Catholyte Modulation and Prussian Blue/Berlin Green Redox Mediator Enabling Efficient High‐Potential Mn2+/MnO2 Reaction for Aqueous Hybrid Batteries

The use of multielectron Mn2+/MnO2 cathode reaction achieves high‐energy aqueous acid–base asymmetric hybrid batteries. However, the ion crossover through the membrane and the accumulation of unreacted MnO2 lead to serve potential and capacity decay of the cathode during cycling. Herein, the catholyte modulation using the KCl additive and the design of a Prussian blue/Berlin green (PB/BG) redox mediator on the cathode current collector are developed to promote high‐potential Mn2+/MnO2 reaction. The addition of saturated KCl greatly suppresses the ion crossover to maintain the catholyte acidity, thus enabling high‐potential Mn2+/MnO2 reaction and stable high‐voltage discharge plateau of ≈2.2 V for the hybrid battery at a prolonged cycling. The reversible MnO2 deposition is effectively improved by the PB/BG redox mediator, which contributes to significant enhancements in the reversible capacity, rate capability, and cycling stability of the cathode. Importantly, the as‐assembled hybrid battery exhibits the highest energy density of ≈260 Wh kg−1 (based on the mass of the aqueous MnCl2 catholyte and a hydrogen storage anode material) among the previously reported Mn‐based acid–base asymmetric batteries.


Introduction
The large-scale application of renewable energy resources is inseparable from an efficient and safe energy storage system, which can help wind, solar, and other intermittent energy to achieve grid integration. [1]Aqueous rechargeable batteries have been regarded as a promising energy storage system.Besides the commercialized lead-acid, [2] nickel-metal hydride, [3] and redox flow batteries [4] with high safety and low cost, novel aqueous batteries based on various charge carriers (e.g., Li-ion, [5] Na-ion, [6] K-ion, [7] Ca-ion, [8] Mg-ion, [9] Alion, [10] Zn-ion, [11] ammonium-ion, [12] and proton batteries [13] ) have also emerged.In these novel aqueous batteries, Mn-based materials, especially MnO 2 , have attracted notable attention as the cathode materials due to their abundant resources, low cost, and high-energy feature. [14]nO 2 realizes energy storage through various electrochemical redox mechanisms including the conversion reaction between MnO 2 and MnOOH, [15] the cation insertion-extraction reactions, [16] and the Mn 2þ /MnO 2 deposition/dissolution reaction.[17][18][19] The Mn 2þ /MnO 2 reaction offers unique merits of a high theoretical capacity of 616 mAh g À1 (MnO 2 ) through the two-electron transfer and a high redox potential of 1.23 V (vs standard hydrogen electrode).[20] However, the Mn 2þ /MnO 2 reaction can only occur in an acidic electrolyte and the reaction reversibility is strongly affected by the electrolyte acidity, [21,22] which poses challenges in matching the anode material with the acidic electrolyte.Only a few anodes such as Pb, [23] hydrogen, [17] and Cu [24] have been paired with Mn-based cathodes in acidic electrolytes to construct stable Mn-based aqueous batteries.Furthermore, the battery voltage is limited due to the high electrode potential of these anodes. Recetly, acid-base asymmetric batteries based on the Mn 2þ /MnO 2 deposition/dissolution reaction have been proposed, including decoupled MnO 2 À Zn, [18] MnO 2 À MH (metal hydride), [25] and MnO 2 Àquinone batteries.[26] The design of acid-base-decoupled electrolytes has extended the application of the Mn 2þ /MnO 2 deposition/dissolution chemistry. Thee hybrid batteries operate through decoupled electrochemical reactions at the Mn-based cathode in the acidic electrolyte (catholyte) and at different anodes in the alkaline electrolyte.An ionselective membrane is used to prevent neutralization reactions of the acidic and alkaline electrolytes. However the catholyte acidity of the hybrid batteries tends to decrease upon cycling because of ion crossover (e.g., H þ and K þ ) between the electrolytes through the membrane, thus leading to degradation of the battery's discharge voltage.