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[Atsunori Ikezawa](https://orcid.org/0000-0002-8857-7159), [KOYAMA, Yukinori](https://orcid.org/0000-0002-7090-4430), Tadaaki Nishizawa, [Hajime Arai](https://orcid.org/0000-0001-6695-637X)

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[A high voltage aqueous proton battery using an optimized operation of a MoO3 positive electrode](https://mdr.nims.go.jp/datasets/4541ab59-91b5-41e2-bd3b-b62fe21cd38c)

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A high voltage aqueous proton battery using an optimized operation of a MoO3 positive electrodeJournal ofMaterials Chemistry APAPEROpen Access Article. Published on 06 January 2023. Downloaded on 2/1/2023 6:24:19 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineView Journal  | View IssueA high voltage aqaSchool of Materials and Chemical TechYokohama 226-8502, Japan. E-mail: ikezawbResearch and Services Division of MateriaInstitute for Materials Science, Tsukuba 305† Electronic supplementary informahttps://doi.org/10.1039/d2ta08581jCite this: J. Mater. Chem. A, 2023, 11,2360Received 3rd November 2022Accepted 3rd January 2023DOI: 10.1039/d2ta08581jrsc.li/materials-a2360 | J. Mater. Chem. A, 2023, 11, 2ueous proton battery using anoptimized operation of a MoO3 positive electrode†Atsunori Ikezawa, *a Yukinori Koyama, b Tadaaki Nishizawaa and Hajime Arai aAqueous proton batteries have attracted increasing attention owing to their potential of high safetystandard, high rate capability, and long cyclability. While some inorganic negative electrode materials forproton batteries have recently been found, inorganic positive electrode materials have rarely beenreported. In this work, we investigate the proton insertion–extraction mechanism of MoO3 usingoperando X-ray diffraction and density functional theory calculation to optimize its operating conditionsas a positive electrode. It is found that the phase transition between MoO3 and phase-I HxMoO3 canreversibly be utilized by preventing irreversible phase transition from phase-I to phase-III involving thechange of proton accommodation from the intralayer to interlayer sites. A MoO3 electrode using thephase transition between MoO3 and phase-I shows an average reduction potential of 0.44 V vs. SHEwith a maximum reversible capacity of 100 mA h g−1. A MoO3j50 wt% H2SO4 aq.jHxMoO3 full-cellexhibits a maximum discharge capacity of 73 mA h g−1 and maintains nominal discharge voltage above0.47 V, which is the highest voltage among aqueous proton batteries composed of insertion-type oxideactive materials.1. IntroductionAqueous proton batteries with two insertion-type electrodes,where protons go back and forth between these electrodesduring charging and discharging, are attractive candidates forpost-lithium-ion batteries.1 These battery systems potentiallysatisfy high safety standards owing to aqueous electrolytesolutions and have high rate capabilities because of protons'high mobility. In addition, the protons' small ionic radiuspossibly contributes to the high cyclabilities of insertion-typeactive materials. However, aqueous proton batteries have notbeen commercialized due to the lack of active materials forpractical applications. Acid electrolyte solutions are generallyused for aqueous proton batteries to ensure high protonconductance and prevent the insertion of other cations.Therefore, active materials for aqueous proton batteries arerequired to have not only the ability of reversible protoninsertion/extraction but also stability in acidic electrolyte solu-tions. Examples of electrode materials for proton batteries areorganic materials2 and cyanide complexes1,3 but they are rela-tively bulky, which is unfavorable for high energy density of thecells. Promising compact materials are oxides, and someinsertion-type oxide negative electrode materials, such asnology, Tokyo Institute of Technology,a.a.aa@m.titech.ac.jpls Data and Integrated System, National-0044, Japantion (ESI) available. See DOI:360–2366MoO3,4 WO3,5 and TiO2,6 have been reported. However,insertion-type oxides as a positive electrode material, which isneeded to establish aqueous proton batteries composed ofinsertion-type active materials for both electrodes, have rarelybeen found.In this study, we focused on MoO3 as a positive electrodematerial for aqueous proton batteries. MoO3 has long beenstudied as a hydrogen insertion material.7,8 Besides pristineMoO3 (space group Pbnm, number 62), four stable phases arereported as the proton-inserted product HxMoO3: phase-I (0.23< x < 0.40; Cmcm, 63), phase-II (0.85 < x < 1.04; C2/m, 12), phase-III (1.55 < x < 1.72; C2/m), and phase-IV (x = 2.0; space groupunknown, monoclinic).7,8 In addition, phase-IIa (0.6 < x < 0.8;C2/m) has also been reported as a metastable phase.9 Althoughthe space group symmetry changes by reduction and oxidation,the layered structure framework of MoO3 is