Grain boundary engineering for efficient and durable electrocatalysis

Abstract

Grain boundaries in noble metal catalysts have been identified as critical sites for enhancing catalytic activity in electrochemical reactions such as the oxygen reduction reaction. However, conventional methods to modify grain boundary density often alter particle size, shape, and morphology, obscuring the specific role of grain boundaries in catalytic performance. This study addresses these challenges by employing gold nanoparticle assemblies to control grain boundary density through the manipulation of nanoparticle collision frequency during synthesis. We demonstrate a direct correlation between increased grain boundary density and enhanced two-electron oxygen reduction reaction activity, achieving a significant improvement in both specific and mass activity. Additionally, the gold nanoparticle assemblies with high grain boundary density exhibit remarkable electrochemical stability, attributed to boron segregation at the grain boundaries, which prevents structural degradation. This work provides a promising strategy for optimizing the activity, selectivity, and stability of noble metal catalysts through precise grain boundary engineering.

Fig. 1. Synthesis of GB-rich nanoassemblies.

a Illustration depicting the formation of GB-rich Au NAs through the collision, attachment, and coalescence of NPs’ surfaces devoid of capping agents facilitated by H₂ gas bubbling. b High resolution HAADF-STEM micrograph of the GBs between the Au NP building blocks within H-Au NAs. c HAADF-STEM image and (d) grain orientation maps from the corresponding 4D-STEM dataset for H-Au NAs. The inset shows the color code for the out-of-plane coordinates. e Histogram plots of the GB types derived from ~600 GBs in the 4D-STEM data. LAGB refers to low-angle GBs with a misorientation of less than 15 degrees, while HAGB denotes high-angle GBs with a misorientation exceeding 15 degrees. Source data are provided as a Source Data file.

Fig. 2. Defects and strain characterization for H-Au NAs.

a HAADF-STEM image of GBs and undercoordinated surface atoms (orange dots) in H-Au NAs. Enlarged regions showing the (b, c) GB surface terminations and structure of (d) a ∑9/(221) and (e) ∑27/(552) GB. f–h Real space strain maps along the x (ε_xx), y (ε_yy) and xy (ε_xy) directions. Notice how on the higher energy GBs elevated values of strain are found. i Virtual annular dark field image from the 4D-STEM data and (j–k) corresponding relative strain maps (ε_xx and ε_yy), showing a similar inhomogeneous strain distribution.

Fig. 3. GB density tuning by varying H₂ flow rate.

TEM images of Au NAs prepared from different gas flow rates, encompassing (a) a low-flow-rate of 30 sccm, denoted as L-Au NAs, (b) a medium-flow-rate of 100 sccm, designated as M-Au NAs and (c) a high-flow-rate of 300 sccm, identified as H-Au NAs. HAADF-STEM image of the GBs in (d) L-Au NAs and (e) M-Au NAs. LAGB indicates low-angle GBs with a misorientation of less than 15 degrees, while HAGB refers to high-angle GBs with a misorientation greater than 15 degrees. f Cyclic voltammograms in 0.1 M HClO₄ at scan rate of 100 mV s⁻¹. g Relationship between gas flow rates and GB surface density (see Supporting Information for more details). GB density measurements were conducted at least three times, with average values reported. Error bars represent the standard deviation. h XRD patterns of L-Au NAs, M-Au NAs and H-Au NAs, where a noticeable shift toward lower angles is observed for the (111) and (200) peaks, as highlighted by the black arrows indicating an expansion of the lattice. i Fourier-transformed EXAFS spectra for Au L3-edge for Au NPs, L-Au NAs, M-Au NAs, and H-Au NAs. j Correlation between GB surface density and lattice expansion/coordination number calculated from the Fourier-transformed EXAFS spectra. Error bars indicate the standard deviation from measurements taken at least three times. Source data are provided as a Source Data file.

Fig. 4. Two electron ORR activity measurements and rationalization.

a Linear sweep voltammetry of Au NPs, L-Au NAs, M-Au NAs and H-Au NAs recorded at 1600 rpm and a scan rate of 5 mV s⁻¹ in 0.1 M HClO₄, together with the detected H₂O₂ currents on the ring electrode (upper panel) at a fixed potential of 1.2 V vs. RHE. b Calculated H₂O₂ selectivity during potential sweep. c Comparison of the onset potential and H₂O₂ selectivity of Au catalysts developed in this study and the state-of-the-art H₂O₂ catalysts in 0.1 M HClO₄ from the literature (Table S4). d Correlation between GB density and the two-electron ORR activity, including onset potential, H₂O₂ selectivity, specific activity, and mass activity. Error bars indicate the standard deviation based on at least three independent measurements. e The calculated ORR activity volcano relationship between the limiting potential (U_L) and the free energy of *OOH (ΔG_*OOH) for the two-electron pathway to H₂O₂. f Calculated reaction coordinate diagrams for the Au and lattice-expanded Au. Source data are provided as a Source Data file.

Fig. 5. Electrochemical and structural stability.

a Durability measurements of Au NPs, L-Au NAs, M-Au NAs and H-Au NAs at a fixed disk potential of 0.35 V. b Calculated H₂O₂ selectivity after durability tests. HAADF-STEM images of (c) L-Au NAs and (d) H-Au NAs after durability tests. A 2 nm thin-sliced tomogram from a 3D atom map (Supplementary Figs. S34, S37) of (e) L-Au NAs and (h) H-Au NAs (iso-composition surface >90 at.% Au). Extracted GB tomogram of (f) L-Au NAs and (i) H-Au NAs of the region delineated by the box (isodensity surface of 150 Au atoms/nm³). The orange spheres represent B atoms. 1D compositional profiles of identified Au and B elements of (g) L-Au NAs and (j) H-Au NAs along the direction indicated by the arrow. Error bars indicate the standard deviation from a minimum of three independent measurements. Source data are provided as a Source Data file.