Full paperTailoring the nanostructure and electronic configuration of metal phosphides for efficient electrocatalytic oxygen evolution reactions
Graphical abstract
The oxygen-incorporated nickel-cobalt phosphide/nitrogen-doped carbon (NiCoPO/NC) nanosheets were synthesized in a controlled manner to tailor nanostructure and electronic configuration for enhancement of mass transport and electronic conductivity and oxygen evolution reactions (OER). The NiCoPO/NC nanosheets possess remarkable stability and electrocatalytic activity with a low over-potential of 300 mV at 10 mA cm−2.
Introduction
The oxygen evolution reaction (OER) is considered a key reaction process of many promising renewable energy conversion and storage systems, including water splitting, metal-air batteries and regenerative fuel cells. The sluggish kinetics of OER requires efficient electrocatalysts to facilitate the catalytic reaction [1]. Commercial iridium oxides (IrO2) and ruthenium oxides (RuO2) are currently the most efficient catalysts for OER due to their low overpotentials and high proton mobility. Unfortunately, large-scale applications have been impeded by their scarcity and high cost [[2], [3], [4]]. Thus, designing efficient, inexpensive and earth-abundant electrocatalysts remains urgent for OER.
To date, non-precious metal electrocatalysts have been widely developed for OER, such as transition-metal phosphides [5], oxides [6], sulfides [7], hydroxides [8] and metal organic frameworks [9]. Among these materials, nickel or cobalt-based metal phosphides (Ni/Co MPs) have attracted enormous attention and are promising OER electrocatalysts due to their abundance and excellent metallic characteristics [[10], [11], [12]]. Although significant progress has been made, the electrochemical performance of Ni/Co MPs need further improvement. It is well-known that designing the electrocatalyst nanostructure and optimizing the composition are two effective strategies to promote the OER electrocatalytic performance of Ni/Co MPs. On the one hand, Ni/Co MPs with diverse morphologies, e.g. one-dimensional (1D) nanoparticles, 2D nanosheets, and 3D hierarchical structures have been synthesized in previous studies [[13], [14], [15], [16]]. Among these nanostructures, 2D nanosheets are of great interest for electrocatalysis due to their high specific surface area, abundant catalytic active sites and rapid interfacial charge and electron transfer [[17], [18], [19], [20], [21]]. On the other hand, some recent studies demonstrated that the incorporation of O atoms into Ni/Co MPs not only regulates the electronic structure but also activates the catalytic sites by elongating the M − P bond, and therefore accelerates the catalytic efficiency of OER [[21], [22], [23], [24], [25]]. In addition, metal doping or hybridization with porous carbon material is a favorable approach to improve the intrinsic activity of monophase Ni/Co MPs toward OER [[26], [27], [28]]. Therefore, a controllable and facile method to synthesize oxygen-incorporated nickel-cobalt phosphide and nitrogen-doped carbon hybrid materials with different morphologies, such as nanosheets and nanocages, for enhanced catalytic performance is highly desired. However, doping with metals or nonmetals and combining carbon materials to modulate the electrochemical properties of Ni/Co MPs is very challenging as the nanostructure is easily damaged.
In recent years, zeolitic imidazolate framework-67 (ZIF-67) assembled from cobalt metal ions and 2-methylimidazole (MeIM) has been used as a promising precursor to obtain metal phosphides for enhanced electrocatalytic performance owing to the large specific surface area, high N and C contents and adjustable diverse structural topologies [[29], [30], [31], [32], [33]]. Generally, metal phosphides can be synthesized from ZIF-67 via carbonization or oxidation reactions followed by a phosphating process [30,32,33]. However, the carbonization or oxidation process is time-consuming with uncontrollable morphology or composition. Nickel-cobalt layered double hydroxide (Ni–Co LDH) with controllable morphology and adjustable composition was synthesized by controlling precipitation and acidic etching during the refluxing process of ZIF-67 and nickel nitrate [34,35]. In addition, Ni–Co LDH/MeIM with sheet-like nanostructures and appropriate composition could be obtained by using deionized water to promote the hydrolysis of Ni(NO3)2·6H2O and controlling the reaction kinetic balance between the precipitation and the acidic etching of the ZIF-67 sacrificial template during the reflux process [[36], [37], [38]]. Moreover, the LDHs can be regarded as rational intermediates to incorporate oxygen atoms into phosphides due to the abundant oxygen elements. Most importantly, Ni–Co LDH/MeIM can provide oxygen atoms and nitrogen-doped carbon for the generated phosphides without destroying the nanosheet structure.
Inspired by the above considerations, we attempted to synthesize NiCoPO/NC nanosheets by controlling the precipitation and acidic etching of ZIF-67 with Ni(NO3)2·6H2O during the reflux process and then conducting the phosphorization process. A small amount of deionized water was added to increase the hydrolysis rate of nitrates during the reflux process, which is essential for the formation of Ni–Co LDH/MeIM nanosheets. The NiCoPO/NC nanosheets were assessed as OER electrocatalysts and show excellent catalytic performance in both activity and stability.
