ABSTRACT
We study the electrocatalytic oxygen evolution reaction using in situ X-ray absorption spectroscopy (XAS) to track the dynamics of the valence state and the covalence of the metal ions of LaFeO3 and LaFeO3/LaNiO3 thin films. The active materials are 8 unit cells grown epitaxially on 100 nm conductive La0.67Sr0.33MnO3 layers using pulsed laser deposition (PLD). The perovskite layers are supported on monolayer Ca2Nb3O10 nanosheet-buffered 100 nm SiNx membranes. The in situ Fe and Ni K-edges XAS spectra were measured from the backside of the SiNx membrane using fluorescence yield detection under electrocatalytic reaction conditions. The XAS spectra show significant spectral changes, which indicate that (1) the metal (co)valencies increase, and (2) the number of 3d electrons remains constant with applied potential. We find that the whole 8 unit cells react to the potential changes, including the buried LaNiO3 film.
ABSTRACT
Exploring earth-abundant catalysts with ultra-high activity and durability are the decisive challenges for oxygen evolution reaction. This work prepared the FeS/FeOxH@Fe nanosheets as the efficient and stable electrocatalysts of oxygen evolution reaction (OER) through a simple one-step co-deposition method. The FeS/FeOxH@Fe exhibited small overpotentials of 245, 376 and 482 mV at the current density of 10, 500 and 1000 mA cm-2 without iR-compensations in 1.0 M KOH solution, respectively. Constructing amorphous structure and the interface between amorphous and crystal can obviously improve the conductivity of FeOxH, which is beneficial to the improvement of catalytic performance. This work may provide an effective and controlled strategy to design highly active OER catalysts with an interface structure between amorphous and crystal by a well-designed co-deposition.
ABSTRACT
The design of electrocatalysts with cost-efficient, highly-efficient and stable for hydrogen evolution reaction (HER) is significant to alleviate energy crisis in the future. Herein, we prepared the Ni11(HPO3)8(OH)3/Mn3O4 (named as NiPi/Mn3O4) composite with crystalline/amorphous interface by electrochemical activation based on Ni3P/MnOOH precursor. The precursor was synthesized by two-step method that firstly electrodeposited on nickel foam and then used anodic oxidation under the applied voltage. The XPS analysis manifested that NiPi/Mn3O4 had a strong interface coupling between Ni11(HPO3)8(OH)3 and Mn3O4 contributed to fast kinetics of hydrogen evolution reaction. The morphology of the nanowires and nanoparticles were provided lager contact area between catalyst and electrolyte. Meanwhile, the obvious interface between crystalline and amorphous also resulted in NiPi/Mn3O4 heterostructureted material with faster electron transport and superior stability. Owing to these characteristics, the NiPi/Mn3O4 displays amazing HER activity with low overpotential of 84â¯mV at 100â¯mAâ¯cm-2 and a long-term stability of over 100â¯h at 200â¯mAâ¯cm-2 in 1.0â¯M NaOH solution. The surprising activity of NiPi/Mn3O4 is better than that of most reported catalysts, similar to the state-of-the-art Pt/C.
ABSTRACT
An enormous challenge in the development of renewable hydrogen sources for electricity-driven water-splitting systems has been the limitation of the earth-abundant and robust highly-active cathode electrocatalysts. In this work, we developed a simple sulfur-anion doping strategy to obtain the S-doped NiCo composite (S-NiCo@50) on Ni foam (NF) via a one-step electrochemical deposition. It was found that doped sulfur plays a crucial role in reducing the overpotential of hydrogen evolution by providing abundant active sites as identified by the XPS spectrum. The formed metallic Ni and Co effectively promoted electron transportation. The synergistic effects between the amorphous CoxNiyS(x+y) substance and crystalline Ni and Co metal seemed to result in enhanced HER activity. In particular, the S-NiCo@50 electrode, featuring a hierarchical morphology, showed an ultralow overpotential of 28 and 125 mV at 10 and 100 mA cm-2, respectively, in 1.0 M NaOH with a large exchange current density (j0) of 4.8 mA cm-2 as well as high conductivity and stability; its catalytic properties are superior to most of the reported alkaline electrocatalysts and are on par with commercial Pt/C. Assembled with the counter electrode (Ni-Fe/NF), the overall water splitting was proved with a low 1.55 V at 10 mA cm-2. Moreover, we built the Ni24Co6S6 cluster as the S-NiCo@50 model and revealed its intrinsic activity by density functional theory (DFT) calculations. This study shows that S-doping and component control can be an exquisite strategy for realizing high-efficiency electrochemical water reduction.
