This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.
This project was stimulated by developments at 3M on nanoporous platinum obtained via electrochemical de-alloying of Ni-Pt particles which showed a four-fold increase in activity for oxygen reduction reaction (ORR) processes at the Ni7Pt3 composition despite the observed lack of Ni in near the surface of the active catalyst. We carried out Reax force field (ReaxFF) reactive molecular dynamics (RMD) to show that leaching out the Ni leaves a jagged nanoporous Pt with 4-fold increased activity. We teamed with the Yu Huang experimental group to make jagged porous Pt nanowires (J-PtNWs) the same way. The J-PtNWs exhibit ultrahigh specific surface area and specific activity, together delivering a record-high mass activity for ORR, a 50 times improvement over Pt/C. The RMD simulations suggest that highly stressed, undercoordinated rhombus-rich surface configurations of the jagged nanowires enhance ORR activity versus more relaxed surfaces. Stressed and undercoordinated crystalline-like surface rhombi can markedly decrease the reaction barrier of the rate-determining steps (RDs) of ORR, thus improving ORR activity.
We consider that the fuel cell catalyst development is hampered by an incomplete understanding of the reaction mechanism underlying the ORR. Several general guidelines based on theoretical insights and experimental observations have been proposed, but there is no consensus of the controlling factors underlying ORR. Despite numerous publications on this subject, the existing fundamental understanding of the ORR mechanism was unable to explain the experiments. We made a major breakthrough in applying quantum mechanics reactive metadynamics (QM-RμD) with explicit solvation (water) model, determining the full ORR mechanism on a Pt(111) surface including the dependence on applied potential. An important discovery is that surface water plays an essential role in donating protons to various ORR steps. The predicted onset potential and activation barriers agree with experiment to 0.05 V and 0.05 eV, respectively. With the ORR mechanism now fully understood, we can use our validated methodology to examine the changes upon alloying and surface modifications to increase the reaction rate by reducing the RDS barriers.
Along with ORR, hydrogen oxidation reaction (HOR) and hydrogen evolution reaction (HER) play important role in electrochemical devices. In fuel cells, HOR provides protons for fuel cell operation. Both HOR and HER are 2 orders slower in alkaline electrolyte than in acidic electrolyte, but no explanation has been provided so far. We performed QMMD with explicit considerations of solvent and applied voltage (U) to in situ simulate water/Pt(100) interface in the condition of under-potential adsorption of hydrogen. We find that as U becomes more negative, the electrode tends to repel water, which in turn increases the hydrogen binding. We predicted a 0.13 eV increase in hydrogen binding from pH = 0.2 to pH = 12.8 with a slope of 10 meV/pH, which is close to the experimental observation of 8 to 12 meV/pH. Thus, we conclude that the changes in water adsorption are the major reason for the pH-dependent hydrogen binding on a noble metal.
The new insight of critical role of surface water in modifying electrochemical reactions provides a guideline in designing advanced ORR and HOR/HER catalysts.
The advances in the theory and software, which we developed in the frame of this project, should impact many other areas. For instance, the approach developed for modeling of de-alloying nanoparticles, the polarizable charge equilibration (PQEq) method and RexPoN force field methodology that includes charge and polarization based on the PQeq method, nonbond interactions and bond breaking, are applicable to other fields of research such as DNA, polymers, inorganic systems for applications to biomolecular, pharma, batteries and other electrochemical devices.
Two undergraduate students were involved in the project and obtained training in electrochemistry and computational modeling, in addition to one post-doc and four graduate students (three of them successfully defended their Ph.D. dissertations). With this NSF grant support, 14 papers were published in high-impact scientific journals such as Science, Journal of American Chemical Society, Energy & Environmental Science, Proceeding of the National Academy of Sciences, Chemical Science, etc. and one book chapter. The results were also reported at over 40 conferences, workshops and in invited talks. A fruitful collaboration was established with the Yu Huang experimental group (UCLA), the Zhifeng Ren group (University of Houston, Texas) and Dr. Alessandro Fortunelli (CNR-ICCOM, Consiglio Nazionale delle Ricerche, Italy).
Last Modified: 01/25/2019
Modified by: WilliamAGoddard
