Enhanced electrochemical performance in microbial fuel cell with carbon nanotube/NiCoAl-layered double hydroxide nanosheets as air-cathode

https://doi.org/10.1016/j.ijhydene.2021.08.168Get rights and content

Highlights

  • In-situ growth of NiCoAl-LDH nanosheets on CNT surface by hydrothermal method.

  • CNT/NiCoAl-LDH catalyst has amazing stability and durability in the MFC cathode.

  • Maximum power density produced by CNT/NiCoAl-LDH-MFC was 433.5 ± 14.8 mW/m2.

  • Fully exposed active sites and high conductivity improve MFC performance.

Abstract

The ternary component NiCoAl-layered double hydroxide (NiCoAl-LDH) and carbon nanotube (CNT) nano-composite (CNT/NiCoAl-LDH) were successfully prepared by a simple hydrothermal method. The NiCoAl-LDH nanosheets were effectively and uniformly grown on CNTs, forming a cross-linked conductive network structure, and stainless steel (SS) mesh was used as the base to load CNT/NiCoAl-LDH for microbial fuel cell (MFC) cathode. X-ray diffraction (XRD) results presented that the CNT/NiCoAl-LDH hybrid exhibited the (003), (006), (012), (015), (018), (110) and (113) crystal planes of hydrotalcite reflection. The surface functional groups C-O, C=O, C-H, C-N and M-O of the hybrid were confirmed. The cross-linked network structure of the hybrid was observed and the content and proportion of each element of the hybrid were found. CNT/NiCoAl-LDH showed excellent catalytic oxygen reduction reaction (ORR) ability by cyclic voltammetry (CV) and linear voltammetry (LSV) due to its abundant electrochemical active sites and excellent conductivity. The maximum output voltage of CNT/NiCoAl-LDH catalyst as MFC cathode was 450 mV, the maximum power density was 433.5 ± 14.8 mW/m2, and the maximum voltage stabilization time was 7–8 d. The results indicated that the CNT/NiCoAl-LDH hybrid was full potential as a high-performance, low-cost MFC cathode catalyst in future.

Introduction

Microbial fuel cell (MFC) could directly convert the chemical energy of organic matter into electrical energy by using the catalysis of microorganisms [1,2]. It was a new type of energy conversion device that integrates electricity generation and environmental protection [3,4]. Because of its advantages, MFC had attracted more and more attention from many researchers in the past decade to achieve promising sustainable energy [[5], [6], [7], [8]]. However, MFC was subject to many restrictions in practical applications due to its high material cost and small-scale low power. Thus, finding a catalyst that could enhance the oxygen reduction reaction (ORR) on the cathode side of the MFC was a crucial task [[9], [10], [11], [12], [13]].

Au and Pt-based catalysts had high electrochemically active and redox properties, but their high cost hindered their large-scale industrial application [14,15]. Some cost-effective alternatives were accessible, such as conductive polymers [16], non-platinum metal catalysts [17,18], transition metal oxide nanoparticles, transition metal sulfides [19], transition metal phthalocyanines and porphyrins [20], transition metal oxides [21,22] or macrocyclic compounds and carbon nanotubes were some examples of cheaper ORR catalysts [23]. Among them, MFC cathode materials of nanosheet layered double hydroxide (LDH) without conductive agents and binders have been proven to have huge advantages in terms of enhanced electrochemical performance [[24], [25], [26], [27]]. The electrode without any additives could more effectively to promote the transfer of charges and protons and simplify the electrode preparation process [28]. In addition, compared with traditional flake materials, two-dimensional nano-flake transition metal compounds had a larger aspect ratio, abundant crystal plates and electrochemically active sites, which help to shorten the transmission distance of ions and further raise the material utilization rate [29,30]. In previous studies, the core-shell nano-materials formed by doping NiFe-LDH in Fe3O4 and Co3O4 were used as the cathode catalyst of MFC, which could improve high power generation capacity and stability [24,25]. Furthermore, it was found that, compared with any binary component NiAl-LDH, NiFe-LDH or CoAl-LDH, the ternary component NiCoAl-LDH containing two transition metals (Co and Ni) showed higher electrochemical performance [[31], [32], [33]]; compared with single metal hydroxides (such as Ni(OH)2), the ORR of NiCo-LDH was mainly due to the introduction of another transition metal basic ion to increase electrochemically active sites [34]. In this regard, the dispersion of LDH materials was one of the key issues that need attention. Hence, various porous substrates such as carbon nanotubes, foamed nickel, carbon fibers, graphene foams, etc. were used as frameworks or scaffolds to disperse nanosheet-like transition metal compounds [[35], [36], [37], [38]] and avoid self-aggregation [[39], [40], [41], [42]], which would be useful to generate additional active centers and ultimately improve electrochemical performance. On the other hand, enhancing the conductivity of LDH was also one of the key issues to be considered [[43], [44], [45], [46]]. Therefore, it was necessary to design excellent composites composed of doped conductive materials and LDH [47,48]. Carbon nanotubes (CNT) had very high conductivity and high specific surface area, which could provide good electron transfer paths and affluent electrochemical active sites [49]. So far, NiCo-LDH/CNT nano-composites has been synthesized by the chemical water bath precipitation method with more electrochemically active sites, on account of outstanding conductivity of CNT, the electrochemical activity and cycle stability have been lifted. Besides, by directly mixing CNT with CoAl-LDH nanosheets, the specific capacitance and long-life performance of CoAl-LDH could be enhanced. In short, CNT could not only avoide the self-aggregation of LDH, but also efficiently boost the conductivity of LDH.

