List of Figures

Figure 1.1 Overview of the book's content. We start with the fundamentals of microbial electrochemical technologies (Chapter 2), electrodes (Chapter 3), and thermodynamics (Chapter 4) to provide basic background knowledge on these aspects. With these basics, we discuss static electrochemical methods (Chapter 5) and, from there, move on to electrode kinetics (Chapter 6) and dynamic electrochemical methods (Chapter 7). Finally, we apply this knowledge to the analysis of full electrochemical cells (Chapter 8). Figure 2.1 A two-chamber electrochemical cell composed of two electrochemical half-cells containing an external electric circuit (1); the electrons are taken from the oxidation reaction (2) at the anode (4) and are used for the reduction reaction (3) at the cathode (5). The anode chamber (6) and the cathode chamber (8) are separated by a membrane (7) through which cations (c+) and/or anions (a-) move to maintain electroneutrality. Figure 2.2 Examples of primary METs: (A) A microbial fuel cell (MFC), in which biological oxidation of acetate at the anode is coupled to the reduction of oxygen at the cathode and electricity is generated; (B) A microbial electrolysis cell (MEC), in which biological oxidation of acetate at the anode is coupled to the reduction of protons to hydrogen at the cathode, for which electric energy is required; and (C) microbial electrochemical synthesis (MES), in which oxidation of water at the anode is coupled to the biological reduction of CO2 to acetate that also requires electric energy. Figure 2.3 Direct and mediated extracellular electron transfer (here shown for microbial anodes) is explained in more detail in Box 2.1. Direct extracellular electron transfer (DEET) can happen via redox-active components located in the cell wall, like cytochromes or flavins (1), or via conductive pili or cytochrome networks (2). Mediated extracellular electron transfer (MEET) occurs via mediators in solution that can shuttle electrons between the microorganisms and the electrode (3). Figure 2.4 Electrochemically active microorganisms are widespread in the environment. Sources where these microorganisms are commonly found are, for example, wastewater treatment plants, animal guts, sediments, and deep sea chimneys. Figure 2.5 Redox towers representing the energy level (potential) of different reactions. The redox bar on the right shows the potential of the electrochemical reactions, expressed versus NHE (see Chapter 3) as reference electrode. At the anode, an oxidation reaction takes place and the oxidation of organic matter like glucose or acetate yields electrons at high energy level (high on the tower). Reduction reactions, in which electrons are taken up, also occur at different heights on the tower, depending on their energy level. Electrons (water) flow spontaneously when the oxidation reaction happens at higher energy level (higher in the tower) than the reduction reaction, e.g. when combining glucose oxidation with nitrate reduction. External energy is needed when the oxidation reaction happens at lower energy level than the reduction reaction, e.g. the combination of water oxidation at the anode with copper reduction at the cathode. Figure 2.6 Connection between an upper and a lower reservoir as analogy for the electric potential,. (A) Water in the higher reservoir contains more energy (potential energy) than water in the lower reservoir, and the pressure difference between the higher reservoir and lower reservoir is measured with a pressure meter. (B) The water represents electrons, with the electrons at higher potential (level) containing more energy than the electrons at lower potential (level). The potential difference between the electrons is measured with a voltmeter. Figure 2.7 Spontaneous reactions, like in a microbial fuel cell, are comparable to water flowing freely from a higher reservoir to a lower reservoir. Non-spontaneous reactions, like those in a microbial electrolysis cell, are comparable to water being pumped uphill and thus requiring electric energy to proceed. Figure 2.8 (A) Flat plate design with alternating anodes (red) and cathodes (blue); (B) tubular design where one electrode is situated in the middle, surrounded by the other electrode. Figure 2.9 Typical current profiles for a microbial fuel cell operated in batch mode (line 1), fed-batch mode (line 2), and continuous mode (3). Figure 2.10 Working principles of different types of membranes that allow certain ions or molecules to pass through, whereas other ions or molecules are blocked. AEM = anion-exchange membrane; CEM = cation exchange membrane; BPM = bipolar membrane, and porous membranes that select based on size. Figure 2.11 Classification of microbial electrochemical technologies, in which primary METs, which are the main focus of this book, rely on DEET and MEET between electroactive microorganisms and electrodes, and secondary METs use indirect interactions between electrodes and microorganisms. Figure 3.1 Donnan potential formation across a semipermeable membrane between phases I and II. X- is denoting a complex anion that cannot cross the membrane. Figure 3.2 Two important reference electrodes: (A) the silver/silver chloride electrode using 3?M KCl with notation Ag (s) | AgCl (s) | KCl (3?mol?l-1, aq) with: 1-silver wire coated with AgCl and immersed here in a 3?M KCl-solution, 2-voltmeter, 3-electrode of interest, 4-salt bridge/ capillary assuring the ionic contact, 5-diaphragm/ membrane/ frit. (B) The standard hydrogen electrode (SHE) Ptblack (s) | H3O+ (aH+ = 1, aq), H2 (1?bar, g) with 1-platinum wire facing a hydrogen saturated solution with 1 bar H2 and activity of H+ being 1, 2-voltmeter, 3-electrode of interest, 4-salt bridge/capillary assuring the ionic contact, 5-diaphragm/membrane/frit. Figure 3.3 The mean sea level is set as a baseline for the altitude of reservoirs, cities, and landmarks on the globe. The heights of reservoirs 1 and 2 can thus be compared to sea level. Similarly, electrochemical reactions can be expressed versus a reference electrode, which indicates a fixed potential. (Please note that in equilibrium no water is flowing.) Figure 4.1 Hydrogen fuel cell. The overall cell reaction is the conversion of hydrogen and oxygen into water, releasing electric energy. This overall cell reaction consists of two half-cell reactions: the oxidation of hydrogen to protons, releasing electrons at the anode, and the reduction reaction, in which electrons react with oxygen and protons to form water at the cathode. Electrons move through the electric circuit from anode to cathode and for electroneutrality the ionic charge, mainly protons, moves through the electrolyte solution from anode to cathode. Figure 4.2 Microbial electrolysis cell that uses acetate and electric energy to produce hydrogen. In the anodic half-cell reaction, electrons are released from the biological oxidation of acetate to HCO3-. Additional external inputs of electric energy drive the hydrogen evolution reaction (HER) at the cathode. Figure 4.Ex1 A microbial fuel cell that combines glycerol (C3H8O3) oxidation at the anode with oxygen (O2) reduction at the cathode. Figure 5.1 Simplified representation of a three-electrode arrangement in a double-chamber electrochemical cell with connection to the potentiostat (P). All electrodes need to be in ionic contact; here, the working electrode (WE) and counter electrode (CE) are separated via a membrane. The potential is set and measured between the WE and reference electrode (RE), and the current flows and is measured between the WE and CE. Figure 5.2 The capacitive current and the electrochemical double layer. (A) showing electrochemical double layer with inner Helmholtz plane (IHP) and outer Helmholtz plane (OHP) as well as the dipole structure of a water molecule; (B) the orientation of the water molecules is depending on the difference in electric potential between electron conductor and electrolyte solution - left: no preferred orientation of water molecules as both electric potentials are identical (point of zero charge), middle: preferred orientation of the O-atom of water to the electron conductor, as its potential is more positive than that of the electrode solution, and finally, right: the reversed situation to the one shown in the...