possible disruptions on the other side, will destabilize the world socially, politically and economically. Which problem will hit earlier is The Stone Age didn’t end because we ran out of stones unclear, the effects of global warming or the CO Recycling: collapse of world economy due to adverse political effects of depending on fossil oil and 2 gas. Most probably we will experience both Technically, the heterogeneous catalysts today less efficient. Photovoltaic conversion of disturbing effects simultaneously. using metallic systems are in use since over solar energy with efficiencies around 20% and Thus, we need renewable, sustainable and hundred years in the industry. Fischer-Tropsch the highly efficient electrolysis systems which

globally available fuels to keep our industrial type catalysts are used to convert CO2/CO go above 60% efficiency for hydrogen, makes The Conversion of development without increasing the atmos- mixtures into hydrocarbons since back in the electrocatalysis systems at the end above pheric CO2 content. Is this possible? Yes! early 1900s in Kaiser Wilhelm Institute for 12-15% efficient from solar to chemical energy. Coal Research (today May Planck Institute for This is clearly more feasible as compared to

CO2 as a new source of carbon Coal Research in Mülheim Germany). In most pure photochemical conversion. Furthermore, feedstock of the available power-to-gas systems today, the fact that wind energy is also directly usable Renewable Energy The carbon source of fossil fuels is nothing the energy is provided in the form of solar by the electrocatalysis systems, makes such else than atmospheric CO2, which was con- and wind hydrogen, which then reacts with systems even more attractive for large scale

verted by natural photosynthesis of green CO2 resulting in methane (CH4). This process implementation. plants and algae over millions of years. In has been first described by Paul Sabatier Many scientists suggest the direct use of

the natural photosynthesis, CO2 and H2O are and awarded with Nobel Prize for Chemistry hydrogen gas as the future chemical fuel and chemically converted to higher hydrocarbons in 1912. energy vector. Jeremy Rifkin in his great work into Chemical Fuels by solar energy input. This makes the discus- The widely used forms of renewable energy “The Hydrogen Economy” describes a future sion highly interesting, since all fossil fuels conversion (solar, wind and hydropower) where hydrogen gas is the global energy 4 by Niyazi Serdar Sariciftci are thus, a form of recorded and stored solar deliver electricity as output. Therefore, we will vector. As of today, the storage of hydro- energy which was shining onto our planet concentrate on electrocatalysis, photo-elec- gen gas as well as feasible supply of liquid We want to bring the idea of conversion of CO2 into synthetic fuels (CO2 recycling) millions of years ago. tro-catalysis as well as bio-electrocatalysis hydrogen are still a problem. In his influential into attention, as a possible approach for transportable storage of renewable energy. After realizing this, scientists can ask the in the passages below. Pure photocatalysis book5 “Beyond Oil and Gas: The Methanol Recycling of CO by homogeneous and/or heterogeneous catalytic approaches have been important question: Can we imitate the nat- is also an active area of research but as of Economy”, George Olah (Nobelprize 1994) 2 ural photosynthesis in an artificial chemical investigated with increasing emphasis within the scientific community. In the last decades, process and get our desperately needed especially using organic and bioorganic systems towards CO reduction has attracted 2 fuels by conversion of CO2 into useful chem- great interest. Chemical, electrochemical, photoelectrochemical and bioelectrochemical icals? The answer is yes and under the dif- approaches are discussed vividly as new routes towards the conversion of CO into ferent key words such as “solar fuels, power 2 to gas, artificial photosynthesis, photon to synthetic fuels and/or useful chemicals in the recent literature. Here we want to especially fuels, electricity to gas” etc essentially this emphasize the new developments in bio-electrocatalysis with some recent examples. process of CO2 conversion to artificial fuels is covered. If realized, the entire energy sec- tor can be converted to these artificial fuels 2 We need renewable and CO2 in this Corona year of 2020. That The existent fossil oil reserves are created by solar CO2 conversion, eventually neutral fuels means we already surpassed a factor localized mostly in politically unsta- outperforming the fossil fuel area. As quoted In his seminal work Svante Arrhenius of 1.5 and are on the best way to ble geographies of the world. Their by the former Saudi oil minister, Sheik Ahmed

