The contribution of subnanometer pores in carbon electrodes to the charge-storage mechanism in supercapacitors has been the subject of intense debate for over a decade. Here, we provide a model system based on graphene oxide, which employs interlayer constrictions as a model for pore sizes that can be both controllably tuned and studied in situ during supercapacitor device use. Correlating electrochemical performance and in situ tuning of interlayer constrictions, we observe a peak in specific capacitance when interlayer constriction size reaches the diameters of unsolvated ions, supporting the hypothesized link between loss of ion solvation shell and anomalous capacitance increase for subnanometer pores.

Depending on the materials employed, MIM devices can exhibit: i) two stable resistive states (often called non-volatile RS), used to fabricate binary electronic memories; [2] and ii) one stable and one unstable resistive state (often called threshold RS), explored for the fabrication of electronic neurons, i.e., devices for signal integration and electrical spike generation in artificial neural networks (ANNs); [3] and iii) multiple stable resistive states (often called analogue RS), explored for the fabrication of electronic synapses, i.e., devices that define the strength of the connection between electronic neurons in ANNs. [3] The fabrication of state-of-theart electronic memories and ANNs using MIM-like RS devices requires high integration density (>10 10 devices/mm 2 ), [4] i.e. the RS must be demonstrated in ultrasmall devices (<100 nm 2 ). [5][6][7] The smallest memristors fabricated to date have sizes ≈2 nm × 2 nm and a Pt/TiO 2 /HfO 2 /Pt structure, [6] but just few RS cycles were demonstrated and the switching voltages presented a high variability (>2 V). Ref.[7] presented a complete statistical analysis in Multiple studies have reported the observation of electro-synaptic response in different metal/insulator/metal devices. However, most of them analyzed large (>1 µm 2 ) devices that do not meet the integration density required by industry (10 10 devices/mm 2 ). Some studies emploied a scanning tunneling microscope (STM) to explore nano-synaptic response in different materials, but in this setup there is a nanogap between the insulator and one of the metallic electrodes (i.e., the STM tip), not present in real devices. Here, it is demonstrated how to use conductive atomic force microscopy to explore the presence and quality of nano-synaptic response in confined areas <50 nm 2 . Graphene oxide (GO) is selected due to its easy fabrication. Metal/GO/metal nano-synapses exhibit potentiation and paired pulse facilitation with low write current levels <1 µA (i.e., power consumption ≈3 µW), controllable excitatory post-synaptic currents, and long-term potentiation and depression. The results provide a new method to explore nano-synaptic plasticity at the nanoscale, and point to GO as an important candidate for the fabrication of ultrasmall (<50 nm 2 ) electronic synapses fulfilling the integration density requirements of neuromorphic systems.

Using reduced graphene oxide (r-GO) as a multifunctional conductive binder, a simple, cost-effective, and environmentally friendly approach is developed to fabricate activated carbon/reduced graphene oxide (AC/r-GO) composite electrodes for supercapacitors with outstanding performance. In such a composite, r-GO provides several much needed critical functions: r-GO not only serves as the binder material improving the AC particle/particle cohesion and electrode-film/substrate adhesion but also improves the electrical conductivity of the composite and provides additional surfaces for ion adsorption. Furthermore, during electrode fabrication, initial GO precursor functions as an effective dispersant for AC, resulting in a stable electrode material slurry. Employing characterization by advanced microscopy techniques, we show that AC and r-GO assemble into an interconnected network structure, resulting in a composite with high specific capacitance, excellent rate capability, and long cycling life stability. Such high-performance electrodes coupled with their relatively simple, scalable, and low-cost fabrication process thereby provide a clear pathway toward large-scale implementation of supercapacitors.

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