[22,27] Furthermore, the electrochemical dissolution of MnO 2 typically follows the MnOOH-mediated The use of multielectron Mn 2þ /MnO 2 cathode reaction achieves high-energy aqueous acid-base asymmetric hybrid batteries.However, the ion crossover through the membrane and the accumulation of unreacted MnO 2 lead to serve potential and capacity decay of the cathode during cycling.Herein, the catholyte modulation using the KCl additive and the design of a Prussian blue/Berlin green (PB/BG) redox mediator on the cathode current collector are developed to promote high-potential Mn 2þ /MnO 2 reaction.The addition of saturated KCl greatly suppresses the ion crossover to maintain the catholyte acidity, thus enabling high-potential Mn 2þ /MnO 2 reaction and stable high-voltage discharge plateau of %2.2 V for the hybrid battery at a prolonged cycling.The reversible MnO 2 deposition is effectively improved by the PB/BG redox mediator, which contributes to significant enhancements in the reversible capacity, rate capability, and cycling stability of the cathode.Importantly, the as-assembled hybrid battery exhibits the highest energy density of %260 Wh kg À1 (based on the mass of the aqueous MnCl 2 catholyte and a hydrogen storage anode material) among the previously reported Mn-based acid-base asymmetric batteries.
pathway.This process leads to the release of Mn 3þ ions into the electrolyte solution.These dissolved Mn 3þ ions then undergo a disproportionation reaction to induce the generation and accumulation of unreacted MnO 2 in the cathode, which lead to a low coulombic efficiency and poor cycling stability of the battery. [28]Therefore, inhibiting ion crossover of the electrolytes and enhancing the Mn 2þ /MnO 2 reaction efficiency are critical for improving the electrochemical performance of acid-base asymmetric batteries.
Herein, the catholyte modulation and the surface structure design of the cathode current collector are employed to eliminate the ion crossover and promote high-potential Mn 2þ /MnO 2 reaction, respectively.The addition of KCl up to a saturated concentration in the MnCl 2 -based acidic catholyte can effectively suppress the crossover between H þ ions in the catholyte and K þ ions in the alkaline electrolyte.The catholyte acidity is therefore well maintained to achieve the high-potential Mn 2þ /MnO 2 reaction of the cathode upon prolonged cycling.Moreover, the efficiency of this reaction is significantly enhanced by constructing a Prussian blue/Berlin green redox mediator on a graphite felt (GF) current collector.As a result, the as-assembled MnÀMH hybrid battery using a MnCl 2 -based acidic catholyte and a hydrogen storage alloy anode demonstrates significant improvements in discharge voltage, reversible capacity, cycling stability, and energy density.The electrochemical properties of the hybrid battery and the working mechanism of the redox mediator are systematically investigated.The charging and discharge test was conducted at a current density of 40 mA g À1 with a fixed charge capacity of 300 mAh g À1 based on the mass of the hydrogen storage alloy in the anode.During the first cycle, all the batteries exhibit a long flat charge voltage plateau at %2.3 V, followed by sloping discharge plateaus of %1.9 V for the batteries with 0-1.3 M KCl additive and of %2.0 V for the battery with 1.85 KCl additive.The first reversible discharge capacities of these batteries are less than 150 mAh g À1 .Upon the activation for five cycles, significant improvements in both the discharge plateau and the reversible capacity are observed.All the batteries demonstrate an increased flat discharge plateau to %2.2 V and the discharge capacities above 200 mAh g À1 .This activation may be ascribed to the structural modification of the cathode current collector and the refinement of alloy particles in the anode during cycling. [25]Nevertheless, upon the further cycling to 20 cycles, an evident discharge voltage decay occurred for the batteries with 0 and 0.45 M KCl additive (Figure 1a,b).This may be related to the ion crossover between the K þ from the anode side and the H þ in the catholyte though the proton exchange membrane (PEM), which reduces the catholyte acidity and thus leads to the formation of an intermediate low-potential electrochemical reaction process at the cathode side, namely, MnO 2 !MnOOH !Mn 2þ , instead of one-step high-potential transformation.Note that this voltage decay was greatly hindered when the higher concentrations of KCl were