calculated in 1896 (!) the increase in doubling the CO2 content as com- sustainable utilization may be a polit- Zaki Yamani, warning his OPEC colleagues: global temperature by several degrees pared to Arrhenius times. The pre- ical challenge. The rapidly growing “The Stone Age didn’t end because we ran

upon increasing the CO2 content in the dicted increase in temperature will economies (China, India and others out of stones.” A new and better technology atmosphere1. The discussion appar- have massive consequences for the in the Asia-Pacific Rim) are increasing always prevails. ently has started way back then, when human life on this planet. their oil consumption parallel to their Two main approaches are being sug- Niyazi Serdar coal was the major energy source and On the other hand, humanity needs economic development. Increasing gested to cope with CO which are Carbon Sariciftci 2 Prof. Sariciftci is Ordinarius air pollutant fueling the early days of for its continued existence not only demand on one side and decreasing Capture and Sequestration (CCS) and Professor for Physical industrial revolution. Arrhenius cal- essentials like food, clean water, or unstable supply of fossil oil with Carbon Capture and Utilization (CCU). CCS Chemistry and the Founding Director of the Linz Institute culates up to 3 degrees increase in shelter and clothing but also large describes the capture of CO2 at its human- for Organic Solarcells (LIOS) global atmospheric temperature upon amounts of energy. Ever since our made origin (factories, power plants etc.) and at the Johannes Kepler University of Linz/Austria. increasing the CO2 content by a factor ancestors, the cavemen, cultivated its sequestration in underground (oil wells, After a PhD in physics from the University of Vienna of 1.5. Furthermore, increase of global fire, we have been using natural car- under ocean and underground bedrocks (Austria) and postdoctoral temperature up to 6 degrees is pre- bon-based energy resources such etc.) without utilizing CO as such. It is not study at the University 2 dicted by Arrhenius upon increasing as wood, coal, oil, gas to create the clear if this storage over long times is sus- Figure 2. Space-Time diagram explaining the relation between transport and storage of energy. For a sustainable use of Stuttgart (Germany), of renewable energies these problems of storage and transport have to be solved. he joined the Institute for the CO content by a factor of 2. The energy we need. The industrial rev- tainable, thus CCS can eventually be com- Polymers and Organic 2 Solids at the University increase of CO2 by a factor of 3 will olution and the rapid development bined with CCU as a concentrated source of California, Santa Barbara, USA. His major take us back to Tertiary times when during the last 300 years burned car- of stored CO2. Differently, CCU approach contributions are in the fields “…arctic temperatures have exceeded bon-based fossil fuels significantly. covers a broad number of processes which of photoinduced optical, magnetic resonance and the present temperature about 8-9 This overusing enabled the very mod- can be applied to address the issue where transport phenomena in degrees.” ern human societies we today are CO is not only captured but also used as a semiconducting and metallic 2 polymers. He is the inventor At those days of Arrhenius, the proud of. More than 95% of the energy feedstock for various chemicals like formic of conjugated polymer Figure 1: This paper of Arrhenius from and fullerene based “bulk CO2 content in the atmosphere was used in transportation comes from oil. acid, carbon monoxide, methanol and meth- heterojunction” solar cells. around 300 ppm.1 By the time we This sector is growing significantly. In 1896 was surely beyond and before any ane. Michele Aresta from Bari and others political discussions of today. Reading this write this article the CO2 in the atmos- a globalized world economy trans- paper carefully can convince even the most have shown many different ways to utilize Figure 3. Different reference electrode potentials vs. NHE. Ag/AgCl electrode potential value is given for 3M KCl aggressive “anti-global-warming hardliners”. 3 phere is exceeding 410 ppm even port of goods is an essential sector. CO2 as chemical feedstock. solution. The vacuum level for the determination of energy bands are set to -4.75eV for NHE.

