Mass Transport Control by Surface Graphene Oxide for Selective CO Production from Electrochemical CO₂ Reduction

Abstract

Electrochemical CO 2 reduction is always accompanied by a competitive hydrogen evolution reaction as water is used as a hydrogen source. In addition to intrinsic activity control, geometrical factors of electrocatalysts such as their porous structure have been demonstrated to affect the reaction selectivity, but understanding its origin is still important. Herein, we demonstrate that reduced graphene oxide layers can effectively control the Faradaic efficiency for CO production of porous zinc nanoparticle electrocatalysts. Simply tuning the coverage of graphene oxide dramatically varies Faradaic efficiency for CO production from 66 to 94% even in the bicarbonate electrolyte at the same biased potential, in which the hydrogen evolution rate was notably suppressed without sacrificing CO 2 reduction to CO production rate unlike many Zn-based electrocatalysts. The graphene oxide layers are revealed to play roles in providing geometric barriers for the mass transport channels of reactants rather than changing the chemical states of the Zn-based electrocatalysts according to in situ X-ray absorption spectroscopic analysis and electrochemical reaction kinetic studies. In addition, computational fluid dynamics simulation studies estimate the Faradaic efficiency dependence on the surface coverage and suggest that the selective suppression of H 2 evolution is associated with the larger increment in local pH compared to that in local pCO 2 at the porous electrocatalyst surfaces. Decoupling between these reactant concentrations is originated from the higher consumption rate and lower bulk concentration of proton compared to those of CO 2 , and the surface coating with graphene oxide can be an effective way to control mass transport channel.

Three-Dimensional Cathodes for Electrochemical Reduction of CO2: From Macro- to Nano-Engineering

“…The type of catalyst material plays an important role in determining catalytic properties, elementary steps, and the overall mechanism and pathway of the reaction. Earlier mechanistic studies have focused on obtaining key catalytic parameters related to performance, total yield, selectivity, and Faradaic efficiency under hard assumptions such as absence of mass transport limitation, low intermediate coverage and steady-state reaction conditions [260,262,263].…”

“…Three-dimensional (3D) porous electrodes have also been used for CO 2 RR, where the porosity of the catalytic medium provides an additional parameter to tune concentration gradients of active species and thereby control the selectivity [255,262]. Experimental studies have shown that the selectivity of CO 2 RR can be considerably improved by increasing electrode porosity.…”

“…The porosity of the electrode can reduce the impact of the HER on the selectivity of the CO 2 RR for producing CO [66,260,262]. The selectivity of CO production has been shown to be insensitive to the thickness of the porous Au catalyst [260].…”

“…[57,257]. By increasing pH at low overpotentials, the current density attributed to production of HCOO − species increases [262]. A reaction mechanism for the electroreduction of CO 2 to CO on a Ag electrode based on a concerted proton-electron transfer (CPET) is depicted in Figure 15 [255,259].…”

“…The majority of modeling studies of mass transport effects in CO 2 RR have explored planar electrodes [115,116,256]. A planar geometry imposes a higher mass transport limitation on the electrochemical cell than that in a 3D porous electrode, which may hinder the transport rate of active species from bulk of electrolyte to the electrode surface [99,262]. Two major designs are proposed in the literature to minimize the effect of mass transport on performance.…”