Palladium-Hydrogen Electrode

The palladium-hydrogen electrode, usually written Pd/H2 or PdH, is a reference electrode made from palladium metal that has absorbed hydrogen. It is a member of the hydrogen electrode family, the group of electrodes built on the reaction between hydrogen ions in solution and hydrogen gas. In use it behaves much like the better-known standard hydrogen electrode. The difference is that palladium can soak hydrogen into its crystal lattice, and that ability lets the electrode hold a steady potential without a stream of hydrogen gas bubbling past it.

That one property explains why the electrode exists. A conventional hydrogen electrode needs a gas cylinder, a bubbler, a platinized platinum surface and careful exclusion of oxygen. A palladium electrode that has been loaded with hydrogen carries its own supply inside the metal. It can be made very small, sealed into a microfluidic chip, or dropped into an autoclave where a gas line would be awkward. It also brings its own quirks: the stored hydrogen slowly leaks away, the potential depends on which hydride phase the metal is in, and the material swells as it absorbs hydrogen.

This article covers what a reference electrode is for, how the hydrogen electrodes relate to one another, the chemistry of palladium hydride that makes the electrode work, why its potential sits on a plateau, how electrodes are prepared and used, where they turn up in research and industry, and what can go wrong.

What a reference electrode does

Potentials are always differences

Nobody can measure the electrical potential of a single electrode on its own. A voltmeter reads the difference between two points, so every electrochemical potential is reported against something. In a typical three-electrode experiment there is a working electrode, where the reaction of interest happens, a counter electrode that carries the current, and a reference electrode that carries almost no current and holds a known, stable potential. The instrument controls or measures the working electrode's potential relative to the reference.

A good reference electrode has a few jobs:

• Its potential should be known, so that results can be converted to a common scale.
• Its potential should stay put over the length of the experiment.
• It should not contaminate the solution being studied.
• It should recover quickly if a small current briefly flows through it.
• It should survive the temperature, pressure and chemistry of the experiment.

No single design does all of these perfectly in every setting. That is why chemists keep a whole catalog of reference electrodes, and why niche designs like the palladium-hydrogen electrode survive.

The common reference electrodes

The table below lists widely used aqueous reference electrodes and their potentials relative to the standard hydrogen electrode, as compiled in Wikipedia's reference electrode article. The palladium-hydrogen electrode is listed there too, but without a single fixed value. That is because its potential is tied to the pH of the solution it sits in, as explained later.

Most of the electrodes in that table depend on a salt solution held behind a porous junction. That design works well at room temperature, but it has drawbacks. Chloride or sulfate can leak into the test solution. A liquid junction potential forms where two different solutions meet. At high temperature, the filling solution can boil or degrade. The hydrogen electrodes avoid some of these problems because they respond directly to the hydrogen ions already present in the solution.

The hydrogen electrode family

Standard and normal hydrogen electrodes

The standard hydrogen electrode is the anchor of the electrochemical potential scale. Its half-reaction is:

2 H⁺(aq) + 2 e⁻ ⇌ H₂(g)

By convention its standard potential is exactly zero at every temperature. The practical setup described in the Wikipedia article uses a platinized platinum electrode, meaning platinum coated with finely divided platinum black, hydrogen gas bubbled over it, an acid solution in which the hydrogen ion activity equals one, a bubbler that keeps out oxygen, and a connection to the second half-cell. The platinum black increases the surface area and speeds up the reaction. Well-made electrodes of this kind agree with each other to within about 10 microvolts.

The older normal hydrogen electrode used a platinum electrode in 1 N strong acid with hydrogen at about 1 atmosphere. The standard hydrogen electrode replaced it with an idealized definition: a hydrogen ion activity of one and a hydrogen pressure of 1 bar. Wikipedia notes that this ideal solution is not physically attainable at those concentrations, which is why the standard hydrogen electrode is a reference point on paper more than a device on the bench.

The same article estimates the absolute electrode potential of the standard hydrogen electrode at 4.44 ± 0.02 V at 25 °C. That number matters to physicists comparing electrochemical and vacuum energy scales, but everyday electrochemistry works with the convention that the standard hydrogen electrode is zero.

