Curriculum · Module 7 · Intermediate–Advanced

Fuel Cells, Electrolysers and Hydrogen

From catalyst to stack: polarisation, EIS, ECSA and high-current measurements.

Fuel-cell testing: polarisation curve, EIS and ohmic-resistance measurement

Frequently searched:fuel cell polarisation curvePEM fuel cell EISmembrane resistance measurementfuel cell test station

1 Theory

Thermodynamics and losses

The reversible voltage of the hydrogen/oxygen fuel cell is 1.23 V at 25 °C; in practice the open-circuit voltage stays at 0.95–1.0 V because of mixed potential and gas crossover. As current is drawn the voltage falls through three losses: activation (at low current, logarithmic; mainly cathode ORR), ohmic (at mid current, linear; membrane + contact) and mass transport (at high current, sharp drop; water management/gas diffusion). The polarisation curve is modelled as E = E₀ − b·log(i) − R·i − m·exp(n·i); the maximum of the power curve defines the nominal operating point.

Measuring the polarisation curve

Galvanostatic steps (with a settling time at each step, typically 1–5 min) are preferred; a slow potentiodynamic scan is also used but creates hysteresis. Temperature, humidity, stoichiometry and pressure must be fixed and reported. High current (tens to hundreds of amperes) requires a booster/electronic load, low cell voltage (0.3–1 V) high-resolution voltage measurement.

Separating the losses with EIS

EIS taken at the operating point (under DC load) gives the membrane+contact resistance (HFR) at the high-frequency intercept, the cathode charge-transfer semicircle at mid frequency and the mass-transport arc at low frequency. Following the HFR with current shows membrane hydration, growth of the low-frequency arc flooding. The current-interrupt method is the fast alternative for HFR. In stacks the EIS of every cell is measured simultaneously to find the weak cell.

Electrolyser and reversible operation

In a PEM water electrolyser the curve is inverted: above 1.23 V the activation (OER, Ir/Ru oxide), ohmic and mass-transport (bubble) overpotentials are added; current densities of the order of A/cm² around 2 V are common. Durability tests follow the long-term voltage rise (hundreds to thousands of hours) at constant current.

2 Application in measurement

The polarisation (I–V) curve consists of activation, ohmic and mass-transport regions; each region represents a different loss. Galvano-EIS at operating points separates membrane resistance, charge-transfer and diffusion contributions; the high-frequency resistance (HFR) is used to monitor membrane humidification.

Control of gas flow, humidity, temperature and pressure is part of the measurement: BluPEM stations combine this control with an integrated potentiostat/EIS. For high current densities the HCP-803 (80 A) or FlexP (200–800 A) is used; the Entek FCT is a domestic two-channel test instrument up to 25 V/100 A.

In stack measurement the voltage and EIS of every cell are taken simultaneously (VMP-3e + SAM-50).

Key equations

Cell voltage: V = E_rev − η_act − I·R_ohm − η_mass

3 Applicability

Where it applies

MEA/catalyst-layer development, stack validation, optimisation of operating conditions (humidity, temperature, stoichiometry), accelerated durability tests.

Limits and caveats

Single-cell results do not scale directly to the stack; flow distribution and thermal gradients create different losses in the stack.
EIS requires steady-state conditions; at high current, where water management fluctuates, NSD rises.

4 What did the application notes find?

Fuel Cell AN31 — Shows the general framework of fuel-cell testing and I/E (polarisation) characterisation with galvanostatic steps; identification of the loss regions on the curve.
Fuel Cell AN32 — Shows the separation of membrane resistance, charge transfer and mass transport by EIS under DC load; interpretation of the spectrum change at different currents.
Electrolyzer AN63 — Shows DC polarisation and EIS measurement at high current (with a booster) on a PEM electrolyser; the change of HFR and OER charge-transfer resistance with current.
Battery AN16 — Shows the simultaneous impedance measurement of series cells in a stack and the detection of the problematic cell; applies directly to fuel-cell stacks.

5 Which instruments perform this measurement?

BluPEM Workstations HCP-803 FlexP Series FCT — Fuel Cell Tester

References

book F. Barbir. PEM Fuel Cells: Theory and Practice (2nd ed.). Academic Press / Elsevier, 2013.
article X. Yuan, H. Wang, J. C. Sun, J. Zhang. AC impedance technique in PEM fuel cell diagnosis — A review. Int. J. Hydrogen Energy 32, 4365–4380, 2007. doi:10.1016/j.ijhydene.2007.05.036
article K. R. Cooper, M. Smith. Electrical test methods for on-line fuel cell ohmic resistance measurement. J. Power Sources 160, 1088–1095, 2006. doi:10.1016/j.jpowsour.2006.02.086

Electrocatalysis and electrolysers: ECSA, HER/OER activity and PEM water electrolysis

Frequently searched:ECSA calculationHER OER catalyst measurementPEM electrolyser testinggreen hydrogen electrolysis

1 Theory

Activity metrics

A 'good' electrocatalyst is defined by three independent metrics: the overpotential needed to reach a given current density (e.g. 10 mA/cm², the solar-fuel benchmark), the Tafel slope (mechanism and scaling at high current) and intrinsic activity — normalisation of the current to the true active area (ECSA) or to the number of active sites (TOF). Mass activity (A/mg Pt) speaks of cost, specific activity (mA/cm²_ECSA) of material quality; current normalised to geometric area depends on loading and is misleading on its own.

