Electrochemical Active Surface Area (ECSA) Testing Principles and Methods

1. Introduction

Electrochemical Active Surface Area (ECSA) refers to the effective surface area of an electrode capable of participating in electrochemical reactions. In electrochemical processes, a larger active surface area indicates greater available sites for reactions, leading to enhanced reaction rates and sustainable current densities. Moreover, precise measurement of ECSA is particularly crucial for studying catalytic activity, as it enables the determination of true reaction kinetics. However, accurately quantifying ECSA in electrode materials poses significant challenges due to multiple factors, including material morphology/structure, physicochemical properties, electrolyte interactions, surface adsorption phenomena, non-Faradaic processes, electrical conductivity, and the intrinsic activity/density of active sites.

2. Electrochemical Active Surface Area Testing Principles and Methods

2.1 Principles

Electrochemical Active Surface Area (ECSA) is a concept relative to geometric area. For conventional electrodes, active sites are considered to be the surface regions contacting the electrolyte. Since many active sites are not simply monolayer structures and are difficult to measure physically, the geometric area is often approximated as ECSA. However, for nanomaterial electrodes - particularly porous electrodes like nickel foam or gas diffusion layer electrodes loaded with nanocatalysts - increased specific surface area exposes more active sites. Using geometric area to calculate current density thus fails to reflect true catalyst activity.

Geometric area critically determines ECSA: even identical materials arranged in vertical arrays on the same substrate (or on nanostructures extending from the substrate) significantly increase exposed catalytic surface area, substantially boosting current.

ECSA can be electrochemically measured via:

• Electrochemical Impedance Spectroscopy (EIS)

• Cyclic Voltammetry (CV)

• Constant Current Discharge (CCD)

Most reliable ECSA methods rely on specific ion/gas adsorption at electrode surfaces (e.g., CO or H₂ adsorption on Pt). Underpotential Deposition (UPD) is also used to assess surface area. Voltage-modulated oxidation state changes provide another measurement approach, though these methods are material-specific.

Common alternatives include:

BET method: Measures gas adsorption under near-vacuum conditions. Requires samples with specific surface areas > several m²/g for sufficient signal intensity. Suitable for fine powders but ineffective for small-area electrodes.

Double-Layer Capacitance (DLC) method: Widely accepted for ECSA estimation. 

(Note: BET and DLC results can differ by orders of magnitude.)

Since electrocatalytic activity involves multifaceted intertwined factors, comprehensive analysis is essential to accurately determine impacts on catalytic mechanisms.

2.2 Testing Methods

CV and EIS are currently the most reliable techniques for calculating ECSA via electrochemical double-layer capacitance (Cdl). Cdl determination approaches:

1.CV method: Measure capacitive currents in the non-Faradaic region at multiple scan rates. Calculate Cdl from the linear slope of current vs. scan rate.

2.EIS method: Derive Cdl from impedance measurements at varying frequencies.

The CV approach is simpler and more commonly employed. The protocol involves performing cyclic voltammetry within potential regions devoid of redox reactions, centering a potential window of approximately 50–100 mV around the open-circuit voltage (OCV). The charging current (IcIc) exhibits a linear relationship with the scan rate (vv) and the double-layer capacitance (CdlCdl) expressed by the equation:

Where Cs is the specific capacitance of a smooth reference surface under identical conditions.

3. Multi-Scan Rate CV Testing Parameters

3.1 Common Parameter Sets

1. Hydrogen Underpotential Deposition (HUPD) Method

Applicable to: Pt, Pt alloys, Ru, Ir

• CV range: 0.0~1.0 V (vs. RHE)

• Scan rate: 50 mV s⁻¹

• Electrolyte: 0.1 M HClO₄ or 0.5 M H₂SO₄

• ECSA calculation: ECSA = QH / 0.21

QH: Integrated charge in hydrogen desorption region (mC)

0.21: Charge required for monolayer hydrogen adsorption/desorption on Pt (mC cm⁻²)

2. CO Stripping Method

Applicable to: Pt, Pd

• CV range: 0.0~1.0 V (vs. RHE)

• Scan rate: 20 mV s⁻¹

• Electrolyte: 0.5 M H₂SO₄

• ECSA calculation: ECSA = Q / 0.42

Q: Integrated charge in CO desorption region (mC)

