Introduction

Electrochemistry is everywhere around us and refers to electrical phenomena concerning charge movement and charge separation associated with chemical phenomena. Thus, the question becomes which phenomena involving chemical species, static and dynamic, cannot be described as electrochemical instead of asking which can.

To be successful in communicating knowledge and critical thinking, the teaching of electrochemistry should reflect society’s needs as well as giving a sufficiently good general grounding in electrochemistry. In this way, the individuals can adapt what they know to newly emerging situations and challenges. On the other hand, chemistry is a basic science i.e. it needs also to be curiosity driven; and curiosity-driven research lies behind many of the advances in applied science. This is what drove giants of electrochemistry such as Galvani, Volta, Davy and Faraday. The International Year of the Periodic Table of Chemical Elements (IYPT) in 2019 showed very clearly that a wider audience, and particularly school children, is full of enthusiasm for the beauty and the beneficial aspects of many chemical elements. This enthusiasm must not be lost.

The all-encompassing electrochemistry definition lay at the heart of the developments in chemistry at the end of the eighteenth century and following this, many experiments throughout the first half of the nineteenth century that were crucial for the isolation and characterisation of the chemical elements and the development of the periodic table. It was not by chance that in the later nineteenth century many universities had a department of physical chemistry and electrochemistry. Teaching electrochemistry was regarded as essential and electrolysis was an important tool. Just two examples from that time: the electrolysis of molten sodium chloride to produce metallic sodium and the production of strong oxidising agents such as potassium permanganate.

In the early twentieth century, monumental advances were made in physical and theoretical chemistry. The large industrial electrolysis processes were implemented and electrical energy was being produced by a variety of technologies from coal-burning power stations to hydroelectric and more recently nuclear power stations. Compared to nowadays, there was not much concern with the environmental effect of the different methods of electrical energy production. From a practical point of view, instrumentation for spectroscopy became highly developed, and there was less focus on electrochemistry research. At that time, in the laboratory most practitioners used “black boxes” to carry out electrochemistry experiments. Very often the experimental results suffered from a lack of constancy and reproducibility, with higher uncertainties compared to spectroscopy. The breakthrough towards what we call modern electrochemistry, which had an important impact not only in electrochemistry but also in analytical chemistry, was the invention of the dropping mercury electrode by Heyrovský [1], for quantitative measurements mainly by reduction of the species, and for which he won the Nobel prize in 1959. For the first time for many, the need to focus on adsorption on the electrode surface, and the transport of electroactive species in solution by diffusion, as well as electrode kinetics for understanding the electrode processes, became clear. The continuous renewal of the electrode surface enabled measurement reproducibility and repeatability.

Whilst the importance of electrochemistry in conceptual terms was realised from that time, both thermodynamic, and kinetic aspects, plus a range of applications, the teaching suffered from some drawbacks. Students need enthusiastic teachers who understand well what they are teaching and demonstrate convincingly that electrochemistry is an integral part of chemistry. Electrochemistry was usually regarded as “different” and, if not more difficult in terms of theory, then certainly in terms of practice.

Experimental electrochemistry encompasses the use of electrical energy to promote chemical transformation (chemical energy) that is electrolysis, in electrolytic cells, and the harnessing of chemical energy to generate electrical energy, in galvanic cells. Research into these phenomena can be done at (dynamic) electron flow equilibrium, by potentiometric measurements where a potential between two electrodes due to different concentrations of a species is measured but no current flows, and away from this value where a potential is applied vs. a reference value and the current measured (or a current is applied and the potential is measured) which are voltammetric or impedance measurements.

How to get accurate and reproducible results in laboratory experiments and avoid the use of mercury? What electrode materials could be used for studying oxidations (mercury itself oxidises at low positive potentials)? Batteries, fuel cells and industrial electrolysis tended to be hidden in the wings, and were not talked about much in general chemistry circles. Of course, there was always important research going on but very much by specialists and not in the mainstream.

