Elsevier

Electrochimica Acta

Volume 259, 1 January 2018, Pages 637-646
Electrochimica Acta

Lignin combined with polypyrrole as a renewable cathode material for H2O2 generation and its application in the electro-Fenton process for azo dye removal

https://doi.org/10.1016/j.electacta.2017.11.014Get rights and content

Highlights

  • Pyrrole was electropolymerized on the GF substrate with lignin as the dopant.

  • PPy/lig composites have abundant Cdouble bondO (including quinone-type) groups.

  • PPy/lig-GF cathode is electrocatalytically effective in oxygen reduction to H2O2.

  • Decolorization and mineralization of AO7 was achieved in the electro-Fenton system.

Abstract

The development of renewable, low-cost, and high-efficiency cathode materials for oxygen reduction to H2O2 is desirable for use in electro-Fenton systems for the oxidative treatment of wastewater. In the present study, we report the synthesis of a renewable biopolymer with lignin (lig) interpenetrated into a polypyrrole (PPy) framework via a simple one-step electropolymerization method. This allows the formation of PPy/lig composites that are uniformly coated on a graphite felt (GF) substrate, having large surface area, increased mechanical stability, and abundant Cdouble bondO (including quinone-type) groups. These features improved the electrocatalytic activity of the PPy/lig-GF cathode for oxygen reduction to H2O2 when an optimal cathodic potential (−0.5 V vs. SCE) was applied, as evidenced by the observed higher H2O2 electro-generation yield and efficiency compared to the raw GF and PPy/ClO4-GF cathodes. Electro-Fenton systems equipped with the GF cathodes (either modified or unmodified) for the decolorization and mineralization of AO7 (an azo dye) followed a pseudo-first-order kinetic model. The PPy/lig-GF cathode achieved substantially higher rate constants and degradation efficiencies. In addition, the incorporation of lignin significantly improved the cycling stability of the cathode material.

Introduction

Lignin is the second most abundant biopolymer after cellulose in the Earth's biosphere as it makes up about 25% of wood dry matter, making it a significant waste component of biomass in the pulp and paper industry [1]. The development of value-added applications for this lignin waste has attracted great attention for the past few decades. A recent promising technology is the incorporation of lignin into a conductive polymer for use as an electrode material with energy storage capability due to the presence of redox active groups (i.e., phenols and quinones) in the lignin [2], [3], [4], [5], [6]. This composite electrode material combines the individual merits of the conductive polymer with sufficient electronic and ionic conductivity to allow charge transfer and a high surface area with many active sites, and the lignin that supplies additional quinonoid groups for the necessary Faradic redox reactions. Motivated by these outstanding advantages, the goal of this work was to explore application of this material as an effective ORR (oxygen reduction reaction) electrocatalyst for generation of H2O2, a process with significant industrial uses. It was previously shown that conductive polymer-based electrodes (e.g., polypyrrole, polyaniline, and polythionphen) exhibited electrocatalytic activities allowing the electro-generation of H2O2 [7]. The two-electron reduction of oxygen to H2O2 can be further accelerated in the presence of quinonoid compounds within the polymer matrix [8]. The diverse mechanisms of oxygen reduction on the quinone-based catalyst have been elucidated, and the following reactions are dominant [9].where Q refers to the attached quinone species, Q•- is the semiquinone radical, and O2•- is the superoxide anion. Reaction (2) has been proposed as the rate-determining step.