Hydrogen electrodes are sensitive. The platinum surface can be poisoned by arsenic, sulfides, colloids, alkaloids and biological material. Easily reduced ions such as Fe³⁺ and chromate interfere, and metal ions such as Ag⁺, Hg²⁺, Cu²⁺, Pb²⁺, Cd²⁺ and Tl⁺ can plate out on the platinum. Organic substances and oxygen adsorb on the surface and shift the reading.

The reversible hydrogen electrode

The reversible hydrogen electrode, or RHE, is a hydrogen electrode placed directly in the solution being studied rather than in a separate acid of unit activity. Because it sees the actual pH of the working solution, its potential moves with pH. At 25 °C and 1 bar hydrogen it follows the relation:

E(RHE) = 0.000 V − 0.0591 × pH (vs. SHE)

Wikipedia's article on the RHE lists its practical advantages. There is no salt bridge, so the electrolyte is not contaminated by chloride or sulfate. There is no liquid junction potential, which matters at temperatures other than 25 °C. And long-term measurements are practical because there is no bridge to maintain.

The RHE scale is popular in research on fuel cells, water electrolysis and electrocatalysis. A potential reported "vs. RHE" tells a reader how far a reaction is from the thermodynamic hydrogen potential in that same solution, which is often more useful than a value against a distant standard.

The dynamic hydrogen electrode

The dynamic hydrogen electrode makes hydrogen on the spot. A small current is passed between a cathode and an anode, so hydrogen is generated at one electrode, which then acts as the reference. According to Wikipedia, it simulates an RHE at a potential about 20 to 40 mV more negative. Dynamic hydrogen electrodes have been used as in-situ references in direct methanol fuel cells and proton-exchange membrane fuel cells.

Where palladium fits

The palladium-hydrogen electrode sits between these options. Like the RHE, it responds to the pH of the solution it is in. Like the dynamic hydrogen electrode, it can avoid a gas cylinder. Its distinguishing trick is that the hydrogen is stored inside the metal itself. Wikipedia's article on the electrode lists this as its main advantage: it does not need a continuous stream of hydrogen gas through the solution, as the standard hydrogen electrode does.

Palladium and hydrogen: the chemistry behind the electrode

A nineteenth-century discovery

The story starts with the Scottish chemist Thomas Graham, who reported in 1866 that palladium can take up hydrogen in large quantities. A report in the Proceedings of the Royal Society of Edinburgh from 1868–69 records palladium heated and cooled in hydrogen absorbing roughly six to seven hundred times its own volume of the gas, and electrodeposited palladium taking up as much as 982 volumes. Wikipedia's palladium hydride article gives a modern figure in the same range: palladium can absorb up to 900 times its own volume of hydrogen at standard temperature and pressure.

Graham interpreted his results in a way that now sounds charmingly wrong. He supposed that hydrogen gas was the vapor of a very volatile metal, which he called "hydrogenium," and that it formed an alloy with palladium. He had a medal struck from this supposed alloy and gave it to John Herschel, his predecessor as Master of the Mint. The Science Museum in London holds an object catalogued as the "Palladium Hydrogenium" medal, dated 1869. Graham used the word "occlusion" for the way palladium held the gas.

The absorption of electrolytically produced hydrogen into a palladium cathode, which is the process used to charge most palladium reference electrodes, was first documented in 1939, according to the palladium hydride article.

Where the hydrogen goes

Palladium has a face-centered cubic crystal structure. When hydrogen enters, it splits into atoms at the surface, and those atoms settle into the spaces between palladium atoms, specifically the octahedral holes in the lattice. At ordinary pressure, absorption tops out near the composition PdH₀.₇, meaning about 70 percent of the octahedral holes are occupied. If every hole were filled, the formula would be PdH₁.

The overall equilibrium is often written in three steps, as Wikipedia's electrode article does:

½ H₂ ⇌ H(adsorbed) ⇌ H(absorbed)

Hydrogen molecules in the gas or in solution split into atoms that sit on the surface, and those atoms move into the bulk of the metal. In an electrochemical setting, hydrogen atoms can also be produced directly on the surface by reducing hydrogen ions from the solution.

Two phases, alpha and beta

Palladium hydride does not absorb hydrogen smoothly and uniformly. At room temperature it forms two distinct solid phases that share the same face-centered cubic palladium framework but differ in how much hydrogen they hold and in the size of the unit cell.

