Electrocatalysts for CO2 reduction

Figure 5.1.9 PEC solar cell. Bottom scheme of the cell with electron microscopy images of a particular of the Ti02-nanotube array electrode and of the Fe nanoparticles on N-doped carbon nanotubes, used as a photocatalyst for water oxidation and an electrocatalyst for CO2 reduction, respectively. It is also shown that it may be possible to use this cell for the production of H2/O2 in separate compartments by water photoelectrolysis. Top photo of the experimental cell and of the assembly of the photoanode with the Nafion membrane. Adapted from [14, 40, 52],...

Re diimine complexes act as photocatalysts and/or electrocatalysts for CO2 reduction to CO. Examples include the tricarbonyl complexes yac-[Re(Q -diimine)(CO)3L]" [n = 0, L = halide n = 1, L = NCMe, P(OR)3 a-diimine = 1,4-disubstituted 1,4-diazabuta-l,3-dienes or bpy and related chelating N-heterocycles], for example, fac-[Re(dmb)(CO)3(NCMe)]+, 5 [Re(dmb)(CO)3]2" and fac-[Re(bpy)(CO)3 P(OPfl)3 ]+. Electron-transfer from an amine electron donor (e g. triethanolamine or triethylamine) to the excited state complex is usually considered as the initiation of the photocatalysis, and metallocarboxylates and metallo-carboxyUc acids have been proposed as intermediates in the formation of CO. The electrocatalytic process is triggered by a 1-electron or a 2-electron cathodically induced chloride (X) or L ligand dissociation to form the catalytic species. ... 

On the other hand, all CV s for TMS HPT s are irreversible, and for that reason TMS HPT s can not be used in practice as electrocatalysts for CO2 reduction in nonpolar solvents. The irreversibility is most likely caused by the lack of protons, which are needed for the reductions. However, the CV s are reversible in aqueous solutions and we decided to investigate another route for CO2 reduction by TMS HPT s. 

There are several reports on the use of MPc complexes as electrocatalysts for the reduction of CO2, but reports on the use of these complexes for the oxidation of CO are scarce. Diffusion electrodes have been employed extensively for the use of MPc complexes as electrocatalysts for CO2 reduction. Using porous gas diffusion electrodes, Fumya and coworkers studied the activity of a series of MPc complexes towards the reduction of CO2 and found the activity to be depended on the nature of the central metal. Table 7.4 . On FePc and PdPc modified electrodes both hydrogen and CO were obtained, on ZnPc and AlPc, the main products were hydrogen, carbon dioxide and formic acid and on H2PC, MgPc, MnPc,... 

Bourrez M, Molton F, Chardon-Noblat S, Deronzier A (2011) [Mn(bipyridyl)(CO)3Br] an abundant metal carbonyl complex as efficient electrocatalyst for CO2 reduction. Angew Chem Int Ed 50(42) 9903-9906. doi 10.1002/anie.201103616... 

DuBois MR, DuBois DL (2009) Development of molecular electrocatalysts for CO2 reduction and H-2 production/oxidatitm. Ace Chem Res 42(12) 1974-1982... 

Finally, it is worth mentioning that nickel or palladium complexes with poly-NHC ligands have been also employed as electrocatalysts for CO2 reduction (Fig. 31). Although the reaction rates are modest, the catalysts, particularly the Pd ones, appear stable against dimerization pathways ultimately leading to catalyst decomposition hence, they hold promise for this reaction provided their activity can be improved. 

Figure 31 Nickel and palladium complexes with poly-NHC ligands employed as electrocatalysts for CO2 reduction.

Os°(bpy)(CO)2] polymer has been characterized by IR, UV-Vis and EPR spectroelectrochemistry. The polymer species is an electron-rich electrocatalyst for CO2 reduction and its fair solubility in polar organic solvents is important for potential use. 

Other metal complexes such as 2,2 -bipyridine complexes of Rh and Ir are efficient electrocatalysts for the reduction of C02 in acetonitrile.134 In the production of formate the current efficiency is up to 80%. Electrochemical reduction catalyzed by mono- and dinuclear Rh complexes affords formic acid in aqueous acetonitrile, or oxalate in the absence of water.135 The latter reaction, that is, the reduction of C02 directed toward C-C bond formation, has attracted great interest.131 An exceptional example136 is the use of metal-sulfide clusters of Ir and Co to catalyze selectively the electrochemical reduction of C02 to oxalate without the accompanying disproportionation to CO and CO2-. 

Since the two-electron reduction to formic acid or CO requires a lower potential, electrolysis using a multielectron transfer catalyst in aqueous or in low-protic media can be carried out at considerably lower voltages. The simplest electrocatalytic system for CO2 reduction is an electrochemical cell that contains a working electrode, a reference electrode, a homogeneous electrocatalyst, the supporting electro-... 

A number of 14-membered tetraazamacrocyclic complexes serve as catalysts for photochemical and electrochemical CO2 reduction. [CoHMD(H20)](C104)2 (HMD = 5,7,7, 12,14,14-hexamethyl-l,4,8,1 l-tetraaza-cyclotetradeca-4,11-diene) [ 1,2] and Ni(cyclam)Cl2 (cyclam = 1,4,8,11-tetraazacyclotetradecane) [3] have been used as electrocatalysts for the reduction of CO2 in H2O or aqueous CH3CN. The ratio for CO/H2 production is 1 for [CoHMD(H20)](C104)2 and >100 for Ni(cyclam)Cl2. Metal(I) complexes, metal(III) hydride complexes, and metallocarboxylates such as [Ni (cyclam)(C02 )] are postulated as intermediates in the electro- and photo-chemical CO2 reduction [4]. 

