Overview of Glassy Carbon Electrodes (GCE)

• Glassy carbon electrodes (GCEs) are self-supporting, conductive, and chemically robust materials derived from activated glassy carbon foam with enhanced surface area and functional groups.
• They exhibit high specific and areal capacitance, leveraging a hierarchical micro- and mesopore architecture to facilitate efficient ion transport for energy storage and sensing.
• Controlled nitric acid activation introduces oxygen-containing groups that boost redox pseudocapacitance while ensuring excellent cycle life and scalability for device fabrication.

Glassy carbon electrodes (GCEs), especially those fabricated from activated glassy carbon foam (AGcf), constitute a class of self-supporting, conductive, and chemically robust electrode materials used extensively in high-performance electrochemical energy conversion and storage devices. These electrodes exhibit high specific capacitance, broad operative voltage windows, scalability, and mechanical stability, making them directly suitable for supercapacitor and sensor platforms.

1. Preparation and Surface Functionalization of Glassy Carbon Electrodes

The protocol for the preparation of AGcf-based GCEs utilizes commercial glassy carbon foam (GCF) as the starting material, characterized by a self-supporting 3D sponge architecture, bulk density of approximately 0.2 g·cm–3, and electrical conductivity exceeding 10 S·cm–1. Untreated GCF presents smooth surfaces and negligible porosity, as determined by scanning electron microscopy (SEM) and Brunauer–Emmett–Teller (BET) analysis (BET = 0.5925 m²·g–1).

Oxidative activation is carried out by immersion of GCF bars (~1 cm × 1 cm × 0.3 cm) in 65 wt % HNO₃ at 90 °C for time periods of 4, 6, or 8 hours, denoted AGcf-4, AGcf-6, and AGcf-8, respectively. Post-treatment steps include thorough rinsing with deionized water to neutral pH and oven drying at 60 °C for 12 h. The AGcf-8 variant yields the highest electrochemical performance and is therefore preferred.

Surface functionalization proceeds via nitric acid-induced oxidation, introducing oxygen-containing groups (hydroxyl, carbonyl, carboxyl) according to the sequence:

• C(graphitic) + HNO₃ → C–OH + NO₂↑ + H₂O
• C(graphitic) + 2 HNO₃ → C=O + NO₂↑ + H₂O
• C(graphitic) + 3 HNO₃ → COOH + NO₂↑ + H₂O

The simplified net surface reaction is:

$$\text{C}_{\text{surf}} + x\,\mathrm{HNO}_3 \longrightarrow \mathrm{C}\text{-O}_x + x\,\mathrm{NO}_2 + x\,\mathrm{H}_2\mathrm{O}$$

2. Structural and Chemical Characterization

Morphological and structural evolution during activation is characterized by:

• SEM: Untreated GCF exhibits ultrasmooth walls with no visible pores. AGcf-8 displays roughened surfaces and nano-etch pits, evidencing enhanced surface texture.
• BET and Pore Distribution: Post-activation, AGcf-8 shows a BET area of 3.5389 m²·g–1 (vs 0.5925 m²·g–1 untreated), with increased micropore (<1 nm) and mesopore (2–50 nm) content. This structure supports improved double-layer capacitance and ion transport.
• XRD: A broad peak at ~25° 2θ confirms the amorphous, glassy nature is retained and no graphitization occurs.
• Raman Spectroscopy: The D:G intensity ratio (I_D/I_G) decreases minimally from 0.99 to 0.94 after activation, suggesting a maintained graphitic backbone with slight defect introduction.
• XPS: Oxygen content (O/(C+O)) increases from 2.1 at% (untreated) to 15.5 at% (AGcf-8). Deconvolution reveals C–C/C=C, C–O, C=O, and O–C=O features as primary constituents, along with enhanced O 1s and minor N 1s incorporation due to HNO₃.

3. Electrochemical Properties and Device Operation

Single-Electrode (Three-Electrode)

AGcf samples are employed as the working electrode (area ~1 cm²) with Ag/AgCl (saturated KCl) as reference and stainless steel counter, in 1 M H₂SO₄ at 25 °C. Cyclic voltammetry (CV) between 0–0.7 V at scan rates 10–100 mV·s–1 yields near-rectangular, quasibox shapes, indicative of dominant electric double-layer (EDL) behavior and contributions from O-functional redox pseudocapacitance.

