An electrochemical cell is any device that converts chemical energy into electrical energy or uses electrical energy to drive a chemical reaction. Every battery in your phone, every hydrogen fuel cell in a bus, and every glucose test strip a diabetic person pricks their finger for relies on the same underlying principle: chemical reactions that shuffle electrons through a circuit. The concept splits into two broad families, one that produces electricity spontaneously and one that needs electricity pumped in, and the sheer range of technologies built on this single idea is surprisingly vast.
The Basic Idea Behind Every Electrochemical Cell
At its core, an electrochemical cell has two electrodes sitting in a substance that allows ions to move between them. One electrode gives up electrons through a chemical reaction (oxidation), and the other accepts them (reduction). When you connect the two electrodes with a wire, electrons flow from one to the other, and that flow of electrons is electrical current. The substance between the electrodes, called the electrolyte, completes the circuit internally by letting charged atoms migrate back and forth.
Cells that generate electricity on their own are called galvanic or voltaic cells. A standard disposable battery is a galvanic cell: you pop it into a flashlight, and the chemical reactions inside push current through the bulb until the reactants are used up. Cells that consume electricity to force a reaction that would not happen on its own are called electrolytic cells. Charging a rechargeable battery is an electrolytic process: you plug it in, and external power reverses the chemistry so the cell can discharge again later. These two modes are mirror images of the same electrochemistry.
Lithium-Ion Batteries and the Intercalation Trick
The rechargeable batteries in phones, laptops, and electric vehicles are lithium-ion cells, and they work through a process called intercalation. During charging, lithium ions slip between the layers of the graphite anode, nestling into the gaps like cards sliding into a deck. During discharge, those ions migrate back to the cathode, and the electrons that accompanied them take the long way around through your device’s circuit, doing useful work along the way.
Graphite is the standard anode material precisely because its layered crystal structure accommodates lithium ions so neatly.1Chemical Engineering Journal: Green and Sustainable. Intercalation-based electrode materials for lithium-ion batteries: structure, chemistry, and performance Researchers have also investigated harder-to-order carbon materials, known as hard carbon, which store ions through a two-step mechanism: first the ions stick to surface defects and edges, then at lower voltages they slide into the graphitic layers themselves.2Advanced Energy Materials. Revealing the Intercalation Mechanisms of Lithium, Sodium, and Potassium in Hard Carbon Hard carbon is interesting partly because it can also host sodium and potassium ions, opening up cheaper alternatives to lithium for future batteries.
Why Batteries Lose Capacity Over Time
If you have ever noticed that an old phone does not hold a charge like it did when it was new, you have experienced the effects of a thin layer called the solid-electrolyte interphase, or SEI. This film forms on the anode surface the very first time a lithium-ion battery charges. It is actually useful at first because it protects the anode from reacting further with the electrolyte. But the SEI keeps growing, slowly, every time you cycle the battery, and each bit of growth consumes a tiny amount of lithium that can never be used for energy storage again.
Modeling work shows that early in a battery’s life, the growth rate is limited by the chemical reaction itself. As the SEI thickens, transport of material through the layer becomes the bottleneck, and the growth rate slows. Over very long cycling, though, the growth can shift back toward a more linear pace, which means capacity fade picks up again.3PubMed Central. Solid–Electrolyte Interphase During Battery Cycling: Theory of Growth Regimes This is one reason that batteries in electric vehicles, which may see thousands of charge cycles over a decade, are designed with careful thermal management and charging protocols to slow SEI growth as much as possible.
Thermal Runaway and Why Batteries Sometimes Catch Fire
The worst-case scenario for a lithium-ion cell is thermal runaway: a self-accelerating chain of reactions that can end in fire or explosion. It usually starts when something goes wrong, such as a manufacturing defect, physical damage, or overcharging, and the internal temperature climbs past a critical threshold. The SEI layer on the anode decomposes first. That exposes the anode to the electrolyte, triggering further heat-generating reactions. Flammable gases build up inside the cell, pressure rises, and if the separator between the electrodes melts, an internal short circuit dumps energy all at once.4Next Energy. Thermal runaway process in lithium-ion batteries: A review
Engineers attack this problem from multiple directions. Battery management systems monitor voltage and temperature to shut down charging before conditions get dangerous. Cell designs include vents and current-interrupt devices. Researchers have modeled thermal runaway in different cell shapes, including prismatic and pouch formats, and tested cooling strategies like running water through tiny channels pressed against the cell walls.5Applied Thermal Engineering. Modeling and analysis of thermal runaway in Li-ion cell These safeguards are why the vast majority of the billions of lithium-ion cells in circulation never have an incident, even though the underlying chemistry is energetic enough to be dangerous if left uncontrolled.
