Insight into the electrocatalytic performance of in-situ fabricated electroactive biofilm-Pd: The role of biofilm thickness, initial Pd(II) concentration and the exposure time to Pd precursor

Highlights

• Key parameters used to indicate the performance of EAB-Pd are proposed.

• Optimum thickness of biofilm is essential to fabricate conductive Pd network.

• The ratio of biomass to Pd(II) concentration affected the morphology of EAB-Pd.

• The distribution of nano-Pd on EAB is crucial to determine the catalytic activity.

Abstract

Biogenic palladium (bio-Pd) nanoparticles have been considered as promising biocatalyst for energy generation and contaminants remediation in water and sediment. Recently, an electroactive biofilm-Pd (EAB-Pd) network, which can be used directly as electrocatalyst and show enhanced electrocatalytic performance, has exhibited tremendous application potential. However, the information regarding to the controllable biosynthetic process and corresponding catalytic properties is scarce. This study demonstrated that the catalytic performance of EAB-Pd could be influenced by Pd loading on bacteria cells (Pd/cells), which was crucial to determine the final distribution characteristic of Pd nanocrystal on EAB skeleton. For instance, the high Pd/cells (over 0.18 pg cell⁻¹) exhibited almost 6-fold and 1.5-fold enhancement over EAB-Pds with Pd/cells below 0.03 in catalytic current toward hydrogen evolution reaction and nitrobenzene reduction, respectively. In addition, the Pd/cells was found to be affected by the synthesis factors, such as the ratio of biomass to initial Pd(II) concentration (cells/Pd_II) and the exposure time of EAB to Pd(II) precursor solution. The Pd/cells increased significantly as the cell/Pd_II ratio decreased from ~5.5 × 10⁷ to ~1.3 × 10⁷ cells L mg⁻¹ or the prolongation of exposure time from 3 h to 24 h. The findings developed in this work extensively expand our knowledge for the in-situ designing biogenic electrocatalyst and provide important information for the development of its catalytic property.

Introduction

Palladium nanoparticles (Pd NPs) have been actively explored as catalysts in recent years due to its unique properties and applications. The notable examples include energy production (Frei et al., 2019; Shiva Kumar and Himabindu, 2020), hydrogenation/dehydrogenation reactions (M. Chen et al., 2018; Qin et al., 2020), drug delivery (Portney and Ozkan, 2006) and catalysts for environmental pollutants remediation (Rajan, 2011; Shi et al., 2019). Remarkably, biosynthesis of Pd NPs attracted more attention for its known economic sustainability, nontoxic and eco-friendly approach especially compared with conventional chemical and physical methods, which often use toxic chemicals under rigid conditions for reactions (Hulkoti and Taranath, 2014; Xiong et al., 2015). Previous studies have demonstrated that Pd NPs could be green-synthesized and recovered on the cell surface through bio-reduction in bacterial cultures, such as Shewanella oneidensis (Dundas et al., 2018; Hou et al., 2020; Ng et al., 2019), Geobacter sulfurreducens (Hernández-Eligio et al., 2020; Pat-Espadas et al., 2014), Escherichia coli (Priestley et al., 2015), Citrobacter freundii (Wang et al., 2020), referred as biogenic palladium (bio-Pd).

Bio-Pd is used effectively in catalysis Heck coupling reaction and several environmental contaminants, namely Cr(VI), azo dyes and organochlorines, degradation/transformation (Ajaz et al., 2019; Hosseinkhani et al., 2013; Martins et al., 2017; Quan et al., 2015; Suja et al., 2014; Zhou et al., 2019). However, the application of bio-Pd is often restricted by the poor conductive nature of microbial cells, when being immobilized on electrode carrier and used as electrocatalyst (Liu et al., 2016; Xiong et al., 2015). In view of that, cell carbonization is usually performed to increase the conductivity (Sun et al., 2012; Yates et al., 2014). But additional energy consumption would be required and those NPs tend to aggregate at high temperature, which would limit their catalytic activity (Jiang et al., 2009; Parker et al., 2014). Recently, graphene oxide coated bio-PdAu is proposed to avoid nanoparticles aggregation with the protection of graphene following a hydrothermal reaction (Liu et al., 2016). However, complicated separation/immobilization procedures and expensive binders are necessary to isolate bio-nanoparticles from the microbial incubation medium to fix on supporters. On the other hand, a novel procedure for Pd(0) in-situ synthesis by employing Geobacter sulfurreducens PCA formed electroactive biofilm (EAB-Pd) has been explored in our previous study, which allows the Pd synthesis, immobilization, and electrochemical application at one single electrode (Hou et al., 2016). More importantly, these cells supported Pd nanoparticles are stable and inter-connected. In the case of application especially in contaminants removal, the synthesized Pd NPs are expected to use together with the biofilm as electrocatalyst. An electric conductive pathway from the electrode to the biofilm surface is able to construct, which enable electrons to be delivered efficiently from electrode to the entire EAB-Pd skeleton and therefore avoid the further cell carbonization and chemical binders. Thus, the Pd NPs immobilization in the EAB based method is more cost-effective and environmentally friendly. Despite of the advancement, the knowledge of the specific synthesis principles underlying the EAB structure and operating conditions, which are essential to scale-up production of this kind of bio-catalyst with well-controlled performance, are still insufficient.

It is generally accepted that the catalytic activity of a catalyst depends markedly on the type of carriers, such as a limitation of diffusion can strongly influenced its activity (Mei et al., 2007). Especially for biofilm substrate, the diffusion resistance is positively correlated to the thickness of biofilm (Marsili et al., 2010; Sun et al., 2016). On the other hand, the morphology and size control of metals nanostructure are found to be critical for the catalytic activity. Through the manipulation of reduction kinetics may allow tailoring the electronic, and catalytic properties of a functional material (Chen et al., 2005; Y. Chen et al., 2018; Lim et al., 2009). For these reasons, this study aims to expend the understanding of the key factors related to in-situ synthesized EAB-Pd. The G. sulfurreducens biofilms at different stages of growth were investigated to provide information for controlling the scale of EAB structure and improving corresponding electrocatalytic activity by regulating the cultivation time of EAB. The effects of initial Pd(II) concentration and different exposure time of EAB to Pd(II) precursor on the performances of EAB-Pd were also studied. In particularly, this article summarized the parameters with respect to Pd/cells, which mainly indicated the characteristic and catalytic performance of EAB-Pd. The electrocatalytic performances of EAB-Pds were evaluated by cyclic voltammetry (CV) and chronoamperometry (CA) methods through hydrogen evolution reaction (HER) and electrocatalytic reduction of typical nitroaromatic contaminants (nitrobenzene, NB). This work also provides a more complete picture of the controllable in-situ synthesis of EAB-Pd and offers important information for extending directly application of other biosynthesized noble metal or alloy nanoparticles catalysts.