The figures come from Wikipedia's palladium hydride article, which gives pure α below x = 0.017 and pure β above x = 0.58; the electrode article puts the α limit at about 0.025 hydrogen atoms per palladium atom and places the β phase near PdH₀.₆. The small differences reflect different sources and conditions rather than a disagreement about the physics.

Between the two limits, the metal is a mixture of α and β regions. As more hydrogen goes in, the β regions grow at the expense of the α regions, but the composition of each phase stays the same. This is the key to the electrode.

The jump in lattice parameter from 388.9 pm to 402.5 pm is large for a solid. Cubing the ratio gives a unit-cell volume increase of about 11 percent ((402.5 / 388.9)³ ≈ 1.109). This calculation is derived from the published lattice constants and is not quoted directly from a source, but it shows why repeated charging can strain and distort palladium.

Other properties worth knowing

Palladium hydride has a few other properties that show up in the literature and occasionally in electrode work:

• Conductivity. Metallic conductivity falls as hydrogen is absorbed, and the solid becomes a semiconductor at around PdH₀.₅.
• Superconductivity. At high loadings near x = 1, palladium hydride becomes superconducting with a transition temperature of about 9 K. Pure palladium is not a superconductor. Reaching x above about 0.75 needs special loading methods, such as cooling during gas loading or electrochemical loading at low temperature.
• Size effects. Hydride does not form in palladium particles smaller than about 2.6 nm, and smaller particles dissolve less hydrogen in their bulk.
• Hydrogen purification. Thin-walled silver-palladium tubes let hydrogen isotopes pass while blocking other gases. Such diffusers are kept above 300 °C to avoid forming the β phase, and silver is added to improve strength and resistance to embrittlement.

Why the potential stays flat

The Nernst relation for a hydrogen electrode

For any hydrogen electrode, the equilibrium potential depends on the hydrogen ion activity in solution and on the hydrogen pressure, or more precisely the hydrogen fugacity. Wikipedia's electrode article writes the relationship as:

E = E⁰ + (RT/F) ln [ a(H⁺) / (p(H₂)/p⁰)^½ ]

Here R is the gas constant, T the absolute temperature, F the Faraday constant, a(H⁺) the hydrogen ion activity and p⁰ the standard pressure of 1 bar. Two things push the potential around: the pH of the solution, and the effective hydrogen pressure at the electrode.

For a gas-bubbled hydrogen electrode, the effective pressure is set by the gas supply. For a palladium electrode, there is no gas supply. The effective hydrogen pressure is instead whatever pressure of hydrogen would be in equilibrium with the hydrogen dissolved in the metal.

The two-phase plateau

Here is where the α and β phases earn their keep. When a binary system such as palladium and hydrogen has two solid phases coexisting at a fixed temperature, the thermodynamic rule known as the Gibbs phase rule leaves no freedom for the hydrogen activity to change. Adding hydrogen just converts more α into β. Removing hydrogen converts β back into α. As long as both phases are present, the chemical potential of hydrogen in the metal, and therefore the equivalent hydrogen pressure, stays fixed.

A fixed equivalent hydrogen pressure means a fixed term in the Nernst equation. The electrode's potential then depends only on the pH of the solution, exactly like an RHE operated at some constant hydrogen pressure. This is why the Wikipedia article reports that palladium charged electrochemically with hydrogen shows a constant potential of about +50 mV relative to the RHE, and that this value is nearly independent of the amount of absorbed hydrogen over a wide range.

Other sources give closely matching numbers:

The last row is a warning as much as a fact. If the palladium is loaded so heavily that only the β phase is left, the plateau disappears and the potential moves toward the RHE value. Charging an electrode correctly means landing in the two-phase region, not simply cramming in as much hydrogen as possible.

Worked example: what the 50 mV offset implies

This example is a calculation from the Nernst equation, not a value reported by the sources.

