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- Sprache: Englisch
Basic Electrochemistry for Biotechnology
Understand the basics of a thriving interdisciplinary research field
Microbial electrochemistry is a subfield of bioelectrochemistry which concerns interactions between microbial organisms and electrically active surfaces such as electrodes. Its growth as a subject of research has been rapid in recent years, and its technological applications are many, particularly as the race to find sustainable organic energy sources accelerates.
Basic Electrochemistry for Biotechnology offers an accessible overview of this interdisciplinary subject and its potential applications. Moving smoothly from the general to the specific, it offers both fundamental principles and some of the most relevant specific examples, such as biofilm electrodes, microbial fuel cells or microbial electrosynthesis cells, making it the ideal choice for building a working knowledge of this exciting new field. Its solid foundation of microbial electrochemical technologies also serves as a starting point for a wide range of applied research areas.
Basic Electrochemistry for Biotechnology readers will also find:
- Carefully designed artistic illustrations
- Hands-on exercises throughout to facilitate entry into laboratory work
- Numerous illustrative examples and calculations designed to demonstrate and reinforce key principles
Basic Electrochemistry for Biotechnology is the perfect point of entry into this growing field for both students and researchers.
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Seitenzahl: 454
Veröffentlichungsjahr: 2023
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Table of Contents
Cover
Table of Contents
Title Page
Copyright
List of Figures
List of Boxes
Preface
Reference
1 A Reader's Guide to Basic Electrochemistry for Biotechnology
2 A Basic Introduction to Microbial Electrochemical Technologies
2.1 Introduction to Microbial Energy Conversion and Microbial Electrochemical Technologies
2.2 Electroactive Microorganisms and Mechanisms of Extracellular Electron Transfer
2.3 Energetics: The Redox Tower and a Water Analogy
2.4 Wastewater Characteristics
2.5 Microbial Electrochemical Technologies: Systems and Design
2.6 Short Alert on Terminology
Questions
References
Note
3 Electrochemical Potential, Electrode Potential, and the Need for Reference Electrodes
3.1 Introduction to Electrochemical Potentials
3.2 Electrodes and Electrode Reactions
3.3 The Relative Electrode Potential and the Need for Reference Electrodes
Questions
References
Notes
4 Reaction Equations and Thermodynamics of Electrochemical Reactions
4.1 Introduction to Oxidation and Reduction Reactions and Thermodynamic Limits
4.2 How to Write and Balance Reaction Equations of (Bio)electrochemical Reactions
4.3 Thermodynamics of Electrochemical Conversions
Questions
References
Notes
5 Static Electrochemical Methods
5.1 Introduction to Static Electrochemical Methods
5.2 What Is a Three‐Electrode Arrangement, a Potentiostat or Power Supply, and for What Are They Needed?
5.3 The Electrochemical Double Layer and Capacitive Current
5.4 Potentiometry, Amperometry, Coulometry, and Constant Current Measurements
5.5 Chronoamperometry
Questions
References
Notes
6 Electrochemical Kinetics
6.1 Introduction to Electrochemical Kinetics
6.2 Basics of Electrochemical Kinetics
6.3 Electrochemical Reversibility
6.4 Overpotentials
6.5 The Overpotential Due to Mass Transfer
6.6 Potential‐Current Plots and Electrode Kinetics
6.7 The Butler–Volmer Equation
6.8 Tafel Equation and Tafel Plots
6.9 Electrocatalysis
Questions
References
Notes
7 Dynamic Electrochemical Methods
7.1 Introduction to Electrochemical Methods with Changing Electrode Potential
7.2 Voltammetry
7.3 Performing Dynamic Electrochemical Methods Using Potentiostats: Discriminating Capacitive and Faradaic Current
7.4 Cyclic Voltammetry
7.5 Redox‐Active Components in Microorganisms
7.6 Acquisition of Polarization and Power Curves Using Stepwise Chronoamperometry and Chronopotentiometry
7.7 Acquisition of Polarization Curves Using External Resistance
7.8 Electrochemical Impedance Spectroscopy
Questions
References
Notes
8 Electrochemical Analysis of Reactors
8.1 Introduction to Characterization of Microbial Electrochemical Cells
8.2 Mass and Electron Balances and Efficiency of Conversions
8.3 Polarization and Power Curves: Analysis of Measured Data
8.4 Internal Resistance and Potential Losses
8.5 Energy Efficiency and Voltage Efficiency
8.6 Ionic Current and Transport Numbers
Questions
References
Notes
9 Seizing the Beauty and Acknowledging the Complexity of Basic Electrochemistry for Biotechnology
Note
Appendix A: Abbreviations
Appendix B: Symbols with Definition and Unit
Appendix C: Solutions to Exercises
Chapter 3
Chapter 4
Chapter 6
Chapter 7
Chapter 8
Appendix D: Tabulated Values
Index
End User License Agreement
List of Tables
Chapter 2
Table 2.1 Overview of important wastewater characteristics, divided into ph...
Table 2.2 COD values and number of electrons exchanged for some compounds t...
Table 2.3 Selection of important parameters to analyze in METs during opera...
Chapter 3
Table 3.Ex1 Details for Exercise 3.1 for calculating the Donnan potential.
Table 3.1 Basic types of electrodes with explanation and examples.