Tryk el al. studied GDEs composed of active carbon fiber and loaded with catalysts Ni, Fe, Pd metals, porphyrins, and phthalo-cyanines. The GDEs gave partial current density of CO2 reduction up to 80 mA cm 2 with production of CO under atmospheric pressure. They presumed that the nanopores present in active carbon fiber may provide quasi high pressure atmosphere due to nanoscale effect." Thus Ni electrocatalyst, which is practically inert for CO2 reduction under atmospheric pressure, may be activated in a similar manner as observed with Ni electrode under elevated pressure. " ... 

This equilibrium is central to acetogenesis, i.e. the mildly exothermic reaction H2 + CO2 —> acetic acid. Complementarily, CODH is utilized by some methano-genic archeons to extract energy by the disproportionation of acetic acid into CO2 and CH4 [126]. Purified CODH from Moordla thermoacetica is an exceptional electrocatalyst for the reduction of CO2 (700 tumovers/hour, 100% conversion, -0.57 V vs. NHE) [127]. 

Dubois DL (1997) Development of transition metal phosphine complexes as electrocatalysts for CO2 and CO reduction. Comments Inorg Chem 19 307-325... 

CO2 reduction. Therefore, they are potential electrocatalysts for the reduction of CO2 (see Scheme 9.16). Two electrons are transferred from the electrode to the active site (buried inside the insulating protein interior) by the iron-sulfur clusters, to reduce CO2 to formate, forming a C-H bond. Conversely, when formate is oxidized, the two electrons are transferred from the active site to the electrode. 

Schwarz et al. incorporated Perovskite-type electrocatalysts Al 8A o 2Cu04 (A = La, Pr. and Gd A = Sr and Th) in GDEs with the electrolyte solution 1 M KOH. They reported that CO2 was reduced to CH3OH, C2H5OH and w-CiHiOH with the total current yield of alcohols 40% of the current density 180 mA cnf They studied various perovskite type oxides, and argued that such oxides without Cu are not active for CO- reduction. 

A number of transition-metal complexes, both in solution and on electrode surfaces, have been shown to be effective in the electrocatalytic reduction of carbon dioxide. All of those complexes significantly decrease the overpotential for reduction of CO2 by up to IV (as compared to a 1-el reduction to the C02 radical), and yield various multielectron reduction products. Known electrocatalysts yield primarily carbon monoxide and formate anion as the major products of the CO2 reduction. Sullivan et al. did detailed mechanistic work on a sales of bipyridine complexes of transition metals, and made several suggestions concerning the design of new electrocatalysts that would be capable of reducing CO2 past the CO and formate step. Their major recommendation is to use as electrocatalysts "electron reservoir" complexes, i.e. compounds capable of storing multiple electrons. 

Functional Models of CODH Molecular Electrocatalysts for Reduction of CO2... 

Shimizu K, Cheng IF, Wang JS, Yen CH, Yoon B, Wai CM. Water-in-supercritical CO2 microemulsion for synthesis of carbon-nanotube-supported Pt electrocatalyst for the oxygen reduction reaction. Energy Fuels 2008 22 2543-9. 

Electrocatalysts for Carbon Dioxide Reduction, Fig. 1 CO2 reduction catalyzed by macrocyclic metal complexes... 

Beley M, Collin JP, Ruppert R, Sauvage JP (1984) Nickel(II)-cyclam an extremely selective electrocatalyst for reduction of CO2 in water. J Chem Soc Chem Commun 1315—1316... 

Agarwal J, Shaw TW, Stanton CJ III, Majetich GF, Bocarsly AB, Schaefer HF III (2014) NHC-containing manganese(I) electrocatalysts for the two-electron reduction of CO2. Angew Chem Int Ed 53 5152-5155... 

Since 1994, pyridinium and its substituted derivatives have been identified as effective and stable homogeneous electrocatalysts for the aqueous multiple-electron, multiple-proton reduction of CO2 to various products, such as formic acid, formaldehyde and methanol. Particularly high Faradaic yields were reached in the reduction of CO2 to methanol in both electrochemical and photoelectrochemical systems under energetically advantageous conditions [149]. 

Alkaline Fuel Cell. The electrolyte ia the alkaline fuel cell is concentrated (85 wt %) KOH ia fuel cells that operate at high (- 250° C) temperature, or less concentrated (35—50 wt %) KOH for lower (<120° C) temperature operation. The electrolyte is retained ia a matrix of asbestos (qv) or other metal oxide, and a wide range of electrocatalysts can be used, eg, Ni, Ag, metal oxides, spiaels, and noble metals. Oxygen reduction kinetics are more rapid ia alkaline electrolytes than ia acid electrolytes, and the use of non-noble metal electrocatalysts ia AFCs is feasible. However, a significant disadvantage of AFCs is that alkaline electrolytes, ie, NaOH, KOH, do not reject CO2. Consequentiy, as of this writing, AFCs are restricted to specialized apphcations where C02-free H2 and O2 are utilized. 

Oxidation of CO is also important in fuel cell applications. By combining the half reaction for the CO2/CO couple, equation (a), with electrochemical reduction of O2, fuel cells may achieve a maximum open circuit potential given by IlSlfCOj/CO) - (02/H20) = 1.34 . In practice, electrocatalysts are required to lessen the normally high kinetic overpotentials for electrodic CO oxidation. An example of a CO/O2 fuel cell which operates at the relatively low T of 80°C by employing [Rh(CO)2Br2] as the electrocatalyst and the Rh couple to mediate CO oxidation is shown below . 

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