If a palladium electrode in the two-phase state sits 50 mV positive of an RHE that is operating at 1 bar of hydrogen, the difference can be expressed as an equivalent hydrogen pressure. From the Nernst relation, the potential shift caused by changing the hydrogen pressure from 1 bar to p is:

ΔE = −(RT/2F) ln (p / 1 bar)

At 25 °C (298.15 K), RT/F is about 0.02569 V. Setting ΔE = +0.050 V gives:

ln p = −2 × 0.050 / 0.02569 ≈ −3.89, so p ≈ 0.020 bar

In other words, the two-phase palladium electrode behaves like a hydrogen electrode fed with hydrogen at roughly 2 kilopascals, about one-fiftieth of an atmosphere. Using the 55 mV figure from the DTU study instead gives about 0.014 bar. The point of the exercise is not the precise pressure but the idea: the hydride acts as a built-in, fixed-pressure hydrogen reservoir.

Worked example: converting a measurement

This example is hypothetical and uses rounded textbook constants.

Suppose a researcher measures a working electrode at +0.300 V against a palladium-hydrogen reference in a buffer of pH 7.0 at 25 °C, and wants to report it against the RHE and the SHE.

1. The Pd/H reference sits about +0.050 V above the RHE in this state. So the working electrode is 0.300 + 0.050 = +0.350 V vs. RHE.
2. The RHE in a pH 7.0 solution sits at −0.0591 × 7.0 = −0.414 V vs. SHE.
3. So the working electrode is 0.350 − 0.414 = −0.064 V vs. SHE.

The same measurement in a pH 1.0 acid would convert differently. The RHE would sit at −0.059 V vs. SHE, and the working electrode would be at about +0.291 V vs. SHE. This pH dependence is the defining feature of RHE-type electrodes, and it is why papers that use a Pd/H reference usually report results on the RHE scale.

How steeply potential tracks pH

The factor 0.0591 V per pH unit is the value of (RT ln 10)/F at 25 °C. It grows with temperature. Computed from the constants:

Real palladium electrodes come close to this ideal but not always exactly. A 2019 study by Xu and colleagues, cited in later literature, reported a palladium electrode with a near-Nernstian response of −56.2 ± 0.2 mV per pH unit between pH 1 and pH 13. That slightly sub-ideal slope is typical of practical sensors and is one reason they need calibration against buffers.

Making and maintaining a palladium-hydrogen electrode

Basic construction

A palladium-hydrogen electrode starts as a piece of palladium: a wire, a foil, a thin film deposited on a chip, or a closed tube. It is loaded with hydrogen in one of two ways.

• Electrolytic charging. The palladium is made the cathode in an electrolyte and a current is passed. Hydrogen ions are reduced at the surface, and the resulting atoms diffuse into the metal. This is the method behind most laboratory and microelectrode designs.
• Gas loading. The palladium is exposed to hydrogen gas from a cylinder. Some high-temperature designs pressurize a palladium or palladium-silver tube with hydrogen from the inside.

A patent survey summarized in recent literature notes that most palladium-hydrogen electrodes are prepared without a secondary power source providing a continuous hydrogen supply. They are charged once, from a non-continuous electrolytic process or a cylinder, and then used.

The leakage problem

A once-charged electrode does not stay charged. Hydrogen escapes from the surface, especially into solutions that contain dissolved oxygen or other oxidizers. As it does, the β phase shrinks. Once the β phase is gone, the two-phase plateau is lost, and the potential starts to drift as the metal reverts toward pure palladium.

How long an electrode lasts depends heavily on geometry. A 2023 paper on pH microelectrodes by Li, Perry and Mauzeroll notes that palladium can hold the α + β state for anywhere from 12 hours to 3 weeks, depending on the ratio of exposed surface to volume. A thin film or fine wire, with a lot of surface and little metal, empties fastest. A thicker piece holds out longer.

Continuous charging

Designers have attacked the leakage problem by feeding hydrogen in continuously. One patent describes an electrode with an internal chamber in which a constant current between a pair of electrodes keeps generating hydrogen. The DTU Energy team behind the 2022 high-temperature study used what they described as a constant electrochemically driven supply of hydrogen to the palladium hydride surface, which kept the mixed α + β phase stable for long periods.

The idea is old. A 1956 study by Schwing and Rogers in Analytica Chimica Acta compared different forms of palladium as pH indicators for single measurements and titrations. They found the most satisfactory form was a palladium membrane on which hydrogen was generated continuously.