The situation nowadays has changed, to a large extent owing to the challenges posed by climate change and humankind’s effect on the environment. The focus of scientific innovation at the moment to address these questions is on energy, materials and sustainability, and all of them include electrochemistry. Each of these three can be illustrated and can be related to many of the objectives of Agenda 2030, Fig. 1. Electrochemistry is recognised by the scientific community to have a central and an essential role in sustainability, even if it sometimes appears without using the word “electrochemistry”. Electrical energy, at the point of use, is clean energy.

Fig. 1
figure 1

The UN Sustainable Development Goals. Electrochemistry is particularly important in goals 3, 4, 6, 7, 9 and 13

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Teaching and giving knowledge to the younger people will help to improve the likelihood that decisions regarding the challenges that the world faces, such as in mitigating the effects of climate change, can be taken more objectively in the future. Additionally, the history of modern chemistry and electrochemistry’s crucial role should also be illustrated and must not be forgotten.

The young person is often looking for immediate answers to questions, without going through all the justifications, descriptions and calculations. Learning through examples, from social media etc. can be a young person’s preferred way, as they want quick insights without having an internal acquired memory bank of “proven” facts to support it. This is difficult since critical thinking is always needed to separate the real from the incorrect or “fake” and is partly the consequence of having so much information available on the internet and in social media. Consideration of the future roles of electrochemistry research [2] and of electrochemical sensing [3] can be a background to rationalising such information.

Thus, an electrochemistry course needs to start with illustrations of electrochemistry’s everyday importance to us all, those applications in the spotlight and those that are normally in the shadows, demonstrating its interdisciplinarity. How can we propose solutions, using electrochemistry teaching, to make the world a better place? The diverse scope and reach of electrochemistry can be seen in the scheme of Fig. 2, always remembering that electrochemical transduction is dynamic: the electrode is itself a tuneable charged reagent as well as a detector of surface phenomena. This is possible by externally applying a controlled potential to a conductor material, the electrode, not only knowing the starting and the finishing point, but enabling following and controlling in time and in situ the mechanism of the reaction occurring at the electrode surface.

Fig. 2
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Electrochemical topics and applications

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The objective of this article is to propose the philosophy behind a full electrochemistry course and its components, which can be adapted in order to be able to reach different audiences: science and/or engineering students and, indeed, the community at large, for whom there is a pressing need to be able to reflect critically on the information provided by different scientific sources so as to be able to take political and economic decisions correctly.

Outline of a course in electrochemistry

The design of a university electrochemistry course, probably at MSc level, and the way that the introduction is given will be influenced by the goals and objectives of the course. It will demonstrate the need for curiosity in the basic sciences and, at the other end, for electrochemical technology applications. There are general electrochemistry textbooks spanning several decades and which we regard as particularly useful [4,5,6,7,8,9]. There are many others available, with a wide choice of styles that may adapt better to the teacher and student as well as many more specific monographs that address particular electrochemistry topics.

Two essential points need to be considered. The first point is to capture the attention of the students through relevant everyday examples of electrochemistry in action and the second is to show how electrochemistry is a fully integrated part of chemistry, with a strongly interdisciplinary nature. The proposed electrochemistry course will be divided into different sections which can be shorter or longer and expanded according to the identified needs, with coexistence of lectures and practical and mini-project work. The best is that one accompanies the other and that there is also ample discussion of experimental results.

Electrochemistry in the modern-day world – brief survey

Illustrations can be given from energy (electrolysis, batteries, fuel cells, photovoltaic cells, hydrogen production), materials (importance of nanomaterials in electrode processes), industrial electrolysis and electrodeposition processes, electrochemical sensors and biosensors, and corrosion. An example, we all see electrochemistry constantly in everyday life, is batteries in mobile phones and in portable computers. Batteries can be very useful for initiating electrochemistry teaching, as rechargeable batteries can be used to immediately introduce the modus operandi and the concept of galvanic and electrolytic cells. This course component can be conducted by direct participation and discussion in the first lecture or asking the students to search for applications between first and second lectures which are then discussed.