Solid-State Batteries and the Interface Problem
One of the most-hyped next-generation technologies replaces the liquid electrolyte in a lithium-ion cell with a solid material. Solid-state batteries promise better safety because there is no flammable liquid to ignite, along with the possibility of higher energy density and longer cycle life.6PubMed. Interface in Solid-State Lithium Battery: Challenges, Progress, and Outlook In principle, you could pack more energy into a smaller, lighter package and sidestep the thermal runaway chain described above.
In practice, the stumbling block is the interface between the solid electrolyte and the solid electrodes. Liquid electrolytes conform to every microscopic bump and valley on an electrode surface, ensuring good contact. A solid electrolyte does not flex the same way, so tiny gaps form, and the resistance at those interfaces can be enormous. Recent modeling work has shown that conventional methods of estimating this resistance using bulk material properties can be off by orders of magnitude, because the chemistry right at the solid-solid boundary behaves differently from the bulk.7PubMed Central. Direct Modeling of the Interfacial Resistance in All-Solid-State Battery Until researchers find reliable ways to engineer low-resistance interfaces, solid-state cells will remain more lab curiosity than mass-market product.
Fuel Cells Turn Hydrogen Into Electricity
A fuel cell is a galvanic electrochemical cell that does not store its reactants internally the way a battery does. Instead, you feed hydrogen (or another fuel) to one electrode and oxygen to the other, and the cell produces electricity, water, and heat for as long as you keep supplying fuel. Proton exchange membrane fuel cells, or PEMFCs, are the type most commonly used in vehicles and portable power systems.
The main cost barrier for PEMFCs is the catalyst. The oxygen side of the cell needs a catalyst to break molecular oxygen apart fast enough to keep up with the hydrogen side, and the standard material for that job is platinum. Platinum is expensive and scarce, so researchers have been developing alternatives based on individual metal atoms dispersed across a nitrogen-doped carbon support, which can perform a similar catalytic function at lower cost.8Advanced Energy Materials. Advanced Atomically Dispersed Metal–Nitrogen–Carbon Catalysts Toward Cathodic Oxygen Reduction in PEM Fuel Cells If these materials can match platinum’s durability over thousands of hours of real-world operation, fuel cells become far more economically viable for cars, trucks, and stationary power.
Water Electrolysis Runs the Reaction in Reverse
If a hydrogen fuel cell combines hydrogen and oxygen to make electricity and water, a water electrolyzer does the opposite: it uses electricity to split water into hydrogen and oxygen. This is the electrolytic side of the electrochemical coin, and it is central to the “green hydrogen” economy because the hydrogen it produces can be carbon-free if the electricity comes from renewables.
Two main electrolyzer technologies compete today. Alkaline electrolyzers use a potassium hydroxide solution as the electrolyte and have been around for more than a century. Proton exchange membrane (PEM) electrolyzers use a solid polymer membrane, the same basic family of material found in PEM fuel cells. Comparative testing has found that PEM electrolyzers consume less energy per unit of hydrogen, roughly 4.1 to 4.3 kilowatt-hours per normal cubic meter compared with 4.6 to 4.8 for alkaline systems.9Applied Energy. Comparative experimental study of alkaline and proton exchange membrane water electrolysis for green hydrogen production PEM systems also handle fluctuating power input better, which matters when the electricity source is solar or wind. Alkaline units, on the other hand, tend to be cheaper and do not need the expensive membrane materials that PEM units require, so the choice depends on the application.
