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Mengyu Yan1 2 Xunbiao` Zhou2 Liqiang Mai2 Jihui Yang1

1, University of Washington, Seattle, Washington, United States
2, Wuhan University of Technology, Wuhan, , China

Nowadays, substantial works have been devoted to electrochemical energy storage and conversion due to the increasing global energy demand. The individual nanowire/sheet electrochemical devices can establish an intrinsic relationship between the electrochemical performance and electronic transport. Meanwhile, it is a fantastic platform to tune the electrochemical behavior (including electrochemical energy storage and electrocatalytic) with external fields, which is meaningful on understanding the reaction mechanism and further developing new optimize strategy.1

In the electrochemical energy storage field, graphene has been widely used to enhance the device-performance due to its high conductivity and large surface area. However, it is still unclear how graphene influences the electrochemical performance and reaction mechanisms of electrode materials. To detect this mechanism in depth, we designed a single-nanowire electrochemical probe to explore the intrinsic mechanisms of the electrochemical reactions in situ.2 It is demonstrated that the high capacity is resulted from increasing MnO2 NWs intercalation capacitance, and the pores in the graphene provide channels for fast ion diffusion without degrading the rate of electron transport. Such devices are further used to detect the ion transport in Li/Na ion based energy storage devices.3 Our results show that the ions would choose the shortest pathway along the radial direction to intercalate into the nanowire structure.

In the electrocatalytic field, we designed an in-situ testing platform with individual Ni-graphene nanosheet based OER devices.4 We demonstrated that the oxygen acts as a barrier to reduce the concentration of OH ions at catalyst surface, slowing down the charge transfer process and OER kinetics. By removing oxygen in the electrolyte, a significant decrease in Tafel slope of over 20% and an early onset potential of 1.344 V vs. RHE are achieved. Afterwards, the individual electrocatalytic devices were applied to understand how does the external electric field tune the HER behavior. Increasing the back gate voltage from 0 to 5 V, the overpotential of MoS2 nanosheet decreases from 240 to 38 mV.5 Such strategy is further extended to VSe2 nanosheet.6 Our results indicate that HER performance improves along with the increasing of the negative back gate voltage. Besides, charge transfer resistance and high-frequency time constant drop dramatically, which demonstrates a much faster charge transfer process.

Reference
1. Mai L, Yan M, Zhao Y. Nature 2017, 546(7659): 469.
2. Hu P, Yan M, Wang X, Han C, He L, Wei X, et al. Nano letters 2016, 16(3): 1523-1529.
3. Xu X, Yan M, Tian X, Yang C, Shi M, Wei Q, et al. Nano letters 2015, 15(6): 3879-3884.
4. Wang P, Yan M, Meng J, Jiang G, Qu L, Pan X, et al.Nature Communications 2017, 8.
5. Wang J, Yan M, Zhao K, Liao X, Wang P, Pan X, et al. Advanced Materials 2017, 29(7).
6. Yan M, Pan X, Wang P, Chen F, He L, Jiang G, et al. Nano Letters 17(7): 4109.

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