Fundamental relations for rational catalyst design in oxygen electrocatalysis
Date of Issue2018-02-06
School of Chemical and Biomedical Engineering
The rapid growth of economy brings about serious environmental and resources problems, which force human search for more efficient and environmental benign technologies for energy conversion. Among the various technologies, fuel cells and soar energy are most attractive. The wide application of the clean energy technologies heavily relies on the efficiency electrocatalytic reactions involved. However, the sluggish kinetics of oxygen electrocatalysis, including oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), seriously limits the efficiency of the clean energy technologies. The kinetics can’t satisfy practical requirements even on the best catalysts of oxygen electrocatalysis, making them highly irreversible reactions. What’s worse, the best catalysts are mainly consist of precious materials and their long-term stability is far below practical requirements, such as RuO2 and IrO2 in OER, Pt and Pd in ORR. On the other hand, earth abundant elements such as Fe, Co, Ni are more stable in oxygen electrocatalysis, but their activity is much lower than the precious catalysts. Therefore, detailed elementary processes in oxygen electrocatalysis and their origin should be identified so that one can rationally design new catalysts to fulfill the critical requirements from activity, stability and cost. A number of important reactions such as the oxygen evolution reaction (OER) are catalyzed by transition metal oxides (TMOs), whereas surface reactivity of which is rather elusive. Therefore, rationally tailoring adsorption energy of intermediates on TMOs to achieve desirable catalytic performances still remains a great challenge. My first research work is the identification of a general and tunable surface structure, coordinatively unsaturated metal cation (MCUS), as a good surface reactivity descriptor for TMOs in OER. Surface reactivity of a given TMO increases monotonically with the density of MCUS, thus increase in MCUS improves the catalytic activity for weak-binding TMOs but impairs that for strong-binding ones. Then, I continued to identify the fundamental relations for catalyst design. The most challenging but critical research in the field of catalysis is to identify the rate determining step and associated with elementary thermodynamic origin. However, sophistication of electrified liquid/solid interface and complexity of catalyst’s structure and composition make it incredibly difficult to derive the surface thermodynamics. Here for the first time, we developed a new kinetic model to give a quantitative description of the electrochemical kinetics of oxygen electrocatalysis with elementary surface thermodynamics. Based on the distinctive features in the kinetics for different surface thermodynamics, a straightforward methodology is developed to identify surface thermodynamics from simple electrochemical tests. Our results show that the mechanistic information derived from one reaction is a critical complement to the other, whereas individual study of either reaction could only provide incomplete mechanistic information. The predictive power of our method in developing better catalysts was successfully demonstrated on α-MnO2. Based on our model, we further answered several questions in oxygen electrocatalysis. For example, what’s the origin of the inconsistence between exchange current density with overall catalytic activity? Exchange current density has been used to represent activity in hydrogen electrocatalysis. However, in oxygen electrocatalysis, exchange current density usually does not correlate with catalytic activity. Through comparison with kinetic behaviour of hydrogen electrocatalysis we prove that kinetics of oxygen electrocatalysis and other highly irreversible reactions are predominantly dependent on Tafel slope, instead of exchange current density. Low Tafel slope of good catalysts originates from the collective contribution from RDS and pre-adsorbed intermediates prior to RDS, which also causes orders decrease in exchange current density predicted from Tafel plots.