| Early Cycle Electrochemical Irreversibility of a MoS2 Electrode in Concentrated Aqueous Ca(NO3)2: Ex Situ Observation of Sulfate-Related Interfacial Evolution |
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Younglim Song1, Sangyup Lee1, Seunga Yang1, Soon-Ki Jeong1,2,3 |
1Department of Future Convergence Technology, Graduate School, Soonchunhyang University, Soonchunhyang-ro 22-gil, Sinchang-myeon, Asan-si 31538, Chungcheongnam-do, Republic of Korea
2Department of Energy Engineering, Soonchunhyang University, Soonchunhyang-ro 22-gil, Sinchang-myeon, Asan-si 31538, Chungcheongnam-do, Republic of Korea
3Advanced Energy Research Center, Soonchunhyang University, Soonchunhyang-ro 22-gil, Sinchang-myeon, Asan-si 31538, Chungcheongnam-do, Republic of Korea |
Correspondence:
Soon-Ki Jeong,
Email: hamin611@sch.ac.kr
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Received: 3 April 2026 • Accepted: 4 May 2026 |
| Abstract |
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An MoS2 electrode was cycled in 6.0 M aqueous Ca(NO3)2 between −1.8 and 1.8 V vs. a saturated calomel electrode (SCE) to examine the effect of early interfacial transformations on its apparent electrochemical response over a wide potential window. Cyclic voltammetry results show a sharp first-cycle anodic peak at ~1.25 V vs. SCE that is strongly attenuated in later cycles. Galvanostatic profiles exhibit a first-cycle plateau at ~1.1–1.2 V vs. SCE, corresponding to an anodic specific capacity of ~2020 mAh g−1, followed by quasi-steady cycling with a discharge capacity of ~157 mAh g−1 and a coulombic efficiency of ~86%. Ex situ X-ray diffraction patterns include a CaSO4-type crystalline signature for the early-cycle samples, with multiple diffraction peaks agreeing well with those of the CaSO4·xH2O-type reference pattern, despite some unmatched peaks. Scanning results further supported the accompanying morphological evolution. Energy-dispersive X-ray spectroscopy results show pronounced Ca/O enrichment after the first charge and partial recovery of the Mo/S signals after discharging, which is consistent with early surface coverage followed by discharge-associated reorganization. A CaSO4·0.5H2O comparison electrode shows lower steady discharge capacity (~77 mAh g−1) and does not reproduce the electrochemical response of the MoS2 electrode under the tested conditions. Taken together, these results indicate that the large early-cycle anodic response of MoS2 under the present 6.0 M aqueous Ca(NO3)2 conditions should be interpreted as arising from both anodic and interfacial processes, including sulfate-related interfacial evolution, rather than reversible Ca2+ storage alone. |
| Keywords:
molybdenum disulfide, aqueous calcium electrochemistry, concentrated nitrate electrolyte, interphase/passivation, calcium sulfate deposition |
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