Activating cobalt(II) oxide nanorods for efficient electrocatalysis by strain engineering

Designing high-performance and cost-effective electrocatalysts toward oxygen evolution and hydrogen evolution reactions in water–alkali electrolyzers is pivotal for large-scale and sustainable hydrogen production. Earth-abundant transition metal oxide-based catalysts are particularly active for oxygen evolution reaction; however, they are generally considered inactive toward hydrogen evolution reaction. Here, we show that strain engineering of the outermost surface of cobalt(II) oxide nanorods can turn them into efficient electrocatalysts for the hydrogen evolution reaction. They are competitive with the best electrocatalysts for this reaction in alkaline media so far. Our theoretical and experimental results demonstrate that the tensile strain strongly couples the atomic, electronic structure properties and the activity of the cobalt(II) oxide surface, which results in the creation of a large quantity of oxygen vacancies that facilitate water dissociation, and fine tunes the electronic structure to weaken hydrogen adsorption toward the optimum region.

The calibration was performed in hydrogen saturated electrolyte with a Pt sheet as the working electrode. Cyclic voltammetry run at a scan rate of 1 mV s -1 , and the average of the two potentials at which the current value was zero was taken as the thermodynamic potential. Therefore, in 1 M KOH, V RHE = V SCE + 1.03 V.

Supplementary Tables
Supplementary Table 1 (Fig. 1a), is advantageous over molecular adsorption by 0.67 eV ( Supplementary Fig. 3). From a catalytic viewpoint, the moderately bounded H-OH groups on {111}-Ov are expected to be more reactive than in the case of {110} facets.

Supplementary Note 2. Analysis of strain on the surface of S-CoO NRs
Geometric phase analysis (GPA) was used to obtain the strain tensor maps of the surface of S-CoO NRs. According to th e GPA method 12 , the derivation of the displacement field gives the strain field: 12 where R is a position in the image (Fig. 2b) where R 1 is the length of unit vector along R 1 shown in Fig. 2b, and Supplementary   Fig. 15).

Supplementary Note 3. Determination the magnitude of tensile strain on the surface of S-CoO NRs
As discussed in the main text, the cation exchange method was employed to fabricate CoO NRs with strained surface. It was found that an increase in the cation exchange temperature results in larger tensile strain on the surface of S-CoO NRs.
The surface-specific strain of S-CoO NRs can be obtained from HADDF-STEM (Strain surface ), while the XRD analysis gives the average information about the whole CoO NRs ( Supplementary   Fig. 16). The relation between the lattice strain measured by HADDF-STEM and XRD can be correlated using the following equation: where surface V and bulk V are the surface-region (below 2 nm from the outermost surface of CoO NRs) and whole volume of S-CoO NRs, R and h is the radius and height of the CoO NRs, respectively. The average radius (R) of S-CoO NRs is estimated as 50 nm ( Supplementary Fig.   11). Based on the XRD analysis (Supplementary Table 2

Supplementary Note 4. Analysis of HER activity differences of S-CoO NRs with varied magnitude of tensile strain
The  Table 2), to obtain the normalized i 0 per active site (i 0/site ). As shown in Supplementary Fig. 22d Table 3). Based on these results, it is reasonable to assume that the outstanding HER activity in 3.0 % S-CoO NRs mainly originate from sucessful strain engineering, which fine-tunes the ΔG H* near to the optimum value that balances hydrogen adsorption and desorption.

Supplementary Note 5. Calibration of Turnover Frequency of 3.0 % S-CoO NRs
As discussed in the theoretical calculations of HER mechanism, O-vacancies on the surface of S- The turnover frequency (TOF) of 3.0 % S-CoO NRs at overpotential of 100 mV was calculated using the following equation:  Fig. 26). As shown in Supplementary Fig. 28, the hydrogen adsorption on Pt (111) surface is optimal with a free energy (ΔG H* ) value of -0.09 eV, which is consistent with the recently reported data 5 . However, for HER in alkaline solutions, the dissociation of water supplies hydrogen and is considered as a key rate determining step 13 . Unfortunately, molecular water adsorption is about 0.78 eV energetically favourable than the complete dissociative adsorption on Pt (111) with an energy barrier of 0.93 eV, indicating that water dissociation is not energetically favourable on this surface ( Supplementary Fig. 26). In contrast, the {111}-Ov surface of S-CoO NRs significantly enhances water dissociation over molecular adsorption ( Supplementary Fig. 3 All calculations were carried out using a plane wave kinetic energy cut-off of 400 eV. All structures in the calculations were spin-polarized and relaxed until the convergence tolerance of force on each atom was smaller than 0.05 eV. The energy converge criteria was set to be 10 -5 eV for self-consistent calculations with a gamma-centre 2x2x1 k-mesh.  Supplementary Fig.   7. Elongating the lattice parameters of the unit cell increased the elastic strain correspondingly. For this unit cell size, the surface O-vacancy concentration is 11.1% (defined as the number of Svacancies divided by the total number of O atoms on the pristine surface), which is in line with the experimental data (~12.5 %) estimated based on the Co-L 2,3 edge XANES spectra (Fig. 3c).
Hydrogen adsorption free energy calculation. The hydrogen adsorption free energy, ΔG H* , can be computed using the following equation 22 : Electrochemical characterization. The Faradaic yield was calculated from the total charge Q(C) passed through the cell and the total amount of hydrogen produced 2 H n (mol). The total amount of hydrogen produced was measured using gas chromatography (Agilent 7890B). Assuming that two electrons are used to produce one H 2 molecule, the Faradaic efficiency can be calculated as follows: where F is the Faraday constant.