Journal of Power Sources 191 (2009) 203–208 Contents lists available at ScienceDirect Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour Electrolyte effects on hydrogen evolution and solution resistance in microbial electrolysis cells Matthew D. Merrill, Bruce E. Logan ∗ Department of Civil and Environmental Engineering, Pennsylvania State University, 212 Sackett Building, University Park, PA 16802, USA article info Article history: Received 12 January 2009 Received in revised form 19 February 2009 Accepted 20 February 2009 Available online 10 March 2009 Keywords: Microbial electrolysis cell Hydrogen evolution reaction Weak acid catalysis Electrolyte kinetics effects Conductivity Solution resistance abstract Protonated weak acids commonly used in microbial electrolysis cell (MEC) solutions can affect the hydro- gen evolution reaction (HER) through weak acid catalysis, and by lowering solution resistance between the anode and the cathode. Weak acid catalysis of the HER with protonated phosphate, acetate, and car- bonate electrolyte species improved MEC performance by lowering the cathode’s overpotential by up to 0.30V at pH 5, compared to sodium chloride electrolytes. Deprotonation of weak acids into charged species at higher pHs improved MEC performance primarily by increasing the electrolyte’s conductivity and therefore decreasing the solution resistance between electrodes. The potential contributions from weak acid catalysis and solution resistance were compared to determine whether a reactor would operate more efficiently at lower pH because of the HER, or at higher pH because of solution resistance. Phosphate and acetate electrolytes allowed the MEC to operate more efficiently under more acidic conditions (pH 5). Carbonate electrolytes increased performance from pH 5 to 9 due to a relatively large increases in conduc- tivity. These results demonstrate that specific buffers can substantially contribute to MEC performance through both reduction in cathode overpotential and solution resistance. © 2009 Elsevier B.V. All rights reserved. 1. Introduction A microbial electrolysis cell (MEC) is a promising new approach for producing hydrogen gas from biodegradable organic matter using exoelectrogenic microbes, but the rates of hydrogen produc- tion need to be improved [1,2]. The use of high surface area anodes, such as carbon fiber brush electrodes, provides a large surface area for microbes, and thus the reactor performance is usually not lim- ited by the rate of oxidation of organic matter by the exoelectrogenic microbes [1,2]. Instead, the hydrogen evolution reaction (HER) on the cathode, and the solution resistance between electrodes, are primarily responsible for limitations in the MEC performance [1,2]. Microbes on the anode grow best under near-neutral pH conditions, and thus both electrodes in a membrane-less system are immersed in solutions at a pH near 7 that is maintained using a phosphate buffer [3], although carbonate buffers have been used in similar situations with microbial fuel cells [4]. The strength of the buffer affects the solution conductivity, and thus it has been well estab- lished that current densities can be increased through a decrease in solution resistance by increasing the concentration of the buffer or other electrolytes [5]. However, protonated weak acids in the solu- tion can also substantially alter the efficiency of the MEC by directly affecting the overpotential for the HER. The relative effects of dif- ∗ Corresponding author. Tel.: +1 814 863 7908; fax: +1 814 863 7304. E-mail address: blogan@psu.edu (B.E. Logan). ferent electrolytes on the HER in concert with solution conductivity have not been systematically explored in prior MEC studies. While strong acids or bases are used for water electrolyzers, it is also known that protonated weak acids can have a catalytic effect on hydrogen evolution [6]. Weak acids (HA) in greater activities than aqueous free protons (H + ) lower the hydrogen overpotential at lower current densities by donating protons to the HER through a weak acid catalytic effect [7–10]. It has been commonly reported that the rates of weak acid deprotonization and conjugate base re-protonization occur much faster than the rates of electron trans- fer [8–10,13,17–18] and so a pH gradient at the electrode surface due to the rates of proton consumption during catalysis would not be expected to occur. A limiting current density (J l ) is eventually reached where there is no increase in current despite the appli- cation of more negative potentials. The magnitude of the limiting current density has been correlated with the weak acid concen- tration [7–18], but prior work has focused almost exclusively on polished hemispherical Pt micro electrodes [7,8,10,11,13,14,17,18]. HER kinetics on larger electrodes are different than on polished hemispherical micro electrodes, where current densities can be ∼10× larger and transport properties can have unusual effects, such as the suppression of bubble formation [7]. There has been little analysis on how the current changes with respect to overpotential below this limiting current density [9,16] (such as the Tafel slope), or on the potential at which the limiting current occurs. This analysis of HER at current densities below the limiting density is impor- tant for practical application of MECs as these systems will operate 0378-7753/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jpowsour.2009.02.077