2.2 and 0.5 μmol, respectively. These results reveal the potential of this rotaxane-type immobilization approach for heterogenized electrocatalysis. Fonds zur Förderung der Wissenschaftlichen Forschung Z222-N19 Version of record

Regarding to the increase in population and industry development, global energy demand tends to increase continuously. However, our main source of energy, the fossil fuels, like coal, oil and natural gas, are limited and start running out. Moreover, the combustion of such fossil-based energy carriers gives carbon dioxide (CO2) back to the atmosphere which is considered as the major cause of the rising of global temperature. Therefore, renewable, cleaner and more sustainable energy sources are urgently needed. Converting inexhaustible energy (such as solar and wind energy) into chemicals or artificial fuels offers transportable fuels which are independent from place and time. One promising approach is to convert exhausted CO2 into fuels, for example, formate, methane and methanol using renewable energies like solar and wind. Developed from such idea, this study is aimed to investigate electrochemical systems for the conversion of CO2 to value-added chemicals products by using bio-origin catalysts such as microorganisms and enzymes. Herein, microbial electrolysis cells containing Methylobacterium extorquens, acetogenic bacteria, were developed and their catalytic activities towards the conversion of CO2 to formate were monitored under applying of a constant potential. In this part, direct electron injection and mediator-assisted approaches were investigated. Neutral red, a biocompatible dye, was introduced into the systems as redox mediator in two ways: homogeneous (dissolved in the solution) and heterogeneous (electropolymerized onto the electrode). Electrochemical characterization of such systems was carried out by means of cyclic voltammetry and electrolysis showing electrocatalytic activities towards the CO2 reduction to formate. The performance was found to be enhanced with the aid of mediator in both systems. Further investigation was performed by combining two bioelectrodes, bioanode and biocathode, which were developed from sewage sludge containing mixed culture microorganism, together in bio-electrochemical systems. The systems were developed serving as a wastewater treatment system where organic substances were oxidized at bioanode and CO2 was reduced to methane (CH4) at biocathode. Such system was successfully built up and system performance was investigated by monitoring chemical oxygen demand values and CH4 amount. Additionally, electrode modifications for both electrodes using poly(neutral red) and chitosan approaches were carried out. The results revealed improved overall performance in both organics degradation and CH4 production. In addition, dehydrogenase enzymes were investigated as electrocatalysts in electrochemical catalytic system. The dehydrogenases are redox enzymes that catalyze redox reactions and require sacrificial electron donor such as nicotinamide adenine dinucleotide, reduced form (NADH). Avoiding using such costly cofactor, electrochemical injection of electrons from electrode directly into enzymes active sites is suggested. Efficient immobilization of enzymes onto the electrode was presented by chemical grafting of conjugated dehydrogenases directly onto graphene (as a conductive nanoplatform). Using an alginate hydrogel matrix, we subsequently coated this bio-nano-electrocatalyst onto the electrode. This immobilization technique was employed for single dehydrogenase for the reduction of acetaldehyde to ethanol and furthermore, of three dehydrogenases for the cascade reduction of CO2 all the way to methanol. The enzyme-graphene bio-nano-electrocatalysts show enhanced reductive current delivered into the system, leading to higher production rate. Author Hathaichanok Seelajaroen Universität Linz, Dissertation, 2020 (VLID)4949873

In this report, we present results on the electrocatalytic activity of conducting polymers [polyaniline (PANI) and polypyrrole (PPy)] toward the electrochemical oxygen reduction reaction (ORR) to hydrogen peroxide (H2O2). The electropolymerization of the polymers and electrolysis conditions were optimized for H2O2 production. On flat glassy carbon (GC) electrodes, the faradaic efficiency (FE) for H2O2 production was significantly improved by the polymers. Rotating disc electrode (RDE) studies revealed that this is mainly a result of blocking further H2O2 to the water reduction pathway by the polymers. PPy on carbon paper (CP) significantly increased the molar production of H2O2 by over 250% at an average FE of above 95% compared to bare CP with a FE of 25%. Thus, the polymers are acting as catalysts on the electrode for the ORR, although their catalytic mechanisms differ from other electrocatalysts. Version of record

Using enzymes as bioelectrocatalysts is an important step toward the next level of biotechnology for energy production. In such biocatalysts, a sacrificial cofactor as an electron and proton source is needed. This is a great obstacle for upscaling, due to cofactor instability and product separation issues, which increase the costs. Here, we report a cofactor-free electroreduction of CO2 to a high energy density chemical (methanol) catalyzed by enzyme–graphene hybrids. The biocatalyst consists of dehydrogenases covalently bound on a well-defined carboxyl graphene derivative, serving the role of a conductive nanoplatform. This nanobiocatalyst achieves reduction of CO2 to methanol at high current densities, which remain unchanged for at least 20 h of operation, without production of other soluble byproducts. It is thus shown that critical improvements on the stability and rate of methanol production at a high Faradaic efficiency of 12% are possible, due to the effective electrochemical process from the electrode to the enzymes via the graphene platform.

We report on a self‐assembled system comprising a molecular copper‐porphyrin photoelectrocatalyst, 5‐(4‐carboxy‐phenyl)‐10,15,20‐triphenylporphyrinatocopper(II) (CuTPP‐COOH), covalently bound to self‐organized, anodic titania nanotube arrays (TiO2 NTs) for photoelectrochemical reduction of oxygen. Visible light irradiation of the porphyrin‐covered TiO2 NTs under cathodic polarization up to −0.3 V vs. Normal hydrogen electrode (NHE) photocatalytically produces H2O2 in pH neutral electrolyte, at room temperature and without need of sacrificial electron donors. The formation of H2O2 upon irradiation is proven and quantified by direct colorimetric detection using 4‐nitrophenyl boronic acid (p‐NPBA) as a reactant. This simple approach for the attachment of a small molecular catalyst to TiO2 NTs may ultimately allow for the preparation of a low‐cost H2O2 evolving cathode for efficient photoelectrochemical energy storage under ambient conditions.

Microbial electrosynthetic cells containing Methylobacterium extorquens were studied for the reduction of CO2 to formate by direct electron injection and redox mediator‐assisted approaches, with CO2 as the sole carbon source. The formation of a biofilm on a carbon felt (CF) electrode was achieved while applying a constant potential of −0.75 V versus Ag/AgCl under CO2‐saturated conditions. During the biofilm growth period, continuous H2 evolution was observed. The long‐term performance for CO2 reduction of the biofilm with and without neutral red as a redox mediator was studied by an applied potential of −0.75 V versus Ag/AgCl. The neutral red was introduced into the systems in two different ways: homogeneous (dissolved in solution) and heterogeneous (electropolymerized onto the working electrode). The heterogeneous approach was investigated in the microbial system, for the first time, where the CF working electrode was coated with poly(neutral red) by the oxidative electropolymerization thereof. The formation of poly(neutral red) was characterized by spectroscopic techniques. During long‐term electrolysis up to 17 weeks, the formation of formate was observed continuously with an average Faradaic efficiency of 4 %. With the contribution of neutral red, higher formate accumulation was observed. Moreover, the microbial electrosynthetic cell was characterized by means of electrochemical impedance spectroscopy to obtain more information on the CO2 reduction mechanism. Refereed/Peer-reviewed Version of record