To design electrochemical interfaces for efficient electric-chemical energy interconversion, it is critical to reveal the electric double layer (EDL) structure and relate it with electrochemical activity; nonetheless, this has been a long-standing challenge. Of particular, no molecular-level theories have fully explained the characteristic two peaks arising in the potential-dependence of the EDL capacitance, which is sensitively dependent on the EDL structure. We herein demonstrate that our first-principles-based molecular simulation reproduces the experimental capacitance peaks. The origin of two peaks emerging at anodic and cathodic potentials is unveiled to be an electrosorption of ions and a structural phase transition, respectively. We further find a cation complexation gradually modifies the EDL structure and the field strength, which linearly scales the carbon dioxide reduction activity. This study deciphers the complex structural response of the EDL and highlights its catalytic importance, which bridges the mechanistic gap between the EDL structure and electrocatalysis.
Electrocatalysis, whose reaction venue locates at the catalyst–electrolyte interface, is controlled by the electron transfer across the electric double layer, envisaging a mechanistic link between the electron transfer rate and the electric double layer structure. A fine example is in the CO2 reduction reaction, of which rate shows a strong dependence on the alkali metal cation (M+) identity, but there is yet to be a unified molecular picture for that. Using quantum-mechanics-based atom-scale simulation, we herein scrutinize the M+-coupling capability to possible intermediates, and establish H+- and M+-associated ET mechanisms for CH4 and CO/C2H4 formations, respectively. These theoretical scenarios are successfully underpinned by Nernstian shifts of polarization curves with the H+ or M+ concentrations and the first-order kinetics of CO/C2H4 formation on the electrode surface charge density. Our finding further rationalizes the merit of using Nafion-coated electrode for enhanced C2 production in terms of enhanced surface charge density.
Single-atom catalysts
(SACs) featuring atomically dispersed metal
cations covalently embedded in a carbon matrix show significant potential
to achieve high catalytic performance in various electrocatalytic
reactions. Although considerable advances have been achieved in their
syntheses and electrochemical applications, further development and
fundamental understanding are limited by a lack of strategies that
can allow the quantitative analyses of their intrinsic catalytic characteristics,
that is, active site density (SD) and turnover frequency (TOF). Here
we show an
in situ
SD quantification method using
a cyanide anion as a probe molecule. The decrease in cyanide concentration
triggered by irreversible adsorption on metal-based active sites of
a model Fe–N–C catalyst is precisely measured by spectrophotometry,
and it is correlated to the relative decrease in electrocatalytic
activity in the model reaction of oxygen reduction reaction. The linear
correlation verifies the surface-sensitive and metal-specific adsorption
of cyanide on Fe–N
x
sites, based
on which the values of SD and TOF can be determined. Notably, this
analytical strategy shows versatile applicability to a series of transition/noble
metal SACs and Pt nanoparticles in a broad pH range (1–13).
The SD and TOF quantification can afford an improved understanding
of the structure–activity relationship for a broad range of
electrocatalysts, in particular, the SACs, for which no general electrochemical
method to determine the intrinsic catalytic characteristics is available.
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