NiCo layered double hydroxides derived Ni0.67Co0.33(PO3)2 as stable and efficient electrocatalysts for overall water splitting
Graphical Abstract
The self-supported Ni0.67Co0.33(PO3)2-12 h electrode with optimization structure exhibits a superior water splitting performance.
Introduction
Recently, the increasing attention has been paid to the electrocatalytic water splitting owing to its high efficiency for the renewable and sustainable energy conversion [1], [2], [3], [4], [5]. There are two distinct parts: oxygen evolution reaction (OER) at anode and hydrogen evolution reaction (HER) at cathode. A high energy expense is needed to achieve an appropriate reaction rate due to the multistep electron transfer in both of the two reactions [6], [7], [8], [9]. Thus, it is pivotal to develop efficient and inexpensive electrocatalysts to reduce the overpotential or accelerate the kinetics of HER and/or OER, and further to reduce the energy cost. Until now, Pt-group is the most effective electrocatalyst for HER and Ru- and Ir-based compounds are highly active electrocatalysts for OER [10], [11], [12], [13]. However, the scarcity, instability and expensiveness severely limit their industrial application. Therefore, it is still highly desired but challenging to design of active, durable and cost-effective electrocatalysts for overall water splitting.
Transition-metal (Mo, Co, Ni, Fe, and W) based catalysts have attracted significant attention as their promising electrocatalytic activity. Such as oxides [14], [15], [16], [17], [18], [19], hydroxide [20], phosphides [21], [22], [23], [24], [25], selenides [26], [27], [28], carbide [29], sulfides [30], [31], [32], [33], nitrides [34], [35], bride [36], layered double hydroxides [37], [38] and other new materials [39], [40], [41], [42], [43], [44] exhibit efficient catalytic activity in both the OER and HER and have been proven to be promising electrocatalysts for industrial application. For the transition metal metaphosphate, only the OER performance has been studied [45], [46]. For instance, Co(PO3)2 nanoparticles needs an large onset overpotential of 310 mV for OER in PBS solution (pH 6.4) [46]. Therefore, the HER performance of transition metal metaphosphate needed to be furtherly explored to expand its application of in water splitting.
The catalytic performance can be improved by morphology designing and electrode structure tuning. Generally, a lot of electrocatalysts were synthesized in the form of powders and tested by coating them onto glass carbon with polymeric binders (Nafion and PVDF). Unfortunately, this method is harmful for active sites and the catalyst is easy to peel off from the glass carbon at high current density which is harmful to the stability. Therefore, direct growth of catalyst on the three-dimensional (3D) porous conductive substrate (such as Ni foam, carbon fiber and NiFe foam) could be an effective method to solve the above problems and make highly active and durable electrodes for water splitting [47], [48], [49]. In addition, the different morphology for the same compound catalyst will have different catalytic activity [50], [51] and the catalytic activity of the electrocatalyst can be tuned by electronic modulation [52].
Encouraged by the above analysis, Ni0.67Co0.33(PO3)2 was firstly synthesized by a simple two-step method (Scheme 1). Owing to the nets with multiscale pores which consist of the micropores with the accelerated electrolyte and gases transfer and the nanopores architecture with more exposing active edge sites, the Ni0.67Co0.33(PO3)2-12 h shows the superior OER performance with a small overpotential of 272 mV at 50 mA/cm2 and the superior HER activity with a small overpotential of 97 mV at 10 mA/cm2. In addition, the alkaline electrolyzer availing of Ni0.67Co0.33(PO3)2-12 h nets as bifunctional catalyst can output a current density of 10 mA/cm2 at 1.46 V and shows over 22 h stability at current density at 10, 50, and 100 mA/cm2, respectively. Such superior catalytic performance enables the potential application of novel Ni0.67Co0.33(PO3)2-12 h catalyst for practical larger-scale electrocatalytic water splitting.
Section snippets
Materials
Ni foam (NF) was purchased from KunShan Kuangxun Ltd. (China). Commercial Johnson–Matthey Pt/C (20 wt% Pt loading on carbon), potassium hydroxide (KOH), cobalt nitrate hexahydrate (Co(NO3)2·6H2O), nickel nitrate hexahydrate (Ni(NO3)2·6H2O), urea, and ammonium fluoride (NH4F, 98.0%) were bought from Aladdin Company. Hydrochloric acid (HCl, ca. 36.0 ~ 38.0% solution in water) and absolute alcohol (Analytical Reagent) were obtained from Beijing Chemical Reagent Factory. The ultrapure water
Characterization
The precursors of NiCo-LDH with different hydrothermal time were firstly studied by XRD. As shown in Fig. S1, the three different precursors are shown similar XRD patterns, which indicates that the crystal structure and phase of the three precursors were same. In order to study the time-dependent evolution of the morphology of NiCo-LDH during the hydrothermal process, the scanning electron microscopy (SEM) was used to observe the morphology of the precursors. As shown in Fig. S2, the morphology
Conclusions
In summary, we have successfully constructed an efficient transition metal metaphosphate catalyst for overall water splitting. The Ni0.67Co0.33(PO3)2-12 h electrode with the optimized nanostructure exhibited high activity and durability toward the overall water splitting in alkaline solution. More importantly, when Ni0.67Co0.33(PO3)2-12 h electrodes were used as both the cathode and anode in a two-electrode system, it needs only a cell voltage of 1.46 V to achieve current density of 10 mA/cm2
CRediT authorship contribution statement
W. Yang and Y. Yu conceived the project and designed the experiments. S. Geng conducted the material synthesis and electrochemical measurements. Y. Huang and F. Akhmat conducted the characterized the material. S. Geng and F. Akhmat co-wrote the paper.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Natural Science Foundation of China under Grant (No. 51871078) and Heilongjiang Science Foundation (No. E2018028).
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