Table 3.2 Overview of selected common reference electrodes and their relativ...
Chapter 4
Table 4.1 Gibbs free energy of formation of components in the MEC reaction....
Table 4.Ex1 Gibbs free energy of formation of components in the glycerol fed...
Table 4.2 Gibbs free energy of formation and enthalpy of formation of compo...
Table 4.Ex2 Gibbs free energy of formation and enthalpy of formation of comp...
Chapter 5
Table 5.1 Overview of static electrochemical methods as discussed in this c...
Chapter 7
Table 7.1 Overview of dynamic methods discussed in this chapter.
Table 7.2 Experimental parameters being set and parameters that can be extr...
Table 7.3 Selected examples of redox‐active compounds and their formal pote...
Table 7.4 Example of steps used in chronoamperometry to record a polarisati...
Table 7.Ex1 Data obtained during chronoamperometry measurements for a bioca...
Chapter 8
Table 8.Ex1 Experimental data obtained from an MFC at different external loa...
Appendix A
Table A.1
Appendix B
Table B.1 Symbols used in
Basic Electrochemistry for Biotechnology
.
Appendix C
Table 4.Ex1 Gibbs free energy of formation of components in the glycerol‐fed...
Table 4.Ex2 Gibbs free energy of formation and enthalpy of formation of comp...
Table 7.Ex1 Data obtained during chronoamperometry measurements for a bioca...
Table 8.Ex1 Experimental data obtained from an MFC at different external lo...
Table 8.Ex1-sol Experimental data for the recording of a polarization curve...
Appendix D
Table D.1
Δ
f
G
θ
values of common reactants and products in microbia...
Table D.2 Selection of typical reactions in microbial electrochemical system...
List of Illustrations
Chapter 1
Figure 1.1 Overview of the book's content. We start with the fundamentals of...
Chapter 2
Figure 2.1 A two‐chamber electrochemical cell composed of two electrochemica...
Figure 2.2 Examples of primary METs: (A) A microbial fuel cell (MFC), in whi...
Figure 2.3 Direct and mediated extracellular electron transfer (here shown f...
Figure 2.4 Electrochemically active microorganisms are widespread in the env...
Figure 2.5 Redox towers representing the energy level (potential) of differe...
Figure 2.6 Connection between an upper and a lower reservoir as analogy for ...
Figure 2.7 Spontaneous reactions, like in a microbial fuel cell, are compara...
Figure 2.8 (A) Flat plate design with alternating anodes (red) and cathodes ...
Figure 2.9 Typical current profiles for a microbial fuel cell operated in ba...
Figure 2.10 Working principles of different types of membranes that allow ce...
Figure 2.11 Classification of microbial electrochemical technologies, in whi...
Chapter 3
Figure 3.1 Donnan potential formation across a semipermeable membrane betwee...
Figure 3.2 Two important reference electrodes: (A) the silver/silver chlorid...
Figure 3.3 The mean sea level is set as a baseline for the altitude of reser...
Chapter 4
Figure 4.1 Hydrogen fuel cell. The overall cell reaction is the conversion o...
Figure 4.2 Microbial electrolysis cell that uses acetate and electric energy...
Figure 4.Ex1 A microbial fuel cell that combines glycerol (C
3
H
8
O
3
) oxidation...
Chapter 5
Figure 5.1 Simplified representation of a three‐electrode arrangement in a d...
Figure 5.2 The capacitive current and the electrochemical double layer. (A) ...
Figure 5.3 Overview of the measurement setup and readouts of (A) chronopoten...
Figure 5.4 (A) Chronoamperogram shows the current as function of time using ...
Figure 5.5 Schematic chronomamperogram of
Geobacter sulfurreducens
grown on ...
Chapter 6
Figure 6.1 Schematic illustration of the change of electrode potential as fu...
Figure 6.2 Illustration of the concentration profiles of (A) Fe
3+
and (B...
Figure 6.3 Sketch of a potential vs. current density plot in the electrochem...
Figure 6.4 Tafel plot showing log|
j
| vs.
η
. The black lines show the me...
Figure 6.Ex1 Tafel plots for the hydrogen evolution reaction using either pl...
Figure 6.5 Schematic pathway of the HER: (I) A proton is reaching the electr...
Figure 6.6 Simplified Tafel plot for the hydrogen evolution reaction on diff...
Figure 6.7 Simplified reaction cascade within a hydrogenase catalyzing React...
Figure 6.8 (A) Tafel plot of a hydrogenase for the hydrogen evolution reacti...
Chapter 7
Figure 7.1 Effect of data sampling during voltammetric measurements in commo...
Figure 7.2 Basic principles of linear sweep voltammetry on the example react...
Figure 7.3 Linear sweep voltammograms showing the current as function of pot...
Figure 7.4 Basic principle of cyclic voltammetry on the example reaction of ...
Figure 7.5 Basic parameters that can be extracted from cyclic voltammetry fo...
Figure 7.6 Illustration of the effect of the mode of mass transfer on the cy...
Figure 7.7 Illustration of the effect irreversibility of in the given condit...
Figure 7.8 Illustration of the effect of coupling an (bio)chemical reaction ...
Figure 7.9 Illustration of ratio of capacitive and Faradic current on the cy...
Figure 7.10 Illustration of cyclic voltammogramms of
Geobacter sulfurreducen
...