ECSA measurement

For platinum-group metals it is calculated from the hydrogen underpotential deposition (H_upd) charge (210 µC/cm² for Pt) or the CO-stripping charge (420 µC/cm²). For oxide and non-metal catalysts the double-layer capacitance method is used: CV is taken at different scan rates in the non-Faradaic region, the slope of capacitive current against scan rate gives C_dl, which is divided by a specific capacitance (typically 40 µF/cm², with large uncertainty). The assumptions and uncertainty of each method must be reported.

HER, OER and the measurement protocol

Hydrogen evolution (HER) is cathodic, oxygen evolution (OER) anodic; together they constitute water electrolysis. iR correction is mandatory in the measurement (at high current most of the overpotential may be ohmic); the reference electrode must be calibrated against RHE, the distinction between CV and LSV and the scan rate stated, and a rotating electrode or stirring used for gas bubbles. Stability is tested by chronopotentiometry (hours at constant current) and successive CVs; dissolved metal ions are verified by ICP. Benchmarking protocols (e.g. η at 10 mA/cm², 2 h at constant current) ensure comparability of results.

From cell to electrolyser

The intrinsic activity measured on an RDE translates into different performance in the membrane electrode assembly (MEA) because of pore structure, ionomer distribution and water/gas management. Measurement at electrolyser level combines DC polarisation and EIS at high current; scanning probe techniques evaluate bipolar-plate and coating homogeneity locally.

2 Application in measurement

Catalyst activity is measured on an RDE with CV/LSV; for Pt the ECSA is obtained from the hydrogen adsorption charge (210 µC/cm²), and area determination is also done by oxide reduction or CO stripping. Mass activity and specific activity are reported at a given overpotential; iR correction and the measurement protocol (Garsany et al.) are mandatory for comparability.

PEM water electrolysers require high current density (2–8 A/cm²) and bidirectional current capability; with the FlexP booster the SP-150e/VMP-3e reaches 200–800 A. Durability tests include long-term constant current and periodic EIS.

SECM provides complementary information on membrane degradation and local activity.

Key equations

ECSA (Pt) = Q_H / (210 µC·cm⁻² · m_Pt)

3 Applicability

Where it applies

Comparison of new catalyst materials, stability screening, mechanism studies (Tafel, RRDE selectivity), electrolyser component validation.

Limits and caveats

C_dl-based ECSA on oxides may have an uncertainty exceeding ±50 %; intrinsic-activity comparisons must be made with caution.
When the RDE 'thin film' condition is violated (thick catalyst layer) the activity is underestimated.

4 What did the application notes find?

Fuel Cell AN11 — Shows the calculation of ECSA from the CV charge of a Pt electrode in the H_upd region, the double-layer correction and suitable scan parameters.
Kinetics AN56 — Shows the extraction of kinetic parameters from RDE data with the Levich/Koutecký–Levich tools; the basis of electron-number determination in reactions such as ORR.
Electrolyzer AN63 — Shows polarisation and EIS measurement at high current on a PEM electrolyser; the booster connection and the separation of HFR/OER resistance.
Scanning Probes AN23 — Shows scanning of fuel-cell bipolar plates with intermittent-contact SECM; the effect of coating and surface treatments on local electrochemical activity.

5 Which instruments perform this measurement?

BluRev RDE-4 — Rotating Disc Electrode System FlexP Series

References

article Y. Garsany, O. A. Baturina, K. E. Swider-Lyons, S. S. Kocha. Experimental Methods for Quantifying the Activity of Platinum Electrocatalysts for the Oxygen Reduction Reaction. Anal. Chem. 82, 6321–6328, 2010. doi:10.1021/ac100306c
article M. Carmo, D. L. Fritz, J. Mergel, D. Stolten. A comprehensive review on PEM water electrolysis. Int. J. Hydrogen Energy 38, 4901–4934, 2013. doi:10.1016/j.ijhydene.2013.01.151
book F. Barbir. PEM Fuel Cells: Theory and Practice (2nd ed.). Academic Press / Elsevier, 2013.

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