Charging in place on a chip

Microfabrication makes in-situ charging especially convenient. In a 2017 paper in Electrochimica Acta, Fanavoll, Harrington, Sunde, Singh and Seland described a thin-film palladium hydride reference electrode made by photolithography and charged with hydrogen in place using a simple current step. The reference electrode sat in a side channel of a microfluidic cell, away from species in the main channel. Placing it upstream of the working and sense electrodes performed significantly better than using an external reference. The authors also analyzed how solution resistance distorts measurements at high currents and gave a formula for the maximum usable current density based on cell geometry and electrolyte resistivity.

Practical charging checklist

The following summarizes general practice drawn from the sources above. Specific currents and times vary with electrode size and are best taken from the paper or manufacturer instructions for a given design.

Where palladium-hydrogen electrodes are used

Microreference electrodes

Because the hydrogen is stored in the metal, a palladium wire can be made tiny without needing a gas supply. That made it an early candidate for microreference electrodes. A short paper titled "A palladium-hydrogen probe electrode for use as a microreference electrode," cited by Wikipedia, appeared in 1968 in the Journal of Scientific Instruments, the series that later became Journal of Physics E. A patent citing the work lists Fleischmann as an author.

Microelectrodes appear in scanning electrochemical microscopy, measurements near surfaces, and biological or corrosion studies where a conventional reference would be far too large. A small palladium-hydrogen element can sit very close to the working electrode, which reduces errors caused by the resistance of the solution between them.

pH sensing and titration

Since the plateau potential tracks pH in the same way as an RHE, a charged palladium electrode can act as a pH indicator. Research on palladium hydride pH sensors continues because palladium is robust and can be miniaturized, unlike the glass membrane of a standard pH electrode.

The same property makes palladium-hydrogen electrodes useful in coulometric titrations, in which acid or base is generated electrically and the end point is detected by a pH-sensitive electrode. Wikipedia cites a University of Otago document on palladium-hydrogen electrodes for coulometric titration, and a 1992 paper by Munasiri and colleagues in the Journal of Electroanalytical Chemistry described palladium-hydrogen electrodes for coulometric titration of acids and bases.

Microfluidic and high-current cells

The Fanavoll design described above shows the attraction in microfluidics. A conventional reference electrode connected through a long, narrow channel suffers from large resistance and slow response. An on-chip palladium film charged in place sits right in the flow path, close to the action.

High temperature and pressure

Many reference electrodes struggle above 100 °C. Filling solutions evaporate, junctions clog, and silver chloride becomes unstable. A palladium-hydrogen electrode has no filling solution, which makes it attractive for high-temperature work.

The 2022 DTU Energy study by P. Leuaa and C. Chatzichristodoulou is a good modern example. They wanted to measure hydrogen evolution kinetics on platinum and nickel in alkaline electrolyte at temperatures from 25 to 150 °C and pressures from 1 to 50 bar, conditions relevant to industrial water electrolysis. They noted that high-temperature, high-pressure studies had been limited by the short life of existing reference electrodes. Their palladium hydride reference, kept in the mixed phase by continuous electrochemical hydrogen supply, was calibrated against the RHE as a function of temperature and pressure and gave about 55 mV vs. RHE in the mixed phase, a value they observed to be independent of temperature and pressure.

Nuclear power plant water

Light-water reactors run at high temperature in water whose chemistry is tightly managed, and engineers measure the electrochemical corrosion potential of steels and alloys to judge the risk of stress corrosion cracking. Patent literature describes hydrogen reference electrodes built from a closed-end tube of palladium-silver alloy pressurized internally with hydrogen and enclosed in a perforated sleeve, intended for high-temperature water and especially nuclear reactor systems. One such patent notes that silver-silver chloride references work in oxidizing conditions but become unstable in reducing environments, which is where a hydrogen electrode is a natural fit.

Related patent work on boiling water reactors shows how much hydrogen chemistry matters in this setting. One patent reports that adding palladium or platinum at levels as low as 0.1 percent to alloys gives a low corrosion potential, about the same as a pure platinum electrode. Without such additions, operators need much higher dissolved hydrogen levels, about 100 to 150 parts per billion, to achieve the same effect. That is a use of palladium's catalytic behavior rather than a reference electrode, but it comes from the same chemistry.

