Metal-organic frameworks induced robust layered Co(OH)2 nanostructures for ultra-high stability hybrid supercapacitor electrodes in aqueous electrolyte

Comparing with intensively studied β-Co(OH)2 with applying on hybrid supercapacitors, α-Co(OH)2 is believed to possess higher electrochemical energy storage performances because of its larger interlayer spacing. However, α-phase is always transforming to β-phase when subject to successive charge-discharge processes with resulting in fast degradation in electrochemical performance. Here, by controlling ZIF-67 hydrolysis with appropriate pH regulating additives, the robust α-Co(OH)2-A nanostructures are obtained with excellent electrochemical energy storing performances, which present higher specific capacity of 87.1 mAh g−1 at 1 A g−1, excellent rate capability of 77% capacity retention at 20 A g−1 and ultra-high cycle stability of over 100% capacity retention over 200, 000 charge-discharge cycles. Through detailed characterizations, such great enhancement is mainly due to the synergistically achieving interlayer crystal water and non-stoichiometric valence states with stable larger interlayer spacing on the robust layered nanostructures. Moreover, the present study also confirms that, instead of most understanding of H+ (de)intercalation, OH− ions significant contributing to pseudocapacitive storage through inserting and reacting with H+ of crystal water and α-Co(OH)2. Thus, the present simple strategy with clear understanding to the energy storage mechanism is beneficial for designing and fabricating mass producible electrode materials of hybrid supercapacitors. •Robust α-Co(OH)2 is obtained by ZIF-67 hydrolysis in pH regulating additives.•Enough external anions is requisite for the forming process of α-Co(OH)2.•α-Co(OH)2 with robust layered structure exhibits excellent cycling stability.•Pseudocapacitive storage by inserting and reacting of OH− with H+ is confirmed.