Co2 Electroreduction To Produce Fuels

PRODUCING FUELS AND ENERGY STORAGE CHEMICALS SPECIES DERIVED FROM CO₂ ELECTRO-REDUCTION

INTRODUCTION

Wind and solar are important sources of energy supply. However, factors such as weather conditions affect bridging demand and production (supply). The biggest challenge faced by societies today is ensuring energy supply for future generations as well as reducing sole dependence on fossil fuels is attained. This owes to the fact that primarily, fossil fuel is gradually extinguishing and reducing its value, and secondly, the amount of CO₂ emissions is on the rise outbalancing its consumption rate on earth¹. These CO₂ emissions from fossil fuels combustions produces approximately billion tons of carbon dioxide annually².

Per research, since 1970s to date, carbon dioxide concentrations in the atmosphere is responsible mainly for the earth s climate change (greenhouse effect) and oceans PH change².

Due to these challenges, considerations are being given to study an efficient way of converting CO₂ to reduced forms which serve as a source energy.

CO₂ electro-reduction being the most efficient and cost effective way of converting CO₂ to harmless reduced forms which serve as a source energy^1-11.

With CO₂ electro-reduction being the most efficient and cost effective way to convert CO₂ to other chemical species, this research will be focusing on this.

CO₂ electro-reduction is the conversion of carbon dioxide to more reduced chemical species using electrical energy². This reduced chemical species can be used as energy storage medium for renewable energy sources or as working fluid.

Electrochemical reduction proceeds with more positive potentials through two-, four-, six- and eight-electron reduction pathways in gaseous and non-aqueous phases at high and low temperatures².

The scheme shown below is the basic half-electrochemical chemical reaction as follows in aqueous/non-aqueous media:

CO₂ + 2H⁺ + 2e CO + H₂O ^1,2,3

Carbon monoxide (CO) as seen in the scheme 1 below is one of the main reduction products obtained in this process.

Other reduction products include formate, formic acid, methane, alcohol and ethylene, aldehyde, oxalate, oxalic acid, methanol. We will be focusing on methane, alcohol and methanol.

In CO₂ electro-reductive processes, ionic liquids (electrolytes) are salts in the liquid state with melting points lower than 100^oC or existing at room temperatures. They are made up of ions and short-lived ion pairs and they have very low vapour pressure which accounts for their conductivity and stability as liquids for longer periods⁴. This property makes them very useful in optical spectrometric methods to detect reaction intermediates.

Ionic liquids sometimes called ionic melts, liquid salts, fused salts are environmentally friendly and are used as good solvents, electrically conducting fluids, useful for CO₂ electro-reduction and prevention of H₂ evolution because of the absence of water. A commonly encountered ionic liquid is 1-butyl-3-methlimidazolium hexafluorophosphate ([BMIM]PF6).

THE RESEARCH ISSUE, AIMS OR QUESTIONS I INTEND TO ADDRESSThere are a lot of challenges currently in CO2 electro-reduction. It chiefly concerns economic feasibility,

1. Low catalytic activity (Degradation of catalyst).

2. Energetic efficiency

3. Current density

4. Faradaic efficiency (Low product selectivity)

5. Insufficient catalyst stability and durability. (Application of technology - far from

satisfactory/quality).

6. Unsatisfactory technology application

7. Optimization of electrode/reactor and system design for practical applications

8. Process costs.

Economic feasibility:

Low catalytic activity:

Per Jinli et al, catalyst developed so far exhibit normally high over-potential for CO₂ electro-reduction, indicating poor examples for practical applications in terms of energy efficiency¹.

Energetic Efficiency: Huei reported difficulty in optimizing the energetic, current density and product selectivity as a leading challenge till date³.

Current Density: Kendra et al reported no desirable products at high current densities (conversion rate) as one of the challenges currently⁶.

Faradaic efficiency (Low product selectivity):

Jinli et al emphasized that few attempts of some catalyst have been made on CO₂ electro-reduction giving desirable product selectivity and stable yields under continuous operation. E.g. the use of Sn, Pb, and Hg metal electrodes to produce formate/formic acid¹.

Insufficient catalyst stability/durability for Industry scale:

Jinli et al proposed single biggest challenge in CO₂ electro-reduction. This is caused either by the active electrode/catalyst surface being slowly covered by the reaction intermediates (carbon films or poisonous species) or by-products poisoning or blocking catalyst active sites leading to rapid catalyst activity degradation¹.

Unsatisfactory technology application:

The technological application is far from satisfactory due to insufficient understanding of the reaction process coupled with insufficient feed quantity (industry scale)^1,6.

Electrode/reactor optimization and system design for practical application:

According to Jinli Qiuo, the main limitation to CO₂ electro-reduction caused by electrode/reactor design is the rapid catalyst degradation, slow transport of CO₂ to electrode surface and insufficient reaction turnover¹.

Process cost: Main challenge in this area is the high cost of production. This includes consumption, capital and energy cost³.

IMPORTANCE OF RESEARCH PROPOSAL

In tackling low catalytic activity, exploring innovative ways of increasing catalytic contact and activity is necessary. In this research, we will focus on creating an enol-like surface intermediate of the copper surface catalyst.

Studies have shown that Copper metal catalyses the formation of C-C bonds generating C₂, C₃ molecules products from the electro-reduction of CO₂ ⁷.