50 | November 2020 www.facs.website www.asiachem.news November 2020 | 51 states: “… the challenges that lie in the way artificial fuels using renewable energies is comparison, readers can refer to conversion first reduction peak of Lehn’s catalyst. to the hydrogen economy are enormous. immediately and direct method of large scale bar given below (Figure 3). This might be explained by the increased Fundamental problems will have to be solved storage. We have well established natural gas Assessing the catalyst performance is π-conjugation. if hydrogen gas is ever to become a practical, pipeline networks in many countries. Such a of high importance for comparing different everyday fuel that can be filled into tanks of our pipeline network has tremendous capacity of catalyst materials. There are several figures Examples of Heterogeneous motor cars or delivered to our homes as easily energy storage. of merit given through the paper namely, Catalysts for CO2 reduction and safely as gasoline…” faradaic efficiency, catalytic rate constantk , Homogeneous catalysis approaches

Therefore, we advocate to focus on CO2 Electrocatalysis of CO2 conversion overpotential and turnover number. for CO2 reduction have been widely used conversion into hydrocarbons as a feasible to hydrocarbons Faradaic efficiency (FE) defines the selec- throughout the history of the field. However, energy vector. By creating hydrogen gas and Below we give some illustrative and recent tivity of a catalyst towards a particular product as seen in the previous chapter electro- immediately converting it to hydrocarbons examples to make photoelectro- and elec- and can be calculated as: (moles product chemical addressing of the catalyst mate- using CO2 (Sabatier process) we will be better trocatalytic conversion of CO2 into synthetic / moles of electrons passes) × (number of rial which is swimming far away from the off, since the infrastructure for hydrocarbon fuels. It is shear impossible to give a compre- electrons needed for conversion) electrochemical double layer might be an fuels is readily available worldwide in form of hensive review of scientific literature here. We Catalytic rate constant (k) can be defined issue. Also recovery of the catalyst or sep- pipelines, tankers, trucks as well as distribu- refer interested readers to the listed books as a coefficient related to the rate of a chem- arating the product from the homogeneous 3 tion networks. and the references therein. The three main ical reaction (in this case reduction of CO2) at mixture with it, is a technological problem. On the other hand, equally important, this sections describe the homogenous (where a given temperature to the concentration of Different mechanisms can lead to degra- method of using CO2 recycling to make arti- the catalyst and CO2 are in same phase), reactant. dation, inhibition and eventual decrease in ficial fuel is an excellent method to store the the heterogeneous catalysis (where the cat- Overpotential = applied potential – thermo- overall efficiency. Bearing these drawbacks renewable energies in a transportable chem- alyst material is in solid phase while CO2 is dynamic (or formal) potential for conversion. in mind, researchers focused on the direct ical fuel (Fig. 2). Since renewable energies dissolved in the electrolyte solution) and the Turnover number (TON) = moles of desired addressing of catalysts using the idea of are supply driven energy sources which are bio-electrocatalysis. products / number of catalytically active sites heterogeneous catalysis. not predictable and highly fluctuating, their Throughout the text, applied potentials and/ (or moles of catalyst). Copper has always been the choice of widespread use is directly related to storage or the potential ranges are reported versus metal when several products and higher capacity. We are talking on Terawatthours reference electrodes like normal hydrogen Examples of Organometallic hydrocarbons like methanol, methane, of energy storage capacity here and it is electrode (NHE), saturated calomel electrode Complexes propanol, formic acid etc. were desired. nearly impossible to supply this with battery (SCE), silver-silver chloride electrode (Ag/AgCl) Without doubt organometallic complexes Readers are highly advised to read the systems. Such a conversion of CO2 into etc. as they are in the original papers. For are the most popular class of materials in detailed work of Hori on the electrochemi-

the field of CO2 reduction. Among them, cal reduction of CO2 using various kinds of Rhenium containing complexes6 reported by metals.8 Jean Marie Lehn group (Nobel Prize 1987) On the organometallic catalysis, the study in 1984. As homogeneous catalyst for elec- from Lieber and Lewis was one of the ear- Figure 7. Three generations of conjugated conducting polymers; a) Polymers with good conductivity but low trochemical reduction of carbon dioxide to liest which addressed a heterogeneous processability, b) Polymers with alkyl chains allowing solubility hence processability, c) Polymers with improved and/or 9 13 carbon monoxide Re(bpy)(CO)3Cl (Fig. 4) approach. Pyrolytic graphite or carbon new physical/chemical and catalytic properties. Reproduced with permission from reference. can produce 32mL of CO when held at a cloth were used as electrodes and they were potential of -1250mV vs. NHE for 14h without modified with Cobalt Phthalocyanine, via any degradation giving a remarkable faradaic drop casting or adsorption from solutions efficiency of 98% and a TON of 300. This in THF. They reached faradaic efficiencies