Fundamentals of electrochemical phenomena

Before embarking on a description of electrode processes some basic fundamentals, with crucial revisits to knowledge acquired previously, will be needed. These include the Nernst equation, standard electrode potentials, activities of chemical species and formal potentials, formulation of cell diagrams and calculation of cell potentials [2], Fundamentals of adsorption, that can be a very fast phenomenon, the importance of the interfacial region at the solid–liquid interface at the atomic level and potential of zero charge of surfaces, should be described.

The interfacial region at the solid–liquid interface and the distribution of ions in solution on the charged solid electrode generating the charge separation, still called the electrical double layer, and the associated capacitive currents, changing with the applied potential and impossible to measure, cannot be neglected [4].

A clear explanation of the mass transport phenomena of diffusion, caused by concentration gradients, of migration due to electric field gradients, and of convection by mechanical movement in the solution, should be given. Diffusion may already exist in a system or appear owing to dynamic depletion of electroactive species due to electrode reactions. Regarding migration, in laboratory experiments, the inert electrolyte ions in solution conduct almost all the current.

Fundamentals of electrode processes

The understanding of the rate and mechanism of electrode processes, in order to be able to control them externally, is crucial. Applying a varying externally controlled potential to an electrode, a charge separation capacitive current, due to changes in charge separation (charge alignment) will flow, and a charge transfer electrode reaction faradaic current, that may be difficult to separate from the capacitive current, will be generated at the electrode surface at potentials corresponding to oxidation or reduction of electroactive species.

The kinetics of electron transfer at the electrode surface should then be addressed. This is usually taught using the Butler-Volmer formulation and connection can be made to the Arrhenius equation. The activation energy is expressed through the potential needed to cause the oxidation or reduction, the difference between this value and the value that is thermodynamically predicted. However, the theory of electron transfer reactions and some of the obstacles to the (adiabatic) electron transfer such as solvent reorganisation and alterations to conformation demonstrate that the Butler-Volmer formulation is not sufficient, especially for large (e.g. biological) molecules where the redox centre is far from the surface. Marcus theory can be introduced at this point.

The interplay between mass transport and kinetics on the observed rate of electrode processes can then be addressed and equations describing this behaviour in the steady-state responses being deduced. In the analysis of the voltammetric experimental data, it should be emphasised that analysis leads to values of formal potentials and not standard electrode potentials, since the equations are formulated with concentrations (and not activities). It must also be stressed that electrocatalysis is due to a decrease in the applied potential necessary for an oxidation or a reduction electrode reaction to occur, caused by faster kinetics, and not to higher currents, which are due to an increase in active surface area.

Electrochemistry in the research laboratory and in the field

The elements of conducting practical electrochemistry experiments then need to be addressed, well described in e.g. [10, 11].

Electrochemistry cannot occur without electrode materials of sufficiently high conductivity (except for zero-current potentiometric measurements) in a conducting medium. Such materials are metallic, carbon, conducting/electroactive polymers, composites or good semiconductors. Their surfaces are often modified with nanomaterials – nanotubes, nanoparticles etc. to increase surface area and possibly to induce an electrocatalytic effect.

The main solvent used is water containing sufficient electrolyte, often an added ionic inert supporting electrolyte that transports the current inside the bulk solution. In the past, nonaqueous solvents were often used but due to their environmental and health toxicity they are now only used if absolutely necessary. Ionic liquids and deep eutectic solvents may solve some problems since they can be both solvent and electrolyte, and they are of lower toxicity.

