Selectivity in Electrochemical CO₂ Reduction

Abstract

Conspectus The electrocatalytic CO2 reduction reaction (CO2RR) to generate fixed forms of carbons that have commercial value is a lucrative avenue to ameliorate the growing concerns about the detrimental effect of CO2 emissions as well as to generate carbon-based feed chemicals, which are generally obtained from the petrochemical industry. The area of electrochemical CO2RR has seen substantial activity in the past decade, and several good catalysts have been reported. While the focus was initially on the rate and overpotential of electrocatalysis, it is gradually shifting toward the more chemically challenging issue of selectivity. CO2 can be partially reduced to produce several C1 products like CO, HCOOH, CH3OH, etc. before its complete 8e–/8H+ reduction to CH4. In addition to that, the low-valent electron-rich metal centers deployed to activate CO2, a Lewis acid, are prone to reduce protons, which are a substrate for CO2RR, leading to competing hydrogen evolution reaction (HER). Similarly, the low-valent metal is prone to oxidation by atmospheric O2 (i.e., it can catalyze the oxygen reduction reaction, ORR), necessitating strictly anaerobic conditions for CO2RR. Not only is the requirement of O2-free reaction conditions impractical, but it also leads to the release of partially reduced O2 species such as O2 –, H2O2, etc., which are reactive and result in oxidative degradation of the catalyst. In this Account, mechanistic investigations of CO2RR by detecting and, often, chemically trapping and characterizing reaction intermediates are used to understand the factors that determine the selectivity in CO2RR. The spectroscopic data obtained from different intermediates have been identified in different CO2RR catalysts to develop an electronic structure selectivity relationship that is deemed to be important for deciding the selectivity of 2e–/2H+ CO2RR. The roles played by the spin state, hydrogen bonding, and heterogenization in determining the rate and selectivity of CO2RR (producing only CO, only HCOOH, or only CH4) are discussed using examples of both iron porphyrin and non-heme bioinspired artificial mimics. In addition, strategies are demonstrated where the competition between CO2RR and HER as well as CO2RR and ORR could be skewed overwhelmingly in favor of CO2RR in both cases.

Spin-control in electrocatalysis for clean energy

“…Therefore, it is crucial to find ways to control selectivity. Recently, Saha et al [57] reported that the spin state of the metal center (M) in catalysts strongly affects the reaction pathway. As shown in Figure 4a, M-COOH is the key intermediate to CO or HCOOH.…”

“…We propose that these studies inspire the exploration of the CISS effect in NRR, for example by using chiral molecules on NRR catalysts. [57] Copyright © 2022 American Chemical Society. c reproduced with permission.…”

Tracking the Oxygen Dynamics of Solid–Liquid Electrochemical Interfaces by Correlative In Situ Synchrotron Spectroscopies

“…The development of advanced energy conversion devices, such as fuel cells and water electrolyzers, is of essential importance for the great blossoming of the modern green and sustainable energy industry. ,− To carry forward the practical deployment of these energy systems, the requirements of active electrocatalysts with high activity and robust long-term durability should be satisfied for efficient fuel cells and water electrolyzers. , Oxygen electrocatalytic reactions, including the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), occur at the solid–liquid electrochemical interfaces of catalytic electrodes and greatly affect the overall efficiency and operational durability of industrial fuel cells and water electrolyzers due to their intrinsically sluggish kinetics. − Therefore, it is very significant to completely understand the essential dynamics of solid–liquid electrochemical interfaces during electrocatalysis to rationally design, synthesize, and screen advanced oxygen electrocatalysts.…”

Immobilization strategies for porphyrin-based molecular catalysts for the electroreduction of CO2

“…[49][50][51][52][53] Additionally, the highly conjugated system of porphyrin-based molecules result in a number of invaluable properties, such as enhanced electronic conductivity and p-p stacking capabilities. [54][55][56][57] Although there exist several overarching reviews of CO 2 electrocatalysts including even more specic reviews on porphyrin/phthalocyanine catalysts, 50,51 a coverage of more recent developments in the eld is required. Herein, major advances involving the functionalization of porphyrin and phthalocyanine catalysts for electrochemical CO 2 RR will be recounted in detail with a focus on recent advances in heterogeneous electrocatalysts and the effects of various immobilization strategies on catalytic performance.…”