Research Article pubs.acs.org/journal/ascecg
Cite This: ACS Sustainable Chem. Eng. 2018, 6, 2023−2036
NiCuCo2O4 Supported Ni−Cu Ion-Exchanged Mesoporous Zeolite Heteronano Architecture: An Efficient, Stable, and Economical Nonprecious Electrocatalyst for Methanol Oxidation Subhajyoti Samanta,† Kousik Bhunia,‡ Debabrata Pradhan,‡ Biswarup Satpati,⊥ and Rajendra Srivastava*,† †
Department of Chemistry, Indian Institute of Technology Ropar, Nangal Road, Rupnagar-140001, India Materials Science Centre, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal-721302, India ⊥ Surface Physics and Material Science Division, Saha Institute of Nuclear Physics, 1/AF, Bidhannagar, Kolkata 700 064, India
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S Supporting Information *
ABSTRACT: A unique strategy to obtain highly efficient electrocatalysts based on the spinel Ni0.5Cu0.5Co2O4 and mesoporous ZSM-5 (Zeolite Socony Mobil-5) is reported here for methanol oxidation in an alkaline medium. To develop an efficient catalyst, Ni2+−Cu2+ ion-exchanged mesoporous ZSM-5 and Ni0.5Cu0.5Co2O4 are prepared. 30 wt % of Ni0.5Cu0.5Co2O4 decorated Ni2+−Cu2+ ion-exchanged mesoporous ZSM-5 exhibits exceptionally higher catalytic activity with reasonably low onset potential than that of Ni0.5Cu0.5Co2O4, NiCo2O4, CuCo2O4, and Ni2+−Cu2+ ion-exchanged mesoporous ZSM-5. Electrochemical oxidation of HCHO and HCOOH also takes place over this catalyst at the same condition. The material exhibits high current density (21.4 mA/cm2) and stable electrocatalytic activity (with 99% retention) even after 1000 potential cycles. However, the benchmark catalyst Pt(20%)/C exhibits low activity (1.6 mA/cm2) and significant deactivation phenomenon (with 25% activity retention) at the identical condition. The cooperative contributions provided by Ni0.5Cu0.5Co2O4 and Ni2+−Cu2+ ion-exchanged mesoporous ZSM-5 support having Brönsted acidity, large external surface area, and intercrystalline mesoporosity deliver excellent electrocatalytic activity. The ease of synthesis, scale-up, stable and exceptional electrocatalytic activity paves the path toward the development of highly efficient alkaline direct methanol fuel cells. KEYWORDS: Methanol fuel cell, Spinel, Mesoporous zeolite, Noble metal free electrocatalyst, Multifunctional catalyst
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application of Pt based DMFC.8 During the methanol oxidation at Pt electrode, CO forms at the electrode surface and deactivates the Pt based catalysts. Noble metal−Pt based alloys have been developed to minimize the deactivation associated with conventional Pt catalysts.11−14 Though the use of noble metal−Pt alloys has undoubtedly curtailed this catalystdeactivation problem, their limited reserves and high costs still continue to restrict their sustainable commercial applications. Alkaline electrolyte based DMFC electrode systems are being developed since the last 10 years. Methanol oxidation is kinetically faster in the alkaline medium than in the acidic medium under electrochemical condition.15,16 The alkaline medium provides many advantages that include retardation in the catalyst poisoning, improved efficiency and stability of the catalyst, and an economical and sustainable choice of electrode materials based on Pd, Ni, Cu, Co, and other economical and
INTRODUCTION
To meet the energy demand and overcome the environmental pollution associated with the fossil fuels and biomass based fuels, efforts are in progress to develop eco-friendly and inexhaustible energy systems such as fuel cells, photoelectrochemical production of H2, Li-ion batteries, and so on.1−3 The use of H2 as a fuel is a major limitation associated with the highly efficient proton exchange membrane fuel cells because H2 is highly flammable and its storage and transportation is not safe.4 The use of methanol as an economical and easily available fuel is attractive and safer than hydrogen because the simple and sustainable storage system for methanol is already in practice.4 These facts have encouraged researchers to design and develop electrode materials and membranes for direct methanol fuel cells (DMFC).4−6 Activity of the anode catalyst defines the efficiency of DMFC. At the conventional electrode, methanol oxidation takes place at high over potential, which demands the development of an efficient anode catalyst.7−10 At present, commercial DMFC uses platinum (Pt) based anode catalyst.7 The high cost, a limited reserve of noble metals, and poor CO tolerance hinder the practical © 2018 American Chemical Society
Received: September 26, 2017 Revised: December 1, 2017 Published: January 2, 2018 2023
DOI: 10.1021/acssuschemeng.7b03444 ACS Sustainable Chem. Eng. 2018, 6, 2023−2036
ACS Sustainable Chemistry & Engineering