Significant advances in electrochemical instrumentation over recent decades have made electrochemistry experiments easy to carry out nowadays, and this explains much of the surge in interest of the whole chemistry community in carrying out the investigation of redox processes. The control instruments, potentiostats, are now nearly all digitally based with microprocessor control; they can acquire very large amounts of accurate voltammetric data, and they are easily interfaced using appropriate software with an external computer, tablet, or even smart phone for recording experimental data and undertaking their analysis. The use of chemometrics and, more recently, Artificial Intelligence (AI) will certainly help in students’ tutorials for experimental planning and enable analyses of a greater quantity of results within an acceptable timeframe, in order to take the best decisions on the data obtained.

Electrochemical methods and techniques

Electrochemical methods encompass (i) potentiometric and (ii) voltammetric measurements. Potentiometric measurements only require two electrodes, a potential difference between two electrodes due to different processes at each of them is measured, but no current flows. Voltammetric measurements, where a potential (vs. a reference electrode) is applied to a working electrode and the current response is measured or a current is applied and the potential is measured, require three electrodes. The importance of using three electrodes for voltammetric and impedance experiments, working electrode, auxiliary electrode and reference electrode, the first two to form the electrical circuit and the reference electrode to enable precise control of the potential applied to the working electrode, must be clearly explained. Typical linear ranges and detection limits for each technique should form part of the lecture material.

Potentiometric methods and cyclic voltammetry, in our opinion, should be taught to all students. Ion-selective electrodes, e.g. pH electrodes, are used widely and need to be recognised as being electrochemical. Similarly, cyclic voltammetry is now often used by materials’ chemists, inorganic, and organic, to carry out a diagnostic first examination, like finger prints, of the redox properties of new compounds and materials.

Pulse techniques are very much used in analytical electrochemistry. The basis of these is the potential step, since pulse techniques involve a succession of step changes to the applied voltage. The theory behind these techniques was studied many years before they were implemented in normal commercial potentiostats, which became easier with the transition to digital potentiostats with microprocessors, since pulses are digital in nature, as well as the ability to store values of current. Chronoamperometric traces are recorded and current sampling of them is done over a small window of time (normally tens of ms) chosen such that capacitive currents can usually be neglected and that the variation in faradaic current during this period is small. The theory of steps and pulse techniques will be learned in order to able to evaluate what happens to the form or height of oxidation or reduction peaks if the experimental control parameters are altered.

Often, the user-friendly software for pulse voltammetric techniques is treated by the user as a black box and simply follows the manufacturer’s control parameter suggestions, but can lead to errors. A drawback of these modern digital instruments is that voltage ramps (as in cyclic voltammetry) are implemented as staircases with small voltage increments between each step, which may not be equivalent to ramps if the step increments are too large. For pulse techniques, differential pulse voltammetry and square wave voltammetry, the correct step widths, potential increments and scan rate should always be selected in agreement with the theory [12, 13].

Electrochemical impedance spectroscopy (EIS) [14] first became widespread in the context of corrosion monitoring by corrosion scientists and engineers. The breadth of use of EIS has increased in recent years in characterisation of materials as electrodes and in application to energy sources and in sensors. To understand impedance results, it is necessary to be able to model the (modified) electrode-solution interfacial region by an equivalent electrical circuit, which reflects the processes that are occurring.

Surface examination and surface analysis

Complementary information from non-electrochemical techniques may be essential for materials’ characterisation. Spectroscopic in situ and ex situ methods, X-ray diffraction and photoelectron spectroscopy, scanning electron and transmission electron microscopies, scanning tunnelling and atomic force microscopies, are important tools. Hybrid techniques such as scanning electrochemical microscopy, photoelectrochemistry, and the electrochemical quartz crystal microbalance should be surveyed and described according to the aims of the electrochemistry course and audience. It should be pointed out which of these techniques can probe surfaces in close to natural environments, without the necessity of a high vacuum.

