Three-dimensional Carbon Nanofibers@ Transition Metal Oxides Composition As Electrocatalysts For Efficient Oxygen Evolution

Electrochemical water splitting provides a pathway for the large-scale conversion of intermittent energy from wind, solar or other renewable sources to produce hydrogen fuel. However, the overall efficiency of the reaction is greatly hindered by the oxygen evolution reaction(OER), which is a kinetically sluggish process. An effective electrocatalyst is needed in order to expedite the reaction and thus enhance the energy conversion efficiency. Ru and Ir oxides are considered to be the most active OER catalysts. However, their large-scale applications are limited owing to their high cost and scarcity. As such, efforts have been made to develop cost-effective and durable alternatives such as non-noble transition metal oxides(hydroxides) which are abundant in nature and have theoretically high catalytic activity. Currently, these OER catalysts are generally prepared as thin films by dip-coating or drop-casting on two-dimensional planar substrates. Although significant progress has been achieved, the activity and stability of these catalysts could be further improved by optimizing their structural and mechanical properties.Bacterial cellulose(BC) networks were used as templates for the in-situ growth of Co(OH)2 nanoplates. A subsequent simple pyrolysis of the nanocomposite generated a bacterial-cellulose-derived carbon nanofibers(CNF) framework which was homogeneously coated with Co nanospheres(denoted as CNF@Co). The CNF@Co has an advanced porous network structure, high surface area and mechanical flexibility. As a three-dimensional(3D) electrode for OER, CNF@Co shown low onset potential(0.458V(vs.Ag/AgCl)), the current density of 10 mA / cm2 achieved at overpotential of 476 mV in 0.1 M KOH, exhibits an excellence OER activity. The continuous measurements of CNF@Co for 11 h showed that only 30 % of the initial current were lost with a constant potential of 0.5 V(vs. Ag /AgCl)). These results strongly suggest that the CNF@Co possesses superior stability in alkaline solutions. The porous structure of CNF@Co both favor the mass transfer and oxygen evolution and the network of the CNF benefit to an efficient electric transfer, thus endow the CNF@Co an excellent performance toward OER.Moreover, NiCo double hydroxides were directly grown on N-doping carbon nanofibers framework(N-CNF-NiCo) by a facile hydrothermal method. The N-CNF-NiCo had a hierarchical porous structure and a high surface area. The advanced 3D structure of the resulting CNF@Co catalyst provides abundant channels and interfaces, which remarkably favors both mass transfer and oxygen evolution. As a 3D electrocatalyst, the N-CNF-NiCo exhibits an excellent catalytic activity and long-time durability toward water oxidation. In addition, this simple strategy was used to prepare other 3D porous CNF@metal or metal oxides(such as Fe, Mn)-based materials for water oxidation.