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easily available transition metals.17−20 To develop a highly efficient and economical catalyst for methanol electrooxidation, transition metal based systems need to be explored. Literature reports suggest that Ni and Cu based catalysts are good choices for this reaction.19−25 The electrocatalytic oxidation of methanol has been compared at NiCo2O4, NiO, and Co3O4 modified electrodes in alkaline medium at the identical condition.23 The methanol oxidation at the NiCo2O4 surface is mediated by the similar species (NiOOH and CoOOH) to that of NiO and CuO catalysts. With increase in the Ni content in Ni−Co hydroxides, response current corresponding to the methanol oxidation was increased.24 Further, to increase the performance of NiCo2O4, Ni(OH)2 was loaded on the external surface of NiCo2O4.25,26 These studies suggest that the incorporation of an optimum amount of nickel in the resultant catalytic system would be important to achieve the best result. Industrially robust zeolites are well explored for their applications as heterogeneous catalysts in energy sectors (especially petrochemical industry) and biomass conversion.27−31 Very recently, Ni/Cu-exchanged mesoporous ZSM5 (Zeolite Socony Mobil-5) and SnO2/CeO2 supported mesoporous ZSM-5 have been explored for the electrochemical oxidation of methanol.32−34 Methanol transportation rate, oxidized products, electrons at the electrode surface and an effective contact at the electrode/electrolyte interface facilitate the electrocatalytic activity of DMFC.35 Therefore, preparation of electrode materials with better textural properties and suitable conductivity is important for DMFC application. Very recently, WO3 based support materials have been reported for their applications in the development of fuel cell electrocatalysts.36,37 Mesoporous ZSM-5 zeolite has all those textural properties that are optimum for its DMFC application, except the required redox property. Applications of mesoporous zeolites in electrochemistry are being explored by our group.38−41 Besides the costly metal (Pd, Pt) based catalysts, systematic design and development of the economical transition metal (Co, Ni, and Cu) based catalysts would be a viable strategy for its practical applications. Integration of suitable redox active metal oxides and metal ion-exchanged Brö nsted acidic mesoporous zeolites could provide enhanced activity in the methanol electro-oxidation. In this study, a novel electrode material based on the nanocomposite of spinel (MCo2O4) and mesoporous ZSM-5 zeolite is reported for the DMFC application. The ion-exchange process was selected to obtain a suitable composition of Ni2+−Cu2+ ions in the mesoporous ZSM-5 (MesoZ). To enhance and fine-tune the electrocatalytic activity, spinels were supported on the external surface of the Ni2+−Cu2+ ion-exchanged MesoZ. In this study, NiCo2O4, CuCo2O4, and Ni0.5Cu0.5Co2O4 materials were prepared. Among these spinels, highly active Ni0.5Cu0.5Co2O4 with optimized amount was supported on the external surface of Ni2+−Cu2+ ion-exchanged MesoZ to develop highly efficient electrocatalyst for methanol oxidation. Further, the activity and stability of the developed catalyst (Ni0.5Cu0.5Co2O4(30%)/ Ni2Cu1-MesoZ) was compared with the benchmark (20%)Pt/ C catalyst under the identical condition to adjudge the efficiency of the presently investigated electrocatalyst. We believe that such a systematic and strategic study for the development of Ni0.5Cu0.5Co2O4 supported Ni2+−Cu2+ ionexchanged MesoZ is reported here for the first time.
Research Article
MATERIALS AND METHOD
Materials, procedure for the catalyst synthesis, details of characterization techniques, electrode fabrication, and electrochemical techniques are provided in the Supporting Information.
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RESULTS AND DISCUSSION As indicated in the Introduction, the content of Nickel plays an important role in the enhancement of the electrocatalytic methanol oxidation activity. Mesoporous ZSM-5 (MesoZ) is not only chosen for the efficient adsorption of methanol at the Brönsted acidic sites42 and enhanced methanol diffusion but also to incorporate active metal ions into the zeolite matrix.43 First, a range of Ni, Cu, and Ni−Cu ions exchanged MesoZ materials were prepared and investigated for electrocatalytic methanol oxidation. Among these materials, Ni2Cu1-MesoZ exhibited the best electrocatalytic activity. The details of electrocatalytic oxidation leading to the optimized support material are not included in this paper. Further, spinels with different compositions of Ni and Cu were prepared and investigated to achieve the highly active Ni0.5Cu0.5Co2O4 spinel for the methanol oxidation. A part of the electrocatalytic activities exhibited by these spinels are provided in this section. Finally, highly active Ni0.5Cu0.5Co2O4 was supported on the best support material Ni2Cu1-MesoZ with different compositions and the complete details of the electrocatalytic methanol oxidation with these heterojunction materials are provided in this study.