Section snippets
Materials
All chemicals were of analytical grade purity and were used without further purification. 2-methylimidazole, cobalt nitrate hexahydrate (Co(NO3)2·6H2O), nickel nitrate hexahydrate (Ni(NO3)2·6H2O), sodium hypophosphite (NaH2PO2) and KOH (90 wt %) were purchased from Aladdin Chemical Reagent Co. Ltd (Shanghai, China). The commercial ruthenium (IV) oxide (RuO2, anhydrous, 99.9%) was purchased from Alfa Aesar Co. Inc. (USA). Nafion solution (5 wt %) was purchased from DuPont Co. Ltd (Circleville,
Composition and structural characterization of the as-prepared catalysts
The synthesis procedure of the porous NiCoPO/NC nanosheets and NiCoPO/NC nanocages are schematically described in Fig. 1. Uniform ZIF-67 nanocrystals were first synthesized by mixing Co(NO3)2·6H2O and 2-methylimidazole in methanol and aging the mixture for 24 h under ambient conditions. Then, Ni–Co LDH/MeIM nanosheets and Ni–Co LDH/MeIM nanocages were successfully obtained via a facile reflux reaction of ZIF-67 with Ni(NO3)2 using deionized water or anhydrous ethanol to control the etching
Conclusion
In summary, we have fabricated NiCoPO/NC nanosheets with porous structure, incorporated oxygen and nitrogen-doped carbon by controllable synthesis of the Ni–Co LDH/MeIM intermediate converted from a ZIF-67 template, followed by a phosphorization process. Moreover, the NiCoPO/NC nanosheets as electrocatalysts for OER show much higher electrocatalytic activity than NiCoPO/NC nanocages, Ni–Co LDH/MeIM nanosheets, Ni–Co LDH/MeIM nanocages and commercial RuO2 due to structural and compositional
Declaration of competing interest
No conflict of interest
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China (No. 21661008) and Guangxi Natural Science Foundation (2018GXNSFBA138002).
Mr. Changshui Wang obtained his master's degree in physical Chemistry at Guangxi Normal University (GXNU), China in 2019 under the supervision of Dr. Dandan Cai. His research mainly focuses on design and synthesis of functional nanomaterials for electrocatalysis.
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Mr. Changshui Wang obtained his master's degree in physical Chemistry at Guangxi Normal University (GXNU), China in 2019 under the supervision of Dr. Dandan Cai. His research mainly focuses on design and synthesis of functional nanomaterials for electrocatalysis.
Miss Weibin Chen received her B.S. degree in Chemistry in 2016 from Lingnan Normal University, China. She is currently studying in Dr. Dandan's group as a master student in the Department of Chemistry and Pharmaceutical Sciences, Guangxi Normal University. Her research focuses on design and synthesis of cobalt-based electrocatalytic materials.
Ding Yuan received her Master degree in Chemistry from Jiangsu University. After graduation, she joined the Centre for Clean Environment and Energy (CCEE) at Griffith University in 2018 as a PhD candidate under the supervision of Prof. Shanqing Zhang. Her current research is focused on the electronic structure engineering of atomically thin nanomaterials for rechargeable batteries and electrocatalysis.
Mr. Shangshu Qian received his Bachelor and Master degrees in Chemistry from Ningbo University, China in 2015 and 2018, respectively. Now he is going to study for his doctorate at Griffith University, Australia, under the guidance of Professor Shanqing Zhang. His main research interests are developing novel polymer-based electrolytes their composites with ceramic-based electrolytes for solid-state batteries.
Dr. Dandan Cai received her Ph.D. in Chemical engineering at South China University of Technology (SCUT) in 2014. Dr. Cai is currently an associate professor in the Department of Chemistry and Pharmaceutical Sciences, Guangxi Normal University (GXNU), China. Her research interests mainly focus on controlled synthesis of two-dimensional electrocatalysts for energy storage and conversion.
Dr. Juantao Jiang received his Ph.D. degree in Chemical Engineering and Technology from Beijing University of Chemical Technology (BUCT) in 2016. Now he is a lecturer in Guangxi Normal University (GXNU), China. His current research interests mainly focus on the catalytic conversion of carbonaceous materials for chemicals and the preparation and application of graphene.
Prof. Shanqing Zhang obtained his Ph. D. degree in electrochemistry in 2001 at Griffith University (GU), Australia. He is currently a program leader in Centre for Clean Environment and Energy (CCEE) at GU. He has developed a series of patented and commercialized nanotechnologies for environment and energy applications based on the functional nanomaterials. His current research interests are mainly focused on synthesis of functional nanomaterials and functionalization of natural polymers for energy storage applications.
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C. Wang and W. Chen contributed equally to this work.