Metal Oxide Electrode Failure Mechanism

The degradation of metal oxide electrodes primarily occurs at two critical interfaces:

• the active coating/electrolyte interface, and

• the substrate/active coating interface.

During prolonged electrolysis, the anode coating gradually deteriorates and detaches, while the titanium substrate becomes locally passivated. As a result, the electrocatalytic activity continuously decreases until the electrode is completely deactivated.

Figure 1.3 illustrates the degradation process:

(a) Before electrolysis

(b) During electrolysis

(c) After deactivation

1 – Titanium substrate

2 – Active coating

3 – TiO₂ passive layer

Main Failure Mechanisms

1. Dissolution of RuO₂ (Electrocatalytic Component Loss)

In ruthenium-based coatings, RuO₂ is the primary electrocatalytic material. Under anodic polarization, RuO₂ can be oxidized into volatile RuO₄:

$$\text{RuO}_2 + 2H_2O \rightarrow \text{RuO}_4 + 4H^+ + 4e^-$$

The generated RuO₄ may dissolve in the electrolyte (e.g., as hydrated species), and further decomposes:

$$\text{RuO}_4 + xH_2O \rightarrow \text{RuO}_2 \cdot xH_2O + O_2$$

This process leads to gradual loss of RuO₂ from the coating, reducing the number of active catalytic sites and causing electrode deactivation.

Studies (including those from Tianjin University) confirm that electrochemical dissolution of Ru components is a dominant failure mechanism.

Two dissolution patterns are typically observed:

• Uniform dissolution across the electrode surface
• Localized dissolution, often at edges or weak regions

When the remaining active coating drops to approximately 18% of the surface, electrode passivation occurs.

Additionally, in acidic environments (e.g., 5 mol/L H₂SO₄ at 40°C), the combined dissolution of RuO₂ and TiO₂ can form a void layer, accelerating coating failure.

2. Loss of Catalytic Activity (Oxygen Vacancy Mechanism)

The RuO₂–TiO₂ coating is typically non-stoichiometric (RuO₂₋ₓ, TiO₂₋ₓ), containing oxygen vacancies that serve as active catalytic sites.

• Higher oxygen vacancy concentration → higher catalytic activity

• Filling of vacancies (oxygen incorporation) → loss of activity

After heat treatment, the coating forms an n-type conductive mixed oxide structure. However, when oxygen vacancies are eliminated:

• Electrical conductivity decreases

• Overpotential increases

• Electrode becomes passivated

Experimental studies show that:

• Removing absorbed oxygen (via vacuum or inert atmosphere treatment) can partially restore electrode activity

• Structural transformation of RuO₂ into less active oxides also contributes to degradation

3. Oxidation and Passivation of Titanium Substrate

During electrolysis, active oxygen species are generated at the anode:

• Part escapes as O₂

• Part diffuses through the coating to the substrate interface

At the substrate:

• Oxygen reacts with titanium to form a TiO₂ passive layer

• This layer acts as a high-resistance barrier (P–N junction-like effect)

• Electron transfer is hindered → electrode potential increases

Additionally, the coating typically exhibits a "mud-crack" (tortoise-shell) structure, which:

• Allows electrolyte penetration

• Exposes the titanium substrate

• Accelerates oxidation and coating detachment

This creates a feedback loop:

1. Substrate oxidation

2. Coating delamination

3. Increased anode potential

4. Accelerated coating dissolution and further oxidation

Coating Failure Morphologies

Scanning electron microscopy (SEM) reveals several typical failure patterns:

1. Fragmented Peeling

Partial detachment along cracks

Deep pits reaching the substrate

Severe structural breakdown

2. Bulging Layer Delamination

Convex "blister-like" peeling

Uneven fracture edges

Visible surrounding crack patterns

3. Crack-Induced Spalling

Multiple microcracks connect into larger cracks

Deep, winding fissures develop

Leads to large-area coating detachment

Summary of Failure Mechanism

The degradation of MMO titanium anodes is the result of combined chemical and physical effects:

Chemical dissolution of active components (RuO₂)

Loss of catalytic activity due to structural changes

Oxidation and passivation of titanium substrate

Mechanical stress and crack propagation

Improvement Strategies

Based on the failure mechanisms, the following optimizations are recommended:

Enhance coating–substrate adhesion (to prevent peeling)

Optimize coating composition to improve corrosion resistance

Control crack formation during coating preparation

Reduce oxygen penetration to protect the substrate

Use multi-layer or gradient coatings for improved durability

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