0.42: Charge required for monolayer CO adsorption on Pd (mC cm⁻²)

3. Redox Peak Method (Lower Accuracy)

Applicable to: Au, Pd, carbon-based materials

• CV range: 0.0~1.6 V (vs. RHE)

• Scan rate: 100 mV s⁻¹

• Electrolyte: 0.1 M KOH

• ECSA calculation: ECSA = Q / 0.42

Q: Integrated charge of reduction peak (AuO → Au) (mC)

0.42: Charge corresponding to oxygen adsorption on Au (mC cm⁻²)

4. Copper Underpotential Deposition (Cu UPD) Method

• Applicable to: Pt, Pd, Pt alloys

• CV range: 0.0~1.6 V (vs. RHE)

• Scan rates: 5 mV s⁻¹ and 10 mV s⁻¹

• Electrolyte: 0.1 M H₂SO₄ + 0.5 mM CuSO₄

• ECSA calculation: ECSA = Q / (0.42 or 0.46)

Q: Integrated charge of Cu stripping oxidation peak (mC)

0.42/0.46: Charge required for monolayer Cu adsorption/desorption (mC cm⁻²)

3.2 Parameter Settings

Multi-scan rate CV testing in the non-Faradaic region requires specific parameterization. The scan initiates at the open-circuit voltage (OCV) and terminates at a predefined endpoint. Critical to this method is selecting an appropriate potential window where only capacitive currents dominate. For oxygen evolution reaction (OER) studies, potentials should remain below 1.23 V (vs. RHE) to avoid water oxidation and potential cation/anion redox processes (e.g., Ni²⁺/S²⁻ oxidation) that may occur within 100 mV above this threshold. While the region below 1.23 V is theoretically non-Faradaic, material-dependent variations necessitate careful window selection. A recommended range of 1.05–1.15 V (vs. RHE) proves generally applicable across noble metals, transition metals, oxides, sulfides, and hydroxides. Avoid OCV-centered windows due to OCV instability and possible inclusion of Faradaic processes.

The stabilization period before scanning typically ranges from seconds to tens of seconds (configurable between 1–100,000 s). Scan rates should span 5–200 mV s⁻¹ across 1–500,000 cycles, with each cycle collecting 2,000 data points by default (total points = 2000 × cycle count).

Non-Faradaic regions exhibit rectangular CV curves symmetric about zero current. Asymmetry within 1.05–1.15 V (vs. RHE) often indicates material charge/discharge imbalance or over-discharge, manifested as significant negative polarization currents. Rectification involves low-scan-rate cycling (10 mV s⁻¹) within this window until equilibrium restores (CV symmetry about zero current). Conductivity dictates cycle requirements: highly conductive materials may stabilize in tens of cycles, whereas poorly conductive counterparts often require >200 cycles due to sluggish charge transfer.

Simultaneously boosting hydrogen production and ethanol upgrading using a highly-efficient hollow needle-like copper cobalt sulfide as a bifunctional electrocatalyst. Journal of Colloid and Interface Science, 2021, 602: 325-333.

4. Application Scenarios

The measurement and calibration of Electrochemical Active Surface Area (ECSA) constitute a critical factor in electrocatalytic mechanism research, with key applications including: catalyst performance evaluation—where ECSA quantifies the catalytically active interface to assess activity and efficiency while revealing active site density for catalyst optimization; fuel cell studies—particularly in proton exchange membrane fuel cells (PEMFCs), where ECSA diagnostics enable catalyst performance understanding and efficiency enhancements; electrocatalytic reaction experiments—providing essential activity data for comparative catalyst analysis, novel catalyst design, and mechanistic investigations; kinetic parameter assessment—enabling current normalization for deriving electrochemical kinetics (e.g., meaningful corrosion rates when normalized by ECSA), especially in energy-related reactions like HER, HOR, OER, and ORR; and comprehensive material performance characterization—evaluating electrochemical activity, ion diffusion rates, and catalytic behavior.

5. Conclusion

ECSA research remains integral to electrocatalytic performance studies. These applications demonstrate how ECSA facilitates catalyst screening under optimal conditions, enables electrochemical activity testing of battery materials, and advances renewable energy technologies such as high-efficiency fuel cells and water electrolysis systems. Consequently, ECSA serves as an indispensable tool in battery and catalyst R&D.