Hello,
A change in allocation has been requested:
Requester: jlow (John J. Low)
Project: DFT_NiOxHy_on_Pt3Ni
Title: First principles studies of stability and reactivity of Ni (hydr)oxide films on Pt3Ni(111) surface in alkaline environments
Description: Electrochemical surface studies on well-defined, single crystal metal and oxide surfaces have provided tremendous understanding of the fundamentals of chemistry and catalysis at interfaces in the past 20 years. The applications of such systems are wide-ranging, from fuel cells to solar fuels production. However, very few studies have rigorously probed the fundamentals of electrocatalytic processes at the interface between oxide/hydr(oxy)oxide films, metal or alloy substrates, and bulk solution phases. Metals and oxides have dramatically different structures, properties, and affinities to reactive species, which bring potentially unique properties to electrocatalytic process at interfaces between these disparate classes of materials. Experimental results by Nenad Markovic’s group (ANL/MSD) have demonstrated that certain electrochemically deposited oxides/hydr(oxy)oxides (nickel, cobalt, iron, and manganese-based, deposited as islands with diameters of 5-10 nan
ometers) on Pt(111) can efficiently catalyze the hydrogen evolution reaction (HER) with activities never before seen on either metal or oxide catalysts in alkaline electrolytes. However, the atomic-scale structures and the underlying fundamentals of the high catalytic activity on these transition metal oxide/Pt(111) interfaces are still not fully understood. In addition, the generalization of previous experimental and theoretical investigations to more complicated metal substrate structures, such as Pt-based alloys, has not been attempted. Indeed, some Pt alloys have shown higher electrocatalytic activity than Pt itself. Investigation of alloy substrates is thus a natural extension of the work on pure Pt substrates and may provide an extra variable to tune the activity of water dissociation and hydrogen evolution reactions at three phase interfaces. As there is still no experiment on these combined systems, the trend predicted from present study could provide a blueprint for
the future search for and preparation of catalysts with higher activity.
Our previous calculations focused on the electrode potential-dependent structures of two dimensional, two ML films of hydroxides (Ni, Co, Fe and Mn) deposited on Pt(111) substrates; this work demonstrated that the Ni and Co-based structures bind hydroxyl groups significantly more weakly than do the Fe and Mn-based structures. Recently, Hubbard U and van der Waals corrections have been added to the calculations due to their importance in describing strongly correlated systems and layered systems with weak interaction between the layers; these results have shown a remarkable level of agreement with experimentally measured bulk oxide/hydr(oxyl)oxide reaction energies. We have tried to use the same approach to study Ni oxide/hydr(oxyl)oxide on Pt(111) surface to generate interface models at relevant potential. The preliminary studies of water dissociation have shown that the barriers at the three-phase boundary edges are much lower than on Pt(111) and step surfaces. In this work,
we will try to firstly refine the study on Ni oxide/hydr(oxyl)oxide films. Besides studying the barrier of water dissociation, we will evaluate the barriers for other elementary reaction steps in HER, together with the coverage-dependent thermodynamics of OH and hydrogen adsorption at these interfaces. We will further extend the work beyond Pt(111) to Pt3Ni(111) alloy substrates, with an ultimate goal of comparing not only the structural and catalytic properties of different oxide/hydr(oxy)oxide films, but also to probe the impact of the substrates on this bifunctional chemistry.
We will initially focus on extending the two dimensional hydroxide thin-film models to accurately describe three phase interfaces of Ni oxides/hydr(oxy)oxides with Pt3Ni(111) substrates and the surrounding bulk medium. The structure and termination of the edges could deviate from the corresponding structures of the two dimensional films, and we will thus need to consider numerous edge structures with different oxidation states. Then, we will test the chemistry of water dissociation at the edges. This will permit us to more thoroughly understand the impact of the nature of the substrate on the structure and chemistry of the three phase boundary, and we fully anticipate that very different physics and chemistry will be observed for films on the Pt3Ni(111) alloy surface in comparing with Pt(111) surface.
Firstly, We will build upon some of our recent work, which has focused on the development of higher computational accuracy and the models of the two dimensional oxide/hydr(oxy)oxide films on Pt(111) and on water dissociation at Ni(OH)2/Pt(111) three-phase interfaces, to extend this work to Pt3Ni alloy substrate .
