| 1. |
- Qiu, Zhen, et al.
(författare)
-
Investigating Redox States and Reaction Dynamics of Ni-based Nano-Catalysts for Alkaline Water Splitting
- 2018
-
Annan publikation (refereegranskat)abstract
- Design and synthesis of highly active and cost-effective electrocatalysts for hydrogen and oxygen generation by water electrolysis can be of paramount importance, as hydrogen has been considered as one of the most promising energy alternatives to traditional fossil fuel-based energy because of its high specific energy density and potentially clean production. Here, we investigate the interfacial cause-effect-relationships in the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) for Ni-based nano-catalysts by modifying the structure and performing in-situ characterization during the reaction. Based on different pH condition during synthesis of NiO nanoflakes (NFs), we show that the crystal growth direction and morphology of the NiO nanostructure can be altered according to the different surface charges, which control the exposure of the surface active sites, resulting in much improvedcatalytic performance. Additionally, by incorporating a redox active dopant (Fe), NiO can be tuned into higher catalytic efficiency and becomes bi-functional with respect to the OER and the HER processes under alkaline conditions. Exploring the 3D synergistic NiFe nano-layered double hydroxide, we find improved catalytic performance after prolonged use, where the current density increases from 9.3 mA cm-2 to 12.7 mA cm-2 during 100 h running at 1.7 V without iR compensation in a 2-electrode system. In order to understand the function and precise mechanism of metal doping and the synergistic effect to improve the catalytic property after prolonged use, we utilize in situ Raman spectroscopic and in situ electrochemical impedance spectroscopy (EIS) to monitor the interfacial redox state and reaction dynamics. As we all know, the material structure plays a vital role on improving the electrocatalytic property and stability. The structural and physical characterization on 3D ultrathin NiFe nano-layered double hydroxide were also investigated with XRD, SEM, TEM and XPS before and after 100 h electrolysis in a two electrode configuration in 1 M KOH at room temperature. The structure changes after the oxygen evolution reaction are minor, while, after the hydrogen evolution reaction, the catalyst undergoes recrystallization as observed by selected area electron diffraction (SAED), with markedly improved electrocatalytic activity. The successful identifications of the underlying reasons for the electrocatalytic improvement open the possibilities for a rational design of Ni-based nano-catalysts, and possibly also for other material systems for use as efficient electrocatalysts for practical alkaline HER and OER processes.
|
|
| 2. |
- Qiu, Zhen, et al.
(författare)
-
The role of interfacial species and nanostructure of Ni-based electrocatalysts for water splitting
- 2018
-
Annan publikation (refereegranskat)abstract
- Given the high specific energy density and potentially clean production, hydrogen is a promising energy carrier to replace the traditional fossil fuel-based energy. Some major application bottlenecks so far are the low hydrogen conversion efficiency and the high cost. It is thus of high interest to design highly active and cost-effective electrocatalysts to increase the hydrogen fuel generation by water electrolysis. Here, we show that by modifying the structure and studying interfacial cause-effect-relationships in the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) in Ni-based nano-catalysts. It is noteworthy that the performance can be drastically improved after the modification. Based on different pH conditions during synthesis of NiO nanoflakes (NFs), the crystal growth direction and morphology of the NiO nanostructure can be altered according to different surface charges, which control the exposure of the surface active sites, resulting in much improved catalytic performance. Additionally, by incorporating a redox active dopant (Fe), NiO can be tuned into higher catalytic efficiency and becomes bi-functional with respect to the OER and the HER processes under alkaline conditions. Exploring the 3D synergistic NiFe nano-layered double hydroxide, we find improved catalytic performance after prolonged use, where the current density increases from 9.3 mA cm-2 to 12.7 mA cm-2 during 100 h running at 1.7 V without iR compensation in a 2-electrode system. In order to understand the function and precise mechanism of metal doping and the synergistic effect to improve the catalytic property after prolonged use, we use in situ Raman spectroscopic and electrochemical impedance spectroscopy (EIS) to monitor the interfacial redox species and reaction dynamics. The structural and physical characterization on 3D ultrathin NiFe nano-layered double hydroxide were also investigated with XRD, SEM, TEM and XPS before and after 100 h electrolysis in a two electrode configuration in 1 M KOH at room temperature. The results show that structure changes after the oxygen evolution reaction are minor, while, after the hydrogen evolution reaction, the catalyst undergoes recrystallization as observed by selected area electron diffraction (SAED), with markedly improved electrocatalytic activity. The successful identification of the underlying reason for the electrocatalytic improvement offers possibilities for a rational design of Ni-based nano-catalysts, and possibly also for other material systems for use as efficient electrocatalysts for practical alkaline HER and OER processes.