Grid-Scale Storage With Flow Batteries
Lithium-ion batteries dominate consumer electronics and electric vehicles, but for storing hours’ worth of electricity on the power grid, a different electrochemical architecture has advantages. Vanadium redox flow batteries store energy in liquid electrolytes held in two external tanks. When you need power, pumps push those liquids through a cell stack where vanadium ions in different oxidation states undergo redox reactions to generate current.10Elsevier. The rise of vanadium redox flow batteries: A game-changer in energy storage
The beauty of this design is that energy capacity and power output are decoupled. If you want more stored energy, you just use bigger tanks with more electrolyte. If you want higher power, you add more cell stacks. Flow batteries also experience essentially no self-discharge: if the pumps are off and the electrolytes sit in their tanks, the energy stays put indefinitely. The trade-off is lower energy density compared to lithium-ion, which makes flow batteries impractical for anything that needs to be lightweight or compact. But for a shipping-container-sized installation behind a solar farm, weight does not matter much.
Electrochemical Sensors in Everyday Medicine
One of the most widespread uses of electrochemical cells has nothing to do with energy storage. Billions of disposable glucose test strips are manufactured each year, and each one is a tiny electrochemical cell. A drop of blood lands on a strip that contains an enzyme, typically glucose oxidase or one of its relatives, along with a chemical mediator. The enzyme catalyzes a reaction with glucose, generating electrons that flow through the mediator to an electrode. The resulting current is proportional to the glucose concentration, and a handheld meter converts that signal into a blood-sugar reading in seconds.11Chemical Reviews. Electrochemical Glucose Sensors and Their Applications in Diabetes Management
Different strip designs use different enzymes, different mediators, and different measurement techniques, such as measuring the current at a fixed voltage or measuring the total charge transferred over a set time. The engineering challenge is making something accurate enough for medical decisions that also costs pennies to produce and works with a single small drop of blood. Electrochemical sensing is also expanding beyond glucose into other analytes: lactate, cholesterol, and environmental contaminants all have prototype or commercial electrochemical sensors that follow the same basic approach.
Microbial Fuel Cells and Bioelectrochemistry
Some bacteria naturally push electrons outside their own cells during metabolism. Researchers have harnessed this ability in microbial fuel cells, which are electrochemical cells where living microorganisms serve as the catalyst at the anode. The bacteria break down organic matter, such as sugars, acetate, or even wastewater, and transfer the electrons generated by their metabolism to the anode electrode. Those electrons travel through an external circuit to a cathode, generating electricity along the way.12Trends in Biotechnology. Microbial fuel cells: from fundamentals to applications
The power output of microbial fuel cells is modest. Isolated exoelectrogenic bacteria from marine water, for instance, have generated voltages up to about 0.34 volts in laboratory setups.13Bioelectricity. Screening of Exoelectrogenic Bacteria from Marine Water Inoculated Microbial Fuel Cell for Electricity Generation You are not going to power a city this way. The real appeal is the dual function: treating wastewater while extracting a bit of energy from it. In remote locations or developing regions where both clean water and electricity are scarce, a system that addresses both problems simultaneously has genuine value, even if the wattage is low.
Industrial Metal Extraction
Long before lithium-ion batteries existed, electrochemical cells were essential in heavy industry. Electrowinning uses electrolytic cells to pull metals out of solution: you dissolve an ore or its processed intermediate in an electrolyte, pass current through it, and the target metal plates out on the cathode as a pure solid. Aluminum production, for example, relies entirely on electrolysis of alumina dissolved in molten cryolite. Copper refining works similarly, with impure copper dissolved from the anode and deposited in pure form on the cathode.
