Pyrolysis of ZIF-Co assisted by a small amount of KNO3: Tuning of Co-CoOx@N, O co-doped carbon hybrid nanostructures for accelerating water splitting

Electrochemical catalytic water splitting is considered as a promising technique for green hydrogen production, and highly active catalysts toward cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER) are required, especially bifunctional ones. In this work, the bifunctional catalysts with Co-CoO x @N, O co-doped carbon (NOC) hybrid nanostructures have been prepared via pyrolysis of ZIF-Co with assistance of a small amount of KNO 3 . KNO 3 plays important roles in tuning the inner and the outer structures of Co-CoO x @NOC hybrid nanostructures. The proposed S Ov , which is defined as the content of oxygen vacancy (O v ) per unit surface area of the inner Co-CoO x nanoparticle, and charge transfer resistance ( R ct ) are used as the descriptors of the inner structures, while hydrophilicity (or contact angle) is applied as the main feature of the outer structures. S Ov , R ct and hydrophilicity are well tuned by the fed mass of KNO 3 , and a moderate fed mass is the key to balancing S Ov , R ct and hydrophilicity and further is beneficial to accelerating specific and intrinsic activity for both HER and OER . Thus, the bifunctional HER/OER specific and intrinsic activity is peaked by Co-CoO x @NOC-0.025KNO 3 with a moderate fed mass of KNO 3 of 0.025 g, such as j s-250-HER of 539.22 μA cm -2 BET and j s-350-OER of 129.83 μA cm -2 BET at the HER and OER overpotentials of 250 and 350 mV, respectively, also corresponding to high turnover frequencies of 6.653 and 0.801 s −1 , respectively. The optimal catalyst also has high apparent activity, such as achieving high HER and OER current densities of 500 mA cm −2 , only requiring low overpotentials of 273.1 and 382 mV, respectively. It promotes overall water splitting with a high current density of 500 mA cm −2 at a cell voltage of ∼ 1.63 V, showing the high HER and OER Faradaic efficiencies of ∼ 98.2 and ∼ 98.0 %, respectively.