Anodic microbial community analysis of microbial fuel cells based on enriched inoculum from freshwater sediment.

The characterization of anodic microbial communities is of great importance in the study of microbial fuel cells (MFCs). These kinds of devices mainly require a high abundance of anode respiring bacteria (ARB) in the anode chamber for optimal performance. This study evaluated the effect of different enrichments of environmental freshwater sediment samples used as inocula on microbial community structures in MFCs. Two enrichment media were compared: ferric citrate (FeC) enrichment, with the purpose of increasing the ARB percentage, and general enrichment (Gen). The microbial community dynamics were evaluated by polymerase chain reaction followed by denaturing gradient gel electrophoresis (PCR-DGGE) and real time polymerase chain reaction (qPCR). The enrichment effect was visible on the microbial community composition both during precultures and in anode MFCs. Both enrichment approaches affected microbial communities. Shannon diversity as well as ß-Proteobacteria and γ-Proteobacteria percentages decreased during the enrichment steps, especially for FeC (p < 0.01). Our data suggest that FeC enrichment excessively reduced the diversity of the anode community, rather than promoting the proliferation of ARB, causing a condition that did not produce advantages in terms of system performance.

Electrodes/electrolyte interfaces in the presence of a surface-modified photopolymer electrolyte: application in dye-sensitized solar cells.

Since hundreds of studies on photoanodes and cathodes show that the electrode/electrolyte interfaces represent a key aspect at the base of dye-sensitized solar cell (DSSC) performances, it is reported here that these interfaces can be managed by a smart design of the spatial composition of quasi-solid electrolytes. By means of a cheap, rapid, and green process of photoinduced polymerization, composition-tailored polymer electrolyte membranes (PEMs) with siloxane-enriched surfaces are prepared, and their properties are thoroughly described. When assembled in DSSCs, the interfacial action promoted by the composition-tailored PEMs enhances the photocurrent and fill factor values, thus increasing the global photovoltaic conversion efficiency with respect to the non-modified PEMs. Moreover, the presence of the siloxane-chain-enriched surface increases the hydrophobicity and reduces the water vapor permeation into the device, thus enhancing the cell's durability.

Comparison of photocatalytic and transport properties of TiO2 and ZnO nanostructures for solar-driven water splitting.

Titanium dioxide (TiO2) and zinc oxide (ZnO) nanostructures have been widely used as photo-catalysts due to their low-cost, high surface area, robustness, abundance and non-toxicity. In this work, four TiO2 and ZnO-based nanostructures, i.e. TiO2 nanoparticles (TiO2 NPs), TiO2 nanotubes (TiO2 NTs), ZnO nanowires (ZnO NWs) and ZnO@TiO2 core-shell structures, specifically prepared with a fixed thickness of about 1.5 µm, are compared for the solar-driven water splitting reaction, under AM1.5G simulated sunlight. Complete characterization of these photo-electrodes in their structural and photo-electrochemical properties was carried out. Both TiO2 NPs and NTs showed photo-current saturation reaching 0.02 and 0.12 mA cm(-2), respectively, for potential values of about 0.3 and 0.6 V vs. RHE. In contrast, the ZnO NWs and the ZnO@TiO2 core-shell samples evidence a linear increase of the photocurrent with the applied potential, reaching 0.45 and 0.63 mA cm(-2) at 1.7 V vs. RHE, respectively. However, under concentrated light conditions, the TiO2 NTs demonstrate a higher increase of the performance with respect to the ZnO@TiO2 core-shells. Such material-dependent behaviours are discussed in relation with the different charge transport mechanisms and interfacial reaction kinetics, which were investigated through electrochemical impedance spectroscopy. The role of key parameters such as electronic properties, specific surface area and photo-catalytic activity in the performance of these materials is discussed. Moreover, proper optimization strategies are analysed in view of increasing the efficiency of the best performing TiO2 and ZnO-based nanostructures, toward their practical application in a solar water splitting device.

The Effect of Sulfur and Nitrogen Doping on the Oxygen Reduction Performance of Graphene/Iron Oxide Electrocatalysts Prepared by Using Microwave-Assisted Synthesis.

