Optimized Kinetics Match and Charge Balance Toward Potassium Ion Hybrid Capacitors with Ultrahigh Energy and Power Densities.

Potassium ion hybrid capacitors (PIHCs) are of particular interest benefiting from high energy/power densities. However, challenges lie in the kinetic mismatch between battery-type anode and capacitive-type cathode, as well as the difficulty in achieving optimized charge/mass balance. These significantly sacrifice the electrochemical performance of PIHCs. Here, strategies including charge/mass balance pursuance, electrolyte optimization, and tailored electrode design, are employed, together, to address these challenges. The key parameters determining the energy storage properties of PIHCs are identified. Specifically, i) the good kinetic match between anode and cathode translates into the very small variation of cathode/anode mass ratio at various rates. This sets general rules for the pursuance of charge balance, and to maximize the electrochemical performance of hybrid devices. ii) A potassium bis(fluoroslufonyl)imide (KFSI)-based electrolyte promotes better electrode kinetics and allows for the formation of more stable and intact solid electrolyte interphase layer, with respect to potassium hexafluorophosphate (KPF6 )-based electrolyte. And iii) hierarchically porous N/O codoped carbon nanosheets (NOCSs) with enlarged interlayer spacing, disordered structure, and abundant pyridinic-N functional groups are advantageous in terms of high electronic/ionic transport dynamics and structural stability. All these together, contribute to the high energy/power density of the activated carbon//NOCSs PIHCs (113.4 Wh kg-1 , at 17,000 W Kg-1 ).

Fe/Fe3C Embedded in N-Doped Worm-like Porous Carbon for High-Rate Catalysis in Rechargeable Zinc-Air Batteries.

Designing low-cost preparation of high-activity electrocatalysts with excellent stability is the route one must take to fully realize large-scale application implementation of zinc-air batteries. 3D nitrogen-doped nanocarbons with transition metals or their derivatives encapsulated in show promising potential in the field of non-precious metal oxygen electrocatalysis. Herein, we report a simple, economical, and large-scale production method to construct worm-like porous nitrogen-doped carbon with in situ-grown carbon nanotubes and uniformly embedded Fe/Fe3C nanoparticles. It not only has high conductivity owing to the nitrogen-doped nature but also has ample active sites and electrolyte diffusion channels benefitting from the uniformly distributed heterostructural Fe/Fe3C nanoparticles and discrete hierarchically porous structures. When used as catalyst materials for a zinc-air battery, an energy density of 719.1 Wh kg-1 and a peak power density of 101.3 mW cm-2 at a 50 mA cm-2 discharge current density is achieved. Additionally, throughout charging and discharging for 200 cycles at a current density of 20 mA cm-2, the charge/discharge voltage gap is nearly constant.

Oxygen-Containing Functional Groups Regulating the Carbon/Electrolyte Interfacial Properties Toward Enhanced K+ Storage.

Oxygen-containing functional groups were found to effectively boost the K+ storage performance of carbonaceous materials, however, the mechanism behind the performance enhancement remains unclear. Herein, we report higher rate capability and better long-term cycle performance employing oxygen-doped graphite oxide (GO) as the anode material for potassium ion batteries (PIBs), compared to the raw graphite. The in situ Raman spectroscopy elucidates the adsorption-intercalation hybrid K+ storage mechanism, assigning the capacity enhancement to be mainly correlated with reversible K+ adsorption/desorption at the newly introduced oxygen sites. It is unraveled that the C=O and COOH rather than C-O-C and OH groups contribute to the capacity enhancement. Based on in situ Fourier transform infrared (FT-IR) spectra and in situ electrochemical impedance spectroscopy (EIS), it is found that the oxygen-containing functional groups regulate the components of solid electrolyte interphase (SEI), leading to the formation of highly conductive, intact and robust SEI. Through the systematic investigations, we hereby uncover the K+ storage mechanism of GO-based PIB, and establish a clear relationship between the types/contents of oxygen functional groups and the regulated composition of SEI.

Changes of EEG phase synchronization and EOG signals along the use of steady state visually evoked potential-based brain computer interface.

OBJECTIVE: The steady-state visual evoked potential (SSVEP)-based brain computer interface (BCI) has demonstrated relatively high performance with little user training, and thus becomes a popular BCI paradigm. However, due to the performance deterioration over time, its robustness and reliability appear not sufficient to allow a non-expert to use outside laboratory. It would be thus helpful to study what happens behind the decreasing tendency of the BCI performance. APPROACH: This paper explores the changes of brain networks and electrooculography (EOG) signals to investigate the cognitive capability changes along the use of the SSVEP-based BCI. The EOG signals are characterized by the blink amplitudes and the speeds of saccades, and the brain networks are estimated by the instantaneous phase synchronizations of electroencephalography signals. MAIN RESULTS: Experimental results revealed that the characteristics derived from EOG and brain networks have similar trends which contain two stages. At the beginning, the blink amplitudes and the saccade speeds start to reduce. Meanwhile, the global synchronizations of the brain networks are formed quickly. These observations implies that the cognitive decline along the use of the SSVEP-based BCI. Then, the EOG and the brain networks related characteristics demonstrate a slow recovery or relatively stable trend. SIGNIFICANCE: This study could be helpful for a better understanding about the depreciation of the BCI performance as well as its relationship with the brain networks and the EOG along the use of the SSVEP-based BCI.