We will follow an analogous procedure to that described for Pt(111) to develop both the two dimensional and three phase interface models on alloys. While it is entirely possible that the structures on alloys will differ substantially from those on Pt(111), we fully anticipate that the experience that we have gained from the Pt work will significantly accelerate our efforts on alloys. Our initial investigations will be on the well-known Pt-skin alloy structures, which should preserve the same qualitative features as the pure Pt(111) substrates, following which we will move to alloys with mixed surface layers, where deviations from the Pt(111) behavior will be more significant.
Ultimately, we would like to compare the water dissociation properties for different oxide/hydr(oxy)oxide films on both pure Pt and alloy substrates. If the hypothesis that H produced from water dissociation moves to metal sites is correct, then the thermodynamics and kinetics of water splitting will be substantially different on the different metal substrates. In combining with the influence of substrates to the thermodynamics and kinetic of hydrogen evolution (2HH2), this could provide significant additional information that would be of use in further searching for and optimizing bifunctional catalysts, by tuning the films and substrates.
Calculations with the VASP code have shown that full convergence of model hydr(oxy)oxide/Pt(111) spin-polarized calculations on the 2×2 super cells require approximately 48 hours per total geometric optimization on 32 cores Intel Xeon-E5. For the Pt3Ni(111) substrate, we will firstly consider well-known Pt skin structure. From low potential (hydrogen evolution potential region) to high potential (oxygen evolution region), we will need to consider a significant number of different stoichiometries and configurations to find optimized potential-dependent interface structures from monolayer up to four layer films; we tentatively estimate that the total number of resulting optimizations will be around 60 (including many possible permutations of O, OH, and vacancy groups), yielding a requirement of ~90,000 core hours. The edge models do not need to consider so many stoichiometries and configurations, but require bigger super cell, such as 6×2 super cells, which may be at least 4-
fold more time consuming than 2×2 super cells on a single geometric optimization. We estimate that there are 10 edge models need to be tested, which may require another 60,000 core hours. To study the kinetics, we need to determine barriers for water dissociation (Volmer step), hydrogen evolution through Heyrovsky mechanism and Tafel mechanism at the three phase boundary edges. We anticipate needing to run approximately six Nudged Elastic Band (NEB) calculations. Due to a single NEB calculation may be 10 fold more time consuming than geometry optimization, we need another 90,000 core hours. Assuming the process could be accelerated through the information accumulated on Pt3Ni Pt skin substrate, the calculations on all alloy substrates with mixed surface layers could be done with comparable core hours. Then our total request comes to 480,000 core hours.
The models of fully two dimensional hydroxide films on Pt(111), which were developed in previous proposal cycles, have been published in Nature Materials. We have also developed a new technique to highly accurately describe the stability of oxides/hydr(oxy)oxides, and have found that using water reference, van der Waals correction and optimized Hubbard U together, the formation energy of oxides and hydr(oxy)oxides can be predicted highly accurately, i.e. the standard deviation with respect the experiment values could be around 0.04 eV. This accuracy has never been reached by standard DFT calculations in previous study, and has strong implication for the application on the extended systems, e.g. thin/nano films listed in the present proposal. It also could be the interest of the broad community and the paper is in preparation.
Current: undetermined amount
Justification:
Requested: 200000
A specific reason has been given:
We have finished the first part of the project, i.e. screening the Ni (hydr)oxide
films/Pt3Ni(111) electrocatalyst. And we also have started the second
part of the project, i.e. evaluation of the solvation effect on the
chemistry at water/electrode interfaces. This part involves ab inito
molecular dynamics (AIMD) simulation on Pt electrodes (about 100 Pt
atoms) covered by about 100 liquid water molecules, and following up
kinetic analysis (NEB) for the water dissociation at the water/electrode
interfaces screened. We have been on half of simulation on the clean Pt
surface covered by water. Besides to finish the ongoing AIMD simulation
and kinetic analysis, we need to do similar simulation on hydrogen
covered electrode and OH covered electrode. Thus, we still need to run
about 8 week-long AIMD simulations with 6 nodes (~140,000 core hours in
total), and 20 two-day-long NEB calculations with 8 nodes (~60,000 core
hours in total).
This needs to be approved and the final allocation amount decided upon.
Thank You,
The LCRC Accounts System
