In situ facile fabrication of Ni(OH)2 nanosheet arrays for electrocatalytic co-production of formate and hydrogen from methanol in alkaline solution

https://doi.org/10.1016/j.apcatb.2020.119510Get rights and content

Highlights

  • Ni(OH)2 nanosheet arrays were formed on nickel foam spontaneously by a simple method.

  • Energy consumption of H2 production was reduced via methanol-water electrolysis.

  • The oxidation product of methanol on the Ni(OH)2/NF was formic acid instead of CO2.

  • The value-added formate and H2 could be co-produced efficiently.

Abstract

Ni(OH)2 nanosheet arrays are in situ prepared on Ni foam through very straightforward ultrasonication in HCl solution and the following rinsing and drying procedures. Value-added formate and hydrogen could be co-produced by selective oxidation of methanol without carbon dioxide emissions and water reduction over Ni(OH)2 nanosheet arrays bifunctional electrocatalysts. Energy depletion for hydrogen evolution from water could be reduced by integrating with selective methanol oxidation rather than oxygen evolution. The prepared Ni(OH)2 nanosheet arrays exhibits high activity for selective methanol oxidation in alkaline methanol-water solution. It gives a low potential of only 1.36 V (vs. reversible hydrogen electrode) to drive the current density of 100 mA cm−2. The faradaic efficiencies of formate are approximately 100% with the current densities from 10 mA cm−2 to 100 mA cm−2. Moreover, it is easy to separate the anodic and cathodic products without help of any membrane, which greatly simplifies the electrolysis system.

Introduction

The worldwide development and growth in population provoke a rapid increase in energy consumption. The depletion of non-renewable fossil energy results in detrimental environmental issues and global warming. Therefore, there is a pressing requirement to explore sustainable and carbon-neutral clean energy [[1], [2], [3]]. Hydrogen (H2), regarded as a promising renewable green energy due to its high energy capacity and environmental friendliness without carbon emission, plays an important role in solving global energy and environment crisis [[4], [5], [6]]. At present, the industrial production of H2 mainly originates from steam reforming of fossil fuels (methane or coal). As one of the most effective techniques, steam reforming not only requires high pressure and high temperature to achieve a relatively good catalytic performance, but also gives rise to the follow-up complex purification of H2 [[7], [8], [9]]. Therefore, developing clean and efficient technology for H2 generation is extremely important to meet the future global energy demands [10]. Electrocatalytic water splitting, an ideal way to produce high-purity H2 with non-pollution from clean and recyclable energy sources [11,12].

Conventional electrocatalytic water splitting involves two half-cell reactions of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) [13]. Unfortunately, a considerable overpotential is commonly required to produce H2 due to the sluggish kinetics of anodic OER. Although many marvelous electrocatalysts have been developed to achieve intriguing OER activities, OER still demands a much higher overpotential to match up with the rate of HER, leading to low overall energy conversion efficiency [12,14]. Moreover, the less valuable product of OER, O2 is generally responsible for the major energy loss of water electrolysis. In addition, the simultaneous production of H2 and O2 are likely to be explosive [15,16]. To avoid the high and unnecessary overpotential of OER, more readily oxidized molecules such as alcohols [[17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27]], hydrazine [[28], [29], [30]], urea [[31], [32], [33]], 5-hydroxymethylfurfural [[34], [35], [36]], and furfural [37] are employed for electrochemical oxidation reaction to replace OER to achieve energy-saving H2 production. Among these readily oxidized molecules, methanol (MeOH) as the simplest alcohol has low cost and huge production capacity [38,39].

