German Edition: DOI: 10.1002/ange.201702160Hydronium Ion BatteriesInternational Edition: DOI: 10.1002/anie.201702160

Hydronium Ion Batteries: A Sustainable Energy StorageSolutionYun-hai Zhu, Xu Yang, and Xin-bo Zhang*

energy storage · hydronium ion batteries · intercala-tion · organic electrodes

Batteries have evolved with a history of more than twocenturies, from the voltaic pile to the lead–acid battery tolithium ion batteries (LIBs). The functionality facilitated byadvanced batteries has transformed our everyday life withdispatchable power sources in communications and recentlyin electrified transportation. Batteries represent an indispen-sable technology to propel the green economy; this would betrue if the energy used to manufacture batteries is comingfrom renewable sources and the energy that is to charge thebatteries is renewable as well. Right now, with the rapidgrowth of the market for electric vehicles, there could be tensof thousands of cars being charged during nights in one city,which would create a significant demand for power. If suchpower comes from power plants that rely on burning fossilfuels, this would not be able to solve the problems ofenvironment pollution and lack of sustainability. Therefore,grid-scale storage must be realized to enable an energyecosystem, which can store the energy from renewable-but-intermittent sources (sun and wind).

Inspired by the tremendous demand, battery scientistshave started to shift their attention to batteries beyondlithium. While lithium is an ideal charge carrier for recharge-able ion batteries, LIBs are ill-suited for large-scale stationarystorage owing to the Earth rarity of lithium (17 ppm in theEarthQs crust) and the geopolitical challenge to mine lithiumminerals at low cost. Recently, the focus of the community hasbeen put on other metal ion carriers for post-LIBs; greatprogress has been made on batteries that operate on Earth-abundant elements, such as Na, K, Mg, and Al, of which thecrust is three orders of magnitude richer than of lithium.[1–4] Incontrast, attention has rarely been given to non-metal ions.Compared with the heavy mass and large radius of metal ions,the proton is an ideal candidate as charge carrier forrechargeable batteries owing to small size, wide availability,and negligible cost. In fact, the proton has been used asa charge carrier for electrodes that form metal hydride alloys

in the conventional nickel hydride batteries.[5] Intercalation ofprotons is also reported for porous graphitic carbon materi-als.[6] Unfortunately, owning to the part of proton consump-tion associated with the hydrogen evolution reaction, most ofthese devices exhibit extremely low coulombic efficiency.Furthermore, almost all of these devices employ an alkalineelectrolyte, where it is the dissociation of water that providesincoming protons. In comparison, protonic acids can directlyproduces protons by themselves when placed in an aqueoussolution. However, owing to the very high dehydration energyof hydronium ions (11 eV) in protonic acids, the cations inprotonic acid electrolytes are hydronium ions, rather thanprotons. Thus, can batteries employ hydronium ions as chargecarriers in acidic electrolytes? Herein, we briefly describelatest approaches to answer this question.

Recently, Ji and co-workers have demonstrated, for thefirst time, hydronium ions can be reversibly stored in anelectrode of crystalline 3,4,9,10-perylenetetracarboxylic dia-nhydride (PTCDA).[7] In that study, they observed a highlyreversible discharge–charge behavior of PTCDA in anaqueous acidic electrolyte of 1m H2SO4 after an initialconditioning process (Figure 1a). The capacity and theoperation potentials are comparable to that of Na-ion storagein the same electrode.[8] They also discovered three pairs ofredox peaks in the cyclic voltammogram (CV) curve ofPTCDA in their system, thus suggesting that multiple redoxreactions occurred during the charge–discharge process (Fig-ure 1b). They tentatively proposed that the H3’/O3’ processcorresponded to the H3O

+ intercalated into the structures,while the H1’/O1’ redox reaction was assigned to the insertion/extraction of H5O2

+ ions. It is worth noting that PTCDA hasserved as a very good model compound for hosting large ions.It has a long-range ordered molecular crystal structure, whichcontains large interstitial sites for ion storage. To clearlyunravel a hydronium storage mechanism in the host structure,the authors employed ex situ X-ray diffraction (XRD) toinvestigate the structural evolution of the PTCDA electrodesat different stages of the electrochemical reaction. Interest-ingly, the results they observed provided substantial evidencethat the structure of PTCDA had experienced dramatic butreversible dilation during cycling, as shown in Figure 2b. Sucha scale of structural change could not possibly be caused byproton intercalation owing to the extremely small size ofprotons. The authors attributed the structural change to theintercalation of hydronium ions into the structure as the size

[*] Y.-h. Zhu, Dr. X. Yang, Prof. Dr. X.-b. ZhangState Key Laboratory of Rare Earth Resource Utilization, ChangchunInstitute of Applied Chemistry, Chinese Academy of SciencesChangchun, 130022 (P.R. China)E-mail: xbzhang@ciac.ac.cn

The ORCID identification number(s) for the author(s) of this articlecan be found under:http://dx.doi.org/10.1002/anie.201702160.