The synthesis of novel catalysts for the oxygen reduction reaction, by means of a fast one-pot microwave-assisted procedure, is reported herein and deeply explained. In particular, the important role of doping atoms, like sulfur and nitrogen, in Fe2O3-reduced graphene oxide nanocomposites is described to address the modification of catalytic performance. The presence of dopants is confirmed by X-ray Photoelectron Spectroscopy analysis, while the integration of iron oxide nanoparticles, by means of decoration of the graphene structure, is corroborated by electron microscopy, which also confirms that there is no damage to the graphene sheets induced by the synthesis procedure. The electrochemical characterizations put in evidence the synergistic catalysis effects of dopant atoms with Fe2O3 and, in particular, the importance of sulfur introduction into the graphene lattice. Catalytic performance of as-prepared materials toward oxygen reduction shows values close to the Pt/C reference material, commonly used for fuel cell and metal-air battery applications.

Consistent static and small-signal physics-based modeling of dye-sensitized solar cells under different illumination conditions.

A numerical device-level model of dye-sensitized solar cells (DSCs) is presented, which self-consistently couples a physics-based description of the photoactive layer with a compact circuit-level description of the passive parts of the cell. The opto-electronic model of the nanoporous dyed film includes a detailed description of photogeneration and trap-limited kinetics, and a phenomenological description of nonlinear recombination. Numerical simulations of the dynamic small-signal behavior of DSCs, accounting for trapping and nonlinear recombination mechanisms, are reported for the first time and validated against experiments. The model is applied to build a consistent picture of the static and dynamic small-signal performance of nanocrystalline TiO2-based DSCs under different incident illumination intensity and direction, analyzed in terms of current-voltage characteristic, Incident Photon to Current Efficiency, and Electrochemical Impedance Spectroscopy. This is achieved with a reliable extraction and validation of a unique set of model parameters against a large enough set of experimental data. Such a complete and validated description allows us to gain a detailed view of the cell collection efficiency dependence on different operating conditions. In particular, based on dynamic numerical simulations, we provide for the first time a sound support to the interpretation of the diffusion length, in the presence of nonlinear recombination and non-uniform electron density distribution, as derived from small-signal characterization techniques and clarify its correlation with different estimation methods based on spectral measurements.

Charge transport improvement employing TiO2 nanotube arrays as front-side illuminated dye-sensitized solar cell photoanodes.

TiO(2) nanotube (NT) arrays with different lengths were fabricated by anodic oxidation of Ti foil and free-standing NT membranes were detached by the metal substrate and bonded on the fluorine-doped tin oxide surface implementing an easy procedure. Morphology of the as-grown material and of the prepared photoanode was investigated by means of electron microscopy, deepening the investigation on the thermal treatment effect. Crystalline orientation and exposed surface area were studied by X-ray diffraction and Brunauer-Emmett-Teller measurements, showing suitable characteristics for the application in dye-sensitized solar cells (DSCs). DSCs were assembled employing a microfluidic housing system. The cell performances and the electron transport properties as a function of the tube length, before and after a TiCl(4) treatment, were characterized by I-V electrical measurements, incident photon-to-electron conversion efficiency, electrochemical impedance spectroscopy and open circuit voltage decay. Fitting the impedance spectra with an equivalent circuit, it was possible to obtain information on the electron diffusion properties into the TiO(2) nanotubes. A comparison with the charge transport properties evaluated in nanoparticle-based photoanodes witnesses a noteworthy increase of electron lifetime and diffusion length, yielding an overall power conversion efficiency up to 7.56%.

A UV-crosslinked polymer electrolyte membrane for quasi-solid dye-sensitized solar cells with excellent efficiency and durability.

Here we report on a novel polymer electrolyte membrane for quasi-solid dye-sensitized solar cells (DSSCs) with excellent efficiency and extended durability. The electrolyte is prepared by an elegant, rapid and cheap UV-induced polymerization method and the chemometric approach is used for the first time, to the best of our knowledge, for the optimization and the fine tuning of the experimental conditions.

Combined experimental and theoretical investigation of the hemi-squaraine/TiO2 interface for dye sensitized solar cells.

A simple hemi-squaraine dye (CT1) has been studied as a TiO2 sensitizer for application in dye sensitized solar cells (DSCs) by means of a combined experimental and theoretical investigation. This molecule is a prototype dye presenting an innovative anchoring group: the squaric acid moiety. Ab initio calculations based on Density Functional Theory (DFT) predict that this acid spontaneously deprotonates at the anatase (101) surface forming chemical bonds that are stronger than the ones formed by other linkers (e.g. cathecol and isonicotinic acid). Moreover an analysis of the electronic structure of the hybrid interface reveals the formation of a type II heterostructure ensuring adiabatic electron transfer from the molecule to the oxide. DSCs containing hemi-squaraine dyes were assembled, characterized and their performances compared to state of the art cells. Experimental results (large incident photon-to-electron conversion efficiency and an efficiency of 3.54%) confirmed the theoretical prediction that even a simple hemi-squaraine is an effective sensitizer for TiO2. Our study paves the way to the design of more efficient sensitizers based on a squaric acid linker and specifically engineered to absorb light in a larger part of the visible range.

