Recent progress in the electrochemical conversion and utilization of CO 2

1. Recent progress in the electrochemical conversion and utilization of CO2 Neil S. Spinner, Jose A. Vega and William E. Mustain*

2. Over the past several years, there has been a growing interest in the capture of carbon dioxideemissions and its chemical conversion to industriallyrelevant products. • Several processes have been developed and studied; however, many of thesemethods are quite expensive since they require either ultra high purity CO2or are energyintensive. Also, many purely chemical methods show low product selectivity. • To address theselimitations, several researchers have initiated activities using electrochemical processes to increasereaction selectivity and reduce cost .

3. bioelectro- chemical electro- chemical advantages & disadvantages photoelectro-chemical

4. population increase & technology advances unprecedented growth in energy consumption natural gas coal wood petroleum release of manycompounds toxicto our environment sulfur oxides CO2 heavy metals nitrous oxides 1. Introduction 1.1 Environmental impact of CO2

5. CO2: On the surface, CO2appears harmless. It is odorless andcolorless. It is non-toxic to humans.However,its a greenhouse gas that absorbsinfrared heat that is reflected off the Earth from the Sun. Thistrapped excess heat has been shown to correlate very well withthe average global temperature, which is shown in Fig. 1 .

6. 1.2 Current efforts to chemically utilize CO2 • Recent years have seen a significant growinginternational interest in both limiting the emission of CO2 and reducing the atmospheric CO2concentration to avoid aglobal catastrophe. • There are several options that haveemerged to control CO2emissions: (i) development of, andconversion toenviron-mentally benign energy sources; (ii)enhancing the energy efficiency andutilization of existingprocesses; (iii) CO2capture/sequestration (iv) conversion of CO2to useful products.

7. Reactions for the synthesis of each ofthese industrially relevant chemicals are presented ineqn (1)–(5), respectively. However, there are several challengesto the chemical conversion of CO2 including: (i) costs of CO2capture, separation, purification and transportation to the usersite; (ii) energy requirements of CO2chemical conversion.

8. 1.3 Using redox processes to facilitate desired reactions • Electrochemical reactors provide three interesting advantagesover pure heterogeneous chemical reactors: (i) It is not limited by traditional thermochemical cycles,meaning that their achievable efficiency is most often significantly higher than their chemical/combustion counterpart; (ii)The reaction rate and pathwayselectivity can be controled preciselythroughthe 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 processindependently. And it enables unique chemistries to occur that wouldnot be possible in conventional systems.

9. 2. Electrochemical CO2conversion 2.1 Background Multiple pathways have been investigated for the electrochemical conversion of CO2, gaseous,aqueous,and non-aqueous phasetechniques atboth high and low temperatures. variations of the solid oxide fuelcell(SOFC)

10. 2.2 Syngas production • significance Conversion of CO2 electrochemically to syngas (CO and H2)isa highly promising pathway for CO2utilization and mitigation.Syngas is an industrially-important precursorused in thesynthesis of methanol and otherhydrocarbons. • Syngas production methods ►In SOFC ►underambient conditions in aqueous solutions.

11. Syngas productionin SOFC • Among the methods reported for syngas production, SOFCs are a popular choice due to high current densities and the potential for power-generating, rather than power- intensive,devices. • Heat produced electrochemically through anodic oxidation is sufficient to sustain the SOFC operating temperature, and, as aresult,the heat required to dissociate gaseous CO2 to CO and surface oxygen species is readily available. This heat utilization enables SOFCs to generate power, compared to low temperature,aqueous systems which require an applied current to electrochemically reduce CO2.

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

13. Dry reforming is a particularly attractive method since it not onlyaddresses improved syngas formation, but also the elimination ofgreenhouse gases and utilization of cheap, abundant, carbon-containing materials. • Performance in an SOFC setup using CH4and CO2 as reactants is also comparable to using H2as fuel.

14. When CO2is used as thefuel without CH4, CO alone can be synthesized at the cathode(eqn (7)) along withoxygen ions, which are transformed to pure oxygen gas at theanode (eqn (8)): Bidrawn et al.demonstratedthis CO2electrolysis with currentdensities over 1 A cm-2, and suggested this technologycould rival efficiencies for similar H2O electrolysis systemsand make a significant impact on greenhouse gas mitigation.

15. 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)):

16. Fortunately, the rate of coke formationis lower than both the rates of reaction for CH4 and CO2 andthe rate of formation of CO.

