Search

Electrochemical reduction of CO2 is a promising method for converting a greenhouse gas into value-added products, utilizing renewable energy. Novel catalysts, electrode assemblies, and cell configurations are all necessary to achieve economically appealing performance. In this talk, I am going to present a zero gap electrolyzer cell, which converts gas phase CO2 to products without the need for any liquid catholyte. I will show how to optimize the cell components, to simultaneously provide industrially relevant performance, such as high partial current density (up to 800 mA cm-2), low cell voltage (−2.8-3.2 V), high conversion efficiency (up to 45 %), and high selectivity for CO production (over 95%). I will also demonstrate our CO2 electrolyzer cell, where multiple layers are connected into an electrolyzer stack, thus scaling up the electrolysis process. The operation of the cell was validated using both silver nanoparticle and copper nanocube catalysts. Evenly distributing the CO2 gas among the layers (parallel connection), the operation of the cell stack was identical to the sum of multiple single-layer cells. When passing the CO2 gas through the layers one after the other (serial gas connection), the CO2 conversion efficiency was increased remarkably. Finally, I will show how the cell can operate with pure water feed on the anode side, therefore avoiding the use of alkaline anolyte. A carefully designed activation protocol will be presented, which allows the cell to operate at high current density, using pure water feed. The effect of temperature and flow rate on the operation will also be presented. Finally, the nature of ion-transport through the anion exchange membrane as a function of the operational conditions will be discussed.

Electrochemical reduction of CO2 is a promising method for converting a greenhouse gas into value-added products, utilizing renewable energy. Novel catalysts, electrode assemblies, and cell configurations are all necessary to achieve economically appealing performance. In this talk, I am going to present a zero gap electrolyzer cell, which converts gas phase CO2 to products without the need for any liquid catholyte. This is the first report of a CO2 electrolyzer cell, where multiple stacks are connected, thus scaling up the electrolysis process. The operation of the cell was validated using both silver nanoparticle and copper nanocube catalysts, and the first was employed for the optimization of the electrolysis conditions. Upon this, CO formation with partial current densities above 250 mA cm−2 were achieved routinely, which was further increased to 300 mA cm−2 (with ~95 % Faradaic efficiency) by pressurizing the CO2 inlet. Evenly distributing the CO2 gas among the stacks (parallel connection), the operation of the multi-stack cell was identical to the sum of multiple single-stack cells. When passing the CO2 gas through the stacks one after the other (serial gas connection), the CO2 conversion efficiency was increased remarkably. Importantly, the presented electrolyzer simultaneously provides high partial current density, low cell voltage (−3.0 V), high conversion efficiency (up to 40 %), and high selectivity for CO production; while operating at up to 10 bar differential pressure. In the second part of the talk I will show how the cell can operate with pure water feed on the anode side, therefore avoiding the use of alkaline anolyte. A carefully designed activation protocol will be presented, which allows the cell to operate at high current density, using pure water feed. The effect of temperature and flow rate on the operation will also be presented. Finally, the nature of ion-transport through the anion exchange membrane as a function of the operational conditions will be discussed.

