Expanding the electrochemical stability window (ESW) of aqueous electrolytes can contribute to the high energy density of electrochemical energy-storage devices and supercapacitors. Here, we demonstrate a unique electrochemical interaction in two- and three-electrode configurations of sodium nitrate (NaNO3) water-in-salt (WiS) electrolyte in a wide range of concentrations (1–12 m) with an electrode made of boron-doped microcrystalline diamond (B-MCDE, grain size ∼1 mm) deposited by a microwave plasma chemical vapor deposition process. Compared to the commercial glassy carbon electrode (C-GCE), the ESW reached ∼3 V. In the frequency range of 10 mHz–100 kHz, the B-MCDE also demonstrated better conductive (down to Rct of 50 Ω) and capacitive (up to Cdl 3.2 μF) properties compared to the C-GCE for all WiS electrolyte concentrations, where the appropriate equivalent circuit was identified. The best performance was obtained for the 8 m WiS concentration at neutral pH and room temperature, while the 12 m WiS was unstable, as confirmed by Raman spectroscopy. The expanded ESW and superior performance of B-MCDE are explained by its high-quality material properties and nanostructured surface (almost nanoporous, feature size <100 nm) formed in the WiS electrolyte. The results reveal that with a well-controlled quality, boron-doped diamond electrodes are advantageous for WiS supercapacitors.
