Structural analysis of the NiO/FNi cathode. Porous NiO nanosheets were in-situ deposited on the Ni foam (NiO/FNi) and employed as a photoelectrode in the magnetic and optical field multi-assisted Li-O2 battery. The schematic diagram of the synthesis strategy for the NiO/FNi cathode is shown in Fig. 2a, which involves two steps, i.e., hydrothermal treatment and subsequent annealing at high temperature 25. The powder X-ray diffraction (PXRD) patterns of the final products are shown in Fig. 2b and all the diffractions are well indexed to cubic NiO. The X-ray photoelectron spectroscopy (XPS) further confirms the composition of the NiO/FNi cathode. In Fig. 2c, the peak located at 531.3 eV is assigned to the O of NiO-based material, and the peak at 532.7 eV is ascribed to the defective O. The peaks appear at 853.8 and 861 eV are attributed to the Ni 2p3/2 main peak and its satellite, and the 872.8 and 879.5 eV corresponded to Ni 2p1/2 main peak and its satellite (Fig. 2d) 26. The energy dispersive spectrum (EDS) element mappings reveal that the Ni and O atoms distribute uniformly across the NiO/FNi cathode (Supplementary Fig. 1). The microstructure of NiO/FNi cathode is investigated by the scanning electron microscopy (SEM, Fig. 2e), which displays the dense NiO nanosheets grown on Ni foam (Supplementary Fig. 2 and 3), as well as the color change from light to dark is displayed (inset). The enlarged SEM image of the NiO/FNi cathode clearly shows the homogeneously aligned and highly porous structure on a large scale (Fig. 2f). It is believed that the interconnected nanosheets can not only provide abundant open space and electroactive surface for electron transfer and electrolyte diffusion, but also enough area for light adsorption. Transmission electron microscopy (TEM) image in low-magnification demonstrates the mesoporous structure of the NiO nanosheets, which can maximize the active surface and enable the achievement of excellent battery performance (Fig. 2g). The high resolution-TEM (HR-TEM) image shows a lattice fringe spacing distance of 0.148 nm, consistent with the (220) planes of face-centered cubic NiO, which is also in consistency with the NiO composition from the PXRD pattern 27. Furthermore, the nitrogen absorption-desorption isotherms suggest the interconnected porous structure with a high specific area of 144.77 m2 g-1 (Fig. 2h). Utilizing the advantages of maintaining good FNi macropores, NiO/FNi cathode could provide a low-resistance pathway for electron and mass transfer via the free-standing structure.
Photoelectrochemical behavior of NiO/FNi. The optical properties of NiO/FNi photoelectrode were investigated with a conventional three-electrode cell. The Mott-Schottky plots of NiO/FNi show a negative slope, which is typical for p-type semiconductors (Fig. 3a), indicating that the photo-induced holes are the dominant charge carriers. As can be seen from the ultraviolet-visible (UV-vis) absorption spectroscopy, the maximum absorption wavelength of NiO/FNi is about 410 nm (Fig. 3b) and the bandgap is further determined to be 3.12 eV by measuring the x-axis intercept from the Tauc plot (Supplementary Fig. 4) 28. The inset of Fig. 3b schematically shows that the CB and VB positions of NiO/FNi (vs. normal hydrogen electrode (NHE)). When NiO/FNi is excited by photons, electrons and holes can be transferred through the suitable positions, thus corroborating the potential of NiO/FNi as an efficient photoelectrode for the consequent ORR and OER processes. To evaluate the illumination effect on the photoelectrochemical ORR and OER process, liner sweep voltammetry (LSV) was detected in O2-saturated 0.5 M Na2SO4 solution. The onset potential of NiO/FNi with illumination for ORR reaches up to -0.28 V vs. Ag/AgCl, suggesting that the effectively improved ORR catalytic performance is originated from the light (Fig. 3c). The photo-assisted ORR process on NiO/FNi is further confirmed by its current responses in Ar atmosphere with and without illumination in Supplementary Fig. 5. To better understand the reaction kinetics of the ORR process, Tafel slopes were calculated based on the LSV curves. The lower Tafel slope of 510 mV dec-1 for NiO/FNi under light indicates that illumination plays a significant role in promoting the reaction kinetics of ORR (Fig. 3d). Additionally, during the OER process, the NiO/FNi under light also exhibits obviously favorable performance comparing to that in dark (Fig. 3e), demonstrating the positive effect of light during OER process. The smaller Tafel slope of NiO/FNi under light (259 mV dec-1) than that in dark (429 mV dec-1) further verifies the positive role on OER process (Fig. 3f). As shown in Supplementary Fig. 6, the EIS results show the reduced charge-transfer resistance of NiO/FNi under light compared to the initial electrode, leading to an increased conductivity, which is also consistent with the above results.