Research investigations has further shown that the pathway leading to the generation of large range of C₂ and C₃ products could occur through an enol-like surface intermediate of the copper surface catalyst⁶.

The surface preparation of the Cu specie electrode (working electrode) would be a perfect morphology of carbon supported sputter/nanoparticle coat for high selectivity towards CO₂ electro-reduction due to the high abundance of uncoordinated sites⁷.

Energetic efficiency, current efficiency and faradaic efficiency:

Optimization of these three parameters have been a subject of concentration which is quite difficult till date. In this research, optimizing these parameters will be key. Suggested usage of optimized Cu electrode surface structure with a special morphology as well as Cu catalyst for high selectivity (faradaic efficiency), energy utilization towards the final product (energetic efficiency), and conversion rate (current density) is deemed.

Research investigations have shown that electro-reduced CO₂ using an electrolytic cell generates methane, ethylene and alcohols used to carry energy in cells. However, in aqueous solutions, the current efficiency for CO₂ electro-reduction is low from high activation barrier. The competing H₂ gas evolution causes difficulties in the electro-reduction and optical spectrometric usage for detection of intermediate reactions. Current research show that ionic liquids used as the electrolyte instead of an aqueous solution can combat hydrogen evolution reducing competing reactions during electro-reduction of CO₂ ⁵.

Electrode-reactor and system design optimization for practical application including unsatisfactory technology application:

With an indebt understanding of different approaches, I would like to consider this approach

The three-dimensional structures of carbon-supported copper will be incorporated into ionic-liquid based electro-polished copper specie electrode using electro-deposition. This copper foam provides both the nanostructured surfaces and cavities necessary to facilitate the reaction between the absorbed CO₂ and hydrogen species to generate high order hydrocarbons from the process.

The electrode working area should be large and a small electrolyte volume in both compartments and a reasonable gas headspace above the electrolyte in each compartment bearing in mind the working electrode is parallel to the counter electrode for uniform voltage.

A study on a special anionic exchange membrane that will be used to separate the working and counter electrode compartments to prevent the oxidation of the products from reduced CO₂ and prevent passage of the anionic products acetate and formate as well⁶.

CO₂ flow through the cell for optimum current efficiency for reduction is key in this process so an insight study on the optimum flow rate of CO₂ transport to the surface as well as prevent interference with bubbles hitting the surface would be carried out.

During CO₂ electro-reduction, some volatile liquid phase products are produced, so study on the optimum temperature of the whole electrolytic cell that prevents evaporation of the volatile products and is optimum simultaneously for the stability of the whole system as a process will be carried out.

Lastly a perfect blend of the ionic liquid for use as an electrolyte to combat challenges of electrochemical window, hydrogen evolution interference, CO₂ solubility and still serve its purpose like an aqueous electrolyte has been lingering for decades, the best so far that has been used are 1-butyl-3-methylimidazolium hexafluorophosphate [BMIM][PF6], 2-hydroxy-N,N,N-trimethylethanaminium (choline ion), etc. despite the uncertainty about their hydrocarbon formation due to the absence of efficient hydrogen ions, stability and sensitivity to oxygen and difficulty in recovering the products from the ionic liquid as extraction with water works for only hydrophilic products, distillation not suitable for poorly volatile or non-thermal labile products, and liquid-liquid extraction not suitable as organic solvents leads to cross-contamination.

CO₂ solubility in that ionic liquid is high reaching a mole fraction of 0.6 at 8 Mpa but both phases are not completely miscible since increase in pressure to 40 Mpa at 25 C showed increase in mole fraction from 1.3 - 7.2% ⁹.

So, this proposed research will consider the synthesis of ionic liquids that can dissolve CO₂ almost completely and still serve its electrolytic abilities efficiently and be stable. Including efficient ways of extracting products from the ionic liquid.

Synthesis of ionic liquids will be based on some characteristics like:

Temperature liquefaction

Anion nature

Large size difference between cation and anion in case of low liquefaction temperature.

Non-flammable liquids with a very low vaporization pressure at temperatures below 150 C.

Large liquid temperature range

High thermal stability (250-300 C)⁴

Process Cost:

Energy (electricity) usage has been another challenge so far Huge amount of electricity required for small product volume in electro-reduction of CO₂. Leading to capital, consumption loss or economical infeasibility. This research will consider cutting down process cost using solar energy approach.

OBJECTIVES OF RESEARCH

The goal of this research is to:

Prepare the most suitable blend of Copper catalyst or any other catalyst for optimum selective electro-reduction of CO₂ to produce the desired amounts of hydrocarbons reducing by product, over potential and in turn increasing current density, energetic efficiency and faradaic efficiency to a desired level.

Delving into the surface chemistry of the catalyst (Cu or other metals), (i.e. improving the surface-intermediate between the catalyst specie electrode and the electrolyte), principally at the enol-like surface intermediate and will make us determine the C₁ and C₂ species involved in C-C bond formation.

Looking at the ionic liquid used as electrolyte for this process and prepare the optimum blend of ionic liquid that reduces all negativities during the process.

Optimize the electrode/reactor and system design for practical application as well improving its technological application using model approach combined with lifecycle analyses.

Lowering cost of production to a minimal level.

And finally, researching ways of incorporating it into large scale.