Electrochemical corrosion

Corrosion is electrochemical by nature, and occurs spontaneously, normally in a non-controlled manner, since the main corrosive agent is oxygen from the atmosphere, normally dissolved oxygen. The fundamentals of electrochemical corrosion need to be explained with clarity, especially the thermodynamic and kinetic aspects, and stressing that the anodic and cathodic reactions are different and take place at different sites on the sample surface, so that electrochemical cells are formed. Methods for investigating corrosion should be indicated: open circuit potential (potential of zero current) and its variation over time, voltammetric methods, electrochemical impedance and electrochemical noise. Evaluation of corrosion will include spectroscopic analysis and examination of surface morphology and some materials’ engineering considerations, e.g. [15].

A description of the types of corrosion: uniform and localised corrosion, pitting corrosion, stress corrosion, corrosion fatigue, hydrogen embrittlement will lead naturally to suggestions as to how to inhibit and protect against corrosion. This includes metallic (more inert metals) inorganic, organic, polymer and conversion coatings. The role of electropainting, sacrificial anodes and anodization can be illustrated. Dynamic anodic and cathodic protection (immunization and passivation) and inhibitors in solution can be described.

Applications of electrochemistry

Examples of the applications of electrochemistry can be selected from the list below, not exhaustive, or all of them could be discussed; only titles of applications have been written and some topics are very extensive. Some of them may have been indicated at the beginning of the course but they can now be understood to a greater depth with the knowledge acquired from the lectures and discussion:

  • Electrochemical sensors. Potentiometric sensors. Amperometric and voltammetric sensors; biosensors and immunosensors. Applications in foods, health and environmental monitoring.

  • Industrial electrochemical processes. Electrochemical reactors; industrial metal extraction. Electrodeposition, electroless deposition. Electromachining. Electrosynthesis including conducting polymers. Water electrolysis (water splitting and hydrogen generation), electrodialysis and electroosmosis.

  • Energy harvesting and electrochemically-generated energy. Batteries and fuel cells – types and applications. Photoelectrochemical solar energy sources, energy storage.

  • Electrochemistry and reducing environmental pollution. Metal recovery, residue treatment and destruction, remediation, recycling.

Laboratory demonstrations, experiments and reports

Laboratory experiments or, if not possible owing to a large number of students, demonstrations are crucial to help the student understand how to set up and carry out electrochemical experiments. This includes the need for the cleaning and pre-treatment of the electrode surface, how to record experimental results, how to analyse them, and how to assess their quality. Data analysis is best done individually by each student, and will need to include treatment of reproducibility and repeatability, and could also include some elementary chemometrics and artificial intelligence. Simulation of electrochemical experiments, and of electrochemical reactor systems could also be included. This practical approach could be applied to, for example, sensors, electrodeposition, battery characterisation, corrosion, etc.

It is also very instructive for the students if each prepares a short monograph on a topic related to the course applications of electrochemistry. In this, besides a literature review, they should explain what is the context of the topic and its importance, what has recently been and is being achieved, with a few illustrative examples, and what are the likely future directions. Consultation of the recent up-to-date literature is expected. The report monographs submitted will be part of the course evaluation and would also be presented to the course colleagues in concise Power Point presentations that, at the same time, are informative for all.

Final remarks

The course proposed includes the large majority of topics that we believe could be necessary for science and engineering students in their future careers. It will almost certainly be necessary to make choices since many of the courses that are given in higher education institutions are not sufficiently long to cover everything. It should be adapted to the background of the students in physical chemistry and electrochemistry.

The course aims are three-fold. First, to show the scope of electrochemistry, how it is an integral part of chemistry and is inherently interdisciplinary with strong links to many other scientific and technological areas of knowledge. Secondly, to give the student sufficient knowledge that they are able to consult the literature and critically analyse what they have found in their research. Thirdly, to impart enthusiasm for electrochemistry and its broad and extensive scope. This last is very important as we look for new ways to carry out industrial processes that are cleaner and more efficient and selective, for monitoring the effects of climate change and toxicity and for using materials more efficiently for energy harvesting and energy storage.