Electrocatalysts play key roles in electrochemical methanol oxidation reaction (MOR). Pt-based materials are the most used electrocatalysts for MOR with high activity [26,27,40,41]. To reduce the cost of electrocatalysts, lots of efforts are also dedicated to develop non-noble metal electrodes such as nickel-based materials [20,[42], [43], [44]]. Over the electrocatalysts, MeOH is usually oxidized to valueless CO2 greenhouse gas in fuel cells to generate power or in electrochemical reformer to produce H2 [42,[45], [46], [47]]. On the other hand, MeOH is also an important C1 raw materials to produce chemicals, for example formic acid (HCOOH) or formate [48]. The price of HCOOH (about 539 € per ton) is much higher than that of MeOH (about 350 € per ton) [[49], [50], [51]]. Currently, HCOOH is industrially produced from MeOH under complex operation and harsh terms even using CO poisonous gas. Thus, it is significant to obtain valuable HCOOH or formate by selective electrochemical oxidation of MeOH. Furthermore, combination of this selective reaction with water electrolysis could not only reduce the voltage of water splitting and improve the efficiency of H2 evolution at the cathode, but also generate high value-added product at the anode to replace CO2. To realize the unique combined reactions, it is important to develop efficient electrocatalysts for selective MOR and HER [[20], [21], [22]]. The electrocatalysts are also expected with low cost and simple preparation method.

Herein, Ni(OH)2 nanosheet arrays are in situ grown on Ni foam (NF) via very simple HCl acid solution ultrasonic immersing and drying treatment. The as-prepared materials are used as bifunctional electrocatalysts for simultaneous HER and selective oxidation of MeOH to co-generate value-added H2 and formate in MeOH-water solution. A low cell voltage of only 1.52 V is required to realize a current density of 10 mA cm−2 over nickel-based nanosheet arrays bifunctional catalysts.

Section snippets

Chemicals and materials

All chemicals used in this study were in analytical reagent (AR) grade without any further purification. Deionized (DI) water with specific resistance of 18.2 MΩ⋅cm was used in all experiments. Chemicals of methanol and HCl were purchased from Xilong Company (China). Potassium hydroxide (KOH) was purchased from Macklin Company (China). Macroporous Ni foam substrate (80 ppi; mass density of 320 g⋅ m−2) was purchased from Liyuan New Materials Company (China).

Preparation of Ni(OH)2 nanosheet arrays

A facile method has been developed to

Morphology of Ni(OH)2/NF nanosheet arrays

Field-emission scanning electron microscopy (FESEM) images of untreated Ni foam (NF) in different magnifications (Fig. 1a and b) demonstrate that the surface of pristine NF is smooth without impurity or attachment. Compared with the pristine NF, a well-covered layer is observed on the surface of NF in the as-prepared material (Fig. 1c). Fig. 1d displays a nanostructure assembled by well-interconnected nanosheet arrays. These nanosheet arrays are stacked on top of each other with different

Conclusions

Ni(OH)2 nanosheet arrays are in situ formed on nickel foam by a very straightforward and economical method, which are applied as efficient bifunctional electrocatalysts for simultaneous HER and selective oxidation of MeOH. High current density can be generated in MeOH-water electrolyte by co-electrolysis with low voltage input. It requires only 1.52 V to drive the current density of 10 mA cm−2, which increases the energy conversion efficiency. The FE of H2 at the cathode is more than 92% even

CRediT authorship contribution statement

Jie Hao: Conceptualization, Methodology, Validation, Formal analysis, Writing - original draft, Project administration. Jianwen Liu: Software. Dan Wu: Writing - review & editing. Mingxing Chen: Software. Yue Liang: Investigation. Qi Wang: Investigation. Lei Wang: Resources. Xian-Zhu Fu: Resources, Writing - review & editing, Visualization, Funding acquisition. Jing-Li Luo: Resources, Supervision.

Declaration of Competing Interest

The authors report no declarations of interest.

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China (No.21975163) and Shenzhen Science and Technology Program (No. KQTD20190929173914967). The authors sincerely acknowledge the Instrumental Analysis Center of Shenzhen University (Xili Campus) for HRTEM measurements and analysis, and thank Yang Chengyu from Shiyanjia Lab (www.shiyanjia.com) for the XPS experiments.

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