Coupling Alkaline Conversion Zn Anode with Acidic Proton-insertion MoO3 Cathode for High-voltage Aqueous Rechargeable Hybrid Battery

Hydrogen ions (H+) and hydroxide ions (OH-) are regarded as ideal charge carriers for rechargeable batteries thanks to their small size, high ion mobility, low cost, and wide exibility compared to the metal ions. However, the implementation of storage of both H+ and OH- in one electrochemical energy device face grand challenge due to incompatibility between H+ and OH-. Herein, we report an alkali-acid Zn-MoO3 hybrid battery that employ H+ and OH- as charge carriers of cathode and anode, respectively, in which the insertion/deinsertion of H+ take place on layer structured MoO3 cathode in acid while OH- are involved in alkaline conversion Zn anode, which offers a promising route to well address the incompatible issues of H+ and OH- in one electrolyte. The as-built hybrid battery can deliver a high open-circuit voltage of 1.85 V, a high rate capability, a high capacity of 158 mAh g-1 at a current density of 5 A g-1, and excellent capacity retention of above 90% over 200 cycles. This work sheds light on the development of aqueous energy devices with high voltage and energy density through materials engineering and device optimization.


Introduction
Electrochemical energy storage and conversion systems play a crucial role in the development of renewable energy for a sustainable future. 1, 2, 3 Among these systems, aqueous-based batteries that employing metal ions as charge carriers, such as Zn-ion battery, 4,5,6 have drawn worldwide attentions due to the advantages of environmental benignity, high safety, high e ciency, and fast kinetics. Whilst nonmetal ions battery, 7,8,9,10 such as hydrogen ions (H + ), hydroxide ions (OH -), NO 3 -, and NH 4 + , receive rare research interest it deserved because they are actually ideal charge carriers for rechargeable batteries, thanks to their small size, high ion mobility, low cost, and wide exibility compared to the metal ions. 11,12 Moreover, it remains a grand challenge for the development of rechargeable battery with dualnonmetal ions as charge carriers of both anode and cathode. 13,14,15 In this regard, it is of great signi cance to explore the feasibility of developing aqueous batteries with dual-nonmetal ions as charge carriers and to clarify the associated electrochemical mechanism. Of various nonmetal ions, H + and OHare the ideal candidates as charge carriers for the dual-nonmetal ions battery with hydrogen insertion/deinsertion in cathode and OHinvolved in conversion anode (e.g., Zn, Al, Mg anode), respectively. To this end, at least two critical issues should be addressed. On one hand, an essential prerequisite for implementing such hybrid electrochemical device is to explore suitable cathode materials with capability for reversible H + insertion/deinsertion. On the other hand, it is a daunting challenge to integrate H + and OHin one electrochemical system because these two ions would react quickly once mixed.
Orthorhombic α-MoO 3 is one of the promising materials for H + storage, 16,17 which is appropriate for the insertion/deinsertion of H + due to its layered structure. 18,19,20 However, the poor conductivity and the limited electrochemical activity of α-MoO 3 greatly restrict its electrochemical H + storage capability and thus behave low speci c capacity and fast decay. 21,22 Accordingly, efforts have been made to improve the conductivity of α-MoO 3 , typically by coating of conductive polymers on the surface of MoO 3 , 23,24,25 such as polyaniline (PANI), polypyrrole (Ppy), owing to their merits of good stability, low cost, and good conductivity, which is bene cial to facilitate electrons transfer and to expedite ions exchange, in this manner can the associated performance be improved to a certain extent, there is great space for further improvement yet. 26,27 The bottleneck for such electrochemical device with non-mental ions storage is the thermal stability windows of 1.23 V, which greatly limits the energy density that is directly related to the working voltage according to the equation (E = C sp * V). 28
Moreover, some atomic-leveled distortion could be clearly observed (remarked by red dots, also see Fig.   S6, supporting information), which is due to the presence of oxygen vacancies. The selected area electron diffraction (SAED) pattern was shown in Fig.1h, indicating the single-crystalline nature of the v-MoO 3 PNB, which is attributed to the (010) zone axis diffraction.
Raman spectroscopy and electron paramagnetic resonance (EPR) spectroscopy were carried out to study the samples.  To evaluate the H + storage capability, cyclic voltammetry (CV) tests were carried out in 1.0 M H 2 SO 4 with a standard three-electrode system with Ag/AgCl as reference electrode, Pt wire as counter electrode, and the MoO 3 modi ed carbon paper (1*1 cm 2 ) as working electrode, respectively. Fig. 3a shows the rst three CV curves of v-MoO 3 PNB at a scan rate of 1 mV s -1 at the potential range from -0.5 to 0.3 V. One can observe three pairs of redox peaks of 0.074 V/0.01 V, 0.032 V/-0.04 V, and -0.371 V/-0.384 V that are related to the insertion and deinsertion of H + into the v-MoO 3 PNB electrode. Notably, the peak current of v-MoO 3 PNB is larger than that of MoO 3 NB (Fig. 3b), implying a higher H + storage capacity for v-MoO 3 PNB, which can be rationally attributed to the higher conductivity (Fig. S8, supporting information) and more activity sites derived from large speci c surface area (Fig. S9, supporting information). Moreover, the reaction kinetics of H + insertion and deinsertion were further studied by recording the CV curves at different scan rates from 0.5 to 10 mV s -1 (Fig. 3c). The peak currents and the peak separations increase along with the scan rate increases, which is due to the enhanced polarization at high scan rates. The Randles-Sevcik equation was employed to determine the relationship between the peak currents (i p ) and scan rate (v). The i p linearly increases with the square root of the scan rate (v 1/2 ) over the scan rate range, with the excellent linearity close to 1 (Fig. 3d), indicating a diffusion process control of the H + insertion and deinsertion on the v-MoO 3 PNB electrode.
