Stable aqueous aluminum-manganese photoelectrochemical cells with high-rate and high-eciency abilities

Aqueous aluminum-ion batteries (AAIBs) are potential candidates for large-scale energy storage devices for their advantages of high energy density, resource abundance, low cost, safety, and environmental friendliness. Due to various redox procedures, good reversibility, and high discharge potential, the aqueous aluminum-manganese oxide battery has drawn wide attention, while the critical issues induced from slow kinetics and undesired soluble Mn 2+ lead to slow charging, poor rate capability, and low energy density. However, there is very limited progress for performance improvement via conventional chemical or physical modification approaches. To overcome these challenges, an efficient photo-regulation strategy has been proposed in terms of direct radiating visible light on the cell during the galvanostatic charging and discharging. The efficient separation and transmission of photoelectrons in the photo positive electrode dramatically improves the dynamics, and fast charging and enhanced rate performance could be achieved. Photo-oxidation behavior can effectively promote the conversion of soluble Mn 2+ , thus further enhancing the energy density of the as-assembled aluminum-manganese battery. Furthermore, a photo-conversion efficiency of up to 1.2% has been acquired. Based on the photo-regulation strategy, the mechanism of the photoelectrochemical coupling system has been understood, which opens a promising route for achieving photoelectrochemical batteries with high energy density and fast charge. illuminated conditions for the photo-actuated aluminum-manganese cells. The inset is a single current pulse with a specific setting parameter of 10 min pulse time, 100 mA g 1 current pulse, and 30 min relaxation time. (I) Nyquist plots of the EIS spectra under dark and illuminated conditions at 0 V bias voltage. (J) The DRT results of the corresponding EIS spectra.


Introduction
The increasing consumption of fossil fuels and critical environmental issues are required to develop eco-friendly and sustainable renewable energy resources and energy storage systems. Aluminum-ion batteries (AIBs) are potential candidates for large-scale energy storage due to the advantages of high energy density, resource abundance, low cost, and safety. However, the current non-aqueous halides ionic liquid electrolytes of high cost, severe corrosivity, air sensitivity, and environmental concerns have significantly restricted their practical applications 1-3 . In contrast, aqueous electrolytes with features of higher safety, lower cost, H2O/O2 insensitivity, and easier fabrication has attracted remarkable attention [4][5][6] . Among such battery prototype, manganese oxides with layered structure and high energy density are widely employed radiates to the surface of the photosensitizers. Subsequently, the generated photoelectron would flow to the external circuit owing to the appropriate matched energy level. On the other hand, the photoexcited electrons can transport along the nanorods, which is beneficial to reduce the recombination rate of separated electronhole pairs. The exposure of α-MnO2 to visible light could effectively lower the charge transport resistance, which is favorable to generate photooxidation behaviors. In the photoelectrochemical battery, consequently, the overall performance including fast recharging, enhanced rate performance, and suppressed soluble Mn 2+ has been achieved.

