The use and optimization of stainless steel mesh cathodes in microbial electrolysis cells
Introduction
Microbial electrolysis cells (MECs) provide a new high-yield approach for hydrogen generation from various organic substrates, such as wastewaters and other biomass. In an MEC, bacteria on the anode oxidize the organic matter and convert energy, available in a biodegradable substrate, into current. By adding a small electrical input (a minimum of 0.14 V compared to 1.23 V needed for water electrolysis) [1], [2], hydrogen can be evolved on the cathode under anoxic conditions, usually with the help of a catalyst.
While many advancements in MEC performance have been made, developing a cost-effective, scalable design is the most critical challenge for the MEC to become a commercialized hydrogen production technology [3]. Rozendal et al. [4] determined that the cathode (including catalyst) could account for the greatest percentage (47%) of the total capital costs for MECs. A precious metal such as platinum (Pt) on the cathode has been used in most studies to catalyze hydrogen evolution [3], [5], [6], [7]. The disadvantages of using platinum include its high cost and poisoning by chemicals such as sulfide (a common constituent of wastewater) [3]. Several researchers have investigated new catalysts such as cobalt and iron cobalt tetramethylphenylporphyrin (CoTMPP & FeCoTMPP) [8], nickel oxide [9], stainless steel (SS) [9], [10], [11], and tungsten carbide [12]. Among these non-Pt catalysts, Ni-based alloys have demonstrated a promising electrocatalytic activity for the hydrogen evolution reaction (HER) in water electrolysis [13], [14]. Hu et al. [15] developed NiW and NiMo catalysts by electrodepositing Ni alloys onto carbon cloth. NiMo produced hydrogen at slightly lower rates that were comparable to Pt in MECs. SS 304 with a high Ni content is cheaper and a commercially available alternative to Ni alloy. Olivares-Ramirez et al. [16] and de Souza et al. [17] reported good catalytic activation of SS 304 in an alkaline solution. In neutral pH conditions, conditions typical of MECs, Call et al. [10] obtained the highest hydrogen production rate of 1.7 m3H2/m3d and overall energy efficiency of 78% using high surface area SS 304 brushes (650 m2/m3 of reactor volume, 0.5 cm electrode spacing) at an applied voltage of 0.6 V. This value was comparable to that obtained with a platinum-catalyzed flat carbon cloth cathode [10], indicating that expensive precious metals are not needed. However, bubble entrapment and a potentially complex construction of an MEC with brush cathodes could limit the application of this approach. In addition, careful design of the system is needed to avoid short circuiting of the SS bristles as the brush cathode must be placed in close proximity to the anode to minimize internal resistance and maximize electrode packing per volume of reactor.
In this study, we examined the use of SS mesh as alternative cathodes to flat carbon cathodes with Pt. SS mesh is flat like the carbon electrodes, allowing closer electrode spacing of the cathode to the anode, but the mesh can produce higher surface areas than a flat sheets of this metal. Mesh are characterized in terms of mesh number (number of lines of mesh per inch), with different wire diameters and pore sizes. While we expect that hydrogen evolution rates would be enhanced by surface area, bubble evolution can also be affected by pore and wire size. We therefore used linear sweep voltammetry to evaluate current densities of different mesh, and observed bubble characteristics such as coverage at different applied voltages. We also measured electrochemically active surface areas using ferrocyanide, and compared these surface areas to those estimated from mesh geometry. The mesh that had the best performance in electrochemical tests was then evaluated in MECs in terms of current densities and hydrogen production rates and recoveries.
Section snippets
Cathodes
SS 304 (0.08%C, 2%Mn, 1%Si, 18-20%Cr, and 8–11%Ni [18]) woven (McMaster-Carr, IL) and expanded mesh (Dexmet Corporation, CT) were evaluated for their suitability as cathodes in MECs (Fig. 1). Mesh were cut into 3.8 cm diameter discs (projected cross sectional area of 7 cm2). Twelve different sized mesh (Table 1), a flat sheet of SS 304, and laboratory-made carbon cloth (type B, E-TEK; Pt, 0.5 mg/cm2) were examined as cathodes. Mesh characteristics are summarized in terms of wire diameter, pore
Evaluation of mesh type
The minimum voltage needed to initiate substantial current (Ve) was similar for all mesh, with Ve = −0.67 ± 0.01 V. For comparison, the Pt/CC cathode required a potential of −0.38 V for current production (Fig. 2; see Table S2). Based on the mesh cathode overpotential obtained from LSVs, a minimum of 0.42 V would be required for hydrogen production in an MEC with an SS 304 mesh catalyst (assumes an anode potential of −0.25 V) [2].
Woven mesh was more effective in increasing current than expanded
Discussion
Mesh #60, which had the densest wire packing and largest active surface area among three mesh tested in MECs (66 m2/m3) and the lowest overpotential (−0.63 V) in LSV tests, achieved the highest area current densities based on projected 7 cm2 cathode area in MECs (12.03 A/m2 at 1.2 V, 8.08 A/m2 at 0.9 V and 3.85 A/m2 at 0.6 V) and volumetric current densities normalized by 30 cm3 reactor volume (281 A/m3 at 1.2 V, 188 A/m3 at 0.9 V and 90 A/m3 at 0.6 V). Very small mesh pores were not as
Conclusions
Stainless steel mesh, as a more scalable and low-cost cathode was first examined in MECs. The results obtained demonstrated that SS woven mesh performed better than expanded mesh as a catalyst for hydrogen evolution. The largest active surface area measured by CV reached specific area of 78 m2/m3 (SS mesh #120), which is three times the active surface area of a flat sheet. An optimum size range existed for different current ranges and bubble coverages. Mesh wire diameter was the dominant factor
Acknowledgements
This study was supported by the King Abdullah University of Science and Technology (KAUST) (Award KUS-I1-003-13).
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