Galvanostatic charge–discharge (GCD) at 10 mA·cm–2:

• Areal capacitance: 22,621.4 mF·cm–2
• Specific capacitance:

$$C_m = \frac{I\,\Delta t}{m\,\Delta V} = 678.6\ \mathrm{F\,g}^{-1}$$

Electrochemical impedance spectroscopy (EIS), sweep 100 kHz–0.01 Hz with 5 mV AC, yields series resistance $R_s ≈ 2.03 \Omega$ and charge-transfer resistance $R_{ct} ≈ 5.34 \Omega$, with nearly vertical low-frequency Nyquist characteristics.

Cycle life testing at 10 mA·cm–2 for 10,000 cycles demonstrates 100 % capacitance retention and Coulombic efficiency.

Symmetric Cell (Two-Electrode)

Devices consist of paired AGcf-8 electrodes (each ~1 cm², ~10 mg carbon·cm–2), separated by PVA/H₃PO₄ or polycarbonate membrane and using either aqueous (3 M H₂SO₄) or ionic liquid (BMIM-NTf₂) electrolytes. CV in the 0–1.6 V window (scan rates 1–40 mV·s–1) retains box-like morphology, with symmetric GCD at 5–20 mA·cm–2 yielding:

• 152.7 F·g–1 at 5 mA·cm–2
• 136.2 F·g–1 at 20 mA·cm–2

EIS reveals dual semicircles indicative of EDL and pseudocapacitive processes. ESR is derived from IR drop via:

$\mathrm{ESR} = \frac{V_{\mathrm{drop}}}{2\,I}$

Cycling durability remains at 100 % capacitance and near 100 % Coulombic efficiency after 10,000 CV cycles at 100 mV·s–1.

4. Absolute, Areal, and Volumetric Performance Metrics

Key performance metrics are summarized as follows:

Metric	Value (Single Electrode)	Value (Symmetric Cell)
Specific capacitance	678.6 F·g–1	152.7 F·g–1 @5 mA·cm–2
Areal capacitance	22,621 mF·cm–2	–
Absolute capacitance	11.3 F per electrode	–
Energy density	–	54.3 Wh·kg–1
Power density	–	38.5 kW·kg–1
Voltage window	0–0.7 V vs Ag/AgCl	0–1.6 V (aq. H₂SO₄)
Cycle life	10,000 cycles, 100 % ret.	10,000 cycles, 100 % ret.

Absolute device power output reaches 0.65 W per unit in two-electrode configuration.

5. Structure–Property–Performance Relationships

Activation via nitric acid etching substantially roughens the GCF surface, increases BET area by approximately sixfold, and introduces oxygen- and nitrogen-containing surface groups. These functional groups (–OH, =O, –COOH) drive reversible faradaic pseudocapacitance, in addition to enhanced electric double-layer capacitance. Minor nitrogen incorporation and oxygen functionalization reduce contact angle, thus improving wettability and facilitating complete electrolyte penetration of inner pores.

The hierarchical pore architecture — incorporating both micro- and mesopores — optimizes the balance between high specific capacitance and rapid ion transport. The preserved graphitic scaffold maintains high electronic conductivity, reflected in consistently low $R_s$ and $R_{ct}$.

6. Recommendations for Fabrication and Application

Post-activation, AGcf substrates must be rinsed extensively to eliminate residual nitrates and dried at 60 °C under vacuum to prevent structural collapse. As self-supporting foams, AGcf electrodes require no binders or additional current collectors; they can be mechanically affixed or directly clamped into cells. Full wettability is ensured by pre-soaking electrodes in electrolyte (8–10 h).

Potential applications include:

• High-voltage aqueous supercapacitors (0–1.6 V)
• Flexible and printable device architectures (custom-shaped foam)
• Electrochemical sensor platforms leveraging high electrochemical surface area (ECSA) and functional groups
• Electrocatalysis supports owing to their acid-stable and conductive properties

7. Scalability and Practical Considerations

The activation procedure supports mass loadings ≥10 mg cm–2 and foam bar thicknesses ≥300 µm, with commercial GCF available in multi-centimeter dimensions. The batch-wise HNO₃ activation process is compatible with large-scale production, suggesting practical scalability without need for supplementary binders or collectors. Such device configurations allow direct translation of single-component AGcf electrodes into advanced supercapacitor and sensor systems with outstanding rate capability, cycle life, and operative voltage windows.

AGcf-8, as established by Wang & Ma (USTC), thus represents a directly applicable glassy carbon electrode platform for next-generation energy storage and conversion technologies (Wang et al., 2019).