Figure 7.Ex1 Cyclic voltammogramms for a nitrate reducing biocathode in (A) ...
Figure 7.11 Schematic representation of the redox cascade in the photosystem...
Figure 7.12 (A) Example of a chronoamperometry measurement in which the anod...
Figure 7.13 The time step selected for each chronoamperometry step (A) will ...
Figure 7.Ex2 Chronoamperometry measurements of potential as function of time...
Figure 7.14 (A) Recording of a polarization curve, where the system is first...
Figure 7.15 During potentiostatic EIS, an alternating voltage is applied at ...
Figure 7.16 (A) Nyquist plot of a basic, ideal system, from which the ohmic ...
Chapter 8
Figure 8.Ex1 Schematic of a MEC where the oxidation of water to oxygen is co...
Figure 8.1 (A) Polarization and power curve for a (microbial) fuel cell with...
Figure 8.2 An electrochemical cell presented as an electric circuit.
E
cell
i...
Figure 8.3 Polarization curve that shows the cell voltage and anode and cath...
Figure 8.4 Schematic overview of the internal resistances in the (microbial)...
Figure 8.5 Energy efficiency as a function of voltage efficiency and Coulomb...
Chapter 9
Figure 9.1 Illustration of some of the fascinating topics in microbial elect...
Appendix C
Figure 4.Ex1 A microbial fuel cell that combines glycerol (C
3
H
8
O
3
) oxidation...
Figure 6.Ex1 Tafel plots for the hydrogen evolution reaction using either pl...
Figure 7.Ex1 Cyclic voltammogramms for a nitrate reducing biocathode in (A) ...
Figure 7.Ex1-sol The formal potential of the more positive EET‐site lies aro...
Figure 7.Ex2 Potential of a biocathode that reduces HCO
3
−
to acetate a...
Figure 7.Ex2-sol Polarization curve with current density versus cathode pote...
Figure 8.Ex1 Schematic of a microbial electrolysis cell (MEC) where the oxid...
Figure 8.Ex2a-sol The polarization curve for the MFC was obtained by plottin...
Figure 8.Ex2b-sol Power curve for the MFC, obtained by plotting the power de...
Guide
Cover
Table of Contents
Title Page
Copyright
List of Figures
List of Boxes
Preface
Begin Reading
Appendix A: Abbreviations
Appendix B: Symbols with Definition and Unit
Appendix C: Solutions to Exercises
Appendix D: Tabulated Values
Index
End User License Agreement
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Basic Electrochemistry for Biotechnology
Falk HarnischTom SleutelsAnnemiek ter Heijne
Authors
Dr. Falk HarnischHelmholtz Centre for Environmental Research GmbH ‐ UFZDepartment of Environmental MicrobiologyLeipzigGermany
and
Leipzig UniversityInstitute for BiochemistryLeipzigGermany
Dr. Tom SleutelsWetsus, European Centre of Excellence for Sustainable Water TechnologyLeeuwardenThe Netherlands
and
University of GroningenFaculty of Science and EngineeringGroningenThe Netherlands
Dr. Annemiek ter HeijneWageningen University & ResearchEnvironmental TechnologyWageningenThe Netherlands
Cover Image: Courtesy of Rikke Linssen
All books published by WILEY‐VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.
Library of Congress Card No.: applied for
British Library Cataloguing‐in‐Publication DataA catalogue record for this book is available from the British Library.
Bibliographic information published by the Deutsche NationalbibliothekThe Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at <http://dnb.d-nb.de>.
© 2024 WILEY‐VCH GmbH, Boschstraße 12, 69469 Weinheim, Germany
All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law.
Print ISBN: 978‐3‐527‐34808‐4ePDF ISBN: 978‐3‐527‐84404‐3ePub ISBN: 978‐3‐527‐84405‐0
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 CO
2
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) Pt
black
(s) | H
3
O
+
(
a
H+
= 1
, aq), H
2
(1 bar, g) with 1‐platinum wire facing a hydrogen saturated solution with 1 bar H
2
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 HCO
3
−
. 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 (C
3
H
8
O
3
) oxidation at the anode with oxygen (O
2
) 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 middle.
Figure 5.3
Overview of the measurement setup and readouts of (A) chronopotentiometry, in which the current is set at zero (open circuit) and the potential is measured as function of time, (B) chronoamperometry, in which the potential of the WE is set against the RE and the resulting current is measured as function of time, and (C) chronocoulometry, in which the total charge is measured as function of time.
Figure 5.4
(A) Chronoamperogram shows the current as function of time using a three‐electrode arrangement. Thereby, the capacitive current decreases much faster than the Faradaic current. (B) Oxidation of immobilized Fe
2+
to Fe
3+
using chronoamperometry. The current decreases to zero after all surface bound Fe
2+
is oxidized and the integral provides the charge (
q
), which can be related to the amount of Fe
2+
being oxidized via Faraday's law (see
Chapter 4
). (C) Chronoamperometric oxidation of dissolved Fe
2+
to Fe
3+
with different ratios of electrode surface area (A) at constant volume of the electrochemical cell or half‐cell (
V
).