Results
The cell configurations and operation principles. In this study, α-MnO2 nanorod was synthesized using a simple solvothermal method, which was simultaneously employed as electrochemically active materials and photosensitizer. The chemical composition and elemental valence of the as-prepared α-MnO2 were firstly verified by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) (Supplementary, Fig.   2A,B). Moreover, the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images revealed that the as-synthesized α-MnO2 presented a homogeneous single crystal rod-like structure with an interplanar spacing of 0.49 nm According to electronic transition characteristics, the optoelectronics would be transmitted to the external circuit along the α-MnO2/rGO/CF while suppressing the migration of holes. Therefore, such feature would ensure the effective separation of the generated electron-hole pairs. In order to simulate the natural sunlight (400-780 nm), an optical filter was introduced to remove the ultraviolet light below wavelength of 400 nm. Meanwhile, a hole with diameter of 8.0 mm was placed in the center of the as-assembled coin-type cell for light radiation (Fig. 1D). The configuration scheme of the aluminum-manganese battery was displayed in Fig. 1E. When applying light to the photo positive electrodes upon the charging or discharging processes, the visible light could radiate directly onto the surface of the positive electrode through the hole. In light excitation, the photosensitizer firstly absorbs photons (Ehv>Eg, Eg: bandgap) to generate electron-hole pairs 14 . Upon absorption of a photon, the electron could be excited from the semiconductor valence band to the conduction band, leaving a hole in the original position. 15 In order to prevent the non-radiative recombination of electron-hole pairs, the reduced graphene oxide (rGO) was employed as an electron transport layer to conduct separated electrons. Compared to a single electrochemical process (Fig. 1F), the photoelectrochemical coupling process would lead to generate optoelectronics to flow out into the external circuit, thereby achieving fast charging and significantly enhanced rate performance (Fig. 1G). Due to the direct radiation of visible light, a photo-oxidation behavior in the photoelectrochemical coupling process could effectively promote the conversion of soluble Mn 2+ , resulting in high utilization of active materials (part 2.4).
To substantially alleviate the overpotential at the pristine Al negative electrode, a Zn/Al alloy negative electrode was also prepared. In the typical process 7,16 , the zinc foil was directly employed as the substrate via an alloying process during the charging stage (Supplementary, Fig. 6A,B). XRD and XPS spectra were performed to confirm the as-  during charging and discharging process. As expected, the high-valence manganese (Mn 4+ ) was gradually decreased with the reduction process. In contrast, a reversible evolution was discovered in the oxidation process, which was consistent with the previous literature 8 . In Fig. 2B, the AAIBs in the dark environment delivered initial discharge capacities of 376.4 and 286.0 mA h g −1 at 100 and 300 mA g -1 , respectively.
After 100 and 200 cycles, the discharge capacities were maintained at 305.0 and 205.4 mA h g −1 , respectively. Compared with the previously reported α-MnO2 positive electrode, the significantly enhanced capacity and stability is mainly attributed to the reversible plating/stripping of Al and lowered overpotential on the alloying negative electrode. In the dark environment and under illumination condition, the corresponding rate performance of the cell was separately evaluated at various rates from 300 to 1000 mA g −1 , as shown in Fig. 2C. In terms of dark environment, the initial discharge capacity 249.5 mA h g −1 (current density of 300 mA g −1 ) was found to decrease to 46 mA h g −1 (current density of 1000 mA g −1 ). Under illuminated conditions, on the contrary, the discharge capacities were significantly increased (203.4 mA h g −1 ) even at current density of 1000 mA g −1 due to the highly efficient photoelectrochemical coupling process. Interestingly, the corresponding charge/discharge curves at different current densities ((Supplementary, Fig. 12) and (Fig. 2D,E)) exhibited prominent potential platforms and considerable capacity increase. Noticeably, the discharge capacity reached 531 mA h g -1 (100 mA g -1 current density) under illumination in comparison with the value ~376.4 mA h g -1 achieved in the dark environment, which corresponds to a substantial capacity increment of 41.3% (Fig. 2D). Due to the existence of photooxidation behavior under light, the significant enhancement of discharge capacity can be understood as the coexistence of photo-charging and electrochemical reduction.
Compared with other AAIBs systems in the previous studies (Table 1), the photoelectrochemical cells under illumination conditions here presented pronounced enhancement in the specific discharge capacity with outstanding rate performance.
In order to study the dynamic behaviors of AAIBs in the dark environment and under light illumination, the electrochemical impedance spectroscopy (EIS, 10 mV bias voltage) was also acquired (Fig. 2F). Apparently, the total charge transfer impedance Compared with the dark condition, D Al 3+ have been improved under illumination during discharge, while D Al 3+ almost remains the same during charge process ( Fig. 2H and Supplementary, Fig. 13).
To demonstrate the contribution of photo-regulation in photoelectrochemical coupling process, the photo-conversion efficiency was calculated to be 1.  Table 2).

