Hydrogen Storage by Reduction of CO2 to Synthetic Hydrocarbons

The storage of renewable energy is crucial for the substitution of fossil fuels with renewable energy. Hydrogen is the first step in the conversion of electricity from renewable sources to an energy carrier. However, hydrogen is technically and economically challenging to store, but can be converted with CO2 from the atmosphere or oceans to hydrocarbons. The heterogeneously catalyzed gas phase reaction and the electrochemical CO2 reduction are reviewed and the application of a new type of reactor is described. The mechanism of the gas phase CO2 reduction on a heterogeneous catalyst is shown in detail and the function of the supported catalyst is explained. Finally, an economic estimation on the cost of synthetic methane is presented which leads to a cost of 0.3 CHF/kWh in CH4.

Decreasing the Stability of Borohydride with Ionic Liquids for Hydrogen Storage and Carbon Dioxide Conversion

Loris Giovanni Lombardo

The world needs to move from a fossil fuel to a renewable energy based society. With the increasing electricity production from wind and PV, short and long term energy storage are becoming one of challenge of the 21st century. While batteries are the preferred method for short term storage, another technology is required for seasonal storage. Hydrogen is a valuable energy carrier owning its high energy density (122 kJ/g). However, compact and safe storage of H2 is challenging. The current solution is to compress hydrogen up to 700 bar for mobile application (fuel cell cars), and metal hydrides for stationary application. Material-based H2 storage avoids using high-pressure cylinder. Complex hydrides are especially attractive for solid-state H2 storage due to their high gravimetric and volumetric hydrogen density. Sodium borohydride (NaBH4) contains 10.6 mass% of H2, but high temperatures are needed to release H2 (505 Â°C) and the reaction is not reversible under moderate conditions. To apply complex hydrides in real applications, they must be modified to improve the kinetics and thermodynamics of H2 desorption. Several possibilities exist such as confinement into scaffolds, catalyst addition, or combination of several hydrides to modify the dehydrogenation pathway. The stability of complex hydrides is determined by the localization of the charge on the central atom of the complex. Therefore, in this thesis the interaction of highly polar or ionic compounds with complex hydrides were investigated. We combine NaBH4 with a special class of organic salts: ionic liquids (IL). Considering their chemical and thermal stability, IL are attractive additive. Using organic additives instead of expensive metal catalysts is another advantage. Two different techniques were employed to combine the IL and NaBH4. The first one consists of mechanochemically milling them to create an interaction between BH4- and IL. The second one is to directly synthesized IL borohydrides ([IL][BH4]) by metathesis reaction. Different ILs, such as vinylbenzyltrimethylammonium chloride ([VBTMA][Cl]), 1-butyl-3-methylimidazolium chloride ([bmim][Cl]), and 1-ethyl-1-methylpyrrolidinium bromide ([EMPY][Br]) were tested. [bmim][BH4] was able to release H2 below 100 Â°C. The hydrogen content ranges between 2.4 mass% and 2.9 mass%. The second challenge of this century is the CO2 problematic. Atmospheric CO2 concentration linearly increases, leading to greenhouse effect. Carbon capture and sequestration (CCS) and utilization (CCU) are primordial research topic to stabilize the CO2 emission. Hydrides are primary reducing agents, thus reduction of CO2 can be accomplished with them. Direct CO2 reduction at ambient conditions is challenging for classical borohydrides. With our [IL][BH4], gaseous CO2 can be captured and reduced under ambient conditions (1 bar, RT). Three CO2 bounds with boron to give triformatoborohydride ([HB(OCHO)3]â. Dilute CO2 (6 vol%) can be used as well. The reaction was followed in real time with a magnetic suspended balance (MSB). Further, CO2 reduction happened when air is used as a carbon dioxide source. Formic acid can be obtained by adding HCl. Thus, ionic liquid borohydrides have great potential as a CO2 absorber and reducer to alternative fuels. In short, we applied ionic liquid to destabilized borohydrides. The resulting materials exhibited potential for solid-state hydrogen storage, as well as CO2 capture and transformation.