retained. Recently,MoO3 has been recognized as a negative electrode material foraqueous proton batteries, which has a relatively high reversiblecapacity of ca. 220 mA h g−1 in sulfuric and phosphoric acidelectrolytes.10,11 In addition, W. Xu et al. have very recently foundthat HxMoO3 can also be utilized as a positive electrodematerialthough its operating potential was relatively low (ca. 0.3 V vs.standard hydrogen electrode (SHE)).12 MoO3 shows two revers-ible redox reactions at around 0.3 and −0.1 V vs. SHE and alsoshows an irreversible reduction reaction around 0.5 V vs. SHE(Fig. S1†). On the other hand, the phase transition behaviorduring the reduction–oxidation processes is not fully under-stood due to ex situ characterization used in most of theprevious research.11,13This journal is © The Royal Society of Chemistry 2023http://crossmark.crossref.org/dialog/?doi=10.1039/d2ta08581j&domain=pdf&date_stamp=2023-01-28http://orcid.org/0000-0002-8857-7159http://orcid.org/0000-0002-7090-4430http://orcid.org/0000-0001-6695-637Xhttps://doi.org/10.1039/d2ta08581jhttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d2ta08581jhttps://pubs.rsc.org/en/journals/journal/TAhttps://pubs.rsc.org/en/journals/journal/TA?issueid=TA011005Paper Journal of Materials Chemistry AOpen Access Article. Published on 06 January 2023. Downloaded on 2/1/2023 6:24:19 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineIn this study, we conducted operando X-ray diffraction (XRD)measurements and density functional theory (DFT) calculationsto investigate the reduction–oxidation mechanism of MoO3both experimentally and theoretically. We clarify the phasetransition process of MoO3 during reduction–oxidation,including irreversible phase transition from phase-I to phase-IIIin the rst reduction by exploring the thermodynamically stablesites for proton insertion in phase-I and phase-III (or phase-II).Furthermore, we show that the reduction reaction around 0.5 Vvs. SHE could reversibly utilize by optimizing the cut-offpotential and successfully construct an aqueous protonbattery composed of insertion-type oxide active materials withan average discharge potential of 0.47 V.2. Experimental methods2.1. Electrochemical measurements and characterizationMoO3 composite electrodes were composed of MoO3 (KantoChemical) : carbon black (Li-250, Denka) : polyvinylidenediuoride (KF polymer L#9305, Kuraray) = 85 : 10 : 5 wt%. Theelectrode slurry in an N-methyl pyrrolidone (Kanto Chemical)solvent was coated on a graphite sheet current collector (Pana-sonic, thickness: 25 mm) and dried at 80 °C for 12 h in a constanttemperature oven (DX302, Yamato).Operando XRD measurements were carried out using anelectrochemical three-electrode cell (Fig. S2(a)†), an electro-chemical measurement system (SP-50, Bio-Logic), and an XRDsystem (SmartLab, Rigaku) using Mo Ka radiation at 60 kV and150 mA. The electrochemical three-electrode cell consisted of theMoO3 composite electrode as the working electrode, a commer-cial AgjAgCljsaturated KCl aq. electrode (RE-1CP, BAS) as thereference electrode, a Pt mesh as the counter electrode, and50 wt% (ca. 7.1 mol dm−3) H2SO4 aq. as the electrolyte solution.The XRD patterns of the MoO3 electrode were recorded from thebackside of the electrode through the graphite sheet currentcollector (Fig. S2(a)†). The geometrical area of the working elec-trode was dened as 1.77 cm2 with a uororubber o-ring.The reduction–oxidation measurements of MoO3 half-cellswere conducted using an electrochemical three-electrode cell(Fig. S2(b)†) composed of the same components as those of theoperando XRD cell and a multichannel charge–discharge system(HJ1001SM8A, Hokuto Denko). The geometrical area of theworking electrode was dened as 0.50 cm2 with a uororubbergasket. The charge–discharge measurements of MoO3j50 wt%H2SO4 aq.jHxMoO3 full-cells were performed using an electro-chemical three-electrode cell with the commercial AgjAgCljsa-turated KCl aq. reference electrode (Fig. S2(c)†) anda multichannel charge–discharge system (EF-7100p, Electro-eld). The geometrical areas of the positive and negative elec-trodes were dened as 0.50 cm2 with a uororubber gasket, andthe loading masses of the positive and negative electrodes wereset to the same value. Before the full-cell construction, thepositive and negative electrodes were electrochemically pre-conditioned with the half cells. The positive electrode wasreduced and oxidized in the potential range from 0.1 to 0.5 V at100 mA g−1 for 30 cycles, and the cycle was terminated in thereduced state. The negative electrode was reduced and oxidizedThis journal is © The Royal Society of Chemistry 2023in the potential range from −0.3 to 0.0 V at 100 mA g−1 for 3cycles, and the cycle was terminated in the oxidized state.All the electrochemical measurements were performed at 25± 2 °C, and the electrodes were exposed to a 5 min open-circuitperiod between charge and discharge.The