Cu2O/SnO2 Heterostructures: Role of the Synthesis Procedure on PEC CO2 Conversion.

Addressing the urgent need to mitigate increasing levels of CO2 in the atmosphere and combat global warming, the development of earth-abundant catalysts for selective photo-electrochemical CO2 conversion is a central and pressing challenge. Toward this purpose, two synthetic strategies for obtaining a Cu2O-SnO2 catalyst, namely co-precipitation and core-shell methods, were compared. The morphology and band gap energy of the synthesized materials were strongly different. The photoactivity of the core-shell catalyst was improved by 30% compared to the co-precipitation one, while its selectivity was shifted towards C1 products such as CO and formate. The stability of both catalysts was revealed by an easy and fast EIS analysis, indicating how the effective presence of a SnO2 shell could prevent the modification of the crystalline phase of the catalyst during PEC tests. Finally, directing the selectivity depending on the synthesis method used to produce the final Cu2O-SnO2 catalyst could possibly be implemented in syngas and formate transformation processes, such as hydroformylation or the Fischer-Tropsch process.

Optimizing the Performance of Low-Loaded Electrodes for CO2-to-CO Conversion Directly from Capture Medium: A Comprehensive Parameter Analysis.

Gas-fed reactors for CO2 reduction processes are a solid technology to mitigate CO2 accumulation in the atmosphere. However, since it is necessary to feed them with a pure CO2 stream, a highly energy-demanding process is required to separate CO2 from the flue gasses. Recently introduced bicarbonate zero-gap flow reactors are a valid solution to integrate carbon capture and valorization, with them being able to convert the CO2 capture medium (i.e., the bicarbonate solution) into added-value chemicals, such as CO, thus avoiding this expensive separation process. We report here a study on the influence of the electrode structure on the performance of a bicarbonate reactor in terms of Faradaic efficiency, activity, and CO2 utilization. In particular, the effect of catalyst mass loading and electrode permeability on bicarbonate electrolysis was investigated by exploiting three commercial carbon supports, and the results obtained were deepened via electrochemical impedance spectroscopy, which is introduced for the first time in the field of bicarbonate electrolyzers. As an outcome of the study, a novel low-loaded silver-based electrode fabricated via the sputtering deposition technique is proposed. The silver mass loading was optimized by increasing it from 116 µg/cm2 to 565 µg/cm2, thereby obtaining an important enhancement in selectivity (from 55% to 77%) and activity, while a further rise to 1.13 mg/cm2 did not provide significant improvements. The tremendous effect of the electrode permeability on activity and proficiency in releasing CO2 from the bicarbonate solution was shown. Hence, an increase in electrode permeability doubled the activity and boosted the production of in situ CO2 by 40%. The optimized Ag-electrode provided Faradaic efficiencies for CO close to 80% at a cell voltage of 3 V and under ambient conditions, with silver loading of 565 µg/cm2, the lowest value ever reported in the literature so far.

High efficiency dye-sensitized solar cells exploiting sponge-like ZnO nanostructures.

Sponge-like nanostructured ZnO layers were successfully employed as photoanodes for the fabrication of highly efficient dye-sensitized solar cells. The sponge-like ZnO layers were obtained by room temperature radio-frequency magnetron sputtering deposition of metallic zinc, followed by thermal oxidation treatment in an ambient atmosphere. The porous films show a 3D branched nanomorphology, with a feature similar to natural coral. The morphological and optical properties of these layers were studied through field emission scanning electron microscopy, specific surface area measurements, ultraviolet-visible transmittance and absorption spectroscopy. The sponge-like ZnO film presents a high density of branches, with a relatively high specific surface area value, and fine optical transmittance. The morphology of the porous structure provides a high number of adsorption sites for the anchoring of sensitizer molecules, making it suitable for the fabrication of ZnO-based photoanodes for dye-sensitized solar cells. The light harvesting performance of the sensitized semiconductor was evaluated by current density vs. voltage measurements, incident photon-to-electron conversion efficiency, open circuit voltage decay and impedance spectroscopy. The modelling of the electrical characteristics evidences a higher electron lifetime and a longer charge diffusion length, if compared to standard TiO(2) nanoparticle based photoanodes. For ZnO films with a thickness up to 18 µm, a photoconversion efficiency as high as 6.67% and a maximum value of the incident photon-to-electron collection efficiency equal to 87% at 530 nm were demonstrated.