EPFL2021

Amorphous Molybdenum Sulfide Films as Hydrogen Evolution Catalysts in Water

Daniel Andreas Merki

Molecular hydrogen is a promising candidate to replace fossil fuels as the energy carrier. Hydrogen does not exist in its molecular form on earth and must therefore be generated, starting from hydrogen-rich compounds. Water would be a renewable resource for hydrogen. It can be split into oxygen and hydrogen. Water splitting, however, needs electrical energy. If this energy is produced from renewable resources, such as solar or wind power, water splitting would be a sustainable and green process. One key step in water splitting is the reduction of protons to form hydrogen. To achieve a high efficiency in this reaction, catalysts are required. This dissertation is devoted to the development of hydrogen evolution catalysts that are made of earth-abundant and inexpensive materials. First, transition metal complexes of tetrathiotungstate and tetrathiomolybdate were studied as potential homogeneous catalysts for hydrogen evolution in organic or aqueous solutions (chapter 1). The redox activity of these complexes was investigated by cyclic voltammetry in the absence and presence of an acid in organic solutions. The first reduction peak of (Pr4N)2[Ni(MoS4)2] became more intense and was shifted to less negative potentials upon addition of an acid, e.g. p-cyanoanilinium tetrafluoroborate. This was considered as a sign for catalytic proton reduction. Electrolysis of a solution containing (Pr4N)2[Ni(MoS4)2] and an acid produced hydrogen. However, the complex turned out to be a precursor of the real catalytically active species, which was deposited on the working electrode’s surface during the electrochemical experiment. Consecutive cyclic voltammetric scans were found to be a good method to deposit the catalytically active film in a controllable manner from an aqueous solution of (Pr4N)2[Ni(MoS4)2]. Furthermore, a catalytically active film could be deposited from a solution of MoS42-, i.e. in the absence of Ni2+. Chapter 2 describes the investigation and characterization of films made using MoS42- as the precursor. The films consist of amorphous molybdenum sulfide. They are efficient hydrogen evolution catalysts in water. The films were characterized by XPS and electron microscopy. Whereas the pre-catalysts could be MoS3 or MoS2, the active form of the catalysts was identified as amorphous MoSx, with x close to 2. Significant geometric current densities were achieved at low overpotentials (e.g., ca. 15 mA/cm2 at η = 200 mV) using these catalysts. The catalysis was compatible with a wide range of pH values. The current efficiency for hydrogen production was quantitative. A Tafel slope of 39 mV/dec was observed, suggesting a rate-determining Heyrovsky step. The turnover frequency per active site was estimated.

EPFL2012

High-pressure NMR spectroscopic and calorimetric studies on formic acid dehydrogenation and carbon dioxide hydrogenation

Cornel Fink

Our current energy production is convenient and simple, but it is not sustainable. The negative impacts on our environment are omnipresent in the form of smog, extinction of species, and global warming. A major challenge of our generation is the transition to new, responsible methods of energy production. The maxim is renewable energy. Hydrogen could serve as an intermediary energy carrier. However, problems arise with the storage of hydrogen due to safety concerns and low volumetric energy density. The reduction of carbon dioxide (CO2) to formic acid (FA) is an elegant process to âcompressâ hydrogen by transforming it into a chemical which is easier and safer to handle. In this dissertation, catalytic FA dehydrogenation, CO2 reduction and calorimetric studies of the involved thermodynamics are described. The first chapter provides a global overview, beginning with general considerations about energy economy, then crossing over to hydrogen storage and infrastructure for a future hydrogen economy, before discussing FA as a hydrogen storage medium and summarizing the developments of FA dehydrogenation and CO2 hydrogenation. Chapter two describes different compounds we have studied as potential catalysts for FA dehydrogenation and CO2 hydrogenation. The focus is on complexes of Ir, Rh, and Ru regarding metals, and on Cp* and an ethylene-spaced diamine as ligands. Other motifs were also explored including Ru-phosphine complexes which afforded unexpected results. The most promising candidates are examined more closely. The third chapter summarizes our findings of formic acid dehydrogenation with homogeneous catalysts, presented in three parts. Each subchapter highlights a different structural feature or metal center. The first one focuses on Ir and Rh metal centers, equipped with Cp* and diamines. In the second section, we explore the catalytic performance of a RAPTA-type precatalyst, and the last one addresses our achievements with a rhodium catalyst containing Cp* and di(1-pyrazolyl)methane ligands. The catalysts were evaluated towards core properties such as stability (TON), activity (TOF) and recycling. Chapter four elucidates the interactions of FA with different solvents and additives, which influence the kinetics and thermodynamics of FA dehydrogenation and CO2 hydrogenation. NMR and FT-IR spectroscopy, conductometry, calorimetric measurements and computational methods were used to analyze the systems thoroughly. In chapter five, high-pressure calorimetric measurements are employed to quantify the actual energy balance during the endothermic process of FA dehydrogenation. For this purpose, we modified an off-the-shelf high-pressure reactor to suit our needs and constructed a thermo-controlled environment. Chapter six describes our research into catalytic CO2 hydrogenation to FA with two ruthenium-based phosphine pre-catalysts. One has three 1,4,7-triaza-9-phosphatricyclotridecane (CAP) ligands coordinated, which is a just recently described phosphine compound. The other complex is prepared via a straightforward synthesis route with an inexpensive, simple ligand. Both are stable in aqueous FA over weeks, making them potential catalysts for large-scale application.

EPFL2018