Probing consequences of anion-dictated electrochemistry on the electrode/monolayer/electrolyte interfacial properties

Altering electrochemical interfaces by using electrolyte effects or so-called “electrolyte engineering” provides a versatile means to modulate the electrochemical response. However, the long-standing challenge is going “beyond cyclic voltammetry” where electrolyte effects are interrogated from the standpoint of the interfacial properties of the electrode/electrolyte interface. Here, we employ ferrocene-terminated self-assembled monolayers as a molecular probe and investigate how the anion-dictated electrochemical responses are translated in terms of the electronic and structural properties of the electrode/monolayer/electrolyte interface. We utilise a photoelectron-based spectroelectrochemical approach that is capable of capturing “snapshots” into (1) anion dependencies of the ferrocene/ferrocenium (Fc/Fc+) redox process including ion-pairing with counter anions (Fc+–anion) caused by differences in Fc+–anion interactions and steric constraints, and (2) interfacial energetics concerning the electrostatic potential across the electrode/monolayer/electrolyte interface. Our work can be extended to provide electrolyte-related structure-property relationships in redox-active polymers and functionalised electrodes for pseudocapacitive energy storage.


Figure S1
Supplementary Figure 1. Schematic of experimental setup comprised of X-ray and ultraviolet photoelectron spectroscopy combined with an electrochemical cell (EC-XPS/UPS). Electrochemical measurements are performed inside a dedicated "EC chamber" using a hanging meniscus configuration under Ar atmosphere followed cell retraction under potential control, chamber evacuation and then sample transfer. The XPS/UPS analysis chamber connected to the EC chamber via gate valve and enables sample transfer back and forth between the analysis chamber and EC chamber.

Supplementary Note 2: Cyclic voltammograms (CVs) and non-ideal behaviour
All of the CVs in Figure 2b of the main text corresponds to a reversible surface-bound redox reaction. This is evidenced by the near negligible anodic to cathodic peak separations (< 5 mV) and peak currents that scale linearly with scan rate. We note that purification of the Fc SAM via silica-gel column chromatography resulted in the same non-ideal behaviour with CVs that were virtually identical indicating that impurities are unlikely the origin of the non-ideal behaviour.
The ideal surface-bound Nernstian reaction exhibits a CV that contains a single peak (FWHM of 90.6/n mV, n is the number of electrons) and negligible peak-to-peak separation. However, Fc SAM at higher coverages commonly exhibit deviations from ideal behaviour with asymmetric/multiple peaks and peak broadening. [8][9][10] In terms of the origin of the non-ideal behaviour, there have been several proposed origins including: (1) Heterogeneity at the local level with different structured/packed domains 11 or so-called isolated and clustered Fc. 9 Rudnev et al 12 performed a CV and scanning probe microscopy investigation of lowindex single crystal and polycrystalline Au, to show that the CV response is related to differences Fc SAM ordering (local disordering and ordered domains), and this further depends on the crystallographic surface structure. The Fc SAM coverage can be diluted with non-electroactive nalkanethiols to yield CVs with characteristics that are more ideal.
(2) Along a similar thread is the nature of the intermolecular interactions experienced by the Fc termini. For surface-bound redox-active monolayers, the CVs can be fitted to phenomenological models based on the Langmuir or Frumkin isotherms to provide insights into the nature of the intermolecular interactions. 11,13 A FWHM of 90.6/n mV corresponds to the absence of lateral interactions (between oxidised and reduced forms of Fc, i.e. O-O, R-R and O-R) whereas an FWHM that is greater or smaller than 90.6/n mV corresponds to attractive and repulsive interactions, respectively.
(3) Buried Fc termini 14 and double-layer effects. 15,16 As experimentally shown by Rowe and Creager 17 and modelled by Smith and White, 18 positional differences between the plane of electron transfer (PET) and the closest approach of ions can lead to so-called double-layer effects causing the multiplicity and broadening of the CV peaks. 16 For instance, it has been suggested that due to the mismatch between the bulkier Fc termini and the alkyl chain, the resulting strain can cause the presence of buried Fc 14 which can result in multiple PET (plane of electron transfer) and thus leads to non-ideal behaviour.
We note that other methods to obtain more ideal CVs include the inclusion of polar functional groups 8 or utilization of a rigid spacer. 19