A green and easy-to-assemble electrochemical biosensor based on thylakoid membranes for photosynthetic herbicides detection.

In this study, we report on an easy-to-assemble amperometric electrochemical biosensor incorporating thylakoid membranes for the detection of photosynthetic herbicides. These molecules interfere with the light-induced photosynthetic electron transport occurring at the level of the photosystems within the thylakoid membranes, thus reducing the current of the associated bioelectrode. Thylakoid membranes isolated from pea plants were adsorbed directly on a bare carbon paper working electrode and placed in the measurement cell in the absence of any electrochemical mediator, obtaining a fully environmental-friendly biodevice capable of photocurrent densities up to 14 µA/cm2. Three photosynthetic herbicides inhibiting Photosystem II and belonging to different chemical classes, namely diuron, terbuthylazine and metribuzin, were detected by measuring the electrode photocurrent, which decreased reproducibly in a concentration-dependent manner in a range between 10-7 - 5 × 10-5 M of each herbicide. The limit of detection for the three herbicides was between 4-6 × 10-7 M. Storage stability tests revealed for the biosensor a half-life longer than 15 days at 4 °C and full stability up to 4 months at -80 °C. This study provides a simple, environmental-friendly and cost-effective procedure for the fabrication of a mediatorless carbon paper-based electrochemical biosensor characterized by high photocurrents, long storage stability, reproducible detections and good sensitivity.

Insights into the Electrochemical Reduction of 5-Hydroxymethylfurfural at High Current Densities.

The electrocatalytic reduction of 5-hydroxymethylfurfural (HMF) is highly selective to 2,5-bishydroxymethylfuran (BHMF) at pH=9.2, diluted HMF solutions, and low current densities. In this work, the electrochemical reduction of 0.05 m HMF solutions was investigated in the 5-50 mA cm-2 current density range over an AgCu foam electrocatalyst. The selectivity towards the formation of BHMF or the dimerization depended on the current density, likely due to differences in the electrode potential, and on the reaction time. Operating at current densities of 40-50 mA cm-2 allowed to find a trade-off between HMF and H2 O activation, achieving 85 % BHMF selectivity and fostering the productivity (0.567 mmol cm-2 h-1 ), though co-producing H2 . The electrochemical characterization by Tafel slopes and electrochemical impedance spectroscopy indicated that the HMF reduction was kinetically favored in comparison to the hydrogen evolution reaction and that the process was limited by charge transfer.

Electrochemical Reduction of CO2 With Good Efficiency on a Nanostructured Cu-Al Catalyst.

Carbon monoxide (CO) and formic acid (HCOOH) are suggested to be the most convenient products from electrochemical reduction of CO2 according to techno-economic analysis. To date, tremendous advances have been achieved in the development of catalysts and processes, which make this research topic even more interesting to both academic and industrial sectors. In this work, we report nanostructured Cu-Al materials that are able to convert CO2 to CO and HCOOH with good efficiency. The catalysts are synthesized via a green microwave-assisted solvothermal route, and are composed of Cu2O crystals modified by Al. In KHCO3 electrolyte, these catalysts can selectively convert CO2 to HCOOH and syngas with H2/CO ratios between 1 and 2 approaching one unit faradaic efficiency in a wide potential range. Good current densities of 67 and 130 mA cm-2 are obtained at -1.0 V and -1.3 V vs. reversible hydrogen electrode (RHE), respectively. When switching the electrolyte to KOH, a significant selectivity up to 20% is observed for C2H4 formation, and the current densities achieve 146 and 222 mA cm-2 at -1.0 V and -1.3 V vs. RHE, respectively. Hence, the choice of electrolyte is critically important as that of catalyst in order to obtain targeted products at industrially relevant current densities.

Living Bacteria Directly Embedded into Electrospun Nanofibers: Design of New Anode for Bio-Electrochemical Systems.

