2.1 Fundamentals of electrocatalysis | Electrochemistry Open Lessons

What an electrocatalyst does

Thermodynamics sets the equilibrium potential of a reaction, for example 1.23 V vs RHE for water oxidation. No material can change that number. What a catalyst changes is how much extra potential, the overpotential η, is needed to drive the reaction at a useful rate. A good electrocatalyst reaches a high current density at a small overpotential and keeps doing so for a long time, while making the product you want.

Reaction	Half reaction (acid form)	E⁰ vs RHE
Hydrogen evolution, HER	2H⁺ + 2e⁻ → H₂	0 V
Oxygen evolution, OER	2H₂O → O₂ + 4H⁺ + 4e⁻	1.23 V
Oxygen reduction, ORR	O₂ + 4H⁺ + 4e⁻ → 2H₂O	1.23 V
CO₂ to CO	CO₂ + 2H⁺ + 2e⁻ → CO + H₂O	about −0.1 V
CO₂ to formate	CO₂ + H⁺ + 2e⁻ → HCOO⁻	about −0.1 V
Nitrate to ammonia	NO₃⁻ + 9H⁺ + 8e⁻ → NH₃ + 3H₂O	about 0.7 V
N₂ to ammonia	N₂ + 6H⁺ + 6e⁻ → 2NH₃	about 0.06 to 0.09 V

Values are rounded. Sources differ by a few tens of millivolts depending on the data and the product state.

Several of these reactions sit close to 0 V vs RHE, so in water they compete with the HER. Much of the work on CO₂, N₂ and nitrate reduction is about suppressing hydrogen, which is why product analysis (Lesson 2.3) matters so much.

Kinetics, the Butler–Volmer equation

For a single electron-transfer step, the net current depends on the overpotential through the Butler–Volmer equation.

j = j₀ [ exp(α_aFη/RT) − exp(−α_cFη/RT) ]j₀ is the exchange current density, the rate of the forward and backward reactions at equilibrium. α_a and α_c are the anodic and cathodic transfer coefficients.

Near η = 0 the curve is a straight line. Its slope gives the charge-transfer resistance, which is what impedance measures as R_ct.
At larger η one exponential dominates, and log|j| becomes linear in η. This is the Tafel region, η = a + b·log|j|, with Tafel slope b = 2.303RT/(αF), about 118 mV/dec for α = 0.5 at 25 °C.
j₀ is the closest single number to intrinsic activity. A catalyst with a hundred times larger j₀ needs one Tafel slope less overpotential for the same current.

Move j₀ by two decades and watch the overpotential at 10 mA/cm² change by two Tafel slopes. Then change α and see how the slope itself changes.

What a Tafel slope can and cannot say

Real reactions have several steps, so the measured Tafel slope depends on which step limits the rate and on how much of the surface is covered by intermediates. For the HER in acid the textbook steps are:

Volmer, H⁺ + e⁻ + * → H*, with a Tafel slope of about 120 mV/dec when it limits the rate.
Heyrovsky, H* + H⁺ + e⁻ → H₂ + *, about 40 mV/dec.
Tafel, 2H* → H₂ + 2*, about 30 mV/dec.

Shinagawa and co-workers (2015) showed with microkinetic models that the same slope can come from different mechanisms and that slopes change with coverage and potential. A Tafel slope is therefore a hint, not proof, of a mechanism. Treat it as one piece of evidence, give the potential range, and make sure it was measured under steady-state, iR-corrected conditions (Lesson 2.2).

The Sabatier principle and volcano plots

A catalyst has to bind the reaction intermediates neither too weakly, or nothing sticks to react, nor too strongly, or the product never leaves. This idea, the Sabatier principle, gives a volcano when activity is plotted against binding energy.

HER. Nørskov and co-workers (2005) showed that the measured exchange current of many metals follows a volcano against the computed hydrogen adsorption free energy ΔG_H*, with the best metals, such as Pt, close to ΔG_H* ≈ 0.
OER. The intermediates OH*, O* and OOH* are linked by scaling relations, because they bind to the surface through the same oxygen atom. Man and co-workers (2011) used this to show why even the best oxides seem to need several hundred millivolts of overpotential, and why breaking the scaling relations is an active research goal.

Volcanoes are a guide for searching, not a law. Real surfaces restructure under reaction conditions, and many OER catalysts turn into oxyhydroxides on their surface during operation.

Making a better catalyst

Seh and co-workers (2017) put the strategies into two groups: increase the number of active sites, or make each site more active. Both can be combined.

More sites	More active sites
Nanostructures with a high surface area	Alloying and doping that tune binding energies
Exposing active edges or facets	Defects and vacancies, strain
Porous electrodes and good dispersion on a support	Mixing many elements, as in high-entropy oxides

Two more things decide whether a good material becomes a good electrode: electron and mass transport (conductivity, gas bubbles, ion supply) and stability (dissolution, restructuring, loss of contact with the support). Always measure them alongside activity.

A note on our own workOur group makes many catalysts from spent batteries and other waste, and from high-entropy and 2D materials. The same rules apply: report activity per geometric area and per active-site estimate, and show that the catalyst, not an impurity, is doing the work.

Replacing the OER

The OER needs a large overpotential and produces O₂, which is often not wanted. In a hydrogen or CO₂ electrolyser, the anode can instead oxidise a cheap organic molecule, such as glycerol, glucose or 5-hydroxymethylfurfural (HMF), at a lower potential and into a more valuable product. The same rules for reporting potentials, currents and Faradaic efficiencies apply, and the liquid products must be measured, for example by HPLC or NMR.

Check your understanding

Quiz for lesson 2.1

Pick one answer per question. You see the explanation straight away.

References and further reading

Z. W. Seh, J. Kibsgaard, C. F. Dickens, I. Chorkendorff, J. K. Nørskov, T. F. Jaramillo, Combining theory and experiment in electrocatalysis, insights into materials design, Science 355 (2017) eaad4998. doi:10.1126/science.aad4998
J. K. Nørskov, T. Bligaard, A. Logadottir, J. R. Kitchin, J. G. Chen, S. Pandelov, U. Stimming, Trends in the exchange current for hydrogen evolution, J. Electrochem. Soc. 152 (2005) J23–J26. doi:10.1149/1.1856988
I. C. Man et al., Universality in oxygen evolution electrocatalysis on oxide surfaces, ChemCatChem 3 (2011) 1159–1165. doi:10.1002/cctc.201000397
T. Shinagawa, A. T. Garcia-Esparza, K. Takanabe, Insight on Tafel slopes from a microkinetic analysis of aqueous electrocatalysis for energy conversion, Sci. Rep. 5 (2015) 13801. doi:10.1038/srep13801
S. Nitopi et al., Progress and perspectives of electrochemical CO₂ reduction on copper in aqueous electrolyte, Chem. Rev. 119 (2019) 7610–7672. doi:10.1021/acs.chemrev.8b00705
P. H. van Langevelde, I. Katsounaros, M. T. M. Koper, Electrocatalytic nitrate reduction for sustainable ammonia production, Joule 5 (2021) 290–294. doi:10.1016/j.joule.2020.12.025
A. J. Bard, L. R. Faulkner, Electrochemical Methods: Fundamentals and Applications, 2nd ed., Wiley, 2001, chapter 3.

← 1.3 Electrochemical impedance spectroscopy 2.2 Reporting electrocatalysis properly →