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PHYSICO-CHEMICAL CHARACTERIZATION Powder X-ray diffraction patterns were recorded to ascertian the framework structure of the material. Diffraction planes (designated in Figure 1a) recorded for NiCo2O4 belong to the cubic NiCo2O4 spinel structure (JCPDS Card no. 43-1003) (Figure 1a).23 Diffraction planes recorded for CuCo2O4 belong to the face centered cubic (FCC) lattices of CuCo2O4 (JCPDS Card no. 78-2177) (Figure 1a).41 Ni0.5Cu0.5Co2O4 exhibits reflections similar to that of NiCo2O4/CuCo2O4, which confirms that the material is crystallized in the FCC structure (Figure 1a). Diffraction planes (designated in Figure 1b) recorded for MesoZ belong to the MFI framework structure.40 XRD patterns of Ni2Cu1-MesoZ and MesoZ are similar, which confirms that the ion-exchange process did not alter the framework structure of the parent material. Diffraction peaks associated with Ni0.5Cu0.5Co2O4 and Ni2Cu1-MesoZ phases are observed in the XRD pattern recorded for Ni0.5Cu0.5Co2O4(30%)/Ni2Cu1-MesoZ (Figure 1b), which suggests that during the loading and heat treatment process, the individual characteristics of both the materials has remained same. The different weight percentage of Ni0.5Cu0.5Co2O4 was loaded on the surface of Ni2Cu1-MesoZ, and the resultant materials exhibit similar diffraction patterns to that of Ni0.5Cu0.5Co2O4(30%)/Ni2Cu1-MesoZ (Figure S1, Supporting Information). N2-sorption analysis exhibits a type IV isotherm and an H2 hysteresis loop for MesoZ (Figure 1c). A steady increase in the adsorption volume in the range of 0.05 to 0.4 (P/P0) is observed, which is followed by the gradual increase in the adsorbed volume in the range of 0.4 to 0.9 (P/P0). This gradual increase in the adsorption volume is due to the capillary condensation of N2 molecules in the intercrystalline mesopore void spaces. Mesopore size distribution, determined from BHJ analysis, is in the range of 2−10 nm for MesoZ. Isotherm and 2024
DOI: 10.1021/acssuschemeng.7b03444 ACS Sustainable Chem. Eng. 2018, 6, 2023−2036
Research Article
ACS Sustainable Chemistry & Engineering
that envelop the external surface of Ni2Cu1-MesoZ resulting in the formation of heterojunction nanocomposite material during the heat treatment process. Surface area (mainly external surface area) and porosity (mainly mesopore volume) determined from N2-sorption analysis are significantly reduced for Ni0.5Cu0.5Co2O4(30%)/Ni2Cu1-MesoZ after the loading of Ni0.5Cu0.5Co2O4 on the surface of Ni2Cu1-MesoZ. Table 1 shows that an increase in the content of Ni0.5Cu0.5Co2O4 over Ni2Cu1-MesoZ, leading to the reduction in their textural properties. The contents of Ni and Cu in the resulting nanocomposite materials influenced the electrocatalytic activity. Therefore, the contents of Ni and Cu were determined using microwave plasma atomic emission spectroscopy (MP-AES) for various materials investigated in this study (Table S2, Supporting Information). Elemental analysis shows that more amount of Ni2+ is ion-exchanged in MesoZ than Cu2+. Ion-exchange of zeolite depends on the exchange capacity as well as the nature of metal ions.44 Moreover, elemental analysis also shows that almost equivalent amounts of Ni and Cu are present in NiCo2O4/CuCo2O4 (Table S1, Supporting Information). A slightly higher amount of Ni is incorporated in Ni0.5Cu0.5Co2O4 when compared to Cu. Elemental analysis also confirms that the input and output quantities of Ni0.5Cu0.5Co2O4 loaded on Ni2Cu1-MesoZ in the nanocomposite materials are almost similar (Table S1, Supporting Information). The morphology of spinels, zeolite, and nanocomposite materials was determined by field emission scanning electron microscopy (FE-SEM). CuCo2O4 and NiCo2O4 exhibit nanorod-like morphology (length (∼1 μm) and diameter (