17. ►syngas production underambient conditions in aqueous solutions. • Due to the lack ofaccessible heat at atmospheric temperaturesand pressures,gaseous CO2cannot be aseasilyelectroreduced as withSOFCs.Dissociation of CO2in aqueoussolutionsrequiresapplied current, andtherefore generallylargeapplied ( negative)potentials. • Yanoet al. converted CO2 to CO and H2at appliedpotentialsashighas -2.4 V over a silver mesh electrode, butconversionswere much lower than those reported for SOFCs. • Delacourt and coworkers achieved current densities up to100 mA. cm-2 over Ag and Pt–Ir gas-diffusion electrodes infuel 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 increasedcell resistance and lacks commercial feasibility.

18. Low CO2solubility in aqueous solutions under ambient conditionsseverely 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 as50 mA cm-2have been reported, however, for applied potentialsup to -2.5 V over various metal-phthalocyanine (M-Pc) gas-diffusion electrodes.The primary product from these electrocatalysts was CO, though H2evolution occurred in nearly allcases as well. • Another potential pathway for electrochemical CO2conversion is through the formation of carbonate anions(CO32-),which has been demonstrated over a Ca2Ru2O7-ypyrochlore cathodeelectrocatalyst in an anion exchangemembrane fuel cell setup.

19. 2.3 Hydrocarbon products • Electrode material • Reaction media

20. Electrode material • copper-basedelectrodes: • The most commonly-used materials forelectrochemical CO2conversion under ambient conditions aretransitionmetals and metal oxides, specifically copper-basedelectrodes . • In aqueous electrochemical CO2reduction methods, using different copper electrodes can found avariety of products formed depending on theelectrode structure and reaction conditions,including ethylene (C2 H4) , CH4 , formic acid (HCOOH), ethane, ethanol, propanol, acetic acid,and et al. • Poisoning of these copper electrodesis a common issue that plagues long-term operation and limitstheir viability for commercialization.

21. 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)

22. Reaction media • In addition to varying the electrode material, some researchershave tried different reaction media to either improve the CO2reduction or tailor specific products selectivity. • aqueous media • non-aqueous media • plasma Yoshida, Yosue,and Nogami demonstrated a direct-current discharge plasmagas chamber with CO2 and H2 as fuels to synthesize CH4 andother 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.

23. 3. Photoelectrochemical CO2Conversion 3.1 Background Photoelectrochemical processes for CO2conversion generallyrequire more complex systems than their purely electrochemicalcounterparts, and are also much more energy intensive due topower requirements both for applied potentials and electrolyteillumination. 3.2 Electrode/electrolyte selection A variety of materials and solutions have been studied as photoelectrocatalysts and electrolytes for CO2reduction. • Aqueous solutions • non-aqueous solutions

24. 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 mostphoto- electrochemical systems isvery low in terms of both current density and faradaicefficiency.

25. 4. Bioelectrochemical CO2conversion 4.1 Background • Though some success has been reported usingmetal electrodesfor the reduction of CO2to fuels, one of its main drawbacksremains the low selectivity of the process where multipleproducts are generally obtained. • In response to low selectivity,several researchers haveinvestigated electrolysis in CO2-saturatedaqueous electrolytes using enzymecatalysts.The high selectivity ofnaturallyoccurringenzymescouldleadtotheformationofaspecific fuel, while minimizing or completely eliminating secondaryreactions.

26. Most of these studies have focused on formate dehydrogenase (FDH), aldehyde dehydrogenase(AldDH) and alcohol dehydrogenase ( ADH). CO2-saturated electrolytescontain bicarbonate anionsthat undergo a series of reactionsto producemethanol. FDH, AldDH and ADH catalyze theformation of formate, formaldehyde and methanol,shown ineqn (20)–(22) respectively.

27. 4.2 Enzyme selectivity • CO2reduction products are highly dependent on the enzymeor enzyme series used, as shown in Fig. 9. It is reported that current efficienciesfor the production of formate can achieve as high as 93% with thisenzyme usingelectrons photogenerated from a p-type indiumphosphide cathode.Therefore , it is possible to reduce CO2tohigher energysubstrates with high current efficiencies and at mildconditions.

28. main problems of biochemical CO2 conversion: overall product yields are very low natural biological enzymes has the following defects: • low activities • poorelectrical conductivity • stability constraints associated with thermal and chemical sensitivities to the reaction conditions

29. 5. Conclusions • Electrochemical techniques are particularly attractive when compared to heterogeneous chemicalmethods in part due to greater potential efficiencies and moretailorable reaction pathways.

31. The end,thank you!