The development of compressible supercapacitors (SCs) that is tolerant to wide temperature range has been severely hindered due to the poor ionic conductivity and absence of extra functions in conventional polymer electrolytes. Herein, a highly conductive and compressible hydrogel polyelectrolyte has been prepared from polyacrylamide cross-linked by methacrylated graphene oxide (MGO-PAM) and the polyelectrolyte can maintain its excellent elasticity at -30 ℃ as well as its original shape at 100 ℃. As a result, the SC based on the MGO-PAM polyelectrolyte outperformed those fabricated with the conventional poly(vinyl alcohol) (PVA)/H₂SO₄ electrolyte over a wide temperature window between -30 and 100 ℃. Meanwhile, the device shows an excellent cycling stability (capacitance retention of 93.3% after 8, 000 cycles at -30 ℃ and 76.5 % after 4, 000 cycles under 100 ℃) and a reversible compressibility (a high capacitance retention of 94.1% under 80% compression). Therefore, the MGO-PAM polyelectrolyte enables the fabrication of compressible SCs with a wide operating temperature, rendering new insights for developing next-generation robust and multifunctional energy-storage devices.

Ultrathin graphitic carbon nitride nanoplatelets (UGCNPs) are synthesized by a facile manner via an efficient and eco-friendly ball milling approach. The obtained UGCNPs are 2–6 nm in size and 0.35–0.7 nm in thickness, with improved specific surface area over that of bulk graphitic carbon nitride. Photochemical experiments show that the UGCNPs are highly active in visible-light water splitting, with a hydrogen evolution rate of 1, 365 μmol·h^-1·g^-1, which is 13.7-fold greater than that of their bulk counterparts. The notable improvement in the hydrogen evolution rate observed with UGCNPs under visible light is due to the synergistic effects derived from the increased specific surface area, reduced thickness, and a negative shift in the conduction band concomitant with the exfoliation of bulk graphitic carbon nitride into UGCNPs. In addition to metal-free visible-light-driven photocatalytic hydrogen production, the UGCNPs find attractive applications in biomedical imaging and optoelectronics because of their superior luminescence characteristics.