Feasibility of quaternary ammonium and 1,4-diazabicyclo[2.2.2]octane-functionalized anion-exchange membranes for biohydrogen production in microbial electrolysis cells

Highlights

• A comparison of 3 anion exchange membranes in MEC for H₂ production is presented.

• The process was evaluated based on electrochemical and H₂ production performances.

• The impact of simple and complex substrates was assessed on MEC behavior.

• PSEBS CM DBC led to the best hydrogen yield and lower overpotential at cathode.

• Geobacter was the predominant genus on the anodes irrespective of membrane type.

Abstract

In this work, two commercialized anion-exchange membranes (AEMs), AMI-7001 and AF49R27, were applied in microbial electrolysis cells (MECs) and compared with a novel AEM (PSEBS CM DBC, functionalized with 1,4-diazabicyclo[2.2.2]octane) to produce biohydrogen. The evaluation regarding the effect of using different AEMs was carried out using simple (acetate) and complex (mixture of acetate, butyrate and propionate to mimic dark fermentation effluent) substrates. The MECs equipped with various AEMs were assessed based on their electrochemical efficiencies, H₂ generation capacities and the composition of anodic biofilm communities. pH imbalances, ionic losses and cathodic overpotentials were taken into consideration together with changes to substantial AEM properties (particularly ion-exchange capacity, ionic conductivity, area- and specific resistances) before and after AEMs were applied in the process to describe their potential impact on the behavior of MECs. It was concluded that the MECs which employed the PSEBS CM DBC membrane provided the highest H₂ yield and lowest internal losses compared to the two other separators. Therefore, it has the potential to improve MECs.

Introduction

In bioelectrochemical technologies, e.g. microbial fuel cells (MFCs) [1], [2], [3], microbial synthesis cells (MSC) [4], [5], microbial desalination cells (MDC) [6] and microbial electrohydrogenesis cells (MEC) [7], [8], the system architecture, in particular the type and properties of the membrane separator applied between the electrode chambers, can play a notable role in terms of process performance [9], [10], [11]. The membrane, as a physical barrier, contributes to the adequate separation of anodic and cathodic reactions while allowing the required passage of ionic species, e.g. H⁺ or OH^–, that maintain charge balancing and operation of the cell [12].

Researchers have shown, e.g. Harnisch and Schröder [13] and Sleutels et al. [14], that the transfer of H⁺ or OH^– across an ion-exchange membrane (IEM) may be suppressed due to competition with other ions, namely sodium, potassium and calcium, present in relatively higher concentrations in the electrolyte solutions. Besides, the transport of both cations and anions other than H⁺ or OH^– across a membrane can develop a pH gradient between the electrodes as well as unfavorable potential losses, which negatively affect the external energy demand of MECs needed to produce hydrogen gas [14]. To mitigate these side effects, a suitable IEM should be chosen. According to the findings by Sleutels et al. [14], MECs installed with AEMs may achieve higher operational efficiencies as a result of the more advantageous ratio of energy (voltage) input to membrane-associated energy losses. Experimental studies by Rozendal et al. [15], [16], Cheng and Logan [17] and Ye and Logan [18] also proposed the deployment of AEM rather than CEM in MECs to reduce the imbalance in pH across the membrane and enhance the process. For example, the volumetric productivity of an MEC unit that employed an AEM was 2.1 L_H2 L⁻¹ d⁻¹, more than 5 times higher than the MEC that employed a CEM which attributed to the lower (internal) ion transport resistance of AEM-MEC [19]. Besides, in our recent work, a bioelectrochemical system (BES) in an MFC configured with PSEBS CM DBC AEM (polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene functionalized with 1,4-diazabicyclo[2.2.2]octane) notably outperformed those that employed either Nafion or AN-VPA 60 CEM [20], indicating the potential of this membrane material to improve microbial electrochemical technology. However, the PSEBS CM DBC AEM has been tested only in MFC-type BESs, where the current densities are generally moderate or low. Hence, it may be worth elaborating on the viability of this separator in applications that apply higher current densities and products other than electricity. In this way, more relevant feedback may be obtained regarding the potential of PSEBS CM DBC AEMs in various BESs. Driven by this motivation, to take a step forward and continue this proposed line of research, a comparative evaluation regarding the H₂ production capacities and electrochemical behavior of MECs in which PSEBS CM DBC is applied was conducted with two commercialized AEMs, namely AMI-7001 and AF49R27 (MEGA, Czech Republic) as references. The comprehensive assessment of these MECs – fed either with a pure or mixed substrates (acetate vs. a mixture of volatile fatty acids (VFAs)) – was carried out by (i) evaluating the performance of the MEC (namely in terms of current density, H₂ production rate and yield, Coulombic efficiency and cathodic H₂ recovery), (ii) microbial community analysis of anodic biofilms and (iii) estimating pH-related as well as ionic voltage losses for the various AEMs. Moreover, all the membranes used were compared based on their operational stability. This is definitely a research gap as papers concerning changes to significant membrane properties before and after use in BESs are few and far between.

In accordance with the above, this work can provide new insights into the significance of membranes in MECs to produce H₂ with an increased degree of efficacy and enhance our understanding of the relationship between the behaviors of MECs and features of membranes.