Supplementary Table 1. XPS-determined stoichiometry for pristine Fc SAM and after polarization at E(anodic) and E(cathodic)
showing the formation of 1:1 Fc + -Xion-pairs upon oxidation to Fc + . The similarity of the Fe 2p area ratios) indicates the reversibility of the EC-XPS/UPS method following electrochemistry and sample transfer. The tabulated data corresponds to the spectra in Figures 3 and S10. The origin of this behaviour is the focus of a future investigation. Nonetheless, the degree of conversion to Fc + as indicated by → in each figure equates to ~4.5-4.7×10 -10 mol cm -2 (theoretical Fc coverage is 4.5×10 -10 mol cm -2 ), which suggests that due to steric constraints of TFSIanions, full conversion to Fc + (seen in Figure 2b of main text) is restricted. On the other hand, in the presence of smaller anions (PF6, ClO4 -) the full conversion to Fc + can proceed.  Change in effective thickness following polarization at E(anodic) and E(cathodic) based on the attenuation differences in the Au 4f spectra (See supplementary note 3 on the method used to determine effective thickness and additional discussion). Error bars indicate SD based on at least 3 independent measurements. The effective thickness of ~18 Å for the pristine Fc SAMs is in line with existing reports utilising other methods. 20,21 The effective thicknesses increases upon E(anodic) for both anions with TFSIat 22.0 ± 0.7 Å and PF6at 19.9 ± 0.5 Å, respectively. Although TFSIis not spherical in nature, the effective ionic radii can be approximated from the van der Waals volume of the anions, where the effective ionic radii then equates to 0.33 and 0.25 nm for TFSIand PF6respectively. 22 Assessing these values, if we assume that the anion is positioned on top of the Fc SAM, the 0.16 nm difference in effective ionic diameters between TFSIand PF6is within the error bar ranges of our effective thickness following E(anodic). However, we note that in reality, the precise position of the anion is not known. We note that the increase in effective thickness should also include a contribution from a change in Fc SAM molecular orientation (i.e. reorientation of Fc + termini or alkyl chain) 23,24 that have been reported to occur upon oxidation to Fc + . In our case, and in particular with TFSI -, the change in effective thickness between E(anodic) and E(cathodic) (~4 Å) is larger than the magnitude of the orientation changes reported ranging from 0.09 to 1.9 Å, 23,25 indicating that the anion should be contributing to the observed increase in effective thickness.

Figure S12
Supplementary Note 4: Theoretical description of interfacial potential distribution Figure S13 In regards to the interfacial potential distribution, the overwhelming majority of the potential drop occurs within the monolayer. This is due to the (1) presence of ion-pairing which neutralises/screens the surface charge, 29 and (2) large difference in the relative permittivity of the SAM (ε = 3) compared to the electrolyte (ε = 78.5). 30 Theoretical descriptions of a surface-bound redox-active monolayer can provide insights into the parameters that affect the interfacial potential distribution. 18 We can observe that in the presence of ion-pairing (Supplementary Figure 13c), the potential drop is predominantly within the monolayer and there is a negligible influence from φPET, φF irrespective of electrode potential. We note that electrolyte concentration does not significantly change the potential distribution in the concentrations of interest (0.1 to 1 M). In terms of the Fc SAM overage, increasing the coverage results in the same tendencies. A key parameter is X as this determines the effectiveness of ionpairing and charge neutralisation while φPET becomes appreciable at X > 0.15.
It is noteworthy to mention the importance of d2. In the presence of buried Fc, the value of d2 will be affected which in turn will influence the interfacial potential distribution. Calvente et al. 16 has theoretically shown the interfacial potential distribution in the presence of multiple redox-planes. Nonetheless, the position of the Fe heteroatom within the monolayer will affect the binding energy shifts and can be related to the deviations from the expected 1 eV/V relationship. . E(anodic) and E(cathodic) correspond to 0.625 and 0 V vs Ag/AgCl, respectively. Note that the different signal-to-noise in (b) is due to different data acquisition settings. Figure S14