Cobalt Water Splitting Catalyst vs Precious Metals

Cobalt-based water splitting catalysts are increasingly competitive with precious metal catalysts like iridium and platinum, offering comparable performance at a fraction of the cost, though they still lag in long-term stability under industrial conditions. This article compares cobalt catalysts to their precious metal peers across activity, durability, cost, and practical applicability, helping you understand where each stands today.

Formula

The calculator estimates the relative activity of a cobalt catalyst compared to iridium dioxide (IrO₂) using the overpotential at a given current density. The formula is:

Relative Activity = (10 / d) × (350 / t)

Where:

• d = current density in mA/cm² (the higher, the more hydrogen/oxygen produced per area)
• t = overpotential in mV (the extra voltage needed above the theoretical minimum to drive the reaction)

The constant 10 is a reference current density, and 350 mV is a typical overpotential for IrO₂ at 10 mA/cm². A relative activity of 1.0 means performance equal to IrO₂; above 1.0 means better, below means worse.

Worked Example

Using the default values (overpotential = 300 mV, current density = 10 mA/cm²):

1. Relative Activity = (10 / 10) × (350 / 300)
2. Relative Activity = 1 × 1.1667
3. Relative Activity = 1.17

This means the cobalt catalyst is about 17% more active than IrO₂ under these conditions. The stability factor (1 – 300/1000) = 0.70, indicating that at this overpotential, the catalyst retains roughly 70% of its initial performance in a simplified model.

Background: Why Cobalt for Water Splitting?

Water splitting (electrolysis) produces hydrogen and oxygen from water using electricity. The oxygen evolution reaction (OER) at the anode is the bottleneck because it requires high energy to break O-H bonds and form O-O bonds. Precious metal oxides like iridium and ruthenium are the benchmark OER catalysts due to their high activity and stability in acidic electrolytes. However, iridium costs roughly $1,500 per troy ounce and is scarce, limiting large-scale deployment. Cobalt, in contrast, is abundant and cheap (about $30 per kilogram), making it an attractive alternative, especially in alkaline electrolyzers where it performs nearly as well as iridium.

How Cobalt Catalysts Work

Cobalt catalysts operate through a similar mechanism to precious metals but with different surface chemistry. In alkaline media, cobalt oxides (Co₃O₄, CoOOH) form active sites that adsorb hydroxide ions and facilitate O-O bond formation. The key steps are:

• Hydroxide adsorption onto cobalt sites
• Oxidation of cobalt to a higher valence state (Co³⁺/Co⁴⁺)
• Oxygen evolution and regeneration of the active site

The overpotential of cobalt catalysts in alkaline electrolytes is typically 300-400 mV at 10 mA/cm², compared to 250-350 mV for iridium. In acidic electrolytes, cobalt dissolves rapidly, so it is rarely used there, while iridium remains stable.

Performance Comparison: Cobalt vs. Iridium vs. Platinum

When comparing catalysts, three metrics matter: overpotential, stability (durability), and cost.

Property	Cobalt oxide	Iridium oxide	Platinum
Overpotential at 10 mA/cm² (alkaline)	300-400 mV	250-350 mV	~400 mV (for OER)
Stability in acid	Poor (dissolves)	Good	Good (but for HER)
Stability in alkaline	Good (thousands of hours)	Good	Good
Relative cost	~$0.03/g	~$50/g	~$30/g

Platinum is the benchmark for the hydrogen evolution reaction (HER) at the cathode, not the OER. Cobalt is rarely used for HER because its activity is lower, though cobalt phosphides have shown promise. For the OER, cobalt is a direct competitor to iridium in alkaline systems.

Choosing Between Cobalt and Precious Metals

The choice depends on the electrolyte and application:

• Alkaline electrolyzers (commercial, large-scale): Cobalt is preferred due to cost and adequate performance. Most commercial alkaline electrolyzers already use nickel or cobalt-based anodes.
• Proton exchange membrane (PEM) electrolyzers (acidic): Iridium is required because cobalt dissolves. PEM systems offer higher current density but cost more.
• Research and lab-scale: Cobalt is often used to test new nanostructures or doping strategies because it is inexpensive and easy to synthesize.
• Photoelectrochemical cells: Cobalt phosphate (Co-Pi) is a popular catalyst for solar-driven water splitting due to its self-healing properties in neutral pH.