The charge and discharge pro les from 1 A g -1 to 20 A g -1 of the v-MoO 3 PNB are shown in Fig. 4a. Even at an ultra-high current of 20 A g -1 , three obvious pairs of charge and discharge plateaus can be observed, which match well with the three-step redox behavior in the CV curves. While the charge and discharge plateaus of MoO 3 NB can't be recognized at large current (Fig. S10, supporting information), implying the high rate capability for the v-MoO 3 PNB thanks to its improved conductivity. Fig. 4b shows the capacity at different current density of the v-MoO 3 PNB, which delivers discharge capacities of approximately 248.2, 245.4, 210.8, and 198.7 mAh g -1 at currents of 1, 5, 10, and 20 A g -1 , respectively. The Coulombic e ciency is in the range of 97 -99% can be obtained when the current is reduced to 1 A g -1 . However, the MoO 3 NB exhibits a poor rate performance with low speci c capacity and inferior capacity recovery (Fig.   S11, supporting information). Fig. 4c shows the stability of the v-MoO 3 PNB for H + insertion and deinsertion. At a current of 10 A g -1 , above 92% of its initial capacity can remain with an average Columbia e ciency of 96% after 200 cycles, which is superior to those of MoO 3 NB (Fig. S12,  In this as-developed hybrid device, the redox reactions of anode and cathode can proceed in their optimal conditions with potential of harvesting the so-called electrochemical neutralization energy (ENE), 44,45,46,47,48 which can signi cantly enhance the voltage and energy density of energy devices. Fig. 5b shows the CV curves of the Zn anode in alkali and the v-MoO 3 PNB cathode acid at a scan rate of 1 mV s -1 . Both electrodes show redox activity at their individual potential windows, which indicates the availability and feasibility for fabricating alkali-acid Zn-MoO 3 hybrid battery. The as-constructed alkali-acid Zn-MoO 3 hybrid battery (Fig. 5b) shows one prominent pair of redox peaks in the potential range of 1.2 V to 1.6 V.
As a result, the battery shows a high open-circuit voltage (OCV) of about 1.85 V (Fig. 5c), higher than most of aqueous battery. 49,50,51,52,53 And the value is constant for 60 mins, implying the decent stability of the hybrid battery. Moreover, a single battery can power a red light-emitting diode (LED, 1.8-2.2 V, Fig.  S13, supporting information), further demonstrating the high voltage of the as-built battery. It is noted that the OCV of the battery varies when the electrolyte is different (Fig. S14, supporting information) because the H + insertion and deinsertion are sensitive to the pH value of electrolytes, including the current and potential (Fig. S15, supporting information). has not been changed after long-term stability test, except for peaks from Na on at 18º and 26.4º, and crystalline carbon paper at about 25º (Fig. S16, Supporting Information). In addition, the morphology and microstructure of the v-MoO 3 PNB also keep almost unchanged, as demonstrated by the SEM images of the material (Fig. S17, Supporting Information). Moreover, the CV curve of Zn anode shifts for high potential, while that of v-MoO 3 PNB cathode shifts to low potential (Fig. S18, Supporting Information), which may be due to the crossover of H + and OH -. For this purpose, we monitor the pH value variation of the catholyte and anolyte during cycling. The concentration of H + and OHdecrease along with cycling ( Fig. S19, Supporting Information), which induces the capacity decay of the alkali-acid Zn-MoO 3 hybrid battery.
To investigate the mechanism of the alkali-acid Zn-MoO 3 hybrid battery, the ex-situ XRD measurements of v-MoO 3 PNB electrode were performed during the charge and discharge process (Fig. 6). As shown in Fig. 6b, all the XRD patterns during the charge-discharge processes show three predominant peaks of (020), (040) and (060), indicating that no phase transition exists in the processes, which suggests a solid solution reaction during H + ions insertion and deinsertion. Moreover, the peak located at about 18º is assigned to the Na on. When discharged to 1.0 V, the peak at 12.7º for the (020)

Characterization
The morphologies of samples were checked by eld-emission scanning electron microscope (FESEM; Hitachi, SU8010, 5 kV) and transmission electron microscope (TEM, FEI, F20, 200 kV). The phase of products was performed on the powder X-ray diffraction (Hitachi, Mini ex600). X-ray photoelectron spectra (XPS) were collected by ESCALAB 250Xi (Thermo Scienti c) XPS spectrometer with an Al Kα as the excitation source (1486.6 eV). The N 2 adsorption-desorption isotherms were accomplished on a Micromeritics Instrument Corporation sorption analyzer at 77 K (Micromeritics TriStar II 3020), from which we can obtain the information of speci c are and pore properties for samples. Declarations Z. W. conceived the research project and revised the manuscript. P. C. contributed to the sample synthesis, experimental measurements, and manuscript writing. Y. D. also contributed to the modi cation of the manuscript. Y. L. contributed to the guidance of the diagram. All authors reviewed and commented on the nal version of the manuscript.