Nitrogen-incorporation activates NiFeOx catalysts for efficiently boosting oxygen evolution activity and stability of BiVO4 photoanodes

Developing low-cost and highly efficient catalysts toward the efficient oxygen evolution reaction (OER) is highly desirable for photoelectrochemical (PEC) water splitting. Herein, we demonstrated that N-incorporation could efficiently activate NiFeOx catalysts for significantly enhancing the oxygen evolution activity and stability of BiVO4 photoanodes, and the photocurrent density has been achieved up to 6.4 mA cm−2 at 1.23 V (vs. reversible hydrogen electrode (RHE), AM 1.5 G). Systematic studies indicate that the partial substitution of O sites in NiFeOx catalysts by low electronegative N atoms enriched the electron densities in both Fe and Ni sites. The electron-enriched Ni sites conversely donated electrons to V sites of BiVO4 for restraining V5+ dissolution and improving the PEC stability, while the enhanced hole-attracting ability of Fe sites significantly promotes the oxygen-evolution activity. This work provides a promising strategy for optimizing OER catalysts to construct highly efficient and stable PEC water splitting devices.

as N:BiVO4), while no evident N 1s peak could be detected from the XPS results ( Supplementary   Fig. 4). However, after N2-plasma treatment on BiVO4/NiFeOx photoanodes under the same conditions, there was an obvious N 1s peak located at 400 eV, suggesting that the nitrogen should be incorporated into the NiFeOx layer instead of BiVO4. Additionally, according to the previously reported literatures, [1,2] this N 1s peak at 397-402 eV should be attributed to the formation of N 3-

Supplementary discussion
The pristine BiVO4 photoanodes were also treated by the N2-plasma for 5 minutes. As shown in Supplementary Fig. 6, compared with pristine BiVO4, the photocurrent basically remained invariability and the stability was still very poor, which further indicates that the enhanced PEC performances of BiVO4 should be mainly attributed to the N:NiFeOx catalysts by accelerating the OER process. Supplementary Fig. 7 The repeatability of LSV (a) and i-t (b) curves for BiVO4/N: NiFeOx photoanodes.

Supplementary discussion
Supplementary Fig. 7 shows the BiVO4/N:NiFeOx photoanodes with excellent repeatability and stability. And the photocurrent density could reach up to 6.4±0.2 mA cm -2 at 1.23 VRHE accompanied with outstanding stabilities. Supplementary Fig. 8 The multiple test LSV curves for N:BiVO4/NiFeOx photoanodes measured at 0.5 M K3BO3 electrolyte.

Supplementary discussion
The PEC performances of N:BiVO4/NiFeOx photoanodes have been evaluated and compared with BiVO4/N:NiFeOx. As shown in Supplementary Fig. 8, a photocurrent density of ~5.3 mA cm -2 (1.23 VRHE) has been obtained on N:BiVO4/NiFeOx photoanodes, which is much lower than that of BiVO4/N:NiFeOx (6.4 mA cm -2 , 1.23 VRHE). This demonstration further confirms that the N incorporation into NiFeOx layer could significantly promote the PEC activity of BiVO4 photoanodes.

Supplementary discussion
To further clarify the effects of N atoms in N:NiFeOx catalysts on PEC activity and stability of BiVO4 photoanodes, a facile O2 plasma treatment process was performed on the BiVO4/NiFeOx photoanodes. As shown in Supplementary Fig. 9.

Supplementary discussion
To further confirm the crucial roles of N-incorporation of NiFeOx for promoting the OER activity and PEC stability of BiVO4 photoanodes, the PEC activities of NiFeB and NiFeP decorated BiVO4 photoanodes have also been measured for comparisons. As shown in Supplementary Fig. 10a, both NiFeB and NiFeP catalysts could enhance the PEC water oxidation activities of BiVO4 to a certain extent (4.7 mA cm -2 and 3.6 mA cm -2 at 1.23 VRHE), which were still lower than that of BiVO4/N:NiFeOx photoanodes (6.4 mA cm -2 ). Besides, the PEC water oxidation stabilities of the two electrodes were gradually decreased with the prolonging of illumination time (Supplementary Fig. 10b). The comparative experiments demonstrate that compared with B and P atoms, the incorporation of N atoms with appropriate electronegativity could more effectively regulate the electronic structure of NiFeOx catalyst and promote the oxygen evolution activity and stability of BiVO4 photoanodes.
Supplementary Fig. 10c and Fig. 10d show the B 1s and P 2p characteristic peaks of BiVO4/NiFeB and BiVO4/NiFeP photoanodes, indicating the successful incorporation of B and P atoms into the NiFeOx catalysts, respectively.

