Constant-distance mode SECM as a tool to visualize local electrocatalytic activity of oxygen reduction catalysts

Abstract

Multidimensional shearforce-based constant-distance mode scanning electrochemical microscopy (4D SF/CD-SECM) was utilized for the investigation of the activity distribution of oxygen reduction catalysts. Carbon-supported Pt model catalyst powders have been immobilized in recessed microelectrodes and compared to a spot preparation technique. Microcavities serve as platform for the binder-free catalyst sample preparation exhibiting beneficial properties for constant-distance mode SECM imaging concerning modified surface area and catalyst loading. The integration of the redox competition mode of SECM into the detection scheme of the 4D SF/CD mode is demonstrated for specifically adapting high-resolution SECM experiments to powder-based catalyst preparations.

Keywords: SECM; electrocatalysis; oxygen reduction; recessed microelectrodes; redox-competition SECM; scanning electrochemical microscopy; shearforce-based constant-distance mode.

Figure 1

4D SF/CD-SECM for the investigation of the catalytic activity towards oxygen reduction. a) Scheme of the competition mode. b) Topographic linescan across a spot of Pt/C catalyst with 20 wt % Pt on a glassy carbon plate obtained in the shearforce-based cd-mode of SECM. The topography is displayed as the z-piezo position of the tip after reaching the predefined stop criterion of the shearforce interaction (point of closest approach). The insert shows an optical micrograph of the investigated spot and the arrow marks the scan position and direction. c) Oxygen consumption profile above the line displayed in b) visualized as a x,Δz,i-image, where Δz represents the distance of the current detection from the point of shearforce interaction. E_sample = −300 mV, E_tip = −600 mV, d_tip = 0.8 µm, f_osc = 356.7 kHz, stop criterion: 5% change of the lock-in signal in bulk.

Figure 2

a) Scheme of a microcavity used as platform for catalyst immobilisation in cd-mode SECM investigations. b) Topography of the cavity obtained by means of shearforce-based cd-scanning and a corresponding linescan shown in c). d) Scheme of the microcavity electrode filled with catalyst powder. Optical micrographs of the cavity electrode before e) and after f) filling with catalyst powder. Residues of the black Pt/C catalyst are visible at the glass insulation and are removed before the filled cavity is inserted in the SECM measuring cell. g) Micrograph of the setup for 4D SF/CD-SECM experiments with the tip positioned next to the microcavity. The glass body of the used reference electrode is visible in the front.

Figure 3

4D SF/CD-SECM experiment at a partly filled microcavity. a) Optical micrograph with a scheme of the catalyst filling, b) topography and c) current image at the point of closest approach. E_sample = −300 mV, E_tip = −600 mV, d_tip = 1.6 µm, f_osc = 299.4 kHz, stop criterion: 15%.

Figure 4

Concept of the 4D SF/CD-RC-SECM. Similar to the 4D SF/CD mode the tip is positioned within the shearforce-interaction regime by means of a z-approach curve (a and b). Data acquisition is performed in subsequent retraction steps (c and d) set out from the point of closest approach. To additionally implement the redox competition mode, a variable potential pulse profile is performed at each tip-to-sample distance. During a competition pulse tip and sample compete for the oxygen inside the gap and a time-dependent current decay curve is recorded at the tip enabling a time-resolved analysis of the oxygen concentration within the gap. After completion of the retraction the tip is moved to the next grid point (e) and the procedure is repeated starting with the shearforce-based positioning (a).

Figure 5

4D SF/CD-RC-SECM experiment at a 100 µm diameter Pt disk electrode as model sample for an ORR catalyst. a) Topography image, b)–e) horizontal current image at the point of shearforce contact and in increments of 5 µm after tip retraction. f) Oxygen consumption profile (vertically-sliced current image). E_sample = −600 mV, E_tip,base = 0 mV, E_tip,pulse1 = 0 mV, t_pulse1 = 0.5 s, E_tip,pulse2 = −600 mV, t_pulse2 = 0.5 s, pictures displayed at t = 0.25 s, d_tip = 3.4 µm, f_osc = 318.2 kHz, stop criterion: 5%.

Figure 6

4D SF/CD-RC-SECM experiment at a microcavity filled with a Pt/C model catalyst. Topography image a) and current images at b) the point of shearforce-contact d, c) d + 4 µm, and d) d + 20 µm. E_sample = −300 mV, E_tip,base = 500 mV, E_tip,pulse1 = 500 mV, t_pulse1 = 0.5 s, E_tip,pulse2 = −600 mV, t_pulse2 = 0.5 s, images displayed at t = 0.25 s, d_tip = 1.9 µm, f_osc = 366.7 kHz, stop criterion: 5%.