Figure 5.5
Schematic chronomamperogram of
Geobacter sulfurreducens
grown on a flat graphite electrode in batch mode in laboratory reactors. The formation of the anode biofilm can be separated into several phases that are shown with the respective current profiles: (A) bare electrode with no current flow, (B) start of electrode colonization with low current, (C) formation of an electroactive biofilm with higher current that, after several batches, is turned into (D) a mature biofilm showing reproducible performance at maximum current. The arrows indicate the moments when acetate is injected.
Figure 6.1
Schematic illustration of the change of electrode potential as function of current density for an anodic electrochemical half‐cell. The most dominant regions of polarization are depicted together with the respectively occurring overpotentials (
Eq. 6.6
and
Eq. 6.7
): (A) open circuit, (B) low current density, (C) medium current density, and (D) high current density.
Figure 6.2
Illustration of the concentration profiles of (A) Fe
3+
and (B) Fe
2+
at an electrode surface during chronoamperometry at a potential that leads to reduction of Fe
3+
with increasing time (see also Figure 5.4C). At
t
= 0, Fe
3+
concentration is maximum, and while current is flowing, the concentration of Fe
3+
close to the electrode will go down until distance
δ
DL
where the bulk concentration is constant. For Fe
2+
, the concentration is zero at
t
= 0 and, as Fe
3+
is reduced to Fe
2+
with time, the concentration in the bulk will slightly increase.
Figure 6.3
Sketch of a potential vs. current density plot in the electrochemically reversible and irreversible cases. For instance, plots A and B show an identical reaction and identical electrochemical half‐cell, meaning the
E
eq
is the same, for different rates of change of the applied potential of the working electrode,
E
WE
. (This change in
E
WE
is for instance performed in voltammetry (see
Chapter 7
)). A slow potential change is leading to an electrochemically reversible system, which is shown in plot A, meaning that a small potential change leads to a big change in current density. When investigating the identical electrochemical half‐cell but with a fast potential change it becomes an irreversible system. This is shown in plot B, where the same change in
E
WE
as used for plot A leads to lower change in current. Further, both plots could depict the same reaction and identical experimental conditions, especially using the same speed for changing the potential for different half‐cells, for instance, using different electrode materials. Again, as the reaction is the same, the
E
eq
is identical. Yet, the material used in plot A would lead to a reversible half‐cell, whereas the one in plot B to an irreversible half‐cell.
Figure 6.4
Tafel plot showing log|
j
| vs.
η
. The black lines show the measured data, and in blue the determination of the exchange current density,
j
0
, and the slopes that allow the determination of the charge transfer coefficient
α
are indicated.
Figure 6.Ex1
Tafel plots for the hydrogen evolution reaction using either platinum or carbon electrodes at acidic conditions or platinum at pH neutral conditions.
Figure 6.5
Schematic pathway of the HER: (I) A proton is reaching the electrode surface, is adsorbed, and reduced to a hydrogen radical (1) that is bound to the electrode surface (2) both combined steps are the so‐called Volmer reaction; (II) (A) Two hydrogen radicals at the electrode surface combine to a H
2
(so‐called Tafel reaction), or alternatively (B) One hydrogen radical at the surface reacts with a proton and an electron to form H
2
(Heyrovsky reaction); (III) The formed H
2
desorbs from the electrode surface.
Figure 6.6
Simplified Tafel plot for the hydrogen evolution reaction on different electrode materials. Please note that the traces represent data from real measurements from different studies, illustrating the challenge in interpretation of Tafel plot data adapted from (see (Vetter 1992)).
Figure 6.7
Simplified reaction cascade within a hydrogenase catalyzing Reaction 6.2: In the reactions site (I) Reaction 6.3 takes place, whereas in the electron transfer site (II) Reaction 6.4 proceeds. Between (I) and (II) intramolecular electron transfer takes place.
Figure 6.8
(A) Tafel plot of a hydrogenase for the hydrogen evolution reaction (HER) and the hydrogen oxidation reaction (HOR) in the presence and absence of H
2
; (B) Tafel plots of four different hydrogenases that differ in their rate constants and suitability to catalyze the hydrogen evolution reaction and the hydrogen oxidation reaction (details see text).
Figure 7.1
Effect of data sampling during voltammetric measurements in common digital potentiostats: (A) generation of the potential ramp (in blue) and the resulting current (orange) for analog devices yielding a truly linear signal excitation (blue dashed line) and linear current (orange dashed line) as well as a common staircase signal by a digital potentiostat (blue solid line) yielding a stepwise potential increase that results in a stepwise response of the current (orange solid line). (B) zoom‐in of a current response for a digital staircase change of potential showing current response of one step of the staircase: (1) total current, (2) capacitive current when the potential is changed in staircase mode, (3) Faradaic currents (
i
=
f
(
E
WE
)
are different during signal acquisition at time intervals a and b. When time interval a is applied for data acquisition, the ratio of
i
c
“masks”
i
F
significantly more than when time interval b is applied for the staircase potential change. For the linear potential change no masking of
i
F
by
i
c
occurs.
Figure 7.2
Basic principles of linear sweep voltammetry on the example reaction of the oxidation of Fe
2+
to Fe
3+
: (A) change of potential of the working electrode,
E
WE
, in the potential window from
E
start
to
E
end
at time the formal potential of the oxidation, , is reached, (B) recording of current with time showing that at time the formal potential of the oxidation, , is reached that results in a Faradaic current due to the oxidation of Fe
2+
to Fe
3+
, after reaching a peak, the current declines due to mass transfer limitation (shown in black) or, in case the reaction is not limited by mass transfer, forms a plateau (in blue) (see also
Figure 7.3
).
