Microbial electrolysis cells turning to be versatile technology: Recent advances and future challenges

Highlights

• Microbial electrolysis cells turning to be versatile technology.

• An up-to-date review on recent application possibilities of MECs.

• A new platform for waste streams treatment and energy and resource recovery.

• Renewable and sustainable power sources are needed to make process cost-effective.

• Application oriented reactor design is necessary to lower upscaling costs.

Abstract

Microbial electrolysis cells (MECs) are an electricity-mediated microbial bioelectrochemical technology, which is originally developed for high-efficiency biological hydrogen production from waste streams. Compared to traditional biological technologies, MECs can overcome thermodynamic limitations and achieve high-yield hydrogen production from wide range of organic matters at relatively mild conditions. This approach greatly reduces the electric energy cost for hydrogen production in contrast to direct water electrolysis. In addition to hydrogen production, MECs may also support several energetically unfavorable biological/chemical reactions. This unique advantage of MECs has led to several alternative applications such as chemicals synthesis, recalcitrant pollutants removal, resources recovery, bioelectrochemical research platform and biosensors, which have greatly broaden the application scopes of MECs. MECs are becoming a versatile platform technology and offer a new solution for emerging environmental issues related to waste streams treatment and energy and resource recovery. Different from previous reviews that mainly focus on hydrogen production, this paper provides an up-to-date review of all the new applications of MECs and their resulting performance, current challenges and prospects of future.

Introduction

The use of fossil fuels in recent years has accelerated the depletion of non-renewable resources. Furthermore, the unprecedented increase in greenhouse gas emissions due to combustion of fossil fuels causes global warming and climate change. A sustainable and carbon-neutral energy source as alternatives to fossil fuels is highly needed to alleviate the global energy crisis and climate change. Bioenergy technologies which use renewable resources such as wastewater to produce biofuels or valuable chemicals will play a role.

Bioelectrochemical systems (BESs) as a young generation of bioenergy technology possesses a tremendous potential for simultaneous wastewater treatment and electric energy generation or valuable chemicals production (Chaudhuri and Lovley, 2003, Aelterman et al., 2006, Jacobson et al., 2011, Logan et al., 2006, Lovley and P.E, 1988, Zhang and Angelidaki, 2012a, Zhang and Angelidaki, 2013). There are two types of BESs according to the way of using electricity. One is known as microbial fuel cells (MFCs) which produce electricity from organic waste streams, while another is known as microbial electrolysis cells (MECs) which require electricity supply for hydrogen production from organic waste streams (Logan et al., 2006, Kundu et al., 2013). MFCs as one of typical BESs have attracted extensive attentions at the early stage of BESs research (Cheng et al., 2006, He and Mansfeld, 2009, Liu et al., 2005a, Logan, 2005, Rabaey et al., 2005, 88). While interesting, researchers are realizing that the economic and environmental value of electricity from MFCs cannot compete with that of other energy sources (e.g., biogas) at this stage. Therefore, a development has been recently initiated to broad the scope of MFCs for more value-added applications, such as hydrogen production by MECs (Fig. 1). The concept of MECs was proposed by two groups almost at the same period (Liu et al., 2005b, Rozendal et al., 2006). This technology was firstly nominated as “electrochemically assisted hydrogen generation”, then “biocatalyzed electrolysis”, “electrohydrogenesis”, and was finally accepted by researchers as “microbial electrolysis cells (MECs)” (Liu et al., 2005b, Cheng and Logan, 2007, Logan et al., 2008, Rozendal et al., 2007, Zhang and Angelidaki, 2012b). MECs have several advantages over other biological hydrogen production processes. Various organic matters such as cellulose, glucose, glycerol, acetic acid, sewage sludge and varied wastewaters can be converted to hydrogen in MECs (Liu et al., 2005b, Cheng and Logan, 2007, Logan et al., 2008, Pant et al., 2012). MECs can even convert the byproducts of dark fermentation (e.g., acetate) into hydrogen with high H₂ yields (e.g., 12 mol-H₂/mol-glucose in theory) (Liu et al., 2005b, Logan et al., 2008). Furthermore, MECs require relatively low energy input (0.2–0.8 V) compared to typical water electrolysis (>2.1 V).

Over the past decade, MECs as a promising platform for H₂ production and alternative applications have drawn much more attention in scientific communities, resulting in rapid advances in the field and extensive journal publications. Fig. 2A shows that the number of publications increased sharply and over 284 articles have been published until January 2013. Furthermore, researchers are distributed in different countries showing that MECs have attracted global attention (Fig. 2B). Similar to the development of MFCs, the research interest of MECs in the early stage lies in onefold direction i.e. H₂ production. Contrary to a mass of research papers, only a few review articles are available. In the first state of the art review on MECs, the reactor architectures, materials, system performance, energy efficiencies and challenges for hydrogen production were reviewed, which offers an insight to the later MECs research works (Logan et al., 2008). Geelhoed et al. (2010) further reviewed the foundation knowledge, technological design concepts and electron transfer mechanisms of electricity-driven hydrogen production. Lee et al. (2010) compared different biological hydrogen production technologies and highlighted the foundation, advantages and challenges of MECs for hydrogen production. More recently, Sleutels et al. (2012) briefly addressed the essential factors affecting the practical application of MECs from the economic point of view. Kundu et al. (2013) summarized the recent efforts on the development of cost-effective cathodes or cathode catalysts for hydrogen generation. These articles indeed provide overview of the MECs with different favor or emphasis. However, these reviews mainly focus on the function of MECs for hydrogen production. The emerging alternative applications of MECs for recalcitrant pollutants removal, resources recovery, chemicals synthesis, bioelectrochemical research platform and biosensor have not yet been reviewed. Therefore, this paper provides a comprehensive review of all the different application possibilities developed so far from the MECs platform. The scientific and technical challenges in the future with respect to different applications are also discussed.