electrodes as-prepared and taken out from the cells werecharacterized using eld emission scanning electron micros-copy (FE-SEM) (SU8230, Hitachi High-Tech) and energydispersive X-ray spectroscopy (EDS) (XFlash FlatQUAD 5060F,BRUKER).2.2. DFT calculationsDFT calculations were performed using the plane-wave basisprojector augmented wave (PAW) method as implemented inthe Vienna ab initio simulation package (VASP) 6.1.14,15 Thegeneralized gradient approximation (GGA) parameterized byPerdew, Burke, and Ernzerhof16 and the Hubbard U extension17with an effective U of 4.38 eV for the Mo-4d orbital were used asthe exchange–correlation functional. The cut-off energy was setat 520 eV. A gamma-centered 8 × 2 × 8 k-point mesh was usedfor the unit cell, and the number of divisions were adjusted forsupercells by their size. The total energy converged to 10−6 eVper atom. Atomic positions and lattice constants were relaxeduntil the total energy converged to 10−5 eV per atom.Proton inserted models were constructed as follows: protonswere attached to O atoms. The OH units were directed to adja-cent O atoms to form hydrogen bonds. The O–H distances wereset to be one third of the O–O distances. All symmetry-independent congurations were employed as initial struc-tures. Atom positions were randomly displaced by 0.01 Å tobreak the symmetry before the structural relaxation. Notes thatare specic to individual models will be described in the Resultsand discussion section. Pymatgen18 and enumlib19–21 packageswere used to construct the structural models.3. Results and discussion3.1. Operando XRDOperando XRD measurement of the MoO3 electrode duringreduction–oxidation was carried out in the potential rangebetween 0.50 and −0.30 V at a current density of 100 mA g−1 toinvestigate the phase transition behavior betweenMoO3 and thefully protonated phase-IV. Fig. 1(a)–(d) show the operando XRDpatterns during the rst and second reduction–oxidationprocesses, while Fig. 1(e) and (f) show the full XRD proles ofselected states, showing that the states (i), (ii), (iii), (iv), (v) and(vi) correspond to MoO3, phase-I, phase-III, phase-IV, phase-II,and phase-IIa, respectively. In the rst reduction, MoO3 showsthree phase-transitions from MoO3 to phase-IV via phase-I andIII (Fig. 1(a), (e) and (f)). First, a MoO3/phase-I biphasic reactionis observed on the potential plateau at around 0.35 V evidencedby the gradual decrease of MoO3 060 (17.7°) diffraction intensityand concurrent increase of phase-I 060 (17.4°) diffraction.Second, a phase-I/phase-III biphasic reaction occurs on theplateau at around 0.05 V indicated by gradual decrease of thephase-I 150 (17.9°) diffraction and concurrent increase of theJ. Mater. Chem. A, 2023, 11, 2360–2366 | 2361http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d2ta08581jFig. 1 Operando XRD patterns of the MoO3 electrode during reduc-tion–oxidation in the potential range from −0.30 to 0.50 V at 100 mAg−1; contour plots of (a) the 1st reduction, (b) the 1st oxidation, (c) the2nd reduction, and (d) the 2nd oxidation; (e and f) line plots withreference patterns.Fig. 2 Operando XRD patterns of the MoO3 electrode during reduc-tion–oxidation in the potential range from 0.10 to 0.50 V; contourplots of (a) the 1st reduction and (b) the 1st oxidation at 100mA g−1 and(c) the 2nd reduction and (d) the 2nd oxidation at 10 mA g−1. (e) AJournal of Materials Chemistry A PaperOpen Access Article. Published on 06 January 2023. Downloaded on 2/1/2023 6:24:19 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinephase-III 510 (18.2°) diffraction. Finally, the phase-III/phase-IVbiphasic reaction on the plateau at around −0.27 V is shownby the gradual decrease of the phase-III 600 (17.8°) diffractionand concurrent increase of the phase-IV 600 (18.1°) diffraction.Monophasic reactions of phase-I and phase-III are alsoobserved aer the MoO3/phase-I and phase-I/phase-III biphasicreactions, respectively, evidenced by the continuous shis ofthe diffraction peaks, such as phase-I 060 and phase-III 600.During the rst oxidation, while the phase-III/phase-IV biphasicreaction is observed on the plateau at around −0.27 V, phasetransitions in higher potential regions are totally different fromthose in the rst reduction process, as shown in Fig. 1(b). Aerthe phase-III monophasic reaction between −0.27 V and 0.05 V,the phase-III/phase-II biphasic reaction on the plateau ataround 0.05 V is shown by the gradual decrease of the phase-III600 (17.5°) diffraction and concurrent increase of the phase-II600 (17.0°) diffraction. Then, the phase-II/phase-IIa biphasicreaction is suggested by the gradual decrease/increase of thephase-II 42-2 (26.3°)/phase-IIa 51-2 (27.8°) diffractions(Fig. S3†). In the second reduction, the change was reverse tothat observed during the rst oxidation, and HxMoO3 showsreversible phase transitions between phase-IIa and phase-IV2362 | J. Mater. Chem. A, 2023, 11, 