Light excitation response.
To fully understand the fast charging and enhanced rate performance of the photo-excited aluminum-manganese battery, it is significant to study the carrier dynamics and transport. Fig. 3A-C shows the transient absorption map, spectra, and dynamics of a spin-coated α-MnO2 thin film using 400 nm pump photons.
As shown in Fig. 3A,B, a strong optical excitation absorption signal was captured around 600 nm, suggesting the generation of electron-hole pairs in the α-MnO2 crystal.
Subsequently, the separated photo-excited electrons can transport with matched energy levels, resulting in the fast-charging rates in the photo-electrochemical battery.
Generally, the recombination rate of electron-hole pairs is linked with carrier lifetime.
Apparently, longer time delay would lead to increased photoinduced electrons. Therefore, the carrier lifetime of both α-MnO2 semiconductor and mixture of α-MnO2 and rGO (α-MnO2-rGO) was evaluated to understand photo-charge carrier dynamics.
Both α-MnO2 and α-MnO2-rGO exhibited only slight signal decay, indicating that their carrier lifetime is much greater than 7.5 ns (Fig. 3C). Additionally, the α-MnO2-rGO presented greater charge carrier dynamics, and it is attributed to the electron transport layer (rGO) that plays a positive effect role in charge separation and transfer. To further confirm the generation and separation process of the photo-induced electrons, the photo-current response upon layer-by-layer transparent photodetector was measured. In the configuration of FTO/rGO/MnO2/Ag photodetector, Ag is the positive electrode and FTO layer is the negative electrode, as shown in Fig. 3D. The photocurrent response (vs. open circuit potential, i.e., 0 V bias voltage) was evaluated via several on/off cycles of the optical radiation, as shown in Fig. 3E. The photocurrent rapidly increased to ~ 0.45 μA cm -2 when the light was turned on. Then, the photocurrent decreased to 0 μA cm -2 when the light was turned off, suggesting good cycling stability and highefficiency extraction of photogenerated electrons. When a 10.0 mV bias voltage was applied to the electrode, the photocurrent was increased to 0.55 μA cm -2 (Fig. 3F), which indicates that the bias voltage could effectively promote the separation and transportation of photogenerated electron-hole pairs. To further study the separation and transfer of photoexcited electrons in the static electrochemical system, the photocurrent response of AAIBs (initial, fully discharged to 0.5 V (D-0.5 V), and fully charged to 2.0 V (C-2.0 V)) was evaluated under on/off state of the visible light illumination (Fig. 3G).
When the light was on, a photocurrent ~44.5 μA cm -2 was obtained in the initial AAIBs.  Photoelectrochemical coupling mechanism. According to the light excitation response behaviors, a large photocurrent was generated due to the high-efficiency separation and transmission of photoelectrons. Therefore, fast charging and enhanced rate performance of the cells could be achieved during the photoelectrochemical coupling process. Therefore, it is significant to understand the mechanism of the photoelectrochemical coupling process. illumination conditions at a current density of 100 mA g -1 . In the charge process, the electrode potential will be positively shifted along with the electrochemical oxidation.
On the contrary, the electrode potential in the discharge process will be negatively shifted upon the electrochemical reduction process. Upon charge in the dark environment, however, the electrode potential was observed to be negatively shifted by ~20 mV (ΔVC=20 mV, Fig. 4B) when the light was applied. Subsequently, the electrode potential continued to shift forward with further illumination. When the illumination was off, this phenomenon was cut-off and the electrode potential was found to shift significantly forward. As expected, the opposite phenomenon was observed during the discharging process. A positive shift (ΔVD=100 mV) was observed in the electrode potential when the cell was exposed to illumination (Fig. 4A). In the charging and discharging process, the shifted potential would be mainly attributed to the characteristic of n-type semiconductors, it may be associated with the negative charge accumulation in the conduction band under illumination. Meanwhile, the results of a significant abrupt change in the electrode potential also confirm that α-MnO2 is highly sensitive to light in the electrochemical process. Upon illumination, an apparent shift in the potential (ΔVD > ΔVC, ΔV =80 mV) was exhibited in the charging and discharge processes. To further probe the shift behavior on electrode potential, density functional theory (DFT) calculation was carried out.  Fig. 15F-H). The corresponding partial density of states (PDOS) (Supplementary, Fig. 15I-L) further confirmed that the intercalated Al 3+ would introduce hybrid level, and whereby the transition barrier of photo-excited electrons from the valence band to the conduction band could be effectively decreased (Fig. 4C). The results from DFT calculation would be used to clarify the shifting phenomenon in the electrode potential, as mentioned in Fig. 4A.
Namely, the introduction of hybrid energy levels significantly promoted the migration of optoelectronics from the valence band to the conduction band during the discharge process, thus leading to a more pronounced significant shift in electrode potential.
In order to determine the chemical changes induced by photoexcitation and clarify the photoelectrochemical coupling mechanism, electron paramagnetic resonance (EPR) was carried out under the conditions with and without applying illumination a photo positive electrode. Fig. 4D shows the EPR spectrum and the corresponding g value distribution is exhibited in Fig. 4E. With or without illumination, six typical characteristic signals were all observed at 300-400 mT (g≈2.0) (Fig. 4E), which was defined as Mn 2+21 . In the EPR spectra, however, there is lacked of signal related to Mn 3+ due to the integer spin 22 . Accompanied with illumination, a higher g-value (g≈3. 8) signal was observed at 100-200 mT (defined as Mn 4+ ), which was in sharp contrast to the signal under dark condition 23 . This phenomenon demonstrates that photo-radiation could promote the transformation of low-valence manganese (Mn 2+ , including partial Mn 3+ ) to high-valence manganese (Mn 4+ ). Therefore, it is reasonable that the photooxidation behavior existed in the photo positive electrode, which can be described as following reaction equation: Under the dark condition, the stable operation of AAIBs could be achieved in a single electrochemical process. Upon illumination, two steps, including photochemical and electrochemical (defined as a photoelectrochemical coupling process), would participate in the electrode process. In addition to the electrochemical process, the photo-oxidation process would also release electrons (photoinduced electrons) and flowed to the negative electrode through the external circuit (Fig. 4F). Meanwhile, the remaining holes would be neutralized by the electrons generated in the process of Al 3+ deintercalation. In the discharge process, the holes were consumed by electrons coming from an external circuit, while the photoinduced electrons would be taken along with the intercalation of Al 3+ .
The dissolution of Mn 2+ was an undesirable chemical behavior in the electrochemical process. Interestingly, such behavior could be suppressed by the photooxidation process. Fig. 4G shows the Raman spectroscopy of the electrolytes after cycles, and the Mn-O (580 cm -1 , in the range of 550-620 cm -1 ) chemical bond was observed. To further verify the inhibitory effect of photo-oxidation behavior on soluble Mn 2+ , inductively coupled plasma mass spectrometry was carried out to track the concentration of Mn 2+ in the electrolyte after different cycles (Fig. 4H). In comparison with dark conditions, the manganese concentration in the electrolyte had been effectively suppressed under the illuminated condition, which indicates that the electrochemically reduced soluble Mn 2+ was subsequently conversed to Mn 4+ by the photo-oxidation process. As a result, unnecessary loss of Mn 2+ could be effectively alleviated. Fig. 4I shows the charging (with illumination) and discharging (with and without illumination) curves at a current density of 100 mA g -1 . Upon photoelectrochemical coupling process, a longer discharge time was obtained, indicating that more Al 3+ was intercalated and greater energy was released. Meanwhile, the charging experiments under light and dark conditions (current density of 100 mA g -1 ) followed by dark galvanostatic discharge cycles at different current densities (100, 200, and 300 mA g -1 ) were also performed (Fig. 4J). In dark discharge, the light-charged cell exhibited a longer discharge time, which fully demonstrated the advantages of photoelectrochemical coupling process.