Figure 7.3
Linear sweep voltammograms showing the current as function of potential for the electrochemical oxidation of Fe
2+
to Fe
3+
during different modes of mass transfer: (1) convective mass transfer with high availability of Fe
2+
that determines the overall rate of oxidation and hence the current; (2) convective mass transfer in which the reaction is more limited by availability of Fe
2+
than by electrochemical kinetics, i.e. in comparison to (1)
η
mass transfer
is limiting the current; (3) and (4) diffusive mass transfer with both showing a
η
mass transfer
with (3) having a higher electrochemical reaction rate than rate of mass transfer compared to (4).
Figure 7.4
Basic principle of cyclic voltammetry on the example reaction of the electrochemically reversible oxidation and reduction of Fe
2+
/Fe
3+
for diffusive mass transfer only: (A) change of potential of the working electrode with time,
E
WE
, in the potential window from
E
start
to
E
rev
and back to
E
start
. The formal potential of the oxidation reaction, , is reached two times (at ) during the scan; (B) corresponding current response to the change of potential shown in (A) with time; (C) shows the combined signal:
i
as function of
E
WE
, combining the data from (A) and (C).
Figure 7.5
Basic parameters that can be extracted from cyclic voltammetry for a reversible electrode reaction and diffusive mass transfer (see
Table 7.2
for details).
Figure 7.6
Illustration of the effect of the mode of mass transfer on the cyclic voltammogram of a fully reversible redox couple with lines (2) and (3) having convective mass transfer in place and oxidized and reduced forms in dissolved solution, whereas the concentration for line (2) is higher than for line (3); for line (1) diffusive mass transfer governs the CV‐signal; and for line (4) the redox couple is only present bound to the electrode surface (and not in solution).
Figure 7.7
Illustration of the effect irreversibility of in the given conditions when diffusive mass transfer governs the CV‐signal with line (1) showing a fully reversible electrode reaction, line (2) showing irreversibility of the oxidation reaction and full irreversibility of the reduction reaction, and line (3) showing full irreversibility of the oxidation reaction and the reduction reaction proceeding at lower rate.
Figure 7.8
Illustration of the effect of coupling an (bio)chemical reaction to an electrode reaction: Line (1) diffusive mass transfer governs the CV‐signal of a fully reversible electrode reaction that is faster than a coupled (bio)chemical reaction if this is in place. Lines (2) and (3), the chemical or biochemical reduction that leads to rereduction of the electrochemically oxidized species is faster than the electrochemical reduction, the difference between lines 1 and 2 in shape and maximum current can be due to, e.g. different scan rates, concentrations of the chemical oxidant, etc.
Figure 7.9
Illustration of ratio of capacitive and Faradic current on the cyclic voltammogram: Line (1) diffusive mass transfer governs the CV signal of a fully reversible electrode reaction for which oxidation and reduction peaks can be well distinguished. When a (slightly) higher capacitive current flows, the CV signal is less visible as the total current increases, as shown in line (2). When the capacitive current prevails, as shown in line (3), the oxidation and reduction signals cannot be observed. In case where no redox‐active compound is present, only a capacitive current is observed, as shown in line (4).
Figure 7.10
Illustration of cyclic voltammogramms of
Geobacter sulfurreducens
biofilms grown on a flat graphite electrode in batch mode in laboratory reactors (see also
Figure 5.4
): (A) CV with high scan rate of 50 mV s
−1
(black line) and low scan rate of 1 mV s
−1
(blue line) in non‐turnover conditions, (B) more detailed view of the CV with low scan rate of 1 mV s
−1
in non‐turnover conditions and assignment of the formal potentials of the four identified redox species being possible extracellular electron transfer sites, (C) CV with low scan rate in turn‐over conditions, (D) first derivative of the voltammogramm shown in (C) with assignment of the actual extracellular electron transfer sites.
Figure 7.Ex1
Cyclic voltammogramms for a nitrate reducing biocathode in (A) blank electrode (blue) as well as biofilm‐covered electrode (black) in the absence of nitrogen species (i.e. neither nitrate nor nitrite) and (B) blank electrode (blue) in the presence of nitrate as well as a biofilm‐covered electrode in the presence of nitrate (black) or nitrate and nitrite (red) as well as only nitrite (orange) (C) cyclic voltammograms and the respective first derivative (inset, D) of a microbial anode oxidizing glucose. CVs of the used working electrode/biofilm (black), blank electrode with resuspended microbial cells (orange), and only supernatant (blue).
Figure 7.11
Schematic representation of the redox cascade in the photosystem. Shown are the steps involving the redox couple Fe
2+
/Fe
3+
.
Figure 7.12
(A) Example of a chronoamperometry measurement in which the anode potential is changed stepwise from −0.2 to 0.2 V vs. NHE. (B) Polarization curve generated from the chronoamperometry measurement by averaging the last few datapoints for current at each potential. The blue line (1) represents a situation where a maximum current is reached; the orange line (2) represents a situation where the current is decreasing beyond a certain anode potential.
