Authors: Jin Suntivich, Hubert A. Gasteiger, Naoaki Yabuuchi, Haruyuki Nakanishi, John B. Goodenough, and Yang Shao-Horn
Journal: Nature Chemistry
Main Affiliation: Materials Science and Engineering Department and Electrochemical Energy Laboratory, Massachusetts Institute of Technology
What did the researchers do?
The group of Yang Shao-Horn and collaborators determined that the oxygen reduction reaction (ORR) activity for metal oxide catalysts depends primarily on σ*-orbital (eg) occupation and secondarily on the metal–oxygen covalency. Essentially, they experimentally elucidated electronic factors that affect the ORR.
Why is this work important?
The experimentally determined factors that affect the ORR comprise a design principle that provides mechanistic insight on how the ORR functions as well as a basis for improving the ORR activity of future metal oxide catalysts.
The ORR is important because it comprises half of the overall reaction that occurs in hydrogen fuel cells. In a hydrogen fuel cell, hydrogen gas and oxygen gas is directly combined to form water and release energy. The ORR is crucial for reducing oxygen gas into water (O2 + 4H+ + 4e– → 2H2O).
However, the ORR reaction in hydrogen fuel cells currently requires the use of platinum as the catalyst. The high cost of platinum represents a barrier to the cost effectiveness of these fuel cells. Therefore, discovering cheap and highly active transition-metal-oxide catalysts is a goal for improving the practical viability of fuel cells.
How did the researchers do it?
The researchers synthesized a number of perovskite materials by mixing rare earth (La), alkaline earth (Ca), and transition metals (Cr, Mn, Fe, Co, and Ni) together with a strong base. The perovskite (having formula ABO3, where A = La/Ca and B = transition metal, see figure left) then precipitated out of solution. Perovskites were used because they have a structure where the transition metal is surrounded by six oxygen atoms in an octahedral coordination environment. As you may expect, these metal-oxygen bonds are well suited for performing reactions that involve oxygen.
These perovskites were then mixed into an “ink”, used to coat the electrodes (made from glassy carbon). Then, they were electrochemically characterized for their ORR activities. In general, these electrochemical experiments involved determining the potential of the ORR for a given current (called chronopotentiometry). If the perovskites were all compared at the same current, the one with the highest potential has the best activity. The reason why the highest potential (and not the lowest potential) is favorable is because the ORR is a reduction reaction (adding electrons to the oxygen gas). Intuitively, we know that reduction reactions occur at more negative potentials. Thus, the ORR with the least driving force (requiring the least amount of energy supplied) is the one with the highest potential.
What did they learn?
By using electrochemical experiments, the researchers were able to plot the performance of the perovskites (where higher potential is better) as a function of the number of eg electrons (see figure right). They discovered a trend where the performance increased up to eg = 1, and then decreased as more electrons are added to the orbital.
In terms of molecular orbital theory, the eg electron in an octahedral environment can be thought of as occupying the dz orbital. Therefore, the eg electrons influence the surface metal to oxygen bond. The eg orbital is also antibonding so as more electrons are added, the metal-oxygen bond weakens.
Now we can use this theory to explain the figure (right). As we start adding electrons to the eg orbital, the metal-oxygen bond weakens and it is easier to release bound oxygen species from the surface. This is good for the ORR process since new oxygen gas (O2) species can displace existing surface sites to bind to the perovskite. However, if we add too many electrons to the eg orbital, then the metal-oxygen bond becomes too weak and it becomes difficult to bind oxygen gas in the first place!
Therefore, to optimize the activity of perovskite materials for oxygen reduction, the material must have just the right number of electrons in the eg orbital. This principle is also qualitatively known as the Sabatier principle. Through the experiments discussed above, the researchers were able to develop a quantitative model of perovskites based on the Sabatier principle which enables the prediction and optimization of ORR perovskite materials.