Photocatalytic properties of NiO/FNi under magnetic field. According to the previous work, Lorentz force can effectively act on photocatalysis process, due to the external MF can produce opposite force on the photoelectrons and holes 19, 21. Thus, we applied NiO/FNi in the photocatalysis process with MF and without MF (MF and NMF, respectively) to understand the influence of MF on the photocatalytic activities. Firstly, the nearly unchanged Mott-Schottky slopes of NiO/FNi under MF (Fig. 4a) imply that the MF has a slight influence on the intrinsic charge carrier density. The LSV curves for ORR at MF and NMF condition nearly show the same trend, suggesting that the Lorentz force cannot facilitate the photocatalytic process without the charge carriers generated via photoelectric conversion (Fig. 4b). Fig. 4c displays the photocatalytic ORR performance of the NiO/FNi under the MF and NMF conditions by using the magnetic photocatalyst setup (inset). The photogenerated current density obviously increased, suggesting that the presence of magnetic field plays a key role in enhancing the photocatalytic performance. In the presence of the external MF, electrons move under the function of the Lorenz force. Besides, driven by the Lorenz force, the free electrons in the system form a polarized distribution that generates a motion-induced electromotive force, which is probably the key reason for the enhancement of the photocatalytic performance. I-t curves were recorded under NMF and MF conditions to analyze the recombination rate of the charge carriers (Fig. 4d). The most stable and highest photocurrent response of the NiO/FNi under light and MF can be assigned to the newly separated photo-charges under MF conditions, which is related to the electromagnetic induction current.
To deeply understand the interaction between the photoelectrode and the magnetic field, the magnetic hysteresis loop (M-H) of NiO/FNi was measured at room temperature. As observed in Fig. 4e, M-H curve exhibits a typical ferromagnetic property with small coercivity and saturated under a magnetic field of 10 KOe, indicating NiO/FNi can be manipulated by an external magnetic field. According to the above experiment results, it is proposed that the electrons and holes can be deviated when they move in MF, leading to better separation of the charge carriers. The generated induced electromotive force under MF can interact with the photogenerated carriers through the mutual attraction of positive and negative charges, contributing to favorable charge separation. The schematic diagram clarified the influence of the MF on the NiO nanostructure under light (Fig. 4f). Herein, COMSOL was further performed to schematically illustrate the influence of the magnetic field on the NiO/FNi (Fig. 4g). Comparing with the Lorentz force of
with the electric force of
the effects of the magnetic field on the charge carriers can be discussed by referring to the electric field 19. It can be clearly seen that a parallel and homogenous electric field presented in the NiO nanosheets (NSs) based on the electromagnetic induction (Fig. 4h). Under magnetic field, the Lorentz force acts on the photo-induced electrons and holes due to the induced current difference between two sides, driving them to opposite directions, thus improving their efficient spatial separation and inhibiting recombination.