Figure 7.13
The time step selected for each chronoamperometry step (A) will influence the shape of the polarization curve (B), since the current may reach a different value. After 300 seconds of equilibration time (blue line), the current is higher than after 1200 seconds of equilibration time (orange line), thus leading to a difference in the polarization curve.
Figure 7.Ex2
Chronoamperometry measurements of potential as function of time after a stepwise increase in current applied to the biocathode that reduces CO
2
to methane.
Figure 7.14
(A) Recording of a polarization curve, where the system is first operated at OCP, after which the external load is changed stepwise from high values to low values or even to zero (short circuit, SC) and then increased back to higher values. (B) The development of cell voltage as function of time, in response to the change in external load. The cell voltage decreases from open circuit potential stepwise down to (almost) zero, and back to higher potential. (C) The development of current density as function of time, in response to the change in external load. The current density is zero at open circuit, and then increases as the external load decreases. Eventually, current decreases again to zero (at open circuit).
Figure 7.15
During potentiostatic EIS, an alternating voltage is applied at different frequencies. The phase shift between the applied voltage and the resulting current yields information on the dominant limiting processes, and this phase shift is different for different frequencies.
Figure 7.16
(A) Nyquist plot of a basic, ideal system, from which the ohmic resistance, total resistance, and charge transfer resistance can be determined. To obtain these values, the data can be fitted to an equivalent circuit. (B) Equivalent Randles circuit (RC) with an ohmic resistance, in series with a parallel capacitance and charge transfer resistance.
Figure 8.1
(A) Polarization and power curve for a (microbial) fuel cell with constant internal resistance. The black line represents the cell voltage; the orange line is the power density. Key performance parameters equilibrium potential (
E
eq
)
, open circuit potential (
E
OCV
), short circuit current (SCC) and maximum power point (MPP) are indicated. Internal resistance is reflected in the slope of the
E
cell
‐
j
curve, with unit Ω m
2
. (B) Polarization and power curve (with power as power density, in W m
−2
) for the same electrochemical system, but internal resistance changes as a function of the current density. The result is a nonsymmetric power curve and is more realistic and of practical relevance. (C) Polarization and power curve for a (microbial) electrolysis cell. The equilibrium potential,
E
eq
, represents the (minimum) thermodynamic energy input, whereas the onset potential (
E
onset
) represents the potential at which current starts flowing. The maximum current density (
j
max
) represents the current where a further increase in potential does not result in an increase in current.
Figure 8.2
An electrochemical cell presented as an electric circuit.
E
cell
is the driving force for current to flow, and the internal
R
int
and external resistance
R
ext
are in series. External resistance represents the load, through which the electrons flow (and where power is generated), whereas internal resistance represents the total internal resistance in the electrochemical system, consisting of different resistances (see
Section 8.4
).
Figure 8.3
Polarization curve that shows the cell voltage and anode and cathode potential as a function of the current density, assuming linear behavior. Cell voltage is defined as cathode potential minus anode potential. With increasing current, the cell voltage of the MFC decreases, until the system reaches a negative cell voltage. The system is now an MEC, and the (applied) voltage is required to drive the reactions.
Figure 8.4
Schematic overview of the internal resistances in the (microbial) electrochemical cell. Note that
R
Ω
is the sum of all ohmic losses that can occur at the contact between wires, current collectors, and electrodes, and in the solution of anolyte and catholyte.
Figure 8.Ex1
Schematic of a MEC where the oxidation of water to oxygen is coupled to the reduction of bicarbonate (HCO
3
−
) to methane.
Figure 8.5
Energy efficiency as a function of voltage efficiency and Coulombic efficiency.
Figure 9.1
Illustration of some of the fascinating topics in microbial electrochemistry and technology you may start to explore further with the help of the knowledge gained from
Basic Electrochemistry for Biotechnology
.
Figure 7.Ex1-sol
The formal potential of the more positive EET‐site lies around −0.22 V.
Figure 7.Ex2-sol
Potential of a biocathode that reduces HCO
3
−
to acetate as a function of time. Second
y
‐axis shows the applied current during stepwise galvanostatic operation.
Figure 8.Ex2a-sol
The polarization curve for the MFC was obtained by plotting the cell voltage as a function of the current density. The arrow to the right shows the first half of the experimental data, and the arrow to the left shows the second half of the experimental data.
Figure 8.Ex2b-sol
Power curve for the MFC, obtained by plotting the power density as a function of the current density. The arrow to the right shows the first half of the experimental data, and the arrow to the left shows the second half of the experimental data.