Common Problems and Misconceptions

A common misconception is that cobalt is always less active than iridium. In alkaline media, nanostructured cobalt oxides can match or even exceed iridium’s activity per geometric area because of higher surface area. However, the stability of cobalt under industrial conditions (high current density, elevated temperature) is often inferior. Cobalt catalysts tend to dissolve slowly in alkaline electrolytes, especially at high potentials, leading to a gradual loss of activity. Another issue is that cobalt can form less active phases (e.g., Co(OH)₂) that reduce performance. Researchers mitigate this by doping with iron or nickel, which enhances activity and stability.

Cost and Scalability

The cost advantage of cobalt is enormous. Iridium costs approximately $50,000 per kilogram, while cobalt oxide costs around $30 per kilogram. For a 1 MW electrolyzer, the anode catalyst cost using iridium could exceed $500,000, while cobalt would cost under $10,000. This price difference drives the push for cobalt in large-scale green hydrogen production. However, cobalt is not entirely without supply concerns—most of it is mined as a byproduct of copper and nickel, and geopolitical factors (e.g., Democratic Republic of Congo) affect supply chains. Still, it is far more abundant than iridium.

Safety and Environmental Considerations

Cobalt compounds are toxic and carcinogenic if inhaled as dust, so handling requires proper PPE (gloves, masks) and ventilation. In water splitting, the catalyst itself is not released into the environment under normal operation, but end-of-life disposal must follow hazardous waste guidelines. Precious metals are less toxic but their mining has significant environmental impacts. From a lifecycle perspective, cobalt’s lower energy cost of production and recyclability make it a more sustainable choice for most applications, provided that mining practices improve.

Alternatives to Cobalt and Precious Metals

Several non-precious, non-cobalt catalysts are being developed:

• Nickel-iron oxides: Very active in alkaline media, sometimes better than cobalt, and cheaper.
• Manganese oxides: Abundant and low-toxicity, but less stable.
• Perovskites (e.g., Ba₀.₅Sr₀.₅Co₀.₈Fe₀.₂O₃): High activity but often unstable.
• Metal-organic frameworks (MOFs): Offer tunable active sites but are expensive to synthesize.

Frequently Asked Questions

Is cobalt as efficient as iridium for water splitting?

In alkaline electrolytes, cobalt can be nearly as efficient as iridium, with overpotentials within 50-100 mV of each other. In acidic electrolytes, cobalt is not usable because it dissolves, so iridium remains the only viable option for PEM electrolyzers.

Why is platinum used in water splitting if it is precious?

Platinum is used for the hydrogen evolution reaction (HER) at the cathode, not the oxygen evolution reaction. It has an extremely low overpotential for HER, and only a small amount is needed (typically 0.5 mg/cm²). Research is finding cheaper alternatives like nickel-molybdenum, but platinum remains the benchmark.

Can cobalt replace iridium in commercial electrolyzers?

Yes, in alkaline electrolyzers, cobalt-based anodes are already used commercially. In PEM electrolyzers, cobalt cannot replace iridium because the acidic environment corrodes cobalt. However, solid-state or anion exchange membrane (AEM) electrolyzers that use alkaline conditions are being developed to allow cobalt use.

How long do cobalt catalysts last?

In lab tests, cobalt oxide catalysts can maintain activity for thousands of hours at moderate current densities (10-100 mA/cm²). Under industrial conditions (1 A/cm²), degradation is faster, often losing 20-30% activity within 1,000 hours. Doping with iron or nickel improves stability significantly.

What is the overpotential of cobalt vs. iridium?

At 10 mA/cm² in 1 M KOH, cobalt oxide typically shows an overpotential of 300-400 mV, while iridium oxide shows 250-350 mV. The difference is usually less than 100 mV, which is why cobalt is considered a viable substitute in alkaline systems.

Is cobalt toxic in water splitting applications?

The catalyst is fixed on the electrode and does not leach into the water under normal operation, so the produced hydrogen and oxygen are not contaminated. However, during manufacturing and disposal, cobalt dust can be toxic if inhaled. Proper handling and recycling are necessary.

Related

• Open Source MCP Server Protection Alternatives
• Get Better at Programming by Playing Games?
• Sort by Custom List: How It Works

in t | Overpotential (mV) | mV | 300 | 0 | 1000
in d | Current density (mA/cm²) | mA/cm² | 10 | 0 | 1000
out eff | Relative activity vs IrO2 | - | (10 / d) * (350 / t) | 2
out note | Stability factor | - | 1 - (t / 1000) | 2