used in the catholyte (Figure 1c,d).Only a single high-voltage discharge plateau appeared and it was almost unchanged for the battery using a saturated KCl addition (%1.85 M) in the catholyte, which indicates that the electrochemical reaction from MnOOH to Mn 2þ at the low potential was eliminated during cycling.The median voltage gap between charge and discharge of the batteries at different cycles is summarized in Figure 1e.The increase in the KCl concentration in the catholyte benefits a gradual decrease in the voltage gap of the batteries.For instance, the battery without the addition of KCl exhibits the median voltage gaps of 0.36 V at the 1th cycle and 0.39 V at the 80th cycle, respectively.In contrast, these values are significantly decreased to 0.2 and 0.12 V for the battery with a saturated KCl addition, respectively, indicating that the ion crossover is greatly suppressed.

Results and Discussion
In order to obtain a quantitative proof regarding the inhibition of ion crossover, the inductively coupled plasma-optical emission spectroscopy (ICP-OES) was used to analyze the variation of the K þ to Mn 2þ molar ratio in the catholytes before and after cycling.For the catholyte without the KCl additive, the K þ content was evidently increased with a K þ /Mn 2þ ratio of 0.036 after 80 cycles of the battery, indicative of severe cation penetration through the PEM membrane from the anode to the cathode side.Meanwhile, the H þ in the catholyte moved from the cathode to the anode side because of the charge balance, which is different from the H þ transfer by the redox reaction of the electrodes.Consequently, the catholyte acidity decreases by a nonelectrochemical process, leading to the voltage decay of the battery during discharge (i.e., the formation of the low-potential electrochemical transformation from MnOOH to Mn 2þ ).Upon the increase of the KCl concentration in the catholyte, the ion crossover was gradually decreased because of a continuous decrease in the K þ /Mn 2þ ratio (Table S1, Supporting Information).When the catholyte is saturated with KCl, the percentage variation of the K þ /Mn 2þ ratio in the catholyte before and after 80 cycles is only 2%, which lies in the error range of the ICP test, thus demonstrating that the addition of saturated KCl in the catholyte effectively inhibits the ion crossover, as illustrated in Figure 1f.Therefore, the high-potential MnO 2 /Mn 2þ dissolution reaction remains during cycling.
The use of the KCl additive maintains the high voltage of the hybrid battery.Nevertheless, the battery still suffers from an evident capacity decay upon cycling.The morphology and composition analysis by the scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) demonstrated that the accumulation of unreacted MnO 2 on the GF current collector started after only the first cycle (Figure S1, Supporting Information).A layer of unreacted MnO 2 was formed after 30 cycles, which is responsible for the discharge capacity decay during cycling.This unreacted MnO 2 is related to the disproportionation reaction of MnOOH. [29]A very recent work using an in situ electrochemical quartz crystal microbalance technique demonstrates that the electrochemical dissolution reaction of MnO 2 follows a MnOOH-mediated pathway in both neutral and acidic electrolytes, instead of a simple two-electron transfer from MnO 2 to Mn 2þ . [22,27]During the discharge of MnO 2 in an acidic electrolyte, it is first reduced to MnOOH, which chemically dissolves into the acidic electrolyte, followed by the disproportionation reaction of Mn 3þ to form Mn 2þ and solid MnO 2 .This is confirmed in our case that the electrochemical reaction of MnOOH/Mn 2þ does not occur in the as-prepared MnCl 2 -based catholyte with a strong acidic environment; instead, the disproportionation reaction of MnOOH occurs.Consequently, it looks like the discharge reaction in the cathode is related to direct transformation from MnO 2 to Mn 2þ .However, the chemically formed MnO 2 from the disproportionation reaction was not well electronically connected, which gave rise to the accumulation of unreacted MnO 2 during cycling.Therefore, it is necessary to introduce a redox mediator to improve the reversibility of the phase transformation reaction on the cathode current collector.