2360–2366aerward, as shown in Fig. 1(c) and (d). These results indicatethat the phase transition from phase-I to phase-III is an irre-versible process.Next, we performed operando XRD to investigate the revers-ibility of MoO3/phase-I biphasic and phase-I monophasic reac-tions for their possible use as high-potential electrodereactions, when there is no phase-III formation with the oper-ating potential range at above 0.10 V. Fig. 2(a)–(d) show theoperando XRD patterns during the rst and second reduction–oxidation processes. Gradual decrease/increase of the MoO3 060(17.7°)/phase-I 060 (17.4°) diffractions and continuous shis ofthe phase-I 060 diffraction are observed in both the rstreduction and oxidation though the appearance of the MoO3060 diffraction does not completely proceed in the fully oxidizedstate (Fig. 2(a) and (b)). In the second cycle, we decreased thereduction–oxidation rate to 10 mA g−1, a tenth of the rate in therst cycle, to reduce kinetic factors to the reversibility. Phase-I060 diffraction shis to a higher diffraction angle and protoncontent, x in HxMoO3, becomes higher at the end of thereduction (Fig. 2(c)) compared to the rst cycle, indicating thatthe proton insertion proceeded further with the decrease in thereduction current. In addition, the phase transition from phase-schematic of the phase transitions of the MoO3 electrode.This journal is © The Royal Society of Chemistry 2023http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d2ta08581jPaper Journal of Materials Chemistry AOpen Access Article. Published on 06 January 2023. Downloaded on 2/1/2023 6:24:19 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineI to MoO3 proceeds more than that in the rst high-rate cycle, asevidenced by the higher intensity of MoO3 060 diffraction andlower proton content x in HxMoO3 aer the second oxidation(Fig. 2(d)). These results clearly show that the phase-I mono-phasic reaction and the MoO3/phase-I biphasic reaction areessentially reversible while the phase transition from phase-I toMoO3 is kinetically limited. To the best of our knowledge, this isthe rst study to report the reversibility of these high-potentialreactions.The phase transition of MoO3 during reduction–oxidation issummarized in Fig. 2(e). The reduction below 0.10 V causes anirreversible phase transition from phase-I to phase-III. Then thereversible phase transitions between phase-IIa and phase-IV, viaphase-II and phase-III, are involved in the subsequent cycleswhen the voltage range is set at−0.3 to 0.5 V. The reversibility ofthe phase transition between phase-IIa and phase-IV is consis-tent with the charge–discharge performances of MoO3 as thenegative electrode.4,22 On the other hand, MoO3/phase-Ibiphasic and phase-I monophasic reactions mostly reversiblyoccur when the voltage range is set at 0.1 to 0.5 V.The appearance of phase-IV in the fully reduced state andthat of phase-II and phase-III in the reverse oxidation processobserved in this study agree with previously reported ex situXRD studies.8,11 R. Schöllhorn et al. have reported based on theirex situ XRD measurement that the reduction of MoO3 in therange of 0.5 < x < 1.67 in HxMoO3, corresponding to the plateauat 0.05 V in this study, yielded a mixture of phases-I, II, and III,which is probably caused by the decomposition of metastablephases I and III to phase-II in the course of ex situ measure-ment. We speculate that phase transition from phase I to III iskinetically more favourable than that from phase I to II.Fig. 1(a) and (b) show that our MoO3 electrode has a redoxplateau at around 0.3 and−0.1 V, which have not been observedin other studies,4,10 resulting in a relatively high reversiblecapacity of ca. 300 mA h g−1 at a 100 mA g−1 rate. This redoxplateau disappears when the charge–discharge rate is increasedto 1 or 2 A g−1 (Fig. S4†), explaining why this process was notobserved in previous studies with high testing rates. A high cut-off oxidation potential of 0.50 V in this study may also inducethis process. We deduce that this process is attributed to somesurface reactions on MoO3, because no structural changes areobserved in operando XRD (Fig. 1(a) and (b)). Further study isneeded to identify it. The low rates used in this study would alsolead to the hydrogen evolution reaction and compositionambiguity of phases II, IIa, III and IV.3.2. DFT calculationsDFT calculations were carried out to obtain theoretical insightsinto the proton accommodation sites in HxMoO3 and the irre-versible phase transition from phase-I to phase-III. For clari-fying the proton accommodation sites, the crystal structure ofMoO3 is schematically illustrated in Fig. S5.