|
|
| 3. |
- Qiu, Zhen
(författare)
-
Tuning of NiO into an Efficient Electrocatalyst for Water Splitting
- 2017
-
Annan publikation (refereegranskat)abstract
- Designing a highly efficient and cost-effective catalysts for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is a significant element for the development of solar hydrogen into a competitive sustainable energy source. In this context, nickel, cobalt, and iron oxides and their double hydroxides have been extensively investigated as electrocatalysts for the OER reaction with a large variation in crystal extensions and exposed lattice faces and thus also in the resulting overpotential. In this work, we show that the surface energies of exposed lattice planes of the growing NiO nanostructures can be altered and an enhanced crystal growth in different crystalline direction can be controlled by a hydrothermal method with a variation of the pH conditions of the precursor solution. The growth direction in between the [111] and [220] directions in cubic NiO with respect to the [200] direction can be altered resulting in NiO nanoflakes (NFs) with controllable extensions. On the basis of the different pH condition during synthesis of the NiO NFs, we find that the morphology of NiO nanostructure is also able to be changed according to the different surface charge, which results in different catalytic performance. By incorporating a redox active dopant (Fe), NiO can be tuned into higher OER efficiency as well as making the electrocatalyst bi-functional with respect to the OER and the HER processes under alkaline conditions. The Fe-NiO system is engineered into a 3D electrode by chemical bath deposition (CBD) method onto a nickel foam framework. In order to identify the composition of the active phase on the surface of Fe-NiO/Ni foam, in situ Raman spectroscopic investigations are carried out during both the OER and HER reactions under water. The results show that the Fe doping plays a critical role for the catalytic property. In support of this, density functional theory (DFT) calculations show that Fe changes the local electron density, shifting the energetically preferable absorption site of H from oxygen in NiO onto Ni in Fe-NiO in the hydrogen evolution reaction.
|
|
| 4. |
- Qiu, Zhen
(författare)
-
Tuning the Overpotential for NiO Nanocatalyst for Water Splitting
- 2017
-
Annan publikation (refereegranskat)abstract
- Designing highly efficient and cost-effective nanocatalysts for water electrolysis by the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) has been considered as a promising route to develop sustainable hydrogen production. In our study, nickel, cobalt, and iron oxides and their double hydroxides/sulfide have been extensively investigated as nanocatalysts for water splitting. We show that the surface energies of lattice planes during synthesizing NiO nanostructures can be altered and an enhanced crystal growth in different crystalline direction can be controlled by a hydrothermal method with variation of the pH conditions of the precursor solution. Based on the different pH, the morphology of NiO nanostructure can also be varied according to the different surface charge, which results in different catalytic performance. Moreover, by Fe doping, NiO could be tuned into higher OER performance by changing the local electronic structure as well as making the nanocatalyst bi-functional with respect to the OER and the HER processes under alkaline conditions. The 3D Fe-NiO nanocatalyst is fabricated by the facile chemical bath deposition (CBD) method on nickel foam templates. In order to identify the composition of the active phase on the surface of Fe-NiO/Ni foam, in situ Raman spectroscopic measurements are carried out for both the OER and HER reactions under alkaline conditions. The results show that the Fe doping plays a critical role for the catalytic property. Density functional theory (DFT) calculations show that Fe change the local electron density, shifting the energetically preferable absorption site of H from oxygen in NiO onto Ni in Fe-NiO in the hydrogen evolution reaction. In addition, the effects of the heteroatoms (S or Se) in the same group as oxygen are investigated as new, efficient nanocatalysts.
|
|