Prussian blue (PB) was electrochemically deposited on the GF current collector by cyclic voltammetry (CV, Figure S2, Supporting Information) in a three-electrode system.The optimized deposition cycle of CV was obtained according to the electrochemical activity test of the as-prepared GF/PB current collectors (Figure S3 and S4, Supporting Information).shows the structure and morphology information of the as-prepared GF/PB current collector, which exhibits a blue surface and excellent hydrophilic feature compared to the pristine GF current collector (Figure 2a).The SEM images show that the GF is well covered by the PB layer (Figure 2b,c).X-Ray diffraction (XRD, Figure 2d) result demonstrates the phase structure of the as-prepared PB according to the characteristic peaks related to (100), (200), and (210) lattice planes at 17.3°3 5.2°, 39.5°, and 53.7°, respectively.X-Ray photoelectron spectroscopy (XPS) result confirms the elements (Figure S5, Supporting Information) and their chemical states (Figure 2e, f ) of the as-prepared PB on the GF surface.The Fe 2p 3/2 peak at 708.4 eV and Fe 2p 1/2 peak at 721.2 eV could be assigned to Fe 2þ in [Fe(CN) 6 ] of PB. [30,31] The Fe 2p 3/2 peak at 708.9 eV and Fe 2p 1/2 peak at 722.4 eV are ascribed to the Fe 3þ of PB.The Fe 2þ -to-Fe 3þ ratio is calculated to be %0.77,which is close to the theoretical value of 0.75.Furthermore, the N 1s spectrum exhibits two peaks located at 397.8 and 398.9 eV, which are assigned to the C≡N and N─Fe bonds of PB, respectively. [31]inear sweep voltammetry tests were carried out to evaluate the effect of PB on the anodic stability of the acidic solution (Figure S6, Supporting Information), which exhibits an enhanced oxidation potential from 1.221 to 1.242 V (vs Ag/AgCl) after using the GF/PB current collector.This indicates that the introduction of PB can reduce the risk of gas evolution on the current collector.Figure 3 shows the electrochemical performance of the Mn-based cathodes and the hybrid batteries using the GF or GF/PB current collectors and the MnCl 2 -based acidic catholyte saturated with KCl.The Mn-based cathodes exhibit similar electrochemical redox processes on both current collectors, as shown in the CV curves, in which both the reduction peaks appear at %1.08 V (vs Ag/AgCl).However, much higher peak currents of the cathode are obtained upon the use of the GF/PB current collector, indicative of significantly enhanced reaction activity of the cathode, in which more manganese species participate in the electrochemical reactions.This reaction activity improvement was further analyzed using the electrochemical impedance spectroscopy (EIS), which was performed using the current collector working electrodes with a previous galvanostatic charge to a potential of 1.05 V (vs Ag/AgCl) to ensure the occurrence of the Mn 2þ /MnO 2 deposition/dissolution reaction.The EIS curves exhibit a capacitive loop at high frequency and a straight line at low frequency.The capacitive loop originates from the Mn 2þ /MnO 2 deposition/dissolution charge transfer resistance (R 2 ) and interfacial capacitance (CPE 1 ), while the straight line is related to the Warburg impedance by ion diffusion.Therefore, the equivalent circuit inset in Figure 3b is used to analyze the impedance data.The samples exhibit similar diffusion behavior because of similar straight lines.However, the use of the PB coating on the current collector benefits a decrease of R 2 from 3.69 to 3.27 Ω, thus facilitating the Mn 2þ /MnO 2 deposition/dissolution reactions.This is consistent with the CV result.Figure 3c shows the evolution of the discharge capacities of the batteries charged at 40 mA g À1 and discharged at various