† MoO3 has anorthorhombic structure of the Pbnm (62) space group with oneMo site and three O sites. Mo atoms are coordinated by six Oatoms forming MoO6 octahedra. MoO6 octahedra share theiredges to form corrugated chains along the c-axis and share theirThis journal is © The Royal Society of Chemistry 2023corners along the a-axis. The edge- and corner-sharing MoO6octahedra form MoO3 layers with intralayer channels, and theMoO3 layers are stacked along the b-axis with van der Waalsgaps. There are two potential sites for inserted protons. One isan O2 site, a bridging site between two corner-sharing MoO6octahedra, and protons are accommodated in the intralayerchannels (intralayer site). The other is an O3 site, a terminal siteon the surface of MoO3 layers, and protons are accommodatedin the interlayer gaps (interlayer site). The O1 site, which isshared by edge-sharing MoO6 octahedra, is hardly accessible forprotons. Calculated structural parameters are summarized inTable S1.† The calculated lattice constants a and c are in goodagreement with the experimental values with an error of ca. 1%,whereas the lattice constant b is overestimated by ca. 12%. Theoverestimation of b is probably due to the use of the conven-tional GGA exchange–correlation functional, in which van derWaals interaction is not properly considered.First, preferable proton accommodation sites in MoO3 wereinvestigated using a 2 × 1 × 2 supercell with an additionalproton (H1Mo16O48 supercell). A proton was attached to an Osite toward one of its adjacent O atoms within 3.1 Å. Allsymmetry-independent congurations were employed as initialstructures, and the structures converged into several groupsaer the structural relaxation. The lowest energy group hasa proton attached to an O2 site toward an adjacent O2 site. Thecalculation results show that two Mo atoms sharing the OH unitare a pair of Mo5+ andMo6+ ions. The second group has a protonattached to an O3 site toward an adjacent O3 site in the nextMoO3 layer, and it is higher in energy by 0.32 eV than the lowestenergy group. Structures having a proton attached on an O1 siteshow 0.58 eV higher energy than the lowest energy group, or theproton wasmigrated to an adjacent O2 site during the structuralrelaxation. These results suggest that the protons are preferablyaccommodated within the MoO3 layers and attached to the O2sites, and this is consistent with the formation of phase-I in theearly stage of the rst reduction.Second, preferable proton congurations in the intralayersites were investigated using a 2 × 1 × 1 supercell of HnMo8O24(n = 2 and 4). The MoO3 framework of phase-I with a spacegroup of Cmcm was used as the initial structure, and protonswere attached to O2 sites toward their adjacent O2 sites within3.1 Å. The lowest energy structure of H4Mo8O24 consists ofMoO3 layers having alternate lled and empty channels asillustrated in Fig. 3(b). The lowest energy structure of H2Mo8O24consists of a MoO3 layer with alternate lled and empty chan-nels, and another MoO3 layer with empty channels only(Fig. 3(a)). This structure has slightly higher energy by 0.02 eVper proton than a mixture of H4Mo8O24 and MoO3. Structureswith protons in different channels have 0.12 eV per proton orhigher energy than the lowest energy structures in bothcompositions. These calculation results suggest that the inser-ted protons are not randomly distributed at the intralayer sitesand that the protons tend to be aligned in the channels. This isconsistent with a previous report.11 Although structures withless-than-half lled channels were not examined in this studydue to a huge number of proton congurations for largersupercells, structures with multiple empty channels betweenJ. Mater. Chem. A, 2023, 11, 2360–2366 | 2363http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d2ta08581jFig. 3 Schematic views of the lowest energy structure models for (a)H2Mo8O24 with intralayer protons, (b) H4Mo8O24 with intralayerprotons, (c) H8Mo8O24 with intralayer protons, (d) H2Mo8O24 withinterlayer protons, (e) H2Mo8O24 with interlayer protons of the secondlowest energy, and (f) H4Mo8O24 with interlayer protons. Purplepolyhedra, red spheres, and white spheres denote MoO6 octahedra, Oatoms, and protons, respectively.Journal of Materials Chemistry A PaperOpen Access Article. Published on 06 January 2023. Downloaded on 2/1/2023 6:24:19 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinethe lled channels are plausible at lower proton contents thanH0.5MoO3, suggesting wide proton solubility in phase-I. Toinvestigate the accommodation sites for further proton inser-tion, a 2 × 1 × 1 supercell of H8Mo8O24 was examined. In thelowest energy structure, all protons are attached to O2 sites.Half of the protons are directed to the adjacent O2 sites,whereas the other half are directed to the second adjacent O2sites (Fig. 3(c)). This suggests that the proton insertion behaviorinto the intralayer sites signicantly changes over the compo-sition of H0.5MoO3.Next, preferable proton congurations in the interlayer siteswas investigated using a 1 × 1 × 2 supercell of HnMo8O24 (n =2, 4). We have our interest in comparison of the protonaccommodation sites between