Fuel cells and electrolyzers

Proton-exchange membrane fuel cells and electrolyzers already contain hydrogen, so hydrogen-type references are natural choices. The dynamic hydrogen electrode is the more common in-situ option in fuel cells. Palladium hydride references show up mainly in research cells where a compact, gas-free reference is needed. Operando studies of palladium in hydrogen have found a bulk hydride of about PdH₀.₆ present at potentials up to 70 mV vs. RHE, which illustrates how readily palladium forms hydride under fuel-cell-like conditions.

Comparison with other reference electrodes

The table shows the palladium-hydrogen electrode's niche. It is not the most stable electrode for routine room-temperature work, where a commercial silver/silver chloride electrode is simpler. It shines where a gas line, a filling solution or a bulky body is impossible or harmful.

Limitations and pitfalls

Hydrogen loss and drift

The most important limitation is that a charged electrode slowly empties. Without a hydrogen supply, the hydride phases lose hydrogen and convert back to palladium, and the potential drifts. The 12-hour to 3-week range reported for the α + β state shows how much this depends on design. Anyone using a once-charged electrode for a long measurement should check it periodically against another reference.

Overcharging

As the DTU work shows, loading the electrode until only the β phase remains drops the potential to about 0 mV vs. RHE. If an operator assumes a +50 mV offset while the electrode is actually overcharged, every reported potential will be off by about 50 mV. A short rest period, or a deliberate partial discharge, can bring the electrode back into the two-phase region.

Oxidizers and redox species

Dissolved oxygen and other oxidizing species react with hydrogen at the palladium surface. That speeds hydrogen loss and can create a mixed potential, a reading that reflects competing reactions rather than true equilibrium. The same list of interferences that troubles platinum hydrogen electrodes, including easily reduced ions and surface poisons, is a reasonable starting point for what to avoid with palladium, though the exact sensitivity depends on the system.

Mechanical strain

The roughly 11 percent unit-cell volume change between pure palladium and the β phase puts the metal under strain when it cycles between phases. The addition of silver to palladium diffuser tubes specifically to resist embrittlement shows that hydrogen damage is a real engineering concern. Electrodes that are charged and discharged many times can distort or crack.

pH dependence

The pH dependence is a feature for some uses and a flaw for others. If the pH of the test solution changes during an experiment, a palladium-hydrogen reference moves with it. In a process where acid or base is consumed or generated, that can shift the reference by tens of millivolts. Researchers who want a fixed reference independent of pH need a different electrode or a separate compartment.

Temperature

The plateau offset reported by the DTU group was independent of temperature and pressure relative to the RHE in their alkaline system, but the RHE itself moves with temperature through the RT/F factor. Converting results to the SHE scale at high temperature requires care, and a calibration at the actual operating conditions is the safest course.

Common misconceptions

"The palladium-hydrogen electrode is the same as the standard hydrogen electrode." It is closely related, but not identical. The standard hydrogen electrode is defined at unit hydrogen ion activity and 1 bar of hydrogen. A palladium-hydrogen electrode sits in the working solution and, in its two-phase state, behaves like an RHE at a reduced effective hydrogen pressure, about 50 mV positive of the RHE.

"Its potential is fixed, so it does not need calibration." The potential is constant only relative to the RHE in the same solution, and only while the electrode stays in the two-phase region. It shifts with pH, drifts as hydrogen leaks out, and drops if the electrode is overcharged.

"More hydrogen means a better electrode." The opposite can be true. A fully β-phase electrode loses the flat plateau that makes the device useful. What matters is having both phases present.

"Palladium only stores a little hydrogen." Palladium can take up hundreds of times its own volume of hydrogen gas. Graham's nineteenth-century figures of six to seven hundred volumes, and up to 982 for electrodeposited metal, match the modern figure of up to 900 volumes.

"Palladium electrodes prove cold fusion." Palladium electrodes loaded with hydrogen or deuterium were central to cold fusion experiments, which were based on the idea that hydrogen packed into the lattice might fuse at low temperature. Wikipedia's palladium hydride article does not present this as established, and it is unrelated to the use of palladium as a reference electrode.

Frequently asked questions

What is a palladium-hydrogen electrode used for?