Discussion
In this study, a prototype of a stable photoelectrochemical coupling system was For substantially overcoming the issues of low energy density and slow charge rate in the aqueous aluminum-manganese batteries, a novel photo-regulation strategy has been proposed and a stable photoelectrochemical coupling system has been established. Based on the electrochemical process of α-MnO2, a photoelectrochemical coupling approach was investigated to achieve fast charging and enhance rate performance. Meanwhile, the photo-oxidation behavior was also observed on the photo positive electrode, and the loss of soluble Mn 2+ was effectively alleviated as expected.
Because of the stable and high-efficiency photoelectrochemical coupling process, a photoelectric conversion efficiency 1.2% has been obtained. Nevertheless, great efforts could be still paid to optimize the photoelectrochemical coupling systems, such as development of a more stable anode and electrolyte. It is believed that rational manipulation approach can be developed to promote the practical applications in various advanced photoelectrochemical engineering and devices.

Synthesis of α-MnO2 nanorod.
Single crystal α-MnO2 nanotubes were prepared by a simple hydrothermal approach. 8 In a typical preparation, 5 mM KMnO4 was firstly dissolved into the solution of 24 mL of 1.0 M HCl with continuous string to obtain a homogeneous solution. Then, the 46 mL of distilled water was slowly introduced abovementioned solution. After stiring at room temperature for 30 min, the solution was transferred into a Teflon-lined stainless-steel autoclave. The reactor was stayed at 140°C and reacted for 18 hours. After cooling to room temperature, the α-MnO2 was separated using a centrifuge from the suspension solution, followed by cleaning with deionized water and alcohol several times and drying at 60 ℃ overnight.
Preparation of aqueous electrolytes. The 2M Al(CF3SO3)3 aqueous electrolyte was prepared in an external environment. Firstly, the 9.48 g of Al(CF3SO3)3 was slowly added into 10 mL deionized water with continuous stirring. Subsequently, the electrolyte was maintained at room temperature for 12 h before use. Germany). The sample was a photo positive electrode that had been discharged to 0.5 V, to obtain a large amount of low-valence manganese. Light illumination was provided by a 300 W Xenon lamp under a low temperature of 70 K.

Data availability
Source data are provided with this paper. The other data that support the findings of this study are available from the corresponding author upon request.
References Wu    Schematic diagram of electron migration in the photoelectrochemical coupling process during charge and discharge.

TOC Graphic
A high efficiency photo-regulation strategy has been proposed and established the stable photoelectrochemical coupling system to overcome the issues that the aqueous aluminum-manganese batteries are suffering. Consequently, the slow kinetics and undesired soluble Mn 2+ have been significantly improved, thus achieving the photoelectrochemical batteries with high energy density and fast charge.