the intralayer and interlayer sitesand getting insights into the irreversible phase transition fromphase-I to phase-III, and thus the proton compositions exam-ined in this study were much less than the actually reportedcompositions of phase-III (1.55 < x < 1.72 in HxMoO3). To reducethe number of congurations, the MoO3 framework of theCmcm space group, which is a common supergroup of the spacegroups of MoO3 (Pbnm) and phase-III (C2/m), was used as theinitial structure, and protons were attached to O3 sites towardtheir adjacent O3 sites of the next MoO3 layer within 3.1 Å. ForH4Mo8O24, congurations were restricted to two protons in eachgap with inter-proton distances more than 1.8 Å to furtherreduce the congurations. This restriction excludes structureshaving a proton facing another proton and those havingmultiple protons attached to a single O3 site. In the lowestenergy structure of H2Mo8O24, two protons are attached todifferent O3 sites forming a chain of hydrogen bonds (Fig. 3(d)).The second lowest energy structure has two protons attached toa single O3 site (Fig. 3(e)). The energy difference between thesetwo structures is 0.002 eV per proton, which is negligible inconsideration of computation accuracy. Their energies are alsoclose to that of the lowest energy structure with intralayerprotons. The lowest energy structure of H4Mo8O24 has OH2units at two O3 sites (Fig. 3(f)), even though such congurations2364 | J. Mater. Chem. A, 2023, 11, 2360–2366are excluded in the initial structures of H4Mo8O24. This struc-ture is lower in energy by 0.09 eV per proton than the lowestenergy structure of H4Mo8O24 with intralayer protons. However,it is still difficult to conclude that the protons are accommo-dated in the interlayer sites more preferably than in the intra-layer sites at H0.5MoO3. This is because the conventional GGAexchange–correlation functional does not properly take van derWaals interaction into account, which could overestimate theenergies of MoO3 and HxMoO3 with intralayer protons (whilehaving no hydrogen bonds between the MoO3 layers). As thelocal structure of the intralayer protons considerably changes inthe H8Mo8O24 model, the interlayer protons would becomestable at x > 0.5 in HxMoO3. Singly attached interlayer protonsseem rather unstable at H4Mo8O24 because all relaxed struc-tures have at least one OH2 unit. The hydrogen bonds towardthe next MoO3 layers cooperatively result in a monoclinicdistortion. The OH2 units at the interlayer sites have beenexperimentally observed from inelastic neutron scatteringspectra for highly reduced HxMoO3.13Even though protons oen moved to other sites during thestructural relaxation, none of the protons moved between theintralayer and interlayer sites. This implies a high potentialbarrier between the intralayer and interlayer sites. Hence, thephase transition between phase-I and phase-III (or other phaseswith interlayer protons) would occur by phase boundarymigration, not by rearrangement of protons in the bulk. Totransform phase-III (or phase-II) into phase-I, it is necessary toremove all the protons from the interlayer sites and to reinsertthem into the intralayer sites. This would require similarpotential to form the MoO3 phase from phase-III. This isa probable reason why phase-I was never observed during theoxidation processes of phase-III.3.3. Charge–discharge properties as a positive electrode forproton batteriesSince the MoO3/phase-I biphasic and phase-I monophasicreactions occur at relatively high potential (ca. 0.5 V vs. SHE),they can be used as a positive electrode material for protonbatteries. Therefore, repeated reduction–oxidation processeswere conducted with MoO3 half-cells in the potential rangefrom 0.10 to 0.50 V to investigate the cyclability as a positiveelectrode material. Fig. 4(a) and (b) show the reduction–oxida-tion curves, reduction–oxidation capacities, and coulombicefficiencies of the MoO3 electrode at 100 mA g−1. The rstreduction and oxidation capacities are ca. 80 and 60 mA h g−1.The irreversible capacity in the rst cycle comes from theincomplete phase transition from phase-I to MoO3 as shown inoperando XRD (Fig. 2). In the subsequent cycles, the reduction–oxidation capacities increase as the cycle number increases andreach a maximum value of ca. 100 mA h g−1 in the 37th cyclewith an average reduction voltage of ca. 0.24 V (0.44 V vs. SHE).The reason for the capacity increase is described in the nextparagraph. When the reduction cut-off voltage was set at 0.05 V,the average electrode potential of 0.24 V (0.44 V vs. SHE)decreased to 0.11 V (0.31 V vs. SHE) (Fig. S6†) due to the irre-versible phase transition from phase-I to phase-III in the rstThis journal is © The Royal Society of Chemistry 2023http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d2ta08581jFig. 4 Reduction–oxidation properties of the MoO3 electrode at 100mA g−1 in the potential range from 0.10 to 0.50 