current densities.The battery using the GF/PB current collector exhibits much higher discharge capacities than the battery using the GF current collector, regardless of the continous increase of the discharge current density.For example, the discharge capacity of the battery was increased from 223.2 to 265.6 mAh g À1 (the capacity contribution of PB is negligible, Figure S7, Supporting Information) at a high discharge current density of 280 mA g À1 upon the use of GF/PB current collector.This indicates that the incorporation of the PB coating effectively inhibits the accumulation of unreacted MnO 2 and improves the reaction reactivity of the cathode.Note that both batteries exhibit an increased discharge capacity with the increase of current density.This may be because the increased current density shortens the reaction time, which reduces the long-distance diffusion of Mn 3þ derived from the dissolution of MnOOH, and thus the chemically formed MnO 2 from the disproportionation reaction of Mn 3þ may be well used for the electrochemical reduction reaction.Moreover, the short reaction time may also reduce the dissolution of MnOOH and the solid MnOOH may directly participate in the electrochemical reduction reaction.Therefore, more manganese species are electrochemically reduced during discharge, resulting in an enhanced discharge capacity.The direct electrochemical reaction from MnOOH to Mn 2þ is reflected in the discharge curve (below 1.6 V) of the battery using the GF/PB current collector (Figure 3d).This conversion reaction can also be observed from the weak redox peaks in the CV curve (below 0.6 V vs Ag/AgCl) of the Mn-based cathode (Figure 3a). Figure 3e shows the long cycling performance of the batteries at 40 mA g À1 , which exhibits an increase in the reversible capacity during the initial cycles.This activation may be ascribed to the structural modification of the cathode current collector and also the the refinement of hydrogen storage alloy particles in the anode. [25]The repeat electrochemical oxidation improved the the hydrophilicity and surface activity of the GF current collector, which facilitated the deposition of manganese oxides and thus enabled an increase in the capacity of the cathode.For the GF/PB current collector, the repeat charge and discharge of the PB particles could refine their grian size, as shown in the XRD pattern that the diffraction peaks of PB were broadened and weakened during cycling, resulting in an increase in the active sites for MnO 2 deposition.Therefore, the intensity of the diffraction peaks of MnO 2 was increased after activation (Figure S8, Supporting Information).The battery using the GF current collector exhibits a continuous capacity decay after the activation, indicating the loss of active manganese species.This capacity decay was significantly improved by the use of the GF/PB current collector, which achieves the enhancement in the capacity retention from 79.5% to 93.4% after 80 cycles.A longer cycling test of the battery using the GF/PB current collector was conducted at a higher current density of 80 mA g À1 .The battery maintains a high discharge voltage plateau at around 2.1 V over 260 cycles (Figure S9, Supporting Information), indicating its stable high-potential electrochemical reaction of the cathode.Moreover, the battery exhibits the maximum energy density of 259.6 Wh kg À1 (based on the mass of the aqueous MnCl 2 catholyte and a LaNi 5 -based hydrogen storage anode material) and a high energy retention of 83.4% after 260 cycles, demonstrating the best energy density performance among those of the previously reported Mn-based acid-base asymmetric batteries (Table S2, Supporting Information).