study was a door opener in the field of carbon up to 60% for CO and 35% for H2. A mag- dioxide reduction and evoked many other nificent turnover number of 370000 was studies thereafter. reached in this study. Hupp and co-workers came up with the Figure 5. (5,5‘-Bisphenylethynyl-2,2‘-bipyridyl)Re(CO)3Cl idea of incorporating a known catalyst, namely Fe-tetraphenyl porphyrin (Fe-TPP), into a metal-organic-framework (MOF)10. Authors note that the choice of MOF facil- itated the access of solvent, reactant and electrolyte solution further into the electro- active sites via their open nanomorphology. Furthermore, the metalloporphyrinic linkers within the MOFs served as both electrocata- lysts and as redox-hopping-based moieties for the delivery of reducing equivalents to catalytic sites. Authors reached faradaic Molecular structure of Re(bpy)(CO) Cl (Lehn’s Figure 4. 3 efficiencies up to ~60% for CO production. catalyst) One of the early studies from Halmann In 2012, Portenkirchner and co-workers used p-type gallium phosphide (GaP) as investigated the effect of extended π-con- the photoelectrode for driving the reduction 11 jugation on the catalytic activity of Lehn of CO2. GaP was immersed in electrolyte catalyst.7 solution together with a graphite rod as the Electrochemical characteristics of (6) counter electrode where saturated calomel were investigated with cyclic voltamme- electrode served as reference electrode. try technique and are displayed in Figure The choice of counter electrode was of stra- 6. It yielded a more positive reduction tegic decision since it was reported that car-

wave around -750mV vs. NHE which is bon does not oxidize formic acid and other Figure 8. Cyclic voltammogram depicting the potentiodynamic polymerization of [3HRe(bpy)(CO)3Cl-Th]. Inset: Photo 13 7 of a very thick film of the polymer in shiny gold color.Reproduced with permission from reference . Figure 6. Electrochemical behavior of (6) under N2 and under CO2. Reproduced with permission from ref. 330mV more positive compared to the carbohydrates back to carbon dioxide. The

52 | November 2020 www.facs.website www.asiachem.news November 2020 | 53 GaP electrode was illuminated using a Hg impregnated the polymer matrix with Pd and products. To utilize them in bio-elec- material on the working electrode can allow arc lamp and was biased with -1000mV vs. which was achieved by consecutive dip- tro-catalysis requires the immobilization of the direct electrical addressing of cata-

SCE. revealing formic acid, formaldehyde ping of the electrode first into K2PdCl4 and the enzymes on the electrodes as well as lyst bypassing the diffusion and methanol after 18 hours of irradiation. then into 0.1M KCl solution and final elec- their feeding with electrons, either directly step as in homogenous cat- Wrighton group reported the immobili- trochemical treatment to yield metallic Pd. or via electron shuttle molecules.14 alysts swimming in the solu- zation of Pd in a bipyridine based polymer After potentiostatic electrolysis in carbonate Schlager et al. demonstrated the immobi- tion. Another advantage of (PQ)2+ and its catalytic activity towards containing de-oxygenated solutions with lization of alcohol dehydrogenase on highly course is the easy recovery of 13 reduction of HCO3- to HCO2- in pres- C enriched carbonate solutions they con- porous carbon felt (CF) electrodes using alg- catalyst material as opposed to 12 ence of H2. Authors first polymerized the firmed the production of formate using NMR inate-silicate hybrid gel as the immobilization homogenous catalysis in a mixed bypridine monomer on W wires and then techniques. A faradaic efficiency of 80% matrix.15 Step-by-step preparation of matrix medium of the educts, products was achieved.12 as well as the enzyme-entrapped electrode and catalysts together. Economics

