In the intricate machinery of a state-of-the-art electrolyzer, a rare metal is vanishing, molecule by molecule, and the race to understand why hinges on capturing events that last mere femtoseconds.
Iridium is one of the rarest elements on Earth, with annual global production of less than ten tons3 . Yet, this obscure metal is the cornerstone of the hydrogen economy, serving as the essential catalyst that generates oxygen in the water-splitting devices that produce green hydrogen. Its extreme scarcity and skyrocketing demand mean that every atom counts. Scientists are now using ultra-fast, time-resolved techniques to unravel a destructive mystery: why does this critically important catalyst spontaneously dissolve during operation, and how can we stop it? This is the story of the battle to save iridium, fought on the frontier of time itself.
Annual global production
Used in electrochemistry
Required recycling rate
Iridium oxide (IrO₂) is the unrivaled champion of the oxygen evolution reaction (OER)—the process that releases breathable oxygen from water during electrolysis. In the harsh, acidic environment of a proton-exchange membrane water electrolyzer (PEMWE), iridium is the only commercially viable catalyst that combines the necessary activity with sufficient stability to not rapidly corrode6 .
The demand for iridium from the electrochemical industry is immense, accounting for nearly half of its global usage3 . According to one analysis, technical end-of-life recycling rates for iridium must reach a minimum of 90% to sustainably support the large-scale industrialization of PEMWE devices3 . This makes understanding and preventing its dissolution not just a scientific curiosity, but an absolute necessity for a clean energy future.
Iridium is the only commercially viable catalyst for the oxygen evolution reaction in acidic PEM electrolyzers.
Not all iridium oxide is created equal. Its resilience against dissolution depends profoundly on its atomic architecture, which primarily exists in two forms:
Characterized by a well-ordered, rigid lattice of IrO₆ octahedra. This structure is highly stable, resisting dissolution by following a "adsorbate evolution mechanism (AEM)" where the OER occurs only on the catalyst's surface, leaving the lattice intact6 .
Has a highly distorted, non-crystalline structure with iridium vacancies. This disorder creates more electrophilic (electron-loving) oxygen species. These species facilitate a "lattice oxygen participation mechanism (LOM)" where oxygen atoms from the crystal lattice itself are directly involved in the reaction6 .
Key Insight: While the LOM pathway makes amorphous IrOx a more active catalyst, it has a devastating trade-off: it generates oxygen vacancies and weakens the oxide structure, making it profoundly more vulnerable to dissolution6 .
A groundbreaking 2025 study published in Nature Communications uncovered a cruel paradox. In electrolyzers that convert carbon dioxide (CO₂) into valuable chemicals, one of the desired products—ethanol—can cross over to the anode and trigger catastrophic iridium dissolution6 .
Scientists designed a key experiment to simulate the environment inside a zero-gap CO₂ electrolyzer. They evaluated the stability of both IrOx and IrO₂ electrodes during the OER in electrolytes containing ethanol.
IrOx electrodes were created via electrodeposition, while crystalline IrO₂ electrodes were formed through thermal treatment6 .
The electrodes were subjected to OER conditions (applying a high voltage) in electrolytes of varying pH, both with and without the presence of ethanol6 .
Using techniques like X-ray absorption spectroscopy, researchers probed the chemical state of iridium in real-time during the reaction6 .
Advanced microscopy and spectroscopy (SEM, XPS, Raman) were used to examine the electrodes, and the electrolyte was analyzed via ICP-MS to measure the quantity of dissolved iridium6 .
The results were stark and alarming. The amorphous IrOx electrode degraded rapidly in the presence of ethanol, with the dissolution rate accelerating as ethanol concentration increased. Remarkably, this accelerated degradation occurred across a wide pH range, from acidic to alkaline conditions6 .
| Electrode Type | Condition | Observed Stability | Key Finding |
|---|---|---|---|
| Amorphous IrOx | Without Ethanol | Moderate | Shows expected, slow dissolution |
| Amorphous IrOx | With 10% Ethanol | Very Low | Near-complete dissolution observed |
| Crystalline IrO₂ | Without Ethanol | High | Stable with minimal dissolution |
| Crystalline IrO₂ | With 10% Ethanol | Moderate | Surface amorphous layer dissolves |
In situ analysis revealed the chemical mechanism: during the OER, ethanol oxidizes first to acetaldehyde. The subsequent oxidation of this aldehyde appears to be the primary driver of dissolution, as it can lead to the incorporation of oxygen from acetate into the iridium oxide lattice, causing its structural collapse6 .
Understanding iridium dissolution requires sophisticated tools that can capture events happening at incredibly fast timescales. Here are the essential instruments researchers use to study this phenomenon:
| Tool or Material | Primary Function | Application in Research |
|---|---|---|
| Online ICP-MS | Quantifies dissolved metal ions in solution in real-time | Precisely measures the rate and amount of iridium dissolution during electrochemical tests9 . |
| In Situ XAS (XANES) | Probes the local chemical state and structure of an element | Tracks changes in iridium's oxidation state and coordination during operation6 . |
| Time-Resolved Spectroscopy | Monitors fast chemical changes using ultrafast laser pulses | Investigates the femtosecond-to-picosecond dynamics of the initial steps of dissolution5 7 . |
| Scanning Electron Microscope (SEM) | Provides high-resolution images of surface morphology | Visualizes physical damage, pitting, and material loss on the electrode surface after testing6 . |
Ultrafast techniques can capture events lasting just 10⁻¹⁵ seconds, revealing the initial steps of dissolution.
Real-time monitoring during electrochemical operation provides insights into dynamic processes.
The story of iridium dissolution is further complicated by the acidity or alkalinity of the environment. Research using online ICP-MS has mapped iridium's behavior from pH 1 to 12.79 .
| pH Regime | Primary Reactant | Proposed Dissolution Mechanism | Stability |
|---|---|---|---|
| Acidic to Neutral | H₂O | H₂O acts as the main reactant, leading to the formation of soluble Ir³⁺ ions9 . | High stability in acidic OER; lower stability in neutral OER depending on buffers9 . |
| Alkaline (pH > 9) | OH⁻ | Hydroxyl ions become the dominant reactant, contributing to the formation of soluble IrO₄²⁻ ions9 . | Reduced stability, with dissolution increasing significantly9 . |
Key Insight: This pH dependence reveals that there is no one-size-fits-all solution. Designing stable catalysts requires a precise understanding of the operational environment.
The insights gained from these time-resolved and operando studies are directly guiding the development of more sustainable technologies.
The clear vulnerability of amorphous IrOx structures underscores the need to engineer catalysts that favor the more stable AEM pathway, even if it means slightly sacrificing initial activity.
The discovery of crossover-induced dissolution means electrolyzer design is just as important as catalyst design. Developing advanced membranes that prevent the transport of organic molecules from the cathode to the anode is now a critical research frontier.
Understanding dissolution mechanisms informs the development of smarter recycling processes. New hydrometallurgical methods are being explored to recover iridium from end-of-life devices using milder, more sustainable chemicals, moving away from traditional, highly corrosive aqua regia3 .
The silent, rapid dissolution of iridium is a formidable obstacle on the path to a hydrogen-powered future. Yet, by using the stroboscopic flash of ultrafast science to freeze this elusive process in time, researchers are not only diagnosing the problem but also lighting the way to its solution. The race to save every atom of this precious metal is, in every sense, a race against time.