V; (a) reduction–oxidation curves; (b) reduction–oxidation capacities and coulombicefficiency; (c) dQ/dV plots. (d) Ex situ XRD patterns of the MoO3electrodes after the 1st and 30th oxidation steps.Fig. 5 Charge–discharge properties of the MoO3j50 wt% H2SO4jHx-MoO3 full-cell at 200 mA g−1; (a) cell voltage; (b) potentials of positiveand negative electrodes; (c) charge/discharge capacities andcoulombic efficiency; (d) average charge/discharge voltages. Thecapacities were calculated based on the mass of MoO3 on one side ofthe electrode.Paper Journal of Materials Chemistry AOpen Access Article. Published on 06 January 2023. Downloaded on 2/1/2023 6:24:19 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinereduction. While the use of the reduction–oxidation processbetween phase-IIa, phase-II, and phase-III has recently beenreported,12 preventing the phase transition from phase-I tophase-III can lead to higher operating potential in aqueousproton batteries.Fig. 4(c) shows the dQ/dV plots in the rst and 37th cycles.The redox peaks at ca. 0.35 and 0.15 V, which are attributed tothe MoO3/phase-I biphasic and phase-I monophasic reactions,are observed in both the rst and 37th cycles, showing that thephase transition behavior is basically unchanged. On the otherhand, the capacities in the high overpotential regions clearlyincrease (shown as blue arrows in Fig. 4(c)), which suggests thatthe promotion of MoO3 formation during oxidation and protoninsertion during reduction causes the capacity increase. Thehigher intensity of MoO3 diffraction aer the 30th oxidationthan that aer the rst oxidation in the ex situ XRD patterns(Fig. 4(d)) supports the facilitation of MoO3 formation duringoxidation in the reduction–oxidation cycles. The ex situ FE-SEM/EDS images of the MoO3 electrodes (Fig. S7†) show almost nomorphological change aer the rst reduction, which indicatesthat the MoO3/phase-I biphasic reaction is an insertion reactionrather than a conversion reaction. In contrast, there are somecracks in MoO3 particles aer the 30th oxidation, which areprobably caused by the repeated anisotropic volume changesduring the reduction–oxidation cycles. Phase transition fromMoO3 to phase-I causes ca. 1.5% and 1.1% increase in the latticeconstants b and c, respectively, and ca. 1.7% decrease in thelattice constant a. We speculate that the particle crackingfacilitates the phase-transition and proton diffusion, resultingin capacity increase, and thus the optimization of themorphology can improve the reduction–oxidation properties.Fig. S8† shows the reduction–oxidation curves of the MoO3electrode at different current densities. The oxidation capacityThis journal is © The Royal Society of Chemistry 2023at 1 A g−1 is ca. 50% of that at 100 mA g−1, which shows that therate capability is not so limited despite the sluggish phasetransition from phase-I to MoO3.3.4. Charge–discharge properties of the MoO3jH2SO4aq.jHxMoO3 full-cellFinally, we constructed a MoO3jH2SO4 aq.jHxMoO3 full-cell, todemonstrate the operation of an aqueous proton batterycomposed of two insertion-type oxide materials. We utilized thephase-III monophasic and phase-III/phase-IV biphasic regions inthe potential range from −0.3 to 0 V for the negative electrodereactions. Fig. 5(a)–(d) show the charge–discharge curves,charge–discharge capacities, and average charge–dischargevoltages of the MoO3jH2SO4 aq.jHxMoO3 full-cell at 200 mA g−1based on the mass of MoO3 on one side of the electrode. Theloading masses of MoO3 in the positive and negative electrodeswere set to the same value as described in the Experimentalsection. The charge–discharge potential curves of the positiveand negative electrodes (Fig. 5(b)) indicate that the designedreactions reversibly occur in the full-cell with the main voltageplateau of 0.6 V (Fig. 5(a)). We therefore conclude that a rocking-chair-type proton battery composed of insertion-type oxide activematerials is successfully established. The highest dischargecapacity is 73 mA h g−1 in the rst cycle, and the capacityretention rate in the 50th cycle is ca. 70%. The optimization ofthe material morphology,23 positive–negative mass ratio, cellfabrication process and cut-off voltage could improve the slightcapacity decrease during cycling. The cell keeps the coulombicefficiency over 98% until the 50th cycle. The average dischargevoltage and the energy density based on the total mass of theactive materials are 0.48 V and 17 W h kg−1, respectively, in theJ. Mater. Chem. A, 2023, 11, 2360–2366 | 2365http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d2ta08581jJournal of Materials Chemistry A PaperOpen Access Article. Published on 06 January 2023. Downloaded on 2/1/2023 6:24:19 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Online1st cycle, which are higher than those of the full cell using theconventional positive electrode reaction, 0.33 V and 15 W h kg−1(Fig. S9 and Table