The aim of this work is the optimization of electrospun polymeric nanofibers as an ideal reservoir of mixed electroactive consortia suitable to be used as anodes in Single Chamber Microbial Fuel Cells (SCMFCs). To reach this goal the microorganisms are directly embedded into properly designed nanofibers during the electrospinning process, obtaining so called nanofiber-based bio-composite (bio-NFs). This research approach allowed for the designing of an advanced nanostructured scaffold, able to block and store the living microorganisms inside the nanofibers and release them only after exposure to water-based solutions and electrolytes. To reach this goal, a water-based polymeric solution, containing 5 wt% of polyethylene oxide (PEO) and 10 wt% of environmental microorganisms, is used as the initial polymeric solution for the electrospinning process. PEO is selected as the water-soluble polymer to ensure the formation of nanofiber mats offering features of biocompatibility for bacteria proliferation, environment-friendliness and, high ionic conductivity. In the present work, bio-NFs, based on living microorganisms directly encapsulated into the PEO nanofiber mats, were analyzed and compared to PEO-NFs made of PEO only. Scanning electron microscopy allowed researchers to confirm the rise of a typical morphology for bio-NFs, evidencing the microorganisms' distribution inside them, as confirmed by fluorescence optical microscopy. Moreover, the latter technique, combined with optical density measurements, allowed for demonstrating that after electrospinning, the processed microorganisms preserved their proliferation capability, and their metabolic activity after exposure to the water-based electrolyte. To demonstrate that the energy-production functionality of exo-electrogenic microorganisms was preserved after the electrospinning process, the novel designed nanomaterials, were directly deposited onto carbon paper (CP), and were applied as anode electrodes in Single Chamber Microbial Fuel Cells (SCMFCs). It was possible to appreciate that the maximum power density reached by bio-NFs, which resulted in being double of the ones achieved with PEO-NFs and bare CP. SCMFCs with bio-NFs applied as anodic electrodes reached a current density value, close to (250 ± 5.2) mA m-2, which resulted in being stable over time and was comparable with the one obtained with carbon-based electrode, thus confirming the good performance of the whole device.

Microwave-Assisted Synthesis of Nitrogen and Sulphur Doped Graphene Decorated with Antimony Oxide: An Effective Catalyst for Oxygen Reduction Reaction.

In this study, we report on the facile synthesis of a novel electrocatalysts for the oxygen reduction reaction (ORR), based on reduced graphene oxide (RGO), functionalized with metallic and non-metallic elements. In particular, thanks to a fast one-pot microwave-assisted procedure, we induced, in the RGO graphene lattice, a combined doping with nitrogen and sulphur, and the simultaneous decoration with antimony oxide nanocrystals. The multi-doped-decorated material shows enhanced catalytic performance towards ORR, with respect to common nitrogen- or sulphur-doped carbon-based materials. The presence of co-doping is confirmed by transmission electron microscopy and X-ray photoelectron spectroscopy analysis. The detailed electrochemical characterization shows the simultaneous effects of dopant atoms on the catalytic behavior. In particular, the importance of nitrogen and sulphur atoms in driving the oxygen absorption, together with the role of antimony in enhancing the electrochemical performance toward the ORR, are discussed.

CuZnAl-Oxide Nanopyramidal Mesoporous Materials for the Electrocatalytic CO2 Reduction to Syngas: Tuning of H2/CO Ratio.

Inspired by the knowledge of the thermocatalytic CO2 reduction process, novel nanocrystalline CuZnAl-oxide based catalysts with pyramidal mesoporous structures are here proposed for the CO2 electrochemical reduction under ambient conditions. The XPS analyses revealed that the co-presence of ZnO and Al2O3 into the Cu-based catalyst stabilize the CuO crystalline structure and introduce basic sites on the ternary as-synthesized catalyst. In contrast, the as-prepared CuZn- and Cu-based materials contain a higher amount of superficial Cu0 and Cu1+ species. The CuZnAl-catalyst exhibited enhanced catalytic performance for the CO and H2 production, reaching a Faradaic efficiency (FE) towards syngas of almost 95% at -0.89 V vs. RHE and a remarkable current density of up to 90 mA cm-2 for the CO2 reduction at -2.4 V vs. RHE. The physico-chemical characterizations confirmed that the pyramidal mesoporous structure of this material, which is constituted by a high pore volume and small CuO crystals, plays a fundamental role in its low diffusional mass-transfer resistance. The CO-productivity on the CuZnAl-catalyst increased at more negative applied potentials, leading to the production of syngas with a tunable H2/CO ratio (from 2 to 7), depending on the applied potential. These results pave the way to substitute state-of-the-art noble metals (e.g., Ag, Au) with this abundant and cost-effective catalyst to produce syngas. Moreover, the post-reaction analyses demonstrated the stabilization of Cu2O species, avoiding its complete reduction to Cu0 under the CO2 electroreduction conditions.