Electrochemical performance of the optical field assisted Li-O2 battery. Applying NiO/FNi as the reversible photocathode, an optical field assisted Li-O2 battery was constructed to investigate the effect of solar energy on the batteries (Fig. 5a). Due to the conversion of light energy to electricity, light-emitting devices can display higher brightness under illumination. Fig. 5b shows the galvanostatic charge and discharge curves of Li-O2 battery with NiO/FNi, the illumination increases the discharge potential (from 2.59 V to 2.69 V) and concurrently decreases the charge potential (from 3.74 V to 2.92 V). Meanwhile, the optical field assisted Li-O2 battery displays promoted energy efficiency from 69.3% to 92.1%, indicating the positive effect of light on both the input and output electric energy conversion. One the one hand, the photogenerated electrons and holes on the surface of NiO/FNi in the optical field can act on the discharging and charging processes, respectively. On the other hand, the deposition and morphology of the discharge products could be adjusted in the optical field 29. The discharge products obtained in the dark densely coated on the nanosheets structure, which would hinder the contact between the electrolyte and the active sites, leading to large polarization during the subsequent charging process (Supplementary Fig. 7a). In sharp contrast, the film-like product deposited on the surface of the cathode in the optical field, which can maintain the active sites after the discharge process and are beneficial for the decomposition process (Supplementary Fig. 7b). Whether under dark or light, the discharge product Li2O2 was mostly decomposed after the 1st recharged (Supplementary Fig. 7c, d). However, the undecomposed Li2O2 accumulated on the cathode surface in the dark after 20th recharged, while the smooth cathode surface is still exposed in the optical field (Supplementary Fig. 8). These results suggesting that the reversible decomposition of Li2O2 was more effective in the optical field.
When suffering the periodic ON-OFF cycles, the Li-O2 battery under light exhibits reproducible and continuous photo-responsiveness during the discharge and charge processes (Fig. 5c). The results suggest that the quick photoexcitation process and fast mass transfer under light could suppress the polarization of the battery. Moreover, the optical field assisted Li-O2 battery presents better rate capability than that in dark (Supplementary Fig. 9). The voltage gaps between charge and discharge process are maintained within 1.85 V even at high current density of 1.2 mA cm-2 and recovered to 0.36 V when returned to 0.01 mA cm-2 after 40 cycles at different current densities, which further implying the facilitated effect of illumination on the rate capability (Supplementary Fig. 10). The oxygen reduction reaction (ORR) curves on the rotating disk electrode show improved kinetics under light, which is responsible for the excellent catalytic performance (Fig. 5d). The photoelectric conversion effect of NiO/FNi mainly acts on reducing the charging voltage and increasing discharge voltage upon illumination, which is corresponding to the position of CB and VB of NiO/FNi (Fig. 5e). During discharge, O2 is reduced on the NiO/FNi surface involving the electron participation, thereby promoting the ORR. Under illumination, the photoelectrons from NiO/FNi transfer to the Li anode through the external circuit and hence reduce the Li+ to Li metals during charging. Meanwhile, holes can be collected on the NiO/FNi surface, and then utilized to oxidize the discharge product. Thus, the charge potential of NiO/FNi can be compensated by the generated photoelectric potential. In short, light plays a key role in achieving the energy storage and conversion from light to electrochemistry.
Electrochemical performance of the magnetic and optical field multi-assisted Li-O2 battery. Inspired by the efficient effect of MF on photocatalysis, a magnetic and optical multi-assisted Li-O2 battery was fabricated with NiO/FNi to study the influence of MF on the photoelectrochemical properties in the batteries. Fig. 6a depicted the charge potential of the battery under optical field and MF with different conditions. Notably, the charge potential of the battery was reduced to 2.73 V for the magnetic induction intensities of 5 mT, indicating that the existence of a permanent MF plays a significant role in enhancing the energy efficiency of the battery. Herein, the optimal magnetic induction intensity is selected to evaluate of the magnetic and optical multi-assisted Li-O2 battery performance. The first discharge and charge profiles of the magnetic and optical multi-assisted Li-O2 battery with NiO/FNi at 0.01 mA cm-2 under MF (5 mT) exhibited an ultra-low polarization voltage of ~ 0.09 V and a high energy efficiency of 96.7% (Supplementary Fig. 11). These results implied that more photogenerated electron-hole pairs existed in the batteries because the rapid recombination of the photo-carriers can be greatly hindered under the MF condition. Even at high current density of 1.2 mA cm-2, the Li-O2 battery under MF (5 mT) and light also shows the expected low charge voltage, which can be recovered to 2.78 V when returned to 0.01 mA cm-2 after 40 cycles at different current densities (Fig. 6b). To further understand how MF facilitate the ion diffusion and improve the battery kinetics, EIS of photo-assisted Li-O2 battery with NiO/FNi under MF was measured (Fig. 6c). The Nyquist plot of NiO/FNi MF (5 mT) light displays a much larger gradient, indicating a faster diffusion rate compared with NiO/FNi MF (0 mT) light. On the basis of the Nyquist plots 30, the Li+ diffusion coefficients’ ratio of NiO/FNi MF light and NiO/FNi NMF light is determined to be 5:1, demonstrating a superior kinetics under MF for Li+ diffusion.
