Accessing the proton storage in neutral buffer electrolytes using an electrodeposited molybdenum phosphate
Graphical abstract
Introduction
Aqueous rechargeable batteries (ARBs) are attracting increasing interest in large-scale electric energy storage due to their intrinsic advantages of high safety, eco-friendliness, and cost-effectiveness [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]. To date, most charge carriers of ARBs are metal ions, such as Li+, Na+, K+, Ca2+, and Zn2+ [1,4,9,22]. Yet, non-metal hydrogen ion (H+) as charge carrier has its unique merits that metal ions cannot achieve [12,13,20]. For example, hydrogen is abundant on Earth, endowing the high sustainability of proton storage [23]. Second, comparing with other charge carriers, H+ has the minimal ionic radius (∼0.84 fm, fm corresponding to 10−15 m) and the lowest ionic mass (1 g mol−1), which promise the fast ion transport and alleviate the structural strain in host materials, leading to high capacity and good cycle life [24], [25], [26], [27], [28]. Third, H+ has ultra-high ionic conductivity in aqueous electrolytes due to the prominent Grotthuss mechanism, where H+ can be rapidly transported along the water molecule chains via the contiguous hydrogen-bonding network, resulting in good rate performance [29,30]. Importantly, H+ can form hydrogen and ionic bonding with the host materials, which is intrinsically different from metal ions that usually interact with the electrode hosts via ionic bonding [12,31]. These interesting features endow proton storage with distinctive battery chemistry, both in thermodynamics and kinetics.
Since 1980s, many electrode materials have been reported for aqueous proton storage, such as Prussian blue analogues (PBAs) [[28], [29], [32], [33], [34], [35]] (e.g., Cu[Fe(CN)6]0.63·□0.37·3.4H2O, copper hexacyanoferrate), organic compounds [[25], [26], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46]] [e.g., pyrene4,5,9,10-tetraone (PTO), 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA), poly(4-vinylcatechol)], metal oxides [[23], [27], [47], [48], [49], [50], [51], [52], [53], [54], [55], [56], [57], [58], [59]] (e.g., MnO2, WO3·xH2O, MoO3, TiO2), and MXenes [60,61] (e.g., Ti3C2Tx). One representative work reported by Wu et al. demonstrated a hydrated Prussian blue analogue material (denoted as CuFe-TBA) which can provide ultrafast proton transportation via the Grotthuss conduction mechanism [29]. The contiguous hydrogen-bonding network constructed by lattice water in CuFe-TBA mediates Grotthuss proton conduction. Results indicate that proton conduction in CuFe-TBA has superior diffusion rate and lower activation energy (Ea < 0.4 eV) compared to other metal ions, leading to the remarkable rate capability and long-life span. Yet, though proton as charge carrier has conspicuous merits, aqueous proton batteries (APBs) still receive relatively meager attention, which could be due to the corrosion and environmental issues of the strongly acidic electrolytes (e.g., 1∼2 M H2SO4) used in APBs. Acidic electrolytes will corrode the electrode materials and current collectors, and may also increase the hydrogen evolution risk during battery operation. Therefore, many research activities have been devoted to other aqueous batteries, e.g., Zn-ion batteries [[11], [18], [62], [63], [64], [65]].
Interestingly, recent research indicates that proton intercalation is commonly involved in aqueous batteries using mild electrolytes. For example, Sun et al. reported a consequent H+ and Zn2+ insertion mechanism for the electrodeposited MnO2 cathode in weakly acidic electrolyte (ZnSO4 + MnSO4, pH5) [66]. The charge transfer and diffusion resistance of H+ in the MnO2 cathode were much smaller than that of Zn2+. A similar phenomenon was also observed in the vanadium oxide cathode for Zn-ion batteries, in which the interactive dual-ion storage mechanism was reported, including sequential H+ and Zn2+ insertion [67]. Galvanostatic intermittent titration technique (GITT) results indicated the faster ion diffusion kinetics of H+ than Zn2+. Though the activity of H+ in these electrolytes is pretty low (10−4∼10−5 mol L−1), Brønsted weak acid [e.g., Zn(H2O)62+ and Mn(H2O)62+] with proton donor effect could provide protons for charge storage [68,69]. Importantly, the fast diffusion rate and the low activation energy of proton conduction also ensure the ubiquitous H+-associated mechanism in aqueous electrolytes. Therefore, we believe that APBs could be realized using mild or even neutral electrolytes. Yet, fundamental research focused on proton storage in neutral electrolytes has rarely been conducted.
