JCAP researchers propose protocol for standardized evaluation of OER catalysts for solar-fuel systems
|Protocol for measuring the electrochemically active surface area, catalytic activity, stability, and Faradaic efficiency of heterogeneous electrocatalysts for OER. Credit: ACS, McCrory et al. Click to enlarge.|
Electro-catalytic water splitting to produce hydrogen and oxygen is a key element of solar-fuels devices; identifying efficient catalysts for the oxygen evolution reaction (OER) is critical to their realization. (The OER is efficiency-limiting for direct solar and electrolytic water splitting, rechargeable metal-air batteries, and regenerative fuel cells. Earlier post.) However, notes a team of researchers from the Joint Center for Artificial Photosynthesis at Caltech, current methods employed to evaluate oxygen-evolving catalysts are not standardized, making it difficult to compare the activity and stability of these materials.
To address this issue, the researchers are proposing a protocol to evaluate the activity, stability, and Faradaic efficiency of electro-deposited oxygen-evolving electrocatalysts. In particular, they focus on methods for determining electrochemically active surface area and measuring electrocatalytic activity and stability under conditions relevant to an integrated solar water-splitting device. A paper on their work is published in the Journal of the American Chemical Society.
… objective evaluation of the efficiency of OER catalysts is complicated by the lack of standardization both in the measurement and reporting of electrocatalytic data. Typical OER catalysts are deposited on a variety of different substrates, and their electrocatalytic activity is measured at a range of pH values, temperatures, and electrolyte compositions and concentrations, making it difficult to compare the performance and stability of different materials. As such, the development and implementation of a benchmarking methodology to test the electrocatalytic efficiency of materials for OER remain a fundamental challenge in solar fuels research.
In the case of components for commercial devices such as photovoltaic cells and polymer-electrolyte membrane fuel cells (PEMFCs), widely accepted testing protocols and figures of merit exist. For instance, one figure of merit that has been used for inexpensive Pt-free oxygen reduction catalysts for PEMFCs is the operating current per cm3 of catalyst material at a fixed overpotential in acidic water at elevated temperature. In this example, the catalyst testing conditions are dictated by the operating conditions necessary for automotive PEMFCs. While the purpose and operating conditions for PEMFCs are significantly different than those for solar water-splitting devices, the testing protocols developed for PEMFCs and other commercial devices nevertheless provide inspiration in the development of a benchmarking methodology for electrocatalysts for solar water-splitting devices.
Herein we report a procedure for evaluating the activity, stability, and electrochemically active surface area for heterogeneous OER catalysts under standard conditions … In particular, the electrochemically active surface area (ECSA) of each catalyst is estimated from measurements of the double-layer capacitance; the activity and stability of each catalyst material are measured using a combination of voltammetry, chronoamperometry, and chronopotentiometry; and the Faradaic efficiency of each material is determined using a rotating ring-disk electrode (RRDE) apparatus.—McCrory et al.
Their goal was two-fold, the researchers said:
to use standard electrochemical procedures and equipment easily accessible to a typical researcher in the field of electrocatalysis; and
to minimize the time and number of experiments necessary to evaluate a catalyst’s activity and short-term stability—an important attribute facilitating the screening of large numbers of catalyst materials.
The relevant figure of merit is the overpotential required to achieve 10 mA cm−2 current density per geometric area at ambient temperature and 1 atm O2. This is approximately the current density expected at the anode in a 10% efficient solar water-splitting device under 1 sun illumination.
To test their protocol, the team evaluated and compared the activity and stability of 10 different electro-deposited Ni- and Co-based metal oxide catalysts for OER under conditions relevant to an integrated solar water-splitting device. From the results of the standardized comparisons, they drew several general observations:
Every non-noble metal catalyst investigated showed similar OER activity, achieving 10 mA cm−2 current density at overpotentials between 0.35 and 0.43 V. An electrodeposited IrOx catalyst under the same conditions achieved 10 mA cm−2 current density at η = 0.32 ± 0.04 V, although it was unstable. The team concluded from this that there is still significant room for improvement in discovering OER catalysts that can operate at high current density and lower overpotential in a stable manner.
Only IrOx showed stability in 1 M H2SO4. Every non-noble metal catalyst investigated was unstable under oxidative conditions in 1 M H2SO4. This result highlights the need for non-noble metal acid-stable OER catalysts in order for solar water-splitting devices operating in 1 M H2SO4 to be feasible.
We note that the benchmarking protocol reported here is considered a first and important step in evaluating catalyst materials; further testing will be needed to truly establish catalyst feasibility. We also note that this benchmarking protocol was specifically designed for testing OER electro-catalysts under conditions relevant to an integrated solar water- splitting device under 1 sun illumination. Other devices that use OER electrocatalysts such as PEM and alkaline water electrolyzers or integrated water-splitting cells under multiple-sun illumination may have significantly different operating parameters and as such will have different figures of merit and may require different testing methods than those reported here.—McCrory et al.
Charles C. L. McCrory, Suho Jung, Jonas C. Peters, and Thomas F. Jaramillo (2013) “Benchmarking Heterogeneous Electrocatalysts for the Oxygen Evolution Reaction,” Journal of the American Chemical Society doi: 10.1021/ja407115p