Electrorefining follows the same principle but starts with an impure metal anode rather than a dissolved ore. Current selectively dissolves the target metal from the anode while leaving impurities behind, and pure metal deposits on the cathode. The electrochemical principles governing these processes, including the relationship between free energy and the voltage required to drive the reaction, are the same ones that govern a household battery, just scaled up to enormous vats of molten salts or acid solutions.14Mineral Processing and Extractive Metallurgy: Transactions of the Institutions of Mining and Metallurgy. Role of electrochemical processes in the extraction of metals and alloys – a review
Diagnosing Battery Health With Impedance Spectroscopy
As batteries age, their internal chemistry changes in ways that are hard to observe directly without cutting the cell open. Electrochemical impedance spectroscopy, or EIS, gives engineers a non-destructive window into what is happening inside. The technique works by applying a small alternating voltage to the cell at many different frequencies and measuring how the cell responds. Different internal processes, such as ion migration through the electrolyte, charge transfer at the electrode surfaces, and diffusion of reactants, each respond on different timescales, so sweeping through frequencies separates their contributions.15Current Opinion in Electrochemistry. Impedance spectroscopy of battery cells: Theory versus experiment
In practice, this means a manufacturer can track how the internal resistance of a battery pack changes over months or years and predict when cells will fall below acceptable performance. It is also used in research to evaluate new electrode materials, new electrolyte formulations, and new cell designs without waiting for full cycle-life testing that could take years.
Solar Water Splitting in a Photoelectrochemical Cell
An especially elegant variation on the electrolytic cell uses sunlight directly as the energy source for splitting water, cutting out the middleman of a separate solar panel and electrolyzer. In a photoelectrochemical cell, a light-absorbing material on one electrode captures photons and uses their energy to drive the water-splitting reaction right at the electrode surface.
One approach bonds a molecule that both absorbs light and catalyzes water oxidation onto a nanostructured electrode. Researchers have demonstrated this concept with a “chromophore-catalyst assembly” attached to a titanium dioxide shell over a high-surface-area conductive oxide core. Under visible light, this system split water into hydrogen and oxygen with an absorbed photon conversion efficiency of about 4.4 percent at peak photocurrent.16PubMed Central. Solar water splitting in a molecular photoelectrochemical cell That efficiency is modest compared with separate solar-plus-electrolyzer systems, but the simplicity and potential for lower cost keep the research active. The broader point is that electrochemical cells are not limited to batteries and industrial vats: they can be designed to harvest energy from light itself.
Recycling the Cells We Have Already Made
With millions of electric vehicles on the road and billions of consumer devices cycling through their battery packs, end-of-life recycling has become an urgent question. Two main industrial routes exist for recovering valuable metals like nickel, cobalt, and lithium from spent cells. Pyrometallurgy smelts the batteries at high temperatures to extract metal alloys. Hydrometallurgy dissolves them in acid solutions and separates the metals chemically. Life-cycle analysis comparing the two approaches for nickel-manganese-cobalt cathode chemistries found that the hydrometallurgical route had about a 24 percent lower climate impact and substantially lower human toxicity burdens.17Journal of Power Sources. Decarbonizing transport through circular battery solutions: Life cycle impacts of hydrometallurgy vs pyrometallurgy in NMC battery recycling
Neither method recovers everything perfectly, and both still consume significant energy. Powering recycling facilities with renewable electricity further reduces their environmental footprint. A longer-term goal in the field is “direct recycling,” where cathode materials are restored to their original crystal structure without fully breaking them down into individual elements first. If that approach scales, it could dramatically cut both the cost and the environmental burden of closing the loop on electrochemical cells.
How Electrolyte Choices Shape Cell Voltage
One detail that surprises people outside the field is how much the electrolyte formulation affects the voltage a cell delivers. Even for the same electrode chemistry, changing the salt concentration in the electrolyte shifts the measured voltage. In lithium primary batteries using fluorinated graphene cathodes, for instance, increasing the lithium salt concentration from 0.5 molar to 2.0 molar dropped the discharge voltage from about 2.89 volts down to about 2.77 volts at 25 percent depth of discharge.18Journal of Power Sources. The influence of electrolyte concentration and solvent on operational voltage of Li/CFx primary batteries elucidated by Nernst Equation That may sound like a small shift, but in a battery pack with dozens or hundreds of cells in series, small per-cell voltage differences add up. The relationship between ion concentration and cell voltage follows directly from the thermodynamics of the reaction, and optimizing electrolyte composition is one of the many levers engineers pull when designing cells for specific applications.
Solvent choice matters too. Different solvents change how easily ions move, how the electrolyte interacts with electrode surfaces, and what temperature range the cell can operate in. The electrolyte is often the least glamorous component of an electrochemical cell, but it quietly determines a surprising share of real-world performance.