Zn- and Ti-Doped SnO2 for Enhanced Electroreduction of Carbon Dioxide.

The electrocatalytic reduction of CO2 into useful fuels, exploiting rationally designed, inexpensive, active, and selective catalysts, produced through easy, quick, and scalable routes, represents a promising approach to face today's climate challenges and energy crisis. This work presents a facile strategy for the preparation of doped SnO2 as an efficient electrocatalyst for the CO2 reduction reaction to formic acid and carbon monoxide. Zn or Ti doping was introduced into a mesoporous SnO2 matrix via wet impregnation and atomic layer deposition. It was found that doping of SnO2 generates an increased amount of oxygen vacancies, which are believed to contribute to the CO2 conversion efficiency, and among others, Zn wet impregnation resulted the most efficient process, as confirmed by X-ray photoelectron spectroscopy analysis. Electrochemical characterization and active surface area evaluation show an increase of availability of surface active sites. In particular, the introduction of Zn elemental doping results in enhanced performance for formic acid formation, in comparison to un-doped SnO2 and other doped SnO2 catalysts. At -0.99 V versus reversible hydrogen electrode, the total faradaic efficiency for CO2 conversion reaches 80%, while the partial current density is 10.3 mA cm-2. These represent a 10% and a threefold increases for faradaic efficiency and current density, respectively, with respect to the reference un-doped sample. The enhancement of these characteristics relates to the improved charge transfer and conductivity with respect to bare SnO2.

AgCu Bimetallic Electrocatalysts for the Reduction of Biomass-Derived Compounds.

The electrochemical transformation of biomass-derived compounds (e.g., aldehyde electroreduction to alcohols) is gaining increasing interest due to the sustainability of this process that can be exploited to produce value-added products from biowastes and renewable electricity. In this framework, the electrochemical conversion of 5-hydroxymethylfurfural (HMF) to 2,5-bis(hydroxymethyl)furan (BHMF) is studied. Nanostructured Ag deposited on Cu is an active and selective electrocatalyst for the formation of BHMF in basic media. However, this catalyst deserves further research to elucidate the role of the morphology and size of the coated particles in its performance as well as the actual catalyst surface composition and its stability. Herein, Ag is coated on Cu open-cell foams by electrodeposition and galvanic displacement to generate different catalyst morphologies, deepening on the particle growth mechanism, and the samples are compared with bare Ag and Cu foams. The chemical-physical and electrochemical properties of the as-prepared and spent catalysts are correlated to the electroactivity in the HMF conversion and its selectivity toward the formation of BHMF during electroreduction. AgCu bimetallic nanoparticles or dendrites are formed on electrodeposited and displaced catalysts, respectively, whose surface is Cu-enriched along with electrochemical tests. Both types of bimetallic AgCu particles evidence a superior electroactive surface area as well as an enhanced charge and mass transfer in comparison with the bare Ag and Cu foams. These features together with a synergistic role between Ag and Cu superficial active sites could be related to the twofold enhanced selectivity of the Ag/Cu catalysts for the selective conversion of HMF to BHMF, that is, >80% selectivity and ∼ 100% conversion, and BHMF productivity values (0.206 and 0.280 mmol cm-2 h-1) ca. 1.5-3 times higher than those previously reported.

Coupled Copper-Zinc Catalysts for Electrochemical Reduction of Carbon Dioxide.

A catalyst plays a key role in the electrochemical reduction of CO2 to valuable chemicals and fuels. Hence, the development of efficient and inexpensive catalysts has attracted great interest from both the academic and industrial communities. In this work, low-cost catalysts coupling Cu and Zn are designed and prepared with a green microwave-assisted route. The Cu to Zn ratio in the catalysts can be easily tuned by adjusting the precursor solutions. The obtained Cu-Zn catalysts are mainly composed of polycrystalline Cu particles and monocrystalline ZnO nanoparticles. The electrodes with optimized Cu-Zn catalysts show enhanced CO production rates of approximately 200 µmol h-1 cm-2 with respect to those with a monometallic Cu or ZnO catalyst under the same applied potential. At the bimetallic electrodes, ZnO-derived active sites are selective for CO formation and highly conductive Cu favors electron transport in the catalyst layer as well as charge transfer at the electrode/electrolyte interface.