7.2 Reading the polarization curve

Three kinds of loss

A polarization curve plots cell voltage against current density. Starting from the open-circuit voltage, three losses pull the voltage down as the current grows.

• Activation loss, a steep drop at low current, mostly from the slow ORR. It follows the Tafel equation.
• Ohmic loss, a straight-line drop in the middle, from the resistance of the membrane, the electrodes and their contacts.
• Mass-transport loss, a fall at high current, when oxygen cannot reach the catalyst fast enough, for example because liquid water floods the pores.

Try the presets. A less active catalyst shifts the whole curve down. A dry membrane makes the middle steeper. A flooded electrode makes the curve collapse earlier. The bar shows where the 1.18 V goes at the operating current.

Power density

Power density is current density times voltage, in W/cm². It peaks at a moderate voltage, often around 0.5 to 0.6 V, but cells are usually run at higher voltage, 0.65 to 0.75 V, where the efficiency is better and the materials last longer. Report the power at a stated voltage as well as the peak.

Telling the losses apart

• High-frequency resistance measured during the test, by impedance or current interrupt, gives the ohmic loss, so the curve can be iR-corrected.
• Tafel analysis of the iR-corrected curve at low current gives the kinetic loss.
• Oxygen gain, the difference between curves in pure O₂ and in air, shows how much is lost to oxygen transport.

Testing ORR catalysts before a fuel cell

New catalysts are usually screened in a three-electrode cell with a rotating disk electrode (RDE) in acid or base, before an MEA is made.

1. Clean and condition the catalyst by cycling in N₂-saturated electrolyte, and measure the ECSA, for Pt from the hydrogen underpotential deposition charge or from CO stripping.
2. Record ORR polarization curves in O₂-saturated electrolyte at several rotation rates, for example 1600 rpm, at a slow scan rate, and subtract the background measured in N₂.
3. Correct the mass-transport part with the Koutecký–Levich relation, 1/i = 1/i_k + 1/i_d, and give the kinetic current at 0.9 V vs RHE, iR-corrected.
4. Report mass activity (A per mg of Pt) and specific activity (per cm² of Pt), as benchmarked by Gasteiger and co-workers (2005). Garsany and co-workers (2010) describe how film preparation changes the result.

RDE results do not always carry over to MEAs, where ionomer, water and oxygen transport play a bigger part, so promising catalysts need to be tested in a real cell.

Durability

Catalysts lose activity as Pt particles dissolve, grow and detach, and as the carbon support corrodes, especially during start-up and shut-down. Accelerated stress tests cycle the potential, for example between 0.6 and 1.0 V for many thousands of cycles, and report the loss of ECSA, of mass activity and of voltage at a fixed current.

Check your understanding

References and further reading

1. H. A. Gasteiger, S. S. Kocha, B. Sompalli, F. T. Wagner, Activity benchmarks and requirements for Pt, Pt-alloy, and non-Pt oxygen reduction catalysts for PEMFCs, Appl. Catal. B 56 (2005) 9–35.
2. 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 (2010) 6321–6328.
3. J. Larminie, A. Dicks, Fuel Cell Systems Explained, 2nd ed., Wiley, 2003.
4. R. O'Hayre, S.-W. Cha, W. G. Colella, F. B. Prinz, Fuel Cell Fundamentals, 3rd ed., Wiley, 2016.