recent progress in the electrochemical conversion and utilization of co 2

Recent progress in the electrochemical Recent progress in the electrochemical conversion and utilization of COconversion and utilization of CO22

Neil S. Spinner, Jose A. Vega and Neil S. Spinner, Jose A. Vega and William E. Mustain*William E. Mustain*

• Over the past several years, there has been a growing interest in the capture of carbon dioxide emissions and its chemical conversion to industrially relevant products.

• Several processes have been developed and studied; however, many of these methods are quite expensive since they require either ultra high purity CO2 or are energy intensive. Also, many purely chemical methods show low product selectivity.

• To address these limitations, several researchers have initiated activities using electrochemical processes to increase reaction selectivity and reduce cost .

advantages advantages &&

disadvantagesdisadvantagesphotoelectrophotoelectro--

chemicalchemical

bioelectrobioelectro--

chemicalchemical

electro-chemical

1. Introduction

1.1 Environmental impact of CO2

population increase & technology advances

unprecedentedgrowth in energy consumption

natural gas coal wood

release of many compoundstoxic to our environment

nitrous oxides

sulfur oxides CO2

heavy metals

petroleum

CO2: On the surface, CO2 appears harmless. It is odorless and colorless. It is non-toxic to humans.However,its a greenhouse gas that absorbs infrared heat that is reflected off the Earth from the Sun. This trapped excess heat has been shown to correlate very well with the average global temperature, which is shown in Fig. 1 .

• Recent years have seen a significant growing international interest in both limiting the emission of CO2 and reducing the atmospheric CO2 concentration to avoid a global catastrophe.

• There are several options that have emerged to control CO2 emissions:

(i) development of, and conversion to environ-mentally benign energy sources;

(ii)enhancing the energy efficiency and utilization of existing processes;

(iii) CO2 capture/sequestration

(iv) conversion of CO2 to useful products.

1.2 Current efforts to chemically utilize CO2

• Reactions for the synthesis of each of these industrially relevant chemicals are presented in eqn (1)–(5), respectively.

However, there are several challenges to the chemicalconversion of CO2 including: (i) costs of CO2 capture, separation, purification and transportation to the user site; (ii) energy requirements of CO2 chemical conversion.

1.3 Using redox processes to facilitate desired reactions• Electrochemical reactors provide three interesting advant

ages over pure heterogeneous chemical reactors: (i) It is not limited by traditional thermochemical cycles,meaning that their achievable efficiency is most o

ften significantly higher than their chemical/combustion counterpart;

(ii)The reaction rate and pathway selectivity can be controled precisely through the electrode potential.

(iii) non-direct reaction between precursors through complementary redox processes on two separate catalysts, which permits researchers to tailor the properties needed for each redox process independently.

And it enables unique chemistries to occur that would not be possible in conventional systems.

2. Electrochemical CO2 conversion

2.1 Background

Multiple pathways have been investigated for the

electrochemical conversion of CO2, gaseous, aqueous,andnon-aqueous phase techniques at both high and lowtemperatures.

variations of the solid oxide fuel cell(SOFC)

2.2 Syngas production

• significance Conversion of CO2 electrochemically to syngas (CO and H

2) is a highly promising pathway for CO2 utilization and mitigation.Syngas is an industrially-important precursor used in the synthesis of methanol and other hydrocarbons.

Syngas production methods ►In SOFC ►under ambient conditions in aqueous solutions.

Syngas production in SOFC

Among the methods reported for syngas production,SOFCs are a popular choice due to high current densitiesand the potential for power-generating, rather than power-intensive,devices. Heat produced electrochemically through anodicoxidation is sufficient to sustain the SOFC operatingtemperature, and, as aresult,the heat required to dissociategaseous CO2 to CO and surface oxygen species is readilyavailable. This heat utilization enables SOFCs to generatepower, compared to low temperature,aqueous systemswhich require an applied current to electrochemicallyreduce CO2.

one typical exaple: CO2 reforming of CH4 in an SOFC setup As an alternative to traditional steam reforming ofmethane(CH4), which is a highly endothermic and energy-intensive process, dry reforming, or CO2 reforming ofCH4,has been reported to produce syngas with morefavorable H2/CO ratios:

• Dry reforming is a particularly attractive method since it not only addresses improved syngas formation, but also the elimination of greenhouse gases and utilization of cheap, abundant, carbon-containing materials.

• Performance in an SOFC setup using CH4 and CO2 as reactants is also comparable to using H2 as fuel.

• When CO2 is used as the fuel without CH4, CO alone can be synthesized at the cathode (eqn (7)) along with oxygen ions, which are transformed to pure oxygen gas at the anode (eqn (8)):

Bidrawn et al.demonstrated this CO2 electrolysis with current densities over 1 A cm-2, and suggested this technology could rival efficiencies for similar H2O electrolysis systems and make a significant impact on greenhouse gas mitigation.

• Disadvantages of SOFC:

One of the biggest challenges facing prolonged SOFC usage is finding solutions to the electrode deactivation that occurs from coke formation and other mechanisms.

Coke formation and deposition can occur through several different reactions,including CH4 decomposition (eqn (10)) and CO disproportion

(eqn (11)):

Fortunately, the rate of

coke formation is lower

than both the rates of

reaction for CH4 and CO2

and the rate of formation

of CO.

►syngas production under ambient conditions in aqueous solutions.

Due to the lack of accessible heat at atmospheric temperatures and pressures,gaseous CO2 can not be as easily electroreduced as with SOFCs.Dissociation of CO2 in aqueous solutions requires applied current, and therefore generally large applied ( negative) potentials.