One of the promising applications of electro-generated H2O2 is in-situ utilization as the Fenton reagent, known as the electro-Fenton process that produces highly reactive radical dotOH radicals for the treatment of wastewater that contains a variety of bio-refractory organic pollutants. The main reactions involved in electro-Fenton technology are described as follows.O2+2H++2eH2O2Fe3++eFe2+

It is apparent that in-situ production of H2O2 can address the challenges of the storage and shipment of concentrated H2O2 required for the traditional Fenton process [10]. H2O2 can be continuously synthesized from the two-electron electrochemical reduction of oxygen in acid media with E0 = 0.695 V (vs. SHE) or in alkaline solutions with E0 = −0.065 (vs. SHE) [11]. The state of the art of H2O2 electro-generation for the electro-Fenton process is documented in acid media [11] because the optimum pH for the Fenton reactions is in the range of 2.8–3.0, at which the catalytic behavior of the Fe3+/Fe2+ redox couple can be propagated. The yield and efficiency of H2O2 electro-generation is an important factor affecting the electro-Fenton process, which is in turn dependent on the type and properties of the cathode materials. The widely used cathodes for the electro-Fenton reaction are made of carbonaceous materials [9], [12], [13], [14] due to their characteristics such as high surface area, low cost and affordability to large-scale applications, but these materials might suffer the problem of relatively low electrode kinetics. Some cathode materials such as Boron-doped diamond (BDD) [15] and mercury [16] can offer favorable electrode kinetics, but might have drawbacks in terms of cost, safety and environmental friendliness. A previous report [17] found that conductive polymers that contained quinonoid compounds were effective in enhancing the electrocatalytic kinetics of ORR, which ultimately improved the efficiency of the electro-Fenton process for pollutant removal. For example, Zhang et al. examined the preparation of conductive polymers (e.g., polypyrrole and polyaniline) doped with quinonoid compounds (e.g., anthraquinone-2,6-disulfoate and anthraquinone-2-sulfonate) that delivered good ORR performance in terms of high H2O2 generation rate and high current efficiency [17], [18], [19].

Here, we make the first attempt to evaluate the ORR performance of polypyrrole/lignin (PPy/lig) composites for H2O2 electro-generation. We further examine the feasibility of using these prepared composites as the cathode material in an electro-Fenton system for azo dye degradation. Graphite felt (GF) was used as the substrate to support the formation of PPy/lig film. Acid Orange (AO7) was chosen as the model azo dye to evaluate the electro-Fenton system function, because it is a widely used synthetic dye that is resistant to biological decomposition and has been widely used to study electro-Fenton reactions [20], [21], [22], [23]. The kinetics of decolorization and mineralization of AO7 were evaluated and the electro-Fenton systems with raw GF and modified GF cathodes were compared.

Section snippets

Preparation of cathode materials

The starting raw graphite felt (GF, Sanye Co. Ltd, China) substrate of size 2.0 cm × 2.0 cm × 0.5 cm was cleaned in a hot H2O2 aqueous solution for 2 h, and then in deionized water at 90 °C for 1 h. The cleaning procedure was repeated twice, followed by thorough rinsing with deionized water that was performed three times and the material was then dried at 60 °C. A titanium wire of 0.6 mm in diameter was folded to string the felts and then twisted together to connect the GF

Characterizations on surface morphology and area

The incorporation of lignin significantly modified the morphology of the conducting PPy. Fig. 1 shows the SEM images of the raw and PPy-modified GF surfaces. When lignin was absent from the electrolyte, the resulting PPy/ClO4-GF exhibited a loose laminar structure in which parts of film were peeling off from the surface. In contrast, the presence of lignin promoted more firmly attached PPy film on the GF surface. This observation was in accordance with previous findings [24], [27], [28], [29]

Conclusions

We have demonstrated that a renewable biopolymer composed of polypyrrole and lignin can be an effective electrocatalyst for the reduction of oxygen to H2O2. The PPy/lig composites synthesized by electrooxidation were homogeneously and firmly coated on the GF substrate, resulting in a rougher surface with increased amounts of surface functional Cdouble bondO groups (including quinone-type groups). With a combination of these characteristics, the PPy/lig-GF cathode achieved higher performance in terms of H2O

Acknowledgements

We gratefully acknowledge financial support from the National Natural Science Foundation of China (nos. 51378216 and 21577041), the Natural Science Foundation of Guangdong Province, China (no. 2016A030311023), and the Fundamental Research Funds for the Central Universities, SCUT (No. 2017ZD066).

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