Introduction & Overview

 
Electrolyzers convert abundant chemicals into more valuable ones by the passage of electricity, normally by breaking down compounds into elements or simpler products. Electrolysis of liquid water [H₂O] into hydrogen gas [H₂] and oxygen gas [O₂] is the classic example of electrochemistry shown to us in science class as teenagers. The principle is easily demonstrated by connecting platinum-tipped wires to a car battery, immersing them in tap water fairly close to each other, and observing the gas bubbles produced. When collected separately, the gases are seen to be different. The H₂ reacts to a glowing splint by burning with a characteristic pop, while the O₂ kindles a flame instead. On a weight basis, H₂ is the most energy-rich fuel known and burns to give almost pure water. Thus, electricalenergy from the battery has been converted into chemical energy contained in the H₂.

Although the efficiency of a typical classroom experiment is very low, state-of-the-art industrial electrolyzers based on polymer-electrolyte membrane [PEM] separators now operate routinely at over 85%. These devices anticipate the imminent development of a renewable energy economy based on electricity and H2 fuel as complementary energy vectors.  The reason for producing H2 from electricity, which of course consumes energy, is to create a form of energy storage useful for indefinite or long time-scales. As necessary storage time increases, the superior economy of chemical storage more than compensates for this energy expenditure.  Just how the PEM electrolyzer achieves these very high efficiencies for electrical-to-chemical energy conversion is described in more technical detail in the next section.

Process Details

The heart of a PEM electrolyzer is shown in the schematic diagram above. Its efficiency is a function primarily of membrane and electrocatalyst performance. This becomes crucial under high-current operation, which is necessary for industrial-scale application.

Membrane: Electrolyte and Gas Separator

The membrane consists of a solid fluoropolymer which has been chemically altered in part to contain sulphonic acid groups, SO₃H, which easily release their hydrogen as positively-charged atoms or protons [H⁺]:

SO₃H -> SO₃^- + H⁺

These ionic or charged forms allow water to penetrate into the membrane structure but not the product gases, molecular hydrogen [H₂] and oxygen [O₂]. The resulting hydrated proton, H₃O⁺, is free to move whereas the sulphonate ion [SO₃^-] remains fixed to the polymer side-chain. Thus, when an electric field is applied across the membrane the hydrated protons are attracted to the negatively-charged electrode, known as the cathode. Since a moving charge is identical with electric current, the membrane acts as a conductor of electricity. It is said to be a protonic conductor. A typical membrane material is sold by Du Pont under the trade name nafion^®. It has several advantages over conventional electrolyzers which normally use an aqueous caustic solution for workable conductivity. Because nafion^® is a solid, its acidity is self-contained and so chemical corrosion of the electrolyzer housing is much less problematic. Because it is an excellent gas separator, allowing water to permeate almost to the exclusion of H₂ and O₂, it can be made very thin; typically only 100 microns, or one tenth of a millimeter. This also improves its conductivity so that the electrolyzer can operate efficiently even at high currents. It is said that the membrane suffers less from internal voltage losses due to a high current passing through a smaller resistance, as given by Ohms Law, viz., V = IR.

However, the membrane also has some disadvantages. Unlike conventional polymers which are water-repellent, nafion^® is a very expensive material. It must also be kept humidified constantly, otherwise its conductivity deteriorates. This last is never a serious problem in an electrolyzer because of contact with hot water, but the PEM fuel cell requires intensive water management for stable, continuous operation. This is described in detail under PEM Fuel Cell.

Electrocatalysts: making electrochemistry easier

A voltage of about 1.5V is supplied to the metal plate electrodes and a unidirectional (DC) electric current is caused to flow. Protons are drawn to the cathode and are discharged as H atoms by combination with electrons (e^-) at the metal cathode surface (M). Pairs of adsorbed H atoms then combine to make molecules of H₂ gas which escape, freeing the electrode surface for more proton discharge:

4H⁺ + 4e^- -> 4M-H

4M- H -> 4M + 2H₂ At the positive electrode or anode, electrons are lost by incoming water molecules creating O ad-atoms, and protons. The electrons are shunted to the cathode, protons enter the membrane, and two O atoms combine to release O₂ gas :

2H₂O -> 2M-O + 4 H⁺ + 4 e^-

2M-O -> 2M + O₂

Although the overall process or mechanism is complex, its sum or balance is simply equivalent to producing two molecules of hydrogen and one molecule of oxygen from two molecules of water:

2H₂O -> 2H₂ + O₂

Since chemical (H₂) energy is being created, a minimum energy must be input to drive the process according to the laws of thermodynamics. In terms of electrical energy, this corresponds to a voltage of 1.23V. In reality, the working voltage necessary to sustain water electrolysis is always greater than this. The extra voltage, generally known as the overvoltage, represents a waste of energy or loss of efficiency. It has two main causes, one of which is the IR loss due to the finite electrical resistance of the electrolyte, or membrane in this case (see above). The second is kinetic in origin, i.e., to do with the overall speed of the process at the electrode surface.

A solid catalyst (M) speeds up chemical reactions due to its surface action. As a simple example, two H atoms held loosely on a surface are much more likely to collide and make H₂ gas than if they are dispersed in a liquid with billions of water molecules in-between. This is a spatial or localized concentration effect. The case of O₂ evolution is much more complex. Two water molecules must be broken into their constituent atoms; then the two O atoms must combine. The electrocatalyst at the anode is a special catalyst which facilitates this process by withdrawing electrons from the water such that the H atoms are ejected as protons, which enter the membrane. Water is said to be activated by charge-transfer. The OH or O atoms are very reactive in their free state. However, when fixed at the surface by chemical bonds, they are much more stable. When more water encounters the surface, its protons are ejected in turn and O atoms are accumulated. These are then able to combine easily by surface diffusion just as described for hydrogen. It is said that the surface provides a low-energy pathway or a new mechanism, which is intrinsically much faster because the speed of the reaction is related exponentially to the energy difference.

It is easy to visualize that if the cathode and anode surfaces, respectively, attract H or O atoms too strongly, the surfaces will become completely covered with these intermediates and the catalytic process stops. On the other hand, if protons or water are not attracted strongly enough, the process never gets going. Only when there is a moderate strength of binding of reactants and intermediates at the electrode surfaces will the right balance be obtained. This is the key factor in determining if a solid catalyst will work efficiently. It is also obvious that the larger is the catalyst surface area available, the more H₂ and O₂ will be produced in a given time, i.e., a higher current will flow in the electrolyzer.

Platinum is long known to be the best catalyst for water electrolysis due to its moderate strength of adsorption of the intermediates of relevance. It has the lowest over-voltage of all metals. Because of its cost, and the preferred operation of the electrolyzer at high current, ingenious ways have been devised to deposit ultra-fine Pt particles either on the electrode support plate, or directly onto the membrane, which is then clamped for good electrical continuity. A current of 1-3 Amperes per square centimeter can be obtained from as little as 3 milligrams of Pt spread over the same area.

written by Jim Highfield
last modified Sunday,  29th August 1999

Glossary

Polymer Electrolyte Membrane [PEM]

Polymer electrolyte membranes [PEMs] were first developed for the chlor-alkali industry. These specialized materials, also known as ionomers, are solid fluoropolymers which have been chemically altered to make them electrically conductive. The fluorocarbon chain or backbone has typically a repeating structural unit, such as - [CF_2¾ CF₂]_n- , where n is very large. Treatments like sulphonation or carboxylation, insert ionic or electrically-charged pendant groups, statistically spaced, into some of these base units giving, e.g., - [CF( SO_3- H+ ) ¾ CF2]_m- , where m can range from n/5 to n/20 depending on the properties most suited for the application. Nafion^Ò , made by Du Pont, is the most commercially successful of such materials, and is available as thin pre-formed membranes in various thicknesses, or as a 5 % solution which may be deposited and evaporated to leave a polymer layer of customized shape.

By coincidence, the letters PE in PEM can stand for "Polymer Electrolyte" or "Proton Exchange" because the last process is linked to the conductivity. Unfortunately, these terms are used interchangeably leading to some confusion for the layman. What is worse, PEM and SPE (Solid Polymer Electrolyte) are also used interchangeably, but the key point is that they all refer to the nafion-type ionomer or its family of compounds.