Given that CO2 is a greenhouse gas, using the energy of sunlight to convert CO2 to transportation fuels (such as methanol or methane) represents a value-added approach to the simultaneous generation of alternative fuels and environmental remediation of carbon emissions. Electrochemistry and photoelectrochemistry have been proven to be a useful avenue for solar water splitting. CO2 reduction, however, is multi-electron in nature (e.g., 6 e- to methanol) with considerable kinetic barriers to electron transfer. It therefore requires the use of carefully designed electrode surfaces to accelerate e- transfer rates to levels that make practical sense. In this talk I will present hybrid photoelectrodes leading to enhanced efficiency, selectivity, and stability. First, I will present the use of electrosynthetic (and photoelectrosynthetic) methods for preparing semiconductors on nanocarbon-modified electrode surfaces. Composites of nanocarbons with both inorganic and organic semiconductors represent an interesting class of new functional materials. Therefore, I will show how electrodeposition can be used to tune composition, crystal structure, and morphology of the nanocomposites for targeted applications. In the second part of my talk, selected examples will be given for how these electrosynthesized hybrid assemblies can be deployed in various photoelectrochemical application schemes, most importantly CO2 conversion. I will present the controlled synthesis and photoelectrochemical behavior of Cu2O/CNT and Cu2O/graphene composites. A carefully designed, multiple-step electrodeposition protocol was developed that ensured homogeneous coating of the CNTs with the Cu2O nanocrystals. TiO2/ graphene nanocomposites were also obtained in a similar manner. This enhanced charge transport property for the hybrids resulted in a drastic increase in the photocurrents measured for the CO2 reduction. In addition to this superior performance, long term photoelectrolysis measurements proved that the Cu2O/nanocarbon hybrids were more stable than the oxide alone. Taking these observations together as a whole, a general model will be presented on the role of the nanocarbon scaffold. Acknowledgements This research was partially supported by the “Széchenyi 2020” program in the framework of GINOP-2.3.2-15-2016-00013 “Intelligent materials based on functional surfaces – from syntheses to applications” project.

Carbon nanomaterials, especially carbon nanotubes and graphene (either alone, or as building blocks of organized 3-D superstructures), are attracting significant attention as large surface area electrode support materials. Their composites with both inorganic and organic semiconductors represent an interesting class of new functional materials. In the first part of my talk I will briefly focus on the use of electrosynthetic (and photoelectrosynthetic) methods for preparing semiconductors on nanocarbon-modified electrode surfaces.1,2I will show how electrodeposition can be used to tune composition, crystal structure, and morphology of the nanocomposites for targeted applications. In the second part of my talk, selected examples will be given for how these electrosynthesized hybrid assemblies can be deployed in photoelectrochemical CO2 conversion. Given that CO2 is a greenhouse gas, using sunlight to convert CO2 to transportation fuel (such as methanol) represents a value-added approach to the simultaneous generation of alternative fuels and environmental remediation of carbon emissions from the continued use of conventional fuels.3 I will present the controlled synthesis and photoelectrochemical behavior of Cu2O/CNT4 and Cu2O/3D graphene composites for the CO2 reduction application. A carefully designed, multiple-step electrodeposition protocol was developed that ensured homogeneous coating of the CNTs with the Cu2O nanocrystals. This enhanced charge transport property for the hybrids resulted in a drastic increase in the photocurrents measured for the CO2 reduction. In addition to this superior performance, long term photoelectrolysis measurements proved that the Cu2O/CNT hybrids were more stable than the oxide alone. Structure property relationships will be shown, which may act as guidelines for the rational design of nanocomposite photoelectrodes. Figure 1. Illustration of the structure of the Cu2O/CNT composite and the increasing photocurrent with the increasing CNT-content. [1] C. Janáky and K. Rajeshwar: The Role of (Photo)electrochemistry in the Rational Design of Hybrid Conducting Polymer / Semiconductor Assemblies: From Fundamental Concepts to Practical Applications, Prog Pol Sci, 43 (2015) 396-435 [2] C. Janáky, E. Kecsenovity, K. Rajeshwar: Electrodeposition of Inorganic Oxide/ Nanocarbon Composites: Opportunities and Challenges, ChemElectroChem, 3 (2016) 181-192 [3] G. Ghadimkhani, N. R. De Tacconi, W. Chanmanee, C. Janaky, K. Rajeshwar: Efficient solar photoelectrosynthesis of methanol from carbon dioxide using hybrid CuO/Cu2O semiconductor nanorod arrays, Chem. Commun. 49 (2013) 1297–1299 [4] E. Kecsenovity, B. Endrődi, Z. Pápa, K. Hernádi, K. Rajeshwar, C. Janáky: Decoration of ultralong carbon nanotubes with Cu2O nanocrystals: a hybrid platform for enhanced photoelectrochemical CO2 reduction, J Mater Chem A, 4 (2016) 3139-3147 Figure 1