List of Boxes
Box 2.1
Energy harvest of electroactive microorganisms
Box 2.2
Normalization and standardization
Box 3.1
Charge can be transported by electrons and ions
Box 3.2
Examples for calculating the Donnan potential
Box 3.3
Examples of electrochemical cells and electrochemical half‐cells
Box 3.4
Conversion of the logarithm of the Nernst equation
Box 4.1
Microbial fuel cell running on glycerol
Box 4.2
Energetics of the glycerol fuel cell at standard conditions (see Figure 4.Ex1)
Box 5.1
Capacitance
Box 5.2
Capacitance or pseudocapacitance
Box 5.3
The measuring principle of pH‐electrodes
Box 5.4
The measuring principle of redox electrodes
Box 5.5
Handling of reference electrodes
Box 5.6
Background current of microbial anodes
Box 5.7
From current to current density
Box 6.1
Rate constants of electrochemical reactions
Box 6.2
What to call the electrode potential for “real” conditions: formal potential
Box 6.3
Side reactions and internal currents
Box 6.4
Catalysis in electrochemistry
Box 6.5
Mechanisms of electrode reactions
Box 6.6
Conversion of natural logarithm to decimal logarithm and back
Box 7.1
Selecting an appropriate scan rate
Box 7.2
Kinetic analysis
Box 7.3
Signal recoding by potentiostats during voltammetry
Box 7.4
The actual metabolic energy gain of electroactive microorganisms
Box 7.5
The need for assuring electrochemical equilibrium in CV and other dynamic methods
Box 8.1
The ohmic drop
Box 8.3
Anaerobic digestion for biogas production
Preface
Research and engineering in both disciplines, microbiology and electrochemistry, that form the foundation of microbial electrochemistry and microbial electrochemical technology look back on a long very successful history and a bright future. Without doubt, technologies like anaerobic digestion and fermentation on the one hand, and hydrogen fuel cells and synthesis of chemicals by electrolysis on the other, will be core elements for a resource‐secure and biobased circular economy. At the same time, processes based solely on microbiological or electrochemical transformations of energy and matter possess inherent constraints that might not be overcome with advancements in the individual fields. These constraints comprise, for instance, the fact that (abiotic) electrochemical reactions involving an increasing number of electrons are limited in selectivity and yield or that biological catalysts cannot exploit electric energy (without a further transformation step). These constraints are promised to be overcome by the interfacing of microbiology and electrochemistry. The ideas, concepts, and research based on microbial electrochemistry as well as the derived prospective technologies are now summarized under one roof (Harnisch and Urban 2018). Below this roof gather pupils, students, and researchers that have received or are receiving training and education from very different disciplines like civil engineering and molecular biology. This diversity in scientific background certainly leads to the flourishing of the entire field. At the same time, however, the knowledge and expertise in the disciplines in which one received no or only partial training are naturally limited. For gathering this knowledge, a learning‐by‐doing approach can be used, but this might be often not the easiest and most straightforward way. Alternatively, scholarly textbooks and tutorial reviews can be excellent sources.
When it comes to fundamentals in electrochemistry in the realm of microbial electrochemistry and microbial electrochemical technologies (METs), we see a crucial lack of a comprehensive and easily accessible source on basic electrochemical knowledge. The existing electrochemical textbooks usually start from the very physical fundamentals and are written for students and researchers, addressing the entire field of electrochemistry with an almost exclusive focus on electrochemical reactions and systems for abiotic catalysts. Further, the focus is often on a detailed mathematical treatise that may set a hurdle to acquiring electrochemical knowledge for students and researchers from other fields than (electro)chemistry and that, at the current stage, cannot be made for METs. From our personal perspective, this leads to an often unnecessarily very limited insight into electrochemical fundamentals and the use of electrochemical methods for researchers in the field of METs, including their potential and limits for data analysis and interpretation.
It is the aim of this textbook to change this. Basic Electrochemistry for Biotechnology will provide an entry to the needed basics in electrochemistry for students and researchers in the interdisciplinary field of microbial electrochemistry and METs. Thereby, the book will cover the needed physical‐chemical fundamentals but limit these to the level that is needed for covering the complexity of the field and mastering the electrochemical basic methods. Thereby, we strived to make it accessible to readers from different disciplines. This did require a simplification of complex phenomena for which we point the interested reader to more specialized sources. For fostering the knowledge gain, we included examples and example calculations to provide easily understandable and applicable information.
We hope that Basic Electrochemistry for Biotechnology can be of use for coursework, self‐education and, maybe even as a reference being used at many desks and lab benches.
This textbook is based on knowledge gathered throughout the years from many books, reviews, research articles, and our own experiences. We did our very best to cite these sources as appropriately as possible. We would like to thank all our colleagues and friends who helped proofreading the manuscript (in alphabetical order): Micaela Brandão Lavender, Margo Elzinga, Paniz Izadi, Aykut Kas, Benjamin Korth, Jörg Kretzschmar, Anne Kuchenbuch, Philipp Kuntke, João Pereira, Shabnam Pouresmail, Katharina Röhring (neè Neubert), Yvonne Schößow, Sanne de Smit, and Xiaofang Yan. Your valuable changes, suggestions, and discussions have lifted this textbook to a higher level. We would especially like to mention Rikke Linssen. In addition to your proofreading of the earliest versions, your artistic talents in translating our scribblings of scientific concepts into understandable but still playful drawings have made this textbook more readable and given it an awesome appearance.
We hope that you enjoy reading!
Leipzig, Germany, and Harenand Wageningen, The Netherlands
Falk Harnisch, Tom Sleutels,
and Annemiek ter Heijne
Reference
Harnisch, F. and Urban, C. (2018). Electrobiorefineries: Unlocking the synergy of electrochemical and microbial conversions.
Angew. Chem.‐Int. Edit.
57 (32): 10016–10023.