S2†).4. ConclusionsThe proton insertion–extraction mechanism of MoO3 wasinvestigated using operando XRD and DFT calculation. Theoperating potential of the MoO3 electrode as a positive electrodefor aqueous proton batteries was optimized based on the ob-tained results, and the MoO3jH2SO4 aq.jHxMoO3 full-cell wassuccessfully constructed.The phase transition behavior of the MoO3 electrode betweenMoO3 and fully protonated phases (phase-IV) was claried byoperando XRD, including irreversible phase transition fromphase-I to phase-III. It was also shown by operando XRD thatphase transition from MoO3 to phase-I could almost reversiblybe utilized by preventing the irreversible phase transition fromphase-I to phase-III below 0.3 V vs. SHE. DFT calculation sug-gested that the stable proton accommodation site changed fromthe intralayer site to the interlayer site at higher proton contentsthan H0.5MoO3 and implied a high potential barrier between theintralayer and interlayer sites. From these results, it was stronglysuggested that the irreversibility of the phase transition fromphase-I to phase-III originated from the change of the protonaccommodation site from intralayer to interlayer.The MoO3 electrode using phase transition between MoO3and phase-I showed a maximum reversible capacity of 100 mA hg−1 and an average reduction potential of 0.44 V vs. SHE. TheMoO3j50 wt% H2SO4 aq.jHxMoO3 full-cell exhibited an averagedischarge voltage of 0.47 V, which is the highest among re-ported aqueous proton batteries (composed of insertion-typeoxide active materials) and maintains a maximum dischargecapacity of 73 mA h g−1.Considering that commercially available MoO3 powder withmicrometer-order size was used in this study and phase tran-sition from phase-I to MoO3 was kinetically sluggish, it isstrongly expected that the charge–discharge characteristics canbe improved by controlling the particle morphology.Author contributionsA. I.: investigation (experiments), methodology (operando XRD),and writing – original dra, Y. K.: investigation (DFT calcula-tions) and writing – original dra, T. N.: investigation (experi-ments), and H. A.: project administration, conceptualization,and writing – review & editing.Conflicts of interestThere are no conicts to declare.AcknowledgementsFE-SEM and EDSmeasurements were supported by Prof. MasaakiHirayama, Prof. Naoki Matsui, Prof. Kota Suzuki, and Prof. RyojiKanno (Tokyo Institute of Technology). Electrochemical2366 | J. Mater. Chem. A, 2023, 11, 2360–2366measurements were technically supported byMs Yuko Narita andMr Teruya Hiramatsu (Tokyo Institute of Technology). Some ofthe DFT calculations were performed on the Numerical MaterialsSimulator at the National Institute for Materials Science.References1 J. Li, H. Yan, C. Xu, Y. Liu, X. Zhang, M. Xia, L. Zhang andJ. Shu, Nano Energy, 2021, 89, 106400.2 X. Wang, J. Zhou and W. Tang, Energy Storage Mater., 2021,36, 1–9.3 X. 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See DOI: https://doi.org/10.1039/d2ta08581j A high voltage aqueous proton battery using an optimized operation of a MoO3 positive electrodeElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2ta08581j A high voltage aqueous proton battery using an optimized operation of a MoO3 positive electrodeElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2ta08581j A high voltage aqueous proton battery using an optimized operation of a MoO3 positive electrodeElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2ta08581j A high voltage aqueous proton battery using an optimized operation of a MoO3 positive electrodeElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2ta08581j A high voltage aqueous proton battery using an optimized operation of a MoO3 positive electrodeElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2ta08581j A high voltage aqueous proton battery using an optimized operation of a MoO3 positive electrodeElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2ta08581j A high voltage aqueous proton battery using an optimized operation of a MoO3 positive electrodeElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2ta08581j A high voltage aqueous proton battery using an optimized operation of a MoO3 positive electrodeElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2ta08581j A high voltage aqueous proton battery using an optimized operation of a MoO3 positive electrodeElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2ta08581j A high voltage aqueous proton battery using an optimized operation of a MoO3 positive electrodeElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2ta08581j A high voltage aqueous proton battery using an optimized operation of a MoO3 positive electrodeElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2ta08581j A high voltage aqueous proton battery using an optimized operation of a MoO3 positive electrodeElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2ta08581j