Figure 4 shows the Fourier-transform infrared (FTIR) spectroscopy and Raman spectroscopy of the GF/PB current collectors at different electrochemical states, which are used to analyze the working mechanism of PB coating.For the as-prepared GF/PB current collector, a distinct absorption peak associated with the CN À stretching vibration in ferrocyanide ([Fe II (CN) 6 ] 4À ) is detected at 2090 cm À1 . [30,32]Upon the charge to 2.2 V, a new peak appears at 2169 cm À1 , which is assigned to the characteristic signal of CN À stretching vibration in ferricyanide ([Fe III (CN) 6 ] 3À ). [33]This means that the PB undergoes the oxidation to Berlin green (BG) during the charging process.Interestingly, at the end of the charge, the characteristic signal of BG disappears in the FTIR spectrum, and only the PB signal can be detected.This structural evolution of the PB material during the charging process is also confirmed by the Raman spectroscopy.The as-prepared PB exhibits a distinct pair of Raman signals of CN À stretching vibration in [Fe II (CN) 6 ] 4À at 2147 and 2091 cm À1 , which shift to higher positions of 2153 and 2096 cm À1 after the charge of 2.2 V, indicative of the phase transformation from PB to BG. [34] However, the CN À vibration peaks return to the original position after the full charge, indicative of the reduction of BG to PB.This reduction process during charge (oxidation) of the cathode indicates that the PB/BG acts as a redox mediator.Its working mechanism based on the above experimental results is illustrated in Figure 4c.During the charging process of the cathode, the PB is first oxidized to BG at a lower oxidation potential of 0.97 V versus Ag/AgCl (Figure S10, Supporting Information) than that of the oxidation from Mn 2þ to MnO 2 (Figure 3a).The strong oxidization of Fe 3þ in BG facilitates the subsequent oxidation of Mn 2þ to MnO 2 , and the BG itself is reduced to PB.In the beginning of the following discharge, the deposited MnO 2 were preferentially reduced to Mn 2þ by Fe 2þ in PB, which transforms to BG.Meanwhile, the BG participates in the electrochemical reaction and is rereduced to PB.The PB/BG redox mediator helps to enhance the reaction activity of the cathode and reduces the accumulation of unreacted MnO 2 on the current collector, thus promoting the Mn 2þ /MnO 2 deposition/dissolution reaction.
The effectiveness of the PB/BG redox mediator in suppressing the accumulation of unreacted MnO 2 is further verified by the SEM and EDS analysis of the current collectors after 80 cycles (Figure 5).The GF current collector is fully covered by the deposit with a porous network structure and the nanosheet and nanoneedle morphology (Figure 5a).This residual deposit indicates the severe accumulation of unreacted MnO 2 on the current collector, which is confirmed by the elemental mapping result (Figure 5b-e).In contrast, the morphology of the residual MnO 2 is not observed for the GF/PB current collector after long cycling; instead, the morphology of PB remains.The elemental mapping of the GF/PB current collector reveals a disproportionate distribution of Fe, N, and Mn elements (Figure 5h-j).The dominant distribution of Fe and N elements indicates a significant reduction in residual MnO 2 on the current collector, and only very few MnO 2 with a nanowire morphology on the PB surface is observed.These findings further support that the PB/BG redox mediator suppresses the generation of unreacted MnO 2 on the current collector and benefits significantly enhanced electrochemical properties of the aqueous Mn-based acid-base hybrid battery.

Conclusion
In summary, the combined strategy using the catholyte modulation via the addition of KCl and the construction of the PB/BG redox mediator on the GF current collector has been developed to effectively promote the Mn 2þ /MnO 2 deposition/dissolution reactions in the acid-base hybrid battery.The use of saturated KCl in the catholyte greatly inhibits the nonelectrochemical ion crossover to maintain the acidic condition of the catholyte, thus stabilizing the high-potential Mn 2þ /MnO 2 reactions for long cycling.Moreover, the PB coating, which was prepared by opotimized electrochemical deposition on the GF current collector, is designed to participate in the cathode reactions via the redox of PB and BG.This redox mediator significantly facilitates the Mn 2þ /MnO 2 deposition/dissolution and thus greatly reduces the accumulation of unreacted MnO 2 on the current collector.Therefore, the reversbile capacity, rate capability, cycling stability, and energy density of the hybrid battery are all evidently enhanced.The highest energy density of the Mn-based acid-base hybrid battery is also achieved.This work provides a promising approach for designing high-performance Mn-based acid-base hybrid batteries.