Third Generation of Conducting Polymers is given in (Fig. 10). of electrocatalytic CO2 reduction is (Figure 7) are synthesized where a solu- Alcohol dehydrogenase was used in a still under discussion whether it will be

tion processable and catalytical side chain control experiment with immobilized enzyme feasible or not in near future. Hybrid sys- Figure 12. Reduction mechanisms for CO2 catalyzed by dehydrogenases. Three-step reduction of CO2 to methanol functionalized conjugated polythiophene in the same matrix (alginate-silicate) driven tems where the catalyst is of bio origin and using NADH as sacrificial coenzyme (A) and via a direct electron transfer to the enzyme without any coenzyme (B). Reproduced with permission from reference 15. structure is achieved.13 only with NADH as the electron and proton the electron source is ideally a photo-elec- Apaydin et al. investigated polythiophene donor to check the activity of the enzymes tro active compound can pave the way for structures with pendant Lehn catalyst over time. They reached a 96% conver- energy efficient and selective conversion of

for the photoelectrochemical reduction sion when NADH was the electron/proton CO2. Biocatalytic systems work at room tem- 13 15 of CO2. donor . Formation of methanol using perature, atmospheric pressure and have This is one of the few studies where the the same matrix but using only electro- superior selectivity. This can play a decisive use of an organic semiconducting polymer chemical energy input without NADH role in the economical calculations of large

and its light absorbing properties are used resulted in a Faradaic efficiency of scale CO2 reduction processes. to drive photoelectrochemical reduction of This avenue of making a cyclic use of car- with low overpotentials (250mV) (Fig. 9). bon will be creating a carbon neutral fuel, which is important to transform our energy Immobilized Enzyme- sector. Our future energy vector shall have

Figure 9. Working principle of the polymeric catalyst Functionalized Electrodes this sustainable, cyclic use of materials and upon irradiation with light. H2 can be observed as a The most selective catalysts are natural 10% at a constant potential of -1200mV processes in accordance with the sustaina- product when the electrolyte medium is protic. F denotes enzymes. They work at ambient conditions vs. Ag/AgCl 15 (Fig. 11 and 12). ble development goals (SDG) of the United the catalyst functionalization on the polymer. Reproduced with permission from reference 13. with a remarkable selectivity toward educts Very recently Hathaichanok Nations. As Arrhenius stated it, our future Seelajeroen et al. demonstrated planetary life may depend on it. the immobilization of three different enzymes on a functionalized graphene. Acknowledgement: 16 In these nano-bio-catalysts we used We acknowledge the European graphene as electrical platform to hook Regional Development Fund (EFRE) three different enzymes thereon and within the project “ENZYMBIOKAT”

perform the biocatalysis from CO2 over (GZ2018-98279-2). Financial sup- formate over formaldehyde all the way port of the Austrian Science to methanol (see Fig. 13). Foundation (FWF) with the This study showed the successful Wittgenstein Prize (Z222 N19) immobilization and electrochemical for Prof. Sariciftci is gratefully ◆ Figure 13: Schematic description of electrically addressing three different enzymes immobilized on a functionalized addressing of enzymes to achieve acknowledged. 16 Figure 10. Experimental procedure: A) Alginate–silicate hybrid matrix solution containing alcohol dehydrogenase, B) graphene sheet as described in reference . bio-electrocatalytic reduction of CO2. gelated as beads in 0.2M CaCl2 or C) soaked with a CF electrode and D) gelated in 0.2M CaCl2 to obtain E) Alginate– enzyme modified carbon felt electrode.Reproduced with permission from reference 15. NADH can be replaced in the reaction cascade with direct electron injection References and suggested reading list: to enzymes. Biocatalytic systems such 1. S. Arrhenius, Phil. Mag. J. of Sci. 1896, 41, 237 6. J. Hawecker, J.-M. Lehn, R. Ziessel, Journal of the Dibenedetto, Rev. Mol. Biotechnol. 2002, 90, 113; as enzymes and bacteria are working 2. https://www.climate.gov/news-features/understanding- Chemical Society, Chemical Communications 1984, M. Aresta, A. Dibenedetto, Dalton Trans. 2007, at mild conditions like room temper- climate/climate-change-atmospheric-carbon-dioxide 984, 328; J. Hawecker, J.-M. Lehn, R. Ziessel, 2975; H. Wang, Z. Ren, “A comprehensive review 3. M. Aresta, Carbon Dioxide as Chemical Feedstock, Chemical Communications 1983, 536–538 of microbial electrochemical systems as a platform ature, atmospheric pressure and are Wiley-VCH, Weinheim, 2010; M. Aresta, G. Fortz, 7. E. Portenkirchner, K. Oppelt, C. Ulbricht, D. a M. technology.”, Biotechnol. Adv., 2013, 31, 1796; H. superior to all other catalysts in their Carbon Dioxide as a Source of Carbon: Biochemical Egbe, H. Neugebauer, G. Knör, N. S. Sariciftci, Li, P. H. Opgenorth, D. G. Wernick, S. Rogers, T.-y. selectivity. and Chemical Uses, D. Reidel Publishing Company, Journal of Organometallic Chemistry 2012, 716, Wu, W. Higashide, P. Malati, Y.-x. Huo, K.-M. Cho, Dordrecht, 1987; M. Aresta, Carbon Dioxide 19–25; K. Oppelt, D. A. M. Egbe, U. Monkowius, M. J. C. Liao, “Integrated Electromicrobial Conversion

