Electrochemical reduction of CO₂ to CO and HCOO⁻ using metal–cyclam complex catalysts: predicting selectivity and limiting potential from DFT

Abstract

Sustainable fuel production from CO2 through electrocatalytic reduction is promising but challenging due to high overpotential and poor product selectivity. Herein, we computed the reaction free energies of electrocatalytic CO2...

Mechanism and Selectivity of Electrochemical Reduction of CO2 on Metalloporphyrin Catalysts from DFT Studies

“…There is a constant difference of 34.88 kJ/mol between ΔG° rxn (HCOO − ) and ΔG° rxn (H 2 ) (H2O) , ΔG° rxn (H 2 ) (H2O) = ΔG° rxn (HCOO − ) + 34.88 kJ/mol [ 20 ], as presented graphically in Figure 1 . This result indicates that HCOO − formation is always more thermodynamically favorable than H 2 formation.…”

“…The Stuttgart–Dresden effective core potentials [ 59 ] were adopted for transition metals, and a 6-31g(d,p) basis set was used for the main group elements. The choice of the basis set has been extensively tested against 6-311++G(d,p) in our previous study [ 20 ]. Our results showed that the computed reduction potentials using 6-311++G(d,p) and 6-31G(d,p) basis sets differed by less than 50 mV.…”

“…The methods of determining the reduction potential of the electron transfer steps and pK a value of the carboxylate adducts ([MP-COOH] 0 and ([MP-COOH] − ) have been presented in our previous study [ 20 ]. The reduction potentials were reported by referencing the standard hydrogen electrode (SHE).…”

“…CO can be fed to the Fischer–Tropsch process [ 12 ] for the synthesis of alkanes, whereas HCOO − can be protonated to formic acid and used as a commodity chemical or fuel in a direct formic acid fuel cell [ 13 ]. Electrocatalysts such as pincer complexes [ 14 , 15 , 16 , 17 , 18 ], cyclam complexes [ 19 , 20 , 21 , 22 ], porphyrins [ 23 , 24 , 25 , 26 , 27 ], phthalocyanines and iron carbonyl [ 28 , 29 , 30 , 31 , 32 ] as well as metal–organic frameworks (MOFs) [ 33 , 34 ] have been tested for CO 2 reduction.…”

“…Desorption of CO from [LM-CO] 0 completes the CO cycle. On the other hand, one-electron reduction of [ML-H] (n−1) leads to metal hydride ([ML-H] (n−1) ), which then reacts with CO 2 or H + to form HCOO − or H 2 [ 14 , 20 , 38 , 39 , 40 ]. The intrinsic property of the metal complex as well as the operating pH and applied potential determines which pathway the reaction will follow and what product will form from the electrochemical reduction of CO 2 .…”