Accelerated interfacial proton transfer for promoting electrocatalytic activity

Interfacial pH is critical to electrocatalytic reactions involving proton-coupled electron transfer (PCET) processes, and maintaining an optimal interfacial pH at the electrochemical interface is required to achieve high activity. However, the interfacial pH varies inevitably during the electrochemical reaction owing to slow proton transfer at the interfacial layer, even in buffer solutions. It is therefore necessary to find an effective and general way to promote proton transfer for regulating the interfacial pH. In this study, we propose that promoting proton transfer at the interfacial layer can be used to regulate the interfacial pH in order to enhance electrocatalytic activity. By adsorbing a bifunctional 4-mercaptopyridine (4MPy) molecule onto the catalyst surface via its thiol group, the pyridyl group can be tethered on the electrochemical interface. The pyridyl group acts as both a good proton acceptor and donor for promoting proton transfer at the interfacial layer. Furthermore, the pKa of 4MPy can be modulated with the applied potentials to accommodate the large variation of interfacial pH under different current densities. By in situ electrochemical surface-enhanced Raman spectroscopy (in situ EC-SERS), we quantitatively demonstrate that proton transfer at the interfacial layer of the Pt catalyst coated with 4MPy (Pt@4MPy) remains ideally thermoneutral during the H+ releasing electrocatalytic oxidation reaction of formic acid (FAOR) at high current densities. Thus, the interfacial pH is controlled effectively. In this way, the FAOR apparent current measured from Pt@4MPy is twice that measured from a pristine Pt catalyst. This work establishes a general strategy for regulating interfacial pH to enhance the electrocatalytic activities.


Preparation of Pt@4MPy and Pt@BT electrodes
The pristine polycrystalline Pt electrodes were cleaned in the 0.5 M H 2 SO 4 solution by repetitive potential scans between surface oxidation and the hydrogen adsorption regions until a stable CV was obtained. Then, the clean Pt electrodes were immersed in ethanolic solutions of 4MPy (or BT) to allow the formation of self-assembled monolayers (SAM) on the surfaces. The coverage of 4MPy on Pt was controlled by varying the adsorption concentration of 4MPy. The samples were rinsed with ethanol and dried under N 2 . Then the samples were directly used for experiments.

Preparation of Pt/C@4MPy electrodes
Commercial 20 wt% Pt/C catalyst (2 mg) were dispersed into a solution consisting of 0.5 mL ultra-pure (18 MΩ) water, 0.5 mL ethanol, and 0.05 mL 5% Nafion. The ink (2 μL) was dropcast on a glassy carbon electrode (diameter: 2 mm) and then dried in vacuum. Then, the Pt/C electrode was immersed in a solution of 2.5-5 μM 4MPy in water for 10-12 hours to allow the formation of a self-assembled monolayer (SAM) of 4MPy on the Pt catalyst surface.

Preparation of 140 nm Au@Pt NPs
140 nm Au nanoparticles were synthesized according to a previously reported method. 1 40 mL of the 140 nm Au colloid was heated to 80 ℃ in a water bath. Then, 1.87 mL solutions of 1 mM H 2 PtCl 6 and 5 mM ascorbic acid were simultaneously, slowly injected into the 140 nm Au colloid over the course of 10 min. After complete injection, the reaction was allowed to continue for 30 min at 80 °C yielding approximately 1 nm Pt shell on the gold nanoparticles. 2

Electrochemical measurements
Electrochemical measurements were performed using a standard three-electrode glass cell and a potentiostat (CHI 660e). The Pt@4MPy samples (prepared by the methods described in 1.2) and the clean Pt electrodes were used as the working electrodes in the respective experiments. Because we are studying the anodic processes, the dissolution of platinum on the cathodic Pt counter electrode can be avoided and therefore a Pt wire can be used as the counter electrode. A mercurous sulfate electrode (MSE, Hg/Hg 2 SO 4 , saturated K 2 SO 4 ) was used as the reference electrode and it can be found from Figure  S1.1 that the MSE is very stable compared with NHE.

In situ-electrochemical SERS measurements
In situ-electrochemical SERS measurements were performed on a WITec Alpha 300 R confocal Raman system. A water immersion objective with a numerical aperture of 1.0 and magnification of 60 was used for laser illumination and signal collection. The 632.8 nm excitation was generated using the output from a He-Ne laser, and the power of the laser on the sample was about 0.2 mW. The spectroelectrochemical cell was equipped with a Pt wire as the counter electrode, and MSE was used as the reference electrode. 3 During the synchronized CV-SERS, EMCCD generated a transistor−transistor logic (TTL) signal right before its exposure, which was sent to the potentiostat to initiate the potential sweep. Thereby, the current and spectral data can be exactly synchronized, and every SERS spectrum in the CV-SERS spectra and every current point in CV can be well correlated. 4

Calculating the coverage of 4MPy on Pt
The coverage of 4MPy ( ) on Pt can be calculated using the following equation:

Estimating the number of 4-MPy molecules on surface compared with the protons released from FAOR
Assuming a value of 1.8 × 10 15 molecules/cm 2 for the surface concentration of chemisorbed 4MPy at most. 5 Therefore, the number of molecules were about 0.56 × 10 14 molecules on a Pt electrode (diameter 2 mm). According to Faradic law, the number of 0.61× 10 16 protons was released even for the lowest activity in pH = 0.61 (seen in Figure 2b). The number of protons by FAOR released is about 2-3 orders of magnitude higher at least than the number of 4MPy molecules on Pt surface.

The derivation of change of Gibbs free energy in proton accepting and donating processes
(1) The above equation describes proton accepting and donating processes by the buffer molecules. Where K a is the acid dissociation constant of molecules. The change of Gibbs free energy can be described as follows: (2)  (6) and (7). is a dimensionless number of conversion factor between concentration and Raman intensity. measured by comparing the intensity ratio, I 4MPy /I 4MpyH + , measured by NMR ( Figure S2.5 a) with that measured by Raman ( Figure   S2.5 b) in a same acidified 4MPy solution partially. The was 3.2.  Table S1. A comparison of FAOR activity of our work with that reported in references. [6][7][8][9] The current densities were normalized by GSA (geometric surface area) and ECSA (electrochemically active surface area)