In this work, for the first time, we fabricated an electrodeposited molybdenum phosphate material (MP) to study the proton storage process using neutral buffer electrolytes [e.g., 0.25 M NH4H2PO4 + 0.25 M (NH4)2HPO4, pH=6.8] for APBs. The MP electrode exhibited enhanced specific capacity compared to its oxide counterpart (MoOx), exhibiting a high areal capacity of 1.96 mAh cm−2 (corresponding to 125.6 mAh g−1, based on the active material loading of 15.6 mg cm−2) with a low cutoff potential of −1.0 V vs. SCE, and excellent cycling stability with 87.8% capacity retention after 2500 charge-discharge cycles at the current density of 1.0 A g−1. In depth mechanism study via electrochemical and spectroscopy measurements indicated that pure proton conduction in the MP electrode was realized using the neutral phosphate buffer electrolyte. Finally, a prototype aqueous battery was assembled using MP as the anode and Prussian blue analogues (copper hexacyanoferrate, denoted as Cu-HCF) as the cathode with neutral electrolyte, displaying a high discharge plateau of ∼1.3 V and good areal energy density of 1.91 mWh cm−2 (55.3 Wh kg−1 based on total active material loading of the electrodes, ∼34.4 mg cm−2).
Section snippets
Materials
All of the chemical reagents were purchased from Sinopharm Chemical Reagent Co., Ltd. and used as received. Graphite foil was purchased from SGL Carbon Gmbh, Germany.
Electrochemical exfoliation of the graphite foil substrate (EG)
The 3D EG substrate was fabricated using a modified electrochemical exfoliation method reported before [70]. Briefly, the electrochemical exfoliation was conducted in a three-electrode cell using a piece of graphite foil [0.8 cm (L) × 0.8 cm (W) × 0.25 mm (H)] as the working electrode, saturated calomel electrode (SCE) and a piece
Results and discussion
A molybdenum phosphate (MP) material was electrodeposited on the exfoliated 3D graphite substrate (EG, Fig. S1a) in the electrolyte containing (NH4)6Mo7O24·4H2O and H3PO4. A MoOx electrode was also fabricated similarly in the solution without H3PO4 for comparison (detailed fabrication process in experimental section). Both the electrodeposited electrodes (MP and MoOx) are amorphous, indicated by the X-ray diffraction (XRD) patterns in Figs. 1a and S2. Except for the diffraction peaks for the
Conclusion
In summary, we have discovered that an electrodeposited molybdenum phosphate material (MP) exhibited a pure proton storage mechanism in neutral buffer electrolytes for the first time. The MP electrode shows superior discharge performance than its oxide counterpart (MoOx), resulting from the mixed-valence nature of Mo and the enhanced surface electronegativity upon PO43− introduction. The aqueous neutral buffer electrolyte provides fast proton diffusion network and acts as the proton-donor to
CRediT authorship contribution statement
Zengming Qin: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Writing – original draft, Writing – review & editing, Funding acquisition. Yu Song: Conceptualization, Methodology, Resources, Writing – review & editing, Supervision, Project administration, Funding acquisition. Yaozhi Liu: Software, Formal analysis. Xiao-Xia Liu: Resources, Writing – review & editing, Supervision, Project administration, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
Y.S. acknowledges the financial support from the National Natural Science Foundation of China (51804066) and the Fundamental Research Funds for the Central Universities (N2205003). X.-X.L. acknowledges the support by the 111 Project (B16009) and the financial support from LiaoNing Science and Technology Development Foundation Guided by Central Government (2021JH6/10500139). Z.Q. and X.-X.L. thank the support by the Fundamental Research Funds for the Central Universities (N2105004).
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