High Throughput Correlative Electrochemistry-Microscopy Analysis on a Zn–Al Alloy

Conventional electrodes and electrocatalysts possess complex compositional and structural motifs that impact their overall electrochemical activity. These motifs range from defects and crystal orientation on the electrode surface to layers and composites with other electrode components, such as binders. Therefore, it is vital to identify how these individual motifs alter the electrochemical activity of the electrode. Scanning electrochemical cell microscopy (SECCM) is a powerful tool that has been developed for investigating the electrochemical properties of complex structures. An example of a complex electrode surface is Zn–Al alloys, which are utilized in various sectors ranging from cathodic protection of steel to battery electrodes. Herein, voltammetric SECCM and correlative microstructure analysis are deployed to probe the electrochemical activities of a range of microstructural features, with 651 independent voltammetric measurements made in six distinctive areas on the surface of a Zn–Al alloy. Energy-dispersive X-ray spectroscopy (EDS) mapping reveals that specific phases of the alloy structure, particularly the α-phase Zn–Al, favor the early stages of metal dissolution (i.e., oxidation) and electrochemical reduction processes such as the oxygen reduction reaction (ORR) and redeposition of dissolved metal ions. A correlative analysis performed by comparing high-resolution quantitative elemental composition (i.e., EDS) with the corresponding spatially resolved cyclic voltammograms (i.e., SECCM) shows that the nanospot α-phase of the Zn–Al alloy contains high Al content (30–50%), which may facilitate local Al dissolution as the local pH increases during the ORR in unbuffered aqueous media. Overall, SECCM-based high-throughput electrochemical screening, combined with microstructure analysis, conclusively demonstrates that structure-composition heterogeneity significantly influences the local electrochemical activity on complex electrode surfaces. These insights are invaluable for the rational design of advanced electromaterials.


Section S2. SEM image of as-cast Zn-Al alloy
Figure S3 SEM image of the hypo-eutectic Zn-4wt.%Alalloy microstructure, showing dendritic Zn-rich islands surrounded by Zn and Al lamellar structures.

Elemental composition of the grain boundary -Area3
Figure S9 EDS analysis conducted on the grain boundary representing Area3, the quantitative elemental composition of Spectrum6 -Spectrum10 are tabulated in Table S3 Table S3 The averaged elemental composition in weight percentages (wt.%) of the grain boundary representing Area3 and the quantitative elemental composition of Spectrum6 -Spectrum10 shown in Figure S8 Microstructural

Elemental composition of Zn rich island -Area6
Figure S11 EDS analysis conducted on lamellar structures representing Area1 & Area4, the quantitative elemental composition of Spectrum1 -Spectrum5 are tabulated in Table S2 Table S5 The averaged elemental composition in weight percentages (wt.%) of the grain boundary representing Area3 and the quantitative elemental composition of Spectrum6 -Spectrum10 shown in Figure S8 Microstructural Figure S1 SEM image of a representative pipette tip with an outer diameter of approximately 400 nm, used for the SECCM scans.

Figure S2
Figure S2Zn-Al alloy sample mounted on a carbon-mixed resin with a resin ring to hold the oil layer for SECCM scanning.

Figure
Figure S4 (a) SEM backscattered electron (BSE) mode image of the selected section of the SECCM scan area, defining areas from Area1 to Area6.The contrasting shades illustrate compositional variations.Red squares denote the locations for quantitative compositional analysis in Figure 6 in the main text and Figure S10 in the Supporting Information.(b) SEM BSE mode image of a complete SECCM scan area highlighting pronounced compositional variations among various microstructures of the Zn-Al alloy.

Figure S5 :
Figure S5: Equipotential frames obtained from the SECCM movie, (a) -0.60 V vs Ag/AgCl in the positive direction of the scan, (b) -0.90 V vs Ag/AgCl in the positive direction and (c) -1.20 V vs Ag/AgCl in the negative direction which do not represent any microstructure-based activity variations.

Figure
Figure S12 (a) SEM image of the selected SECCM scan of Area2 as in Figure7.Note that Type1 is marked with red square, Type 2 is marked with blue square and Type 3 is marked with magenta square.(b) Individual CVs corresponding to Points 1 -6 of Type 1 category.(c) Individual CVs corresponding to Points 7 -12 of Type 2 category.(d) Individual CVs corresponding to Points 13 -15 of Type 3 category.

Table S1
The corrosion potential (E corr ) or corrosion current density (I corr ) from Area1 to Area6 obtained by analyzing Tafel plots in Figure4in the main text.

Table S2
The averaged elemental composition in weight percentages (wt.%) of the lamellar structures representing Area1 & Area4 and the quantitative elemental composition of Spectrum1 -Spectrum5 shown in FigureS7

Table S4
The averaged elemental composition in weight percentages (wt.%) of the grain boundary representing Area3 and the quantitative elemental composition of Spectrum6 -Spectrum10 shown in FigureS8