Mediator-Free SECM for Probing the Diffusion Layer pH with Functionalized Gold Ultramicroelectrodes

Abstract

Probing pH gradients during electrochemical reactions is important to better understand reaction mechanisms and to separate the influence of pH and pH gradients from intrinsic electrolyte effects. Here, we develop a pH sensor to measure pH changes in the diffusion layer during hydrogen evolution. The probe was synthesized by functionalizing a gold ultramicroelectrode with a self-assembled monolayer of 4-nitrothiophenol (4-NTP) and further converting it to form a hydroxylaminothiophenol (4-HATP)/4-nitrosothiophenol (4-NSTP) redox couple. The pH sensing is realized by recording the tip cyclic voltammetry and monitoring the Nernstian shift of the midpeak potential. We employ a capacitive approach technique in our home-built Scanning Electrochemical Microscope (SECM) setup in which an AC potential is applied to the sample and the capacitive current generated at the tip is recorded as a function of distance. This method allows for an approach of the tip to the electrode that is electrolyte-free and consequently also mediator-free. Hydrogen evolution on gold in a neutral electrolyte was studied as a model system. The pH was measured with the probe at a constant distance from the electrode (ca. 75 μm), while the electrode potential was varied in time. In the nonbuffered electrolyte used (0.1 M Li₂SO₄), even at relatively low current densities, a pH difference of three units is measured between the location of the probe and the bulk electrolyte. The time scale of the diffusion layer transient is captured, due to the high time resolution that can be achieved with this probe. The sensor has high sensitivity, measuring differences of more than 8 pH units with a resolution better than 0.1 pH unit.

Figure 1

(a) Voltammetry (0.1 M H₂SO₄, 100 mV s^–1) and schematic representation of the conversion of (b) 4-nitrothiophenol (4-NTP) to 4-hydroxiaminothiophenol (4-HATP), and (c) the two proton–two electron transfer reaction of the redox couple 4-HATP/4-NSTP.

Figure 2

(a) Characterization of the electroactive redox couple 4-HATP/4-NATP in 0.1 M H₂SO₄ at 200 mV s^–1, and (b) calibration of the functionalized Au-UME in 0.1 M Li₂SO₄ solutions adjusted to different pH and saturated with argon or hydrogen.

Figure 3

(a) Capacitive approach configuration and (b) approach curve obtained (blue circles) with its fit to eq 3 (red line).

Figure 4

Cyclic voltammogram of hydrogen evolution taking place at the gold sample in 0.1 M Li₂SO₄ (pH = 3.2) recorded at 100 mV s^–1.

Figure 5

(a) pH measurement during hydrogen evolution in 0.1 M Li₂SO₄ (pH = 3.2) with the sample at −0.75 V vs Ag/AgCl; (b) chronoamperometry recorded at the sample.

Figure 6

pH measurements in the diffusion layer during hydrogen evolution in 0.1 M Li₂SO₄ (pH = 3.2) at different sample potentials. The measurement was performed in duplicate.

Figure 7

pH measurements in the diffusion layer during hydrogen evolution in 0.1 M Li₂SO₄ (pH = 3) performed in a wider potential range. The inset shows the small pH differences recorded when the sample potential was −0.65 and −0.60 V vs Ag/AgCl.