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of hydronium ions is very similar to that of Na-ions (ca.102 pm).

Based on the result of the first-principles density func-tional theory (DFT) calculation, the authors further con-firmed the change of XRD patterns was due to the hydroniumion insertion in PTCDA. And they also identified the sites ofinserted hydronium ions in the unit cell of the PTCDA crystal,where the two hydronium ions were coordinated by three andtwo carbonyl groups, respectively (Figure 2c). On the otherhand, to maintain the charge balance in the electrolyte,activated carbon serves as the positive electrode, electrostati-cally absorbing/desorbing SO4

2@ from/into the electrolyte inan electrical-double-layer manner (Figure 2d). The evidenceis compelling that hydronium ions serve as the charge carrierin this system, thus providing the perspective for the futurehydronium ion batteries.

Besides the sustainability benefit owing to the lack ofreliance on any metal ion carriers, there may exist uniqueadvantages of hydronium ion batteries, particularly regardingthe power performance. The hydration in H3O

+ may provide

a shielding effect for the proton, which may lower theactivation energy for its migration in the host structure. This isevident in this reported results, where at 1 Ag@1, a very largecurrent density for battery electrodes, hydronium intercalatesin PTCDA with very small polarization in contrast to thebehavior of Na- and K-ions in PTCDA. It is reasonable tobelieve that the fast kinetics of hydronium in aqueouselectrolytes may render the hydronium ion batteries highpower capability.

The work of the Ji group is both of fundamental value aswell as potentially large practical impact. As mentioned in thepaper, it is likely that additional to H3O

+, H5O2+, or other

higher-order-hydrated hydronium ions may be intercalatedinto the structure. This provides a unique opportunity toinvestigate the hydration of protons—one of the mostfundamental phenomena in a confined environment. Withrespect to battery materials, to store hydronium ions presentsa new paradigm for materials chemistry to design newmaterials, which will not only meet the needs of this areabut possibly bring unexpected discoveries in the future. In

Figure 1. a) Galvanostatic charge–discharge profiles of the 10th cycle of the PTCDA electrode in 1m H2SO4. b) CVs curves at a scan rate of1 mVs@1 after 50 galvanostatic charge–discharge cycles at 1 Ag@1.

Figure 2. a) Charge–discharge (reduction/oxidation) profiles of the PTCDA electrode in the 50th cycle. b) XRD patterns (no. 1 to 6) of the PTCDAelectrode corresponding to different stages of charge (points 1 to 6 in (a)). The patterns 7, 8, and 9 are simulated XRD patterns from pristinePTCDA, PTCDA with one H3O

+ inserted per unit cell, and PTCDA with two H3O+ ions intercalated per unit cell, respectively. c) Simulated PTCDA

unit cell incorporating two H3O+. d) An illustration of the working principles of the hydronium ion battery.

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terms of the applications, even though the Ji paper focuses onone electrode, there could be ample room of scientificendeavor for future full cells. As already demonstrated by Jiet al. , the PTCDA electrode can be coupled with a capacitivecounter electrode as the cathode, thus forming a hybriddevice. It is also possible that new high-potential materialswill be discovered or identified for hydronium ion storage,where these materials may be of inorganic nature, such asmetal oxides and phosphates, or open-framework structureslike Prussian blue. There is a great chance that otherintercalation electrodes will be identified so that a full-cellhydronium ion battery can be investigated.

This new direction promises remarkable opportunities,but of course there may also be some caveats. When exploringthe functions of organic materials, attention should be paid totheir solubility in the electrolyte; indeed the Ji paper hasshown that the PTCDA electrode when being reduced doesdissolve in the aqueous electrolyte to some extent. Thestructural stability of hydronium-insertion electrodes andtheir consequent long-term cycling stability should receivegreat attention for future studies. Furthermore, water doeshave a narrower stable voltage window compared to a non-aqueous electrolyte; therefore, it is also important to push thepotential of the hydrogen evolution reaction and of theoxygen evolution reaction by introducing overpotential.

In summary, the work of Ji et al. indeed pushes theboundaries of the intercalation chemistry for battery purpos-es. There are significant opportunities in both fundamentaland applied research, where this work serves as a goodstarting point.

Acknowledgement

This work is financially supported by National NaturalScience Foundation of China (51401084); The Jiangsu Prov-

ince Basic Research Program (BK20140267); Ministry ofScience and Technology of the PeopleQs Republic of China(2016YFB0100100); Technology and Industry for NationalDefence of the People’s Republic of China(JCKY2016130B010).

Conflict of interest

The authors declare no conflict of interest.

How to cite: Angew. Chem. Int. Ed. 2017, 56, 6378–6380Angew. Chem. 2017, 129, 6476–6478

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Manuscript received: February 28, 2017Revised manuscript received: March 10, 2017Version of record online: April 18, 2017

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