The Li-O2 battery still shows stable discharge and charge platform without the increase in polarization after 120 h, which is attributed to the fact that the photo-induced electrons and holes can be separated in a long time under MF (Fig. 6d). The influence of the MF on the photo-assisted Li-O2 battery is proposed and illustrated in Fig. 6d (inset). For a typical NiO/FNi photoelectrode, photogenerated electron-hole pair can easily recombine because the formed excited state obeys the conservation laws for energy and momentum, which results in limited participation of the photogenerated carriers in the charge transport. A potential or force to drive the positive and negative carriers towards opposite directions would significantly enhance the charge carrier density. MF was present during the photoelectrochemical process, according to the left-hand rule, both the moving electrons and holes experience a Lorentz force vertical to the direction of movement. Moreover, the forces acting on the negative electron and positive hole of the electron-hole pair are in opposite directions, leading to the deviation of the electrons and holes in opposite directions. Therefore, the photo-induced electron-hole pairs in moving semiconductor particles can be separated in MF, even at the initial generation of an electron-hole pair.
To further examine the stability of the magnetic and optical multi-assisted Li-O2 battery upon cycling, the morphology changes of the recharged O2 cathodes after the 30th cycle were examined. SEM image was carried out to investigate the discharge products under MF (5 mT) and light. The film-like morphology is similar to that under MF (0 mT) and light, indicating that MF has negligible effect on the morphology of the discharge products (Supplementary Fig. 12). The discharge product is evidenced by the Li 1s signal at 54.7 eV in the XPS spectra of NiO/FNi under MF (0 mT) and MF (5 mT) light, which reveals that Li2O2 is the only discharge product after the 30th discharged (Supplementary Fig. 13). After the 30th recharged, XRD patterns demonstrate that the Li2O2 can be oxidized completely during the charging progress (Fig. 6e). As shown in the SEM image, partial residues are still remained on the cathode after the 30th recharge under MF (0 mT) and light (Fig. 6f). This continuous accumulation of the undecomposed products upon cycling would hinder the transportation of the oxygen, electrons and lithium ions within the cathode during subsequent cycling. In sharp contrast, the film-like discharge products have disappeared, and the homogenously nanosheets structure of the whole cathode is almost fully recovered, even after the 30th recharge under MF (5 mT) and light, implying good structure reversibility and stability of the magnetic and optical multi-assisted Li-O2 battery (Fig. 6g). The Li 1s signal at 54.7 eV in the XPS spectra of NiO/FNi further reveals the reversible Li2O2 decomposition under MF (0 mT and 5 mT) light after the 30th recharged (Fig. 6h). Moreover, gas chromatography was performed on the batteries to demonstrate the gas evolution on recharge. As shown in Supplementary Fig. 14, the results imply that O2 are detected as the predominant gas over the recharge process. Tiny amounts of CO2 gas (μL) are traced in the battery under light and magnetic field, suggesting the positive effect of the external field on the decomposition of Li2O2. All the above-mentioned results support that the optical and magnetic field play an active and important role in enhancing the Li-O2 battery performance.