1A Reader's Guide to Basic Electrochemistry for Biotechnology
This book on Basic Electrochemistry for Biotechnology is meant for readers working in the field of microbial electrochemistry and microbial electrochemical technologies. It is meant for readers with different professional training and scientific backgrounds, who are at different stages of their careers and working on the interface between biotechnology, electrochemistry, and engineering. Because of these different backgrounds and levels of expertise, we have to assume a certain knowledge base. This book explains the basic principles of electrochemistry for people with a background in biotechnology, and therefore we assume a certain level of knowledge in microbiology throughout the book.
The book is structured in such a way that the consecutive chapters provide step‐by‐step insights into the basics of electrochemistry (Figure 1.1). Chapter 2 introduces microbial electrochemical technologies and illustrates why we, the authors, are so fascinated by working and spending our professional lives at the interface of microbiology and electrochemistry. Chapters 3 and 4 then provide an insight into the thermodynamic fundamentals of electrochemistry, which is followed by an introduction to basic electrochemical methods in Chapter 5. After the details on electrochemical kinetics in Chapter 6, you will learn how to use and understand dynamic electrochemical methods in Chapter 7. Until then, we have focused almost exclusively on individual electrodes (that we call electrochemical half‐cells). Yet, full reactors are also of great interest to the community. Thus, we will get a glimpse of how to characterize full electrochemical cells in Chapter 8. In Chapter 9, we provide an outlook on the topics covered in this book, and share considerations on the design and execution of experiments and on reporting results.
We designed the individual chapters in such a way that they can also be used independently, e.g. for addressing one specific aspect or refreshing knowledge. Therefore, for the general convenience of the reader, every chapter starts with its key messages. To provide easy access for the reader and to avoid misinterpretation, the stringent use of symbols and units is essential. All abbreviations are summarized in Appendix A, and used symbols, units, and constants are summarized in Appendix B. Furthermore, we believe that well‐designed examples can substantially foster understanding, and since repetition is the mother of learning, we included examples as well as exercises for self‐study, with the answers and solutions shown at the end of the book (Appendix C), and the needed data summarized in Appendix D. In addition, each chapter includes boxes, which provide additional information on important topics.
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).
The figures in this book are presented as an artistic representation of reality. This means that in some cases, some details might be missing from the representation. Nonetheless, the figures are an accurate description of microbial electrochemical systems and graphs. In the case of graphs, you might notice that units are lacking from the axis. Although this might not be considered good scientific practice to some, this has been done intentionally as in many cases the units can have multiple normalizations. The units of the symbols used in graphs can be found in Appendix B.
There will probably be topics that are relevant for microbial electrochemical systems that are not covered in this book; we did our best to include the background and aspects that we find most important. We hope that you enjoy reading!
2A Basic Introduction to Microbial Electrochemical Technologies
Key Messages:
Primary
microbial electrochemical technologies
(
METs
) are based on the use of electroactive microorganisms on anodes (for oxidations) or cathodes (for reductions).
Electroactive microorganisms wire their metabolism to anodes and cathodes by different modes of
extracellular electron transfer
(
EET
).
The flow of electrons in electrochemical reactions can be compared to the flow of water in pipes between reservoirs at different altitudes.
Wastewater is a complex source of energy that is characterized by several physical and chemical sum parameters (most important being the
chemical oxygen demand
[
COD
]) and that can be exploited in METs that use a microbial anode, like the
microbial fuel cell
(
MFC
) and the
microbial electrolysis cell
(
MEC
).
2.1 Introduction to Microbial Energy Conversion and Microbial Electrochemical Technologies
Learning objective: To understand the basic principles ofmicrobial electrochemical technologies (METs).
In this chapter, we will go through the basics of METs. We will learn why wastewater is such an important source for these technologies, and what are typical characteristics of wastewater. We will also introduce the most important components of METs and get basic knowledge of how these microbial electrochemical systems are constructed, operated, and analyzed. This chapter sets the foundation, and further on, in Chapters 3–9, these topics will be explained more in‐depth.
2.1.1 Microbial Conversions
Microbial conversions are widespread in nature and are applied in daily life and industries. In microbial conversions, microorganisms gain energy from the chemical transformation of reactants into products. Examples of microbial conversions include the production of methane from organic material (e.g. in rice paddies or anaerobic digestion (AD) plants that produce biogas, see also Box 8.2), the production of yogurt or kimchi, the production of ethanol from sugar, and the reduction of metals in soils (Madigan et al. 2018).
Generally, a microbial conversion consists of the following two half‐cell reactions (Chapter 3): an oxidation reaction and a reduction reaction (Chapter 4). In an oxidation reaction, electrons are released, while in a reduction reaction, electrons are consumed. In microbial cells, oxidation reactions and reduction reactions are coupled via metabolism. This coupling leads to an exchange of electrons, by which energy is released (see Chapter 4). Microorganisms can use (part of) this released energy to maintain and grow (see Box 2.1). This exchange of electrons occurs until the electrons reach a terminal electron acceptor (TEA). We all know aerobic respiration – which we as human beings also perform – during which the electrons are finally released to oxygen (Madigan et al. 2018). In aerobic respiration, oxygen is the TEA. Oxygen is water‐soluble, which also holds true for many other TEAs like nitrate or sulfate. These TEAs can enter the cell and be reduced intracellularly. In contrast, in some cases, the electrons from an oxidation reaction are transferred to a solid TEA (Gescher and Kappler 2013