Experimental Section
Electrode and Electrolyte Preparation: The 4 cm Â 5 cm commercial graphite felt (Gansu Hoshi Carbon Fiber Co., LTD., China) was soaked in acetone to remove grease before use, following a hydrothermal reaction with the concentrated nitric acid at 80 °C for 8 h.The deposition of PB on the GF current collector was accomplished using a CV method in an aqueous acidic solution, [35] which included K 3 Fe(CN) 6 (2.5 mM), FeCl 3 (2.5 mM), and KCl (0.5 M).The pH value of the solution was adjusted to 2-3 with 5% HCl.The anode was prepared by cold pressing the mixture of commercial LaNi 5 -based hydrogen storage alloy powders (15 mg, MlNi 3.65 Co 0.75 Mn 0.4 Al 0.2 , Ml = mischmetal; Ningbo Shenjiang Sci-Tec Co., Ltd.) and conductive agent of nickel powder (30 mg).The 6 M KOH solution was used as the electrolyte for the anode.The catholyte was prepared by dissolving the manganese salt (5.5 M MnCl 2 ), acid (0.2 M HCl), and potassium salt (0-1.85M KCl) in deionized water.
Materials Characterization: XRD data was collected on a diffractometer with Cu Kα radiation.Field-emission SEM and EDS were conducted using the Zeiss Gemini Ultra55 equipment to characterize the morphology and composition of the samples.ICP-OES data was collected on Agilent-7700.
The FTIR spectrometer of Thermofisher Nicolet IS5 and the Raman spectrometer with a wavelength of 532 nm (Horiba scientific) were used.
Cell Assembly: The cell consisted of two glass electrolytic cells separated by a fixed PEM (Dupont N-117).The cathode chamber included a GF current collector immersed in a 30 mL acid catholyte.The anode chamber contained a hydrogen storage alloy electrode immersed in 6 M KOH alkaline electrolyte.
Electrochemical Measurement: Electrochemical tests were carried out at room temperature using the BioLogic-VMP3 multichannel electrochemical workstation.CV curves were measured in a half cell consisting of three electrodes, in which the Ag/AgCl electrode was used as the reference electrode and the Pt electrode was used as the counter electrodes.Charge and discharge tests were carried out using the Neware Battery Test System (Neware Co., Ltd.) at a fixed charge capacity of 300 mAh g À1 (based on the mass of the hydrogen storage alloy).

Figure
Figure 1a-d shows the charge and discharge curves of the Mn-MH hybrid batteries using the MnCl 2 -based acidic catholytes with different concentrations of the KCl additive.

Figure 1 .
Figure 1.a-d) Charge and discharge curves of the Mn-MH hybrid batteries with different catholytes at selected cycles.e) Median voltage gap between charge and discharge of the batteries with various KCl additions at different cycles.f ) Schematic illustration of the ion crossover variation in the batteries without or with the KCl additive in the catholytes.

Figure 2 .
Figure 2. a) Optical images and b,c) SEM images of the GF and GF/PB current collectors.d) XRD patterns of the pristine GF and GF/PB current collectors.XPS spectra of the GF/PB current collector: e) Fe 2p and f ) N 1s.

Figure 3 .
Figure 3. a) CV curves (1 mV s À1 ) and b) EIS patterns of the Mn-based cathodes using different current collectors in the electrolyte with saturated KCl.c) Rate capability and d) charge and discharge curves of the hybrid batteries charged at 40 mA g À1 and discharged at different current densities.e) Cycling stability (40 mA g À1 ) of the hybrid batteries using different current collectors.

Figure 4 .
Figure 4. a) FTIR and b) Raman spectra of the GF/PB current collectors in different electrochemical stages at the first cycle.c) Schematic of the working mechanism of the PB coating in the catholyte during cycling.

Figure 5 .
Figure 5. SEM and EDS elemental mapping images of the GF a-e) and GF/PB f-j) current collectors after 80 cycles.