Quantification of Active Site Density and Turnover Frequency: From Single-Atom Metal to Nanoparticle Electrocatalysts

Abstract

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.

Improving the intrinsic activity of electrocatalysts for sustainable energy conversion: where are we and where can we go?

“… 10 A few approaches have been used by our community for the explicit determination of the “true” number of active sites, including (1) electrochemical oxidation that yields distinct electrochemical features corresponding to the active edge sites and the inert basal plane sites for MoS 2 catalysts for the HER, 11 (2) the integration of the area below the redox peak for a redox reaction (M ( n +1)+ /M n + ) just before the onset of the OER to estimate the concentration of active sites (M n + ) for 3d transition metal based electrocatalysts, 12 and (3) the use of surface probes like Pb, Cu, CO and CN that selectively adsorb on certain types of active sites ( e.g. undercoordinated sites) that have been used to quantify active site densities for single atom catalysts for the ORR, 13 WS 2 nanosheets for the HER 14 and Au catalysts for the CO 2 RR. 15 …”

Time‐Resolved Potential‐Induced Changes in Fe/N/C‐Catalysts Studied by In Situ Modulation Excitation X‐Ray Absorption Spectroscopy

“…The latter often poses a limit when it comes to signal intensity, especially for electrochemical in situ measurements requiring suitable electrode loadings (commonly <5 mg catalyst cm -2 , i.e., <0.15 mg metal cm -2 -a comparatively low sample amount for spectroscopic measurements) to achieve the high catalyst utilizations needed for reliable spectroscopic results. [53,54] Most crucially, only a low fraction of these MN x C y sites are typically located within the operando-relevant catalystelectrolyte interface (possibly as little as ≈10-20 %, according to recent nitrate-/cyanide-poisoning and in situ MS measurements [36,37,[55][56][57][58] ) thus jeopardizing the interpretation of in situ spectroscopic results that are likely to be preponderantly representative of the materials' bulk composition.…”

Dual‐Metal Atom Electrocatalysts: Theory, Synthesis, Characterization, and Applications

“…Progress has been made on TOF calculations of SACs by measuring active site density using ex situ cryo CO pulse chemisorption and temperature-programmed desorption, [120,121] in situ electrochemical NO 2stripping [122] and, recently, in situ cyanide anion probe. [123] Lower TOF values are obtained with temperature-programmed desorption of CO, as the ex situ adsorbed CO overestimates the number of electrochemically accessible active sites, resulting in a lower calculated TOF. Mössbauer spectroscopy can also probe the total density of specific sites, although deviating from the number of sites accessible for adsorption of O 2 or CO, due to being a bulk characterization technique (Section 4.2.3).…”