Authors: James B. Gerken , J. Gregory McAlpin , Jamie Y. C. Chen , Matthew L. Rigsby , William H Casey , R. David Britt , and Shannon S. Stahl
Journal: J. Am. Chem. Soc.
Published: August 1, 2011
Main Affiliations: Department of Chemistry, University of Wisconsin-Madison and Department of Chemistry, University of California, Davis.
What did the researchers do?
The researchers investigated cobalt-catalyzed water oxidation over a wide pH range from pH 0 to 14. They used a combination of electrochemical and electron paramagnetic resonance (EPR) methods to determine the phase (meaning composition) of the cobalt catalyst in various pH regimes (< pH 3, between pH 3 and 5.5, and pH > 5.5). They then proposed mechanisms for the cobalt-oxide catalysts for two cases: below and above pH 3.5.
Why is this work important?
Water oxidation is half of the process that occurs in water splitting, where hydrogen and oxygen gas is formed from water. This process is a promising for high density energy storage, an area of interest for its potential replacement of carbon-based liquid fuels.
Although cobalt-oxide materials as electrocatalysts for water oxidation have been reported in the 1950s and then, more recently, studied more extensively by Nocera and coworkers, those studies have focused on the neutral and alkaline pH regimes. This work is important in that it extends the characterization of cobalt-oxide water oxidation catalysts into the acidic pH regime. In doing so, the researchers found a new mechanism at low pH where hydrogen peroxide (H2O2) is generated, which then disproportionates into water and oxygen.
How did the researchers do it?
The cobalt-oxide catalysts were made by electrodeposition on fluorine-doped tin oxide (FTO) glass plates. Electrodeposition, in this study, is a process where a positive potential is applied to the FTO plate immersed in a solution containing dissolved cobalt ions (in the Co(II) state). The positive potential oxidizes the cobalt ions into a cobalt-oxide material where the cobalt atoms are in a Co(III/IV) state. FTO is a transparent conductive material that serves as a good electrode and substrate because of its strong stability in solution and under oxidizing conditions.
The deposited films were then placed in various buffer solutions that spanned the pH range from 0 to 14. Buffers were used because they are, by definition, solutions that maintain their desired pH.
Three kinds of electrochemical experiments were run. The first involved using the catalysts to split water and then monitor the amount of oxygen generated. This amount was compared to the theoretical amount of oxygen that should be generated if 100% of the supplied electrons were used to oxidize water. Ideally, the measured percentage (called the faradiac efficiency) should be very close to 100%. Indeed, the measured faradiac efficiency was >95%.
Interestingly, they noticed that under acidic conditions, the detected amount of oxygen was initially sluggish. The researchers then added some iodide and detected peroxide formation. Thus, they proposed that in acidic conditions, water is not directly oxidized to oxygen. Instead, hydrogen peroxide is first generated, which then forms oxygen.
The second experiment involved taking Tafel data, a measure of the potential needed to supply a certain amount of current during the water oxidation process. These Tafel plots are very useful for determining the mechanism for water oxidation on the cobalt-oxide films. The Tafel plots can also give a qualitative feel for how well a film performs over a wide range of currents. The researchers’ Tafel data showed two different mechanisms: one for < pH 3.5 and the other for > pH 3.5.
The third experiment took cyclic voltammograms (CV) of the cobalt-oxides over a wide pH range. The peaks in the CVs correspond to oxidation and reduction of the cobalt species. These peaks were then correlated with a type of phase diagram called a Pourbaix diagram. A Pourbaix diagram shows the thermodynamically stable phase of a species for a given pH and potential. Because the peaks in the CVs matched the boundaries of the cobalt-oxide phases well, this provided a basis for identifying the phase of the cobalt-oxide catalysts.
A large part of the paper used EPR spectroscopy to verify the oxidation and coordination of cobalt atoms in the cobalt-oxide catalysts. These studies were important in providing a rigorous identification of the cobalt-oxide phases in the films and solution. However, the hyperfine details are beyond the scope of this summary.
What did they learn?
The researchers learned: a) There are two different mechanistic regimes for cobalt-oxide films (below and above pH 3.5). For each regime, the researchers proposed a possible mechanism for water oxidation. b) The phases of generated catalyst closely match the cobalt Pourbaix diagram. c) The mechanism for the acidic regime involves the generation of hydrogen peroxide that disproportionates into oxygen and water.