Yanoet al. converted CO2 to CO and H2 at applied potentials as high as -2.4 V over a silver mesh electrode, but conversions were much lower th

an those reported for SOFCs. Delacourt and coworkers achieved current densities up to100 mA. cm-2

over Ag and Pt–Ir gas-diffusion electrodes in fuel cell and modified aqueous-fuel cell experimental setups;however, they primarily reported

H2 evolution with low CO formation, and insertion of an aqueous buffer layer increased cell resistance and lacks commercial feasibility.

• Low CO2 solubility in aqueous solutions under ambient conditions severely limits the output for these devices, and imposes the need for extremely large applied potentials to obtain a reasonable amount of product.

Promising current densities a s high as 50 mA cm-2 have been reported, however, for applied potentials up to -2.5 V over various metal-phthalocyanine (M-Pc) gas-diffusion electrodes.The primary product from these electrocatalysts was CO, though H2 evolution occurred in nearly all cases as well.

Another potential pathway for electrochemical CO2 conversion is through the formation of carbonate anions (CO3

2-),which has been demonstrated over a Ca2Ru2O7-y pyrochlore cathode electrocatalyst in an anion exchange membrane fuel cell setup.

2.3 Hydrocarbon products

• Electrode material

• Reaction media

Electrode material• copper-based electrodes: The most commonly-used materials for electrochemical C

O2 conversion under ambient conditions are transition metals and metal oxides, specifically copper-based electrodes .

In aqueous electrochemical CO2 reduction methods, using different copper electrodes can found a variety of products formed depending on the electrode structure and reaction conditions,including ethylene (C2 H4) , CH4 ,

formic acid (HCOOH), ethane, ethanol, propanol, acetic acid,and et al.

Poisoning of these copper electrodes is a common issue that plagues long-term operation and limits their viability for

commercialization.

• other types of electrodes M-Pc complexes as gas-diffusion electrodes Monel metal (an alloy of nickel, cobalt, copper, and iron) stainless steel (chromium , nickel , and iron) numerous pure transition metals boron-doped diamond (BDD)

Reaction media• In addition to varying the electrode material, some resear

chers have tried different reaction media to either improve the CO2 reduction or tailor specific products selectivity.

aqueous media non-aqueous media plasma

Yoshida, Yosue,and Nogami demonstrated a direct-current discharge plasma gas chamber with CO2 and H2 as fuels to synthesize CH4 and other hydrocarbon gases over copper and iron electrodes.However, due to the extreme applied potentials needed to create the excited plasma state, conventional efficiencies for this cell are very low, and these electrodes also suffer from poisoning caused by graphitic carbon deposition over prolonged usage.

3. Photoelectrochemical CO2 Conversion

3.1 Background Photoelectrochemical processes for CO2 conversiongenerally require more complex systems than their purelyelectrochemical counterparts, and are also much moreenergy intensive due to power requirements both for applied potentials and electrolyte illumination.3.2 Electrode/electrolyte selection A variety of materials and solutions have been studied asphotoelectrocatalysts and electrolytes for CO2 reduction. Aqueous solutions non-aqueous solutions

Many commonly-used catalysts are bimetallic,

n- or p-type electrodes, some of which are additionally

doped.

Although a few reports do show moderate current

densities, overall performance of most photo-

electrochemical systems is very low in terms of

both current density and faradaic efficiency.

4. Bioelectrochemical CO2 conversion

4.1 Background Though some success has been reported using metal

electrodes for the reduction of CO2 to fuels, one of its main

drawbacks remains the low selectivity of the process where

multiple products are generally obtained. In response to low selectivity,several researchers have i

nvestigated electrolysis in CO2-saturated aqueous electrolytes using enzyme catalysts.The high selectivity

of naturally occurring enzymes could lead to the formation of a specific fuel, while minimizing or completely eliminating secondary reactions.

• Most of these studies have focused on formate

dehydrogenase (FDH), aldehyde dehydrogenase (AldDH)

and alcohol dehydrogenase ( ADH). CO2-saturated

electrolytes contain bicarbonate anions that undergo a

series of reactions to produce methanol. FDH, AldDH and

ADH catalyze the formation of formate, formaldehyde and

methanol, shown in eqn (20)–(22) respectively.

4.2 Enzyme selectivity

• CO2 reduction products are highly dependent on the enzyme or enzyme series used, as shown in Fig. 9.

It is reported that current efficiencies for the production of formate can achieve as high as 93% with this enzyme using electrons photogenerated from a p-type indium phosphide cathode.Therefore , it is possible to reduce CO2 to higher energy substrates with high current efficiencies and at mild conditions.

main problems of biochemical CO2 conversion:

overall product yields are very low

natural biological enzymes has the following defects:• low activities • poor electrical conductivity • stability constraints associated with thermal and

chemical sensitivities to the reaction conditions

5. Conclusions

• Electrochemical techniques are particularly attractive when compared to heterogeneous chemical methods in part due to greater potential efficiencies and more tailorable reaction pathways.

Advantages Disadvantages

electro-chemicalconversion

High temperature solid oxide fuel cell devices (SOFC)

High current densities and efficiencies;low power requirement

Electrode degradation

Low temperature aqueous and non-aqueous reactors

Facil operating conditions;Broader products scope

Comparatively low currentdensity and efficiency;Low product selectivity;High power requirement

Photoelectrochemical conversion High product selectivity

Immense energy demandLow product yields ;

bioelectrochemical conversion High product selectivity

Low product yields;Stability constraints.

The end,thank you!The end,thank you!