 

Solid Fluoropolymer

Normal polymers found in the amazing variety of plastics so useful today are hydrocarbon-based, i.e., the repeating unit or monomer consists of carbon (C) and hydrogen (H) atoms. However, for some specialized applications the hydrogen component is not ideal and is replaced by fluorine (F). Perhaps the best-known example is teflon, the non-stick coating used in cooking utensils. Teflon is polytetrafluoroethylene (PTFE), which can be written in chemical shorthand as -[CF_2¾CF₂]_n- , where n can range typically from 100 to 100,000. Fluoropolymers are more chemically and thermally inert than conventional polymers, but are more expensive to manufacture. Their chemical stability is the key to their usefulness as base materials in the manufacture of polymer electrolyte membranes for electrolyzers and fuel cells.

 

Hydrated Proton

The hydrated proton is a proton, H⁺, associated with a water molecule, H₂O, and is usually written as H₃O⁺ although even this is an oversimplification! All chemical species with like electrical charges, i.e., minus/minus or plus/plus, suffer strong repulsion when in close proximity. Only solvent molecules with a strong dipole like water encourage the dissolution of compounds as ions. For example, common salt, NaCl, will dissociate into its constituent ions Na⁺ and Cl^- much more easily in water than in oil. Sodium cations are neutralized or electrically shielded to some extent by attracting the partial negative charge on the O atom of water, whilst the chloride anion is masked by attracting the positively charged H atoms.

In a polymer electrolyte membrane, protonic conductivity or motion is only made possible by the presence of water. Otherwise, the proton prefers to stay close to the sulphonate anion and they coexist as an ion-pair. Depending on the charge and size of the ion, a different number of water molecules will coordinate to it. This can be as high as 6 in some cases, but for the proton it is usually 1 or 2. Obviously, the solvent "sheath" exerts a "drag" and tends to slow down the proton's movement through the membrane but this is unavoidable.

 

Sulphonate ion

There is a small problem of nomenclature when similar combinations or groups can exist. In this case, the sulphonate anion, SO₃^- , is distinct chemically from related alternatives like sulphate, SO₄^2- , and even sulphite, SO₃^2- .

 

Solid Catalyst

Catalysts come in various forms but the vast majority useful in industrial processes are solids because they can be easily manipulated, e.g., periodically separated from the reactants and products for replacement or regeneration. Since most of these are used for the conversion of gases or liquids, they are said to be heterogeneous. An electrocatalyst is a specialized heterogeneous catalyst in that it must often double as electrode material, i.e., it must be an electrical conductor in addition to being an active catalyst. This restricts the choice to metals or occasionally semi-conductors. Some homogeneous processes exist in which the catalyst is an individual chemical species or complex dissolved in the reaction mixture. Enzymes are a natural example of these.

Catalysis literally means "speeding up breakdown" although much industrial catalysis involves synthesis, i.e., making more complex products from simpler reactants. A catalyst is usually defined as a material which accelerates a chemical process while remaining intact or unchanged, such that it can be recovered later. However, substantial chemical exchange of the surface atoms can sometimes occur, as in partial oxidation of hydrocarbons over oxide catalysts. Here, O atoms are inserted into the hydrocarbon directly from the catalyst surface, or even sub-surface, but these are quickly replaced by O atoms from molecular oxygen, O₂, present as co-reactant.

A good catalyst must be active, selective, and stable. Activity is usually defined in terms of molecular turnover, i.e., how many reactant molecules are converted in a given time per unit mass of catalyst, or more usefully, per active site at the surface. This may be an individual atom but is more often clusters or ensembles of several atoms. Because of complex electronic and geometric factors associated with the solid surface, not all surface atoms behave in the same way, such that the number of active sites in any specific reaction may be less than 1%. Obviously, the task of solid catalyst development is to tailor the surface to increase the number, or density, of these active sites as well as their intrinsic or individual activity.

Selectivity becomes important when there are several products which can form but only one of these is desirable. Industrial synthesis of methanol from carbon oxides and hydrogen over supported copper catalysts is a classic example of this. Chemical thermodynamics predicts that the products most likely to form are hydrocarbons whereas the selectivity to methanol in practice is often better than 95%.

Industrial catalysts are unusually stable in the sense that they can be used on-stream for years at a time if necessary, performing billions of turnovers before they must be replaced or regenerated. Regeneration often involves controlled "burning-off" of coke or carbon, which poisons the active sites or fouls the catalyst by plugging pores in the catalyst network, thereby preventing physical access of reactants. Plant economic factors eventually decide what is a suitable catalyst. The most active one is not necessarily the best.