Supplementary discussion
It was considered that after the PEC stability tests, the increased noise of N1s spectrum ( Supplementary Fig. 12f) should be mainly attributed to the formation of hydroxyl (OH) groups on the photoanode surfaces. More specifically, compared with the fresh samples, the detection sensitivity on the photoanode surface has been relatively decreased, especially for the light and low-content elements. To further confirm the above speculation, Fourier transform infrared (FTIR) spectroscopy has been employed to explore the changes of OH groups on the photoanode surface ( Supplementary Fig. 13). Obviously, after the PEC stability tests, the stretching vibration peak of OH groups at 3430 cm -1 has been significantly increased, which clearly reveal the OH formation on the photoanode surface.
Supplementary Table 1 The elemental composition analysis from XPS for BiVO4/N:NiFeOx photoanodes before and after PEC measurements.

Supplementary discussion
The SEM, TEM, XRD and XPS results for the BiVO4/N:NiFeOx photoanodes after the PEC tests have been performed. As shown in Supplementary Fig. 11, after the stability test, the morphology and crystalline structures of BiVO4/N:NiFeOx have no evident changes compared with the fresh samples (Fig. 1). Additionally, the HR-TEM images ( Supplementary Fig. 11c) also clearly indicate that the N:NiFeOx cocatalyst layers were still uniformly covered on BiVO4 surfaces with the thickness of ~4 nm, indicating it's excellent structural stability. Moreover, as shown in Supplementary Fig. 12 and Table 1, the XPS results reveal that the characteristic peaks of all elements could still be detected after the PEC stability tests, but their contents have been slightly decreased compared with the pristine samples, especially for V element, which should be due to the photo-induced V 5+ dissolution from the BiVO4 lattices. [3][4][5] Supplementary Fig. 14 Calculated photocurrent density curves by integrating IPCE curves in (Fig. 2d) with the standard solar spectrum.

Supplementary discussion
The estimated photocurrent densities (Jc) were calculated by integrating the IPCE values with the standard solar spectrum (ASTMG-173-03) using the following equation: (1) Specifically, λ and E(λ) represent the light wavelength (nm) and the corresponding power density (mW cm -2 ), respectively. According to the above equation, the calculated photocurrent densities for BiVO4, BiVO4/NiFeOx and BiVO4/N:NiFeOx photoanodes were 0.17, 1.39 and 2.77 mA cm -2 at 0.6 VRHE, respectively, which are all close to the measured values (0.13, 1.14 and 2.65 mA cm -2 for BiVO4, BiVO4/NiFeOx and BiVO4/N:NiFeOx, Fig. 2a). These demonstrations clearly reveal that the simulated AM 1.5G solar light was well matched with the standard solar spectrum.
Supplementary Table 2 The fitted results of EIS data using the equivalent circuit in Fig. 2e  interfaces. [6,7] Supplementary

Supplementary discussion
The ultrafast transient absorption spectroscopy (fs-TAS) has been performed and shown in Supplementary Fig. 17. It can be clearly observed that compared with pristine BiVO4 photoanodes, the incorporation of N:NiFeOx cocatalysts could significantly increase the hole carriers lifetime from ~2.352 to ~2.569 ns. [8][9][10][11] The fs-TAS results demonstrated that N:NiFeOx cocatalysts could effectively promote charge separation and extend the carriers lifetimes, which is consistent with the ns-TAS results shown in Fig. 3c.