Phase structure tuning of graphene supported Ni-NiO Nanoparticles for enhanced urea oxidation performance
Graphical abstract
Introduction
Urea-containing wastewater currently is a serious problem for the living environment because of the continuous effluent from the industrial wastewater, agricultural production, and a large amount of natural excretion of humans and animals every day (the mass fraction of urea 2–2.5%) [1], [2], [3]. It will be harmful to the environment when a large amount of urea is untreated because the urea can be easily converted into ammonia that will be released into the atmosphere as a main source of the acid rain [4,5]. The electrochemical technique is considered as a valid approach for the urea conversion and utilization. As a rich hydrogen molecular, (hydrogen content, 10.1wt.%) [6], the hydrogen can be produced by the electro-oxidation of urea; it can also be directly fed to the fuel cells system to generate electricity with the benign gaseous products discharge [7,8]; moreover, the urea-electrolysis also becomes a research hotspot for the hydrogen generation as an alternative reaction for water oxidation [9,10]. Urea electro-oxidation is a significant topic in electrochemistry because of the varied applications relevant to energy conversion and utilization, while the serious challenge is met for this reaction due to the complex process involving a six-electron transfer process [11].
Till now, many studies have been done to develop efficient catalysts including noble and non-noble catalysts for the urea oxidation [12], [13], [14]. Among them, obviously, the noble catalyst is not available for the large-scale application due to the high price and low amount; transition nickel-based electrocatalysts have been found to have a modest capability for urea oxidation and the earth-abundant property makes it very promising for real application [15], [16], [17]. For example, a NiF2/Ni2P hybrid showed high performance for the energy-relevant electrooxidation of urea due to the synergistic effect of co-existence of covalent and ionic bonds [18]. The catalytic ability can be further increased by the conductivity improvement by introducing conductive carbon into the Ni-based catalyst system and the phase structure tuning [14,15,19]. For example, NiO nanoparticles supported on graphene sheets annealed at lower temperature exhibited superior electrocatalytic activity for urea oxidation in alkaline solution [20]; meanwhile, Ni-decorated graphene sheets were reported as an effective and stable electrocatalyst for urea oxidation resulting from the synergistic effect of nickel and the high specific surface area of graphene sheets [21]. The influence of several conductive carbon support materials on the electrocatalytic activity of nickel oxide nanoparticles for urea electro-oxidation was comparatively studied and graphene/graphite was more efficient compared with the traditional carbon black and carbon nanotube supports [22]. Freeze-drying is considered as a novel and efficient approach for nanomaterials fabrication. It is different from ordinary thermal-drying, which has a freezing phase, primary drying and secondary drying process, that is separate, unique and interdependent. The basic principle is to “lock-in” the composition and structure of the material by drying and skipping the liquid phase entirely without applying the heat necessary for the evaporation process. It is a facile and simple approach for the advanced catalysts materials fabrication [23,24].
Hereby, it was tentatively employed to fabricate the nickel-based catalyst for urea oxidation. The precursors of nickel (II) acetate tetrahydrate were mixed with graphene completely and the freeze-dried powder was thermally annealed at different temperatures for the catalyst materials fabrication. The crystalline structure, morphology, and elemental composition of the obtained materials were probed by some physical spectroscopic techniques, and an easy phase structure tuning for the obtained hybrid phase of Ni/NiO was found by increasing the annealing temperature. The as-fabricated catalysts were investigated by a series of electrochemical approaches in the alkaline solution for urea oxidation. The annealing temperature was found to have a great influence on the physical and electrochemical properties of the synthesized Ni-based nanocomposites. More metallic Ni was formed in the Ni-NiO system by increasing the annealing temperature, and the sample obtained at 450 °C showed the best performance for urea oxidation resulting from the efficient Ni-NiO synergistic effect and conductivity improvement. Efficient kinetics was also discussed in terms of the Tafel slope, electrochemical impedance spectrum analysis and bipotential chronoamperometry for urea oxidation on the Ni-NiO system. The results might be helpful in understanding the Ni-based catalyst for urea oxidation in hydrogen production and wastewater treatment.
Section snippets
Results and discussion
In order to optimize the annealing temperature, the mass change of the freeze-dried powder of nickel (II) acetate tetrahydrate mixed with graphene was measured by the thermal gravimetric analyzer as a function of temperature; The details of the thermal stability and mass change of the freeze-dried materials were shown in Fig. 1a. The first weight loss of about 7% happened between 25 °C and 150 °C was due to the loss of physically adsorbed water of crystallization; the second rapid weight loss
Conclusion
In summary, a freeze-drying/annealing approach for Ni-precursors/graphene was reported for Ni-NiO catalyst fabrication and the annealing temperature was found significant to influence crystal structure and catalytic performance for urea oxidation. More metallic Ni was formed in the Ni-NiO system because of the NiO reduction by graphene with the increase of the annealing temperature, and the sample obtained at 450 °C exhibited the highest catalytic activity and stability for urea oxidation due
CRediT authorship contribution statement
Shuli Wang: Conceptualization, Methodology, Investigation, Validation, Writing - original draft. Peixuan Xu: Methodology, Investigation, Writing - original draft. Jingqi Tian: Supervision, Writing - original draft, Writing - review & editing. Zong Liu: Formal analysis, Writing - review & editing. Ligang Feng: Funding acquisition, Formal analysis, Supervision, Writing - review & editing.
Declaration of Competing Interest
There are no conflicts of interest to declare.
Acknowledgments
The work is supported by the National Natural Science Foundation of China (21972124, 21603041), the Priority Academic Program Development of Jiangsu Higher Education Institution and Top-notch Academic Programs Project of Jiangsu Higher Education Institutions. L Feng also appreciates the support of Six Talent Peaks Project of Jiangsu Province (XCL-070-2018).
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