Recovery and Utilization, Kluwer Academic List, M. Zabel, N. S. Sariciftci, G. Knör, Journal of of CO2 to Higher Alcohols”, Science, 2012, 335, SUMMARY AND OUTLOOK Publishers, Dordrecht, 2003; Martin M. Halmann, Organometallic Chemistry 2011, 696, 2252–2258. 1596; S. Kuwabata, R. Tsuda, H. Yoneyama, In this general review, we attempted Chemical Fixation of Carbon Dioxide, CRC Press 8. Y. Hori, H. Wakebe, T. Tsukamoto, O. Koga, “Electrochemical Conversion of Carbon Dioxide 1993; Balasubramanian Viswanathan, Vaidyanathan Electrochimica Acta 1994, 39, 1833–1839. to Methanol with the Assistance of Formate to make the case for the idea of CO2 Subramanian, Jae Sung Lee (eds.), Materials and 9. C. M. Lieber, N. S. Lewis, J. Am. Chem. Soc 1984, Dehydrogenase and Methanol Dehydrogenase as conversion to artificial fuels using Processes for Solar Fuel Production, Springer 2014; 106, 5033–5034 Biocatalysts”, J. Chem. Soc., 1994, 116, 5437. renewable energies. Using metallic, M. Kaneko, I. Ochura, Photocatalysis: Science and 10. J. T. Hupp, ACS Catalysis 2015, 5, 6302–6309 15. S. Schlager, H. Neugebauer, M. Haberbauer, Technology, Springer, Heidelberg, 2002; 11. M. Halmann, Nature 1978, 275, 115 –116 G. Hinterberger, N. S. Sariciftci, ChemCatChem organometallic, organic and bioorganic 4. Jeremy Rifkin, The Hydrogen Economy, Polity Press 12. C. Stalder, S. Chao, M. S. Wrighton, J. Am. Chem. 2015, 7, 967–971; S. Schlager, L. M. Dumitru, M. catalysts one can initiate electrocat- 2002, ISBN 0-7456-3041-3 Soc 1984, 3673–3675 Haberbauer, A. Fuchsbauer, H. Neugebauer, D. alytic, photo electrocatalytic and bio- 5. George A. Olah, Alain Goeppert and G.K. 13. D. H. Apaydin, E. Tordin, E. Portenkirchner, G. Hiemetsberger, A. Wagner, E. Portenkirchner, N. S. electrocatalytic reactions to achieve Surya Prakash, Beyond Oil and Gas: The Aufischer, S. Schlager, M.Weichselbaumer, K. Sariciftci, ChemSusChem 2016, 9, 631–635. Methanol Economy, Wiley-VCH 2006, Oppelt, N. S. Sariciftci, ChemistrySelect 2016, 1, 16. H. Seelajaroen, A. Bakandritsos, M. Otyepka, R. this goal. ISBN 978-3-527-31275-7 1156 –1162 Zboril, N. S.Sariciftci, Representation of the electrochemical CO reduction using enzymes. Electrons are injected directly into Figure 11. 2 CO reduction using heterogene- 14. B. E. Logan, Microbial Fuel Cells, Wiley, New Jersey, 17. ACS Applied Materials & Interfaces 2020, 12, 250 the enzymes, which are immobilized in an alginate–silicate hybrid gel (green) on a carbon-felt working electrode. 2 ous catalytic approach has advan- 2008; M. Aresta, A. Dibenedetto, C. Pastore, CO2 is reduced at the working electrode. Oxidation reactions take place at the counter electrode. Reproduced with Environ. Chem. Lett. 2005, 3, 113; M. Aresta, A. permission from reference 15. tages: The immobilization of catalyst

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