In this study, CNT as the active substrate, a binder-free hydrothermal method was used to grow NiCoAl-LDH in situ on the CNT surface. The CNT/NiCoAl-LDH catalyst was supported on the stainless steel mesh (SS) as the cathode substrate, and the CNT/NiCoAl-LDH catalyst was used as the cathode of the MFC. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Scanning electron microscope (SEM) and Energy spectrum analysis (EDS) were used to characterize the structure and morphology of the material. Cyclic voltammetry (CV) and linear voltammetry (LSV) were employed to evaluate the electrocatalytic activity of the CNT/NiCoAl-LDH. The power generation capacity of the CNT/NiCoAl-LDH -MFC in the power density curve, polarization curve and voltage-time curve were tested. The results would provide some evidences that the CNT/NiCoAl-LDH catalyst was of vital importance in enhancing the electrochemical performance of MFC cathode.

Section snippets

Fabrication of the CNT/NiCoAl-LDH

The in-situ growth of NiCoAl-LDH nanosheets on the CNT surface by a simple and efficient hydrothermal method [50,51]. The preparation process of the CNT/NiCoAl-LDH composite was shown in Fig. 1. Using CNT as the active substrate, the NiCoAl-LDH was grown in-situ on the surface of CNT by a binder-free hydrothermal method. First, 0.0713 g NiCl2•6H2O and 0.1665 g of CoCl2•6H2O (Ni:Co molar ratio = 3:7) were dispersed in 60 mL of deionized water, stirred for 20 min, and then 0.3800 g of CO(NH2)2

Physicochemical properties of the CNT/NiCoAl-LDH

The CNT/NiCoAl-LDH was prepared through the above operations (Fig. 1), XRD and FT-IR were used to analyze the structure of the CNT/NiCoAl-LDH, and SEM and EDS were used to analyze the morphology of the CNT/NiCoAl-LDH. The typical XRD pattern of CNT, NiCoAl-LDH and CNT/NiCoAl-LDH was shown in Fig. 2a. The diffraction pattern of NiCoAl-LDH and CNT/NiCoAl-LDH represented a classic hydrotalcite-like structure, exhibiting reflections of (003), (006), (012), (015), (018), (110) and (113) crystal face

Conclusions

The CNT/NiCoAl-LDH composite material as the cathode of MFC showed excellent ORR activity and electrochemical performance. The highly conductive cross-linked network structure of CNT/NiCoAl-LDH fully exposed the electrochemical active center, had a higher charge transfer rate, and improved oxygen reduction catalytic activity and power generation capacity. The maximum output voltage of CNT/NiCoAl-LDH was 450 mV, and the maximum power density was 433.5 ± 14.8 mW/m2. Also, the MFC cathode modified

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.

Acknowledgement

The authors were very grateful for the financial support provided by the National Natural Science Foundation of China (NO. 31901188), Shandong Provincial Natural Science Foundation (ZR2020QC048 and ZR2019BB040), China Postdoctoral Science Foundation (2021M691850), NSFC (NO. 31971503), Shandong Provincial Agricultural Fine Species Project (2019LZGC020), and Experimental Teaching Reform Research Project of Qufu Normal University (SJG201921).

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