It is used as a reference electrode, especially where a conventional reference is too large, needs a filling solution, or cannot survive the conditions. Typical uses include microelectrodes, microfluidic cells, pH sensing, coulometric titrations, and high-temperature or high-pressure electrochemistry.

What potential does it have?

In the two-phase α + β state, about +50 mV relative to the reversible hydrogen electrode in the same solution. One 2022 study measured about 55 mV at 25 °C and 1 bar. Because the RHE itself moves by about 59 mV per pH unit at 25 °C, the potential against the SHE depends on pH.

Why does it not need a hydrogen gas supply?

The hydrogen is absorbed into the palladium lattice before or during use. As long as both hydride phases are present, the metal acts as a reservoir with a fixed hydrogen activity, which plays the role that the gas supply plays in a standard hydrogen electrode.

How long does a charged electrode last?

It depends on the shape and size of the palladium and on what is in the solution. One review reports the two-phase state lasting from 12 hours to 3 weeks depending on the exposed surface-to-volume ratio. Designs with continuous hydrogen generation can run much longer.

How is it charged?

Usually by making the palladium a cathode in an electrolyte and passing a current, so hydrogen ions are reduced and absorbed. It can also be loaded with hydrogen gas. On microfabricated chips, a simple current step in place has been used.

Can it be used in nonaqueous solvents?

Hydrogen electrodes in general are poorly suited to many nonaqueous solvents, partly because catalytic metal surfaces can be poisoned and the potential drifts. Nonaqueous work more commonly uses quasi-reference electrodes calibrated with an internal standard such as ferrocene, which IUPAC recommended as a standard redox couple in 1984.

Is palladium hydride dangerous?

The electrode itself holds a small amount of hydrogen. Larger quantities of hydrogen-loaded palladium, and the hydrogen gas used for loading, carry the usual fire and explosion hazards of hydrogen, and should be handled under normal laboratory safety rules.

Who discovered that palladium absorbs hydrogen?

Thomas Graham reported it in 1866. He believed the hydrogen formed an alloy with palladium and called the hypothetical metallic form "hydrogenium."

Summary

The palladium-hydrogen electrode turns an old discovery, palladium's remarkable appetite for hydrogen, into a practical tool. When palladium holds enough hydrogen to contain both its dilute α phase and its concentrated β phase, the hydrogen activity in the metal is pinned, and the electrode behaves like a reversible hydrogen electrode shifted about 50 mV positive. That makes it a compact, gas-free, junction-free reference whose potential follows the pH of the solution it sits in.

Its strengths are size, simplicity of construction and tolerance of conditions that defeat salt-filled references, including temperatures up to at least 150 °C and pressures up to 50 bar in recent work. Its weaknesses are slow hydrogen loss, the need to stay in the two-phase region, sensitivity to oxidizers, and mechanical strain from the lattice expansion. Used with those limits in mind, and checked against a known reference from time to time, it remains a useful member of the electrochemist's toolkit.

Sources

• Palladium-hydrogen electrode (Wikipedia)
• Palladium hydride (Wikipedia)
• Reversible hydrogen electrode (Wikipedia)
• Standard hydrogen electrode (Wikipedia)
• Reference electrode (Wikipedia)
• Dynamic hydrogen electrode (Wikipedia)
• Leuaa and Chatzichristodoulou, HER kinetics on Pt and Ni at elevated temperatures and pressures employing palladium hydride as reference electrode, 241st ECS Meeting, 2022
• Fanavoll et al., A microfluidic electrochemical cell with integrated PdH reference electrode for high current experiments, Electrochimica Acta 225 (2017) 69–77
• Shields, On the nature of palladium hydrogen, Proceedings of the Royal Society of Edinburgh
• Palladium Hydrogenium medal, Science Museum Group collection
• US Patent 4,290,872, High temperature reference electrode
• US Patent 5,135,709, Method for reducing corrosion of components exposed to high-temperature water
• Palladium-hydrogen pH electrode patent record
• Li, Perry and Mauzeroll, pH microelectrodes review, Journal of the Electrochemical Society 170 (2023) 087509
• Schwing and Rogers, Comparison of different palladium-hydrogen electrodes as pH indicators (catalog record)