The pH Dependence of Oxygen Evolution Reaction

Adam Eddy Author

08/03/2021 Added

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The origin of the super Nernstian behavior of the oxygen evolution reaction on the Ruthenium Oxide catalyst is explored in this video. Density functional theory, as implemented in VASP, was used to determine the surface phase diagrams of multiple Ruthenium Oxide facets as a function of potential and pH.

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[00:00:00.870]Hello. My name is Adam Eddy, and I did research with Dr.
[00:00:04.860]Vitali Alexandrov's research group this summer with the UCARE program.
[00:00:08.820]And I studied the pH dependence of the oxygen evolution reaction,
[00:00:13.860]a larger motivation for me and many young scientists is addressing the effects
[00:00:18.060]of climate change.
[00:00:19.530]Many current processes that use nonrenewable resources must be decarbonized to
[00:00:24.120]reduce emissions. Electrolysis of water,
[00:00:26.940]or more simply put splitting water into oxygen.
[00:00:29.640]And hydrogen gas is one way to decarbonize.
[00:00:33.180]Electrolysis of water is a promising approach to storing energy from
[00:00:36.750]intermittent, renewable sources, such as wind and solar,
[00:00:40.380]and a more sustainable approach to producing hydrogen gas.
[00:00:44.550]The electrolysis of water is an electro chemical reaction.
[00:00:47.850]It consists of two half redox reactions.
[00:00:51.410]At either the cathode or anode. Oxygen is formed at the anode,
[00:00:55.160]and the electrons are transferred to the cathode where hydrogen is formed.
[00:00:59.120]The oxygen.
[00:00:59.630]Evolution reaction occurring at the anode consists of many proton,
[00:01:03.030]couple of the electron transfer steps and is thermodynamically the more energy
[00:01:07.400]intensive reaction.
[00:01:09.890]My research focused on the oxygen evolution reaction.
[00:01:13.100]There's a wide variety of electro- catalysts used to perform OER,
[00:01:17.090]but ruthenium oxide shows high activity and it's considered a benchmark material.
[00:01:22.310]Despite a large amount of research on this material.
[00:01:25.100]One factor that is still perplexing is the effect of pH. The ruthenium
[00:01:29.840]oxide root tile.
[00:01:30.740]Lattice shown in this figure can be cut to expose different faces of the lattice
[00:01:35.690]called facets.
[00:01:37.430]The OER performance of some facets response to changing pH in an expected
[00:01:42.320]fashion and our termed Nernstian,
[00:01:45.500]while their other facets don't respond as expected and are called Super-Nernstian
[00:01:49.760]The potential required to perform OER is measured relative
[00:01:54.320]to the standard hydrogen electrode.
[00:01:58.130]The reaction and potential of the SHE depends on the equilibrium between
[00:02:02.240]hydrogen ions and a hydrogen gas,
[00:02:04.790]and therefore the potential change with pH.
[00:02:08.000]So potential changes with the pH according to the Nernst equation,
[00:02:11.270]which changes by 0.059 volts per unit pH as shown in the
[00:02:15.950]Nernst equation on this slide,
[00:02:18.410]since the potential needed to perform OER is in reference to the SHE.
[00:02:23.480]The OER potential should also change by 0.059 volts per unit
[00:02:28.280]pH. However, this is not true for [inaudible] gene facets.
[00:02:33.680]The purpose of this research is to better understand the effect of pH on the
[00:02:37.190]oxygen evolution reaction, and
[00:02:39.370]to determine why some facets exhibit Super Nernstian behavior.
[00:02:43.790]Another objective is to better understand the OER mechanism.
[00:02:46.820]So more efficient and durable catalysts can be predicted.
[00:02:51.080]The first step to better understanding OER is determining the surface atomic
[00:02:55.070]structure.
[00:02:57.110]Identifying these structures will allow us to better hypothesize how water and
[00:03:01.960]intermediates interact with the Ruthenium Oxide catalyst.
[00:03:06.240]A key goal is computing the surface structures for multiple facets at high.
[00:03:10.390]And low pH levels.
[00:03:12.610]To determine the most stable surface structures.
[00:03:15.220]The Gibbs free energy of formation of each surface structure was compared the
[00:03:19.420]Gibbs free energy change was calculated by the standard method of finding the
[00:03:23.050]difference in energy between the products and reactants one example,
[00:03:26.920]Gibbs free energy calculation for lids option of OH onto the surface of the
[00:03:31.030]catalyst is shown in this slide.
[00:03:33.340]The active catalyst sites are denoted by a star to determine the
[00:03:37.960]energies needed for the Gibbs free energy calculations.
[00:03:41.650]The configurations where model at the atomic scale using a quantum mechanical
[00:03:45.970]model,
[00:03:46.690]the main quantum mechanical model used was density functional theory,
[00:03:51.640]and it was implemented using the Vienna AB Initio
[00:03:55.990]simulation package,
[00:03:57.970]a thorough explanation of DFT and how it is implemented in VASP is beyond the
[00:04:02.410]scope of this presentation.
[00:04:04.240]But the VASP program essentially optimizes an input structure to a minimum
[00:04:08.530]energy ground state by moving the atoms and changing the spatial density of
[00:04:12.730]electrons.
[00:04:15.430]The results I produce for my research this summer are surface phase diagrams.
[00:04:19.750]These diagrams show the Gibbs free energy of
[00:04:23.140]the most stable surface configuration
[00:04:27.400]as the potential of the standard hydrogen electrode changes. A mix of two Nernstian
[00:04:31.690]and three Super Nernstian facets were analyzed,
[00:04:35.140]which are listed in this slide.
[00:04:38.260]First facet analyzed was (001). In this diagram,
[00:04:41.740]the vertical axis shows the Gibbs free energy and the horizontal axis shows the
[00:04:45.610]OER potential versus the standard hydrogen electrode.
[00:04:50.110]The pH values for alkaline neutral and acidic conditions are also shown on
[00:04:55.090]the horizontal axis with the pH values found using the Nernst equation.
[00:05:00.190]Each colored line on the diagram shows a different surface configuration.
[00:05:04.570]These structures are labeled in the legend and visual representations are shown
[00:05:08.530]below the diagram with figures that are bordered by their respective line
[00:05:12.850]colors. The surface,
[00:05:15.940]are labeled with cus and or B R I,
[00:05:19.630]which specifies what site on the catalyst is being utilized.
[00:05:24.100]C U S or cus stands for coordinated unsaturated sites and
[00:05:29.050]are the oxygen atoms that are bonded to only one ruthenium the
[00:05:33.700]Bri stands for bridge and are the oxygen atoms that are bonded to two ruthenium
[00:05:38.620]atoms. For the (001) facet. We can see that at high pH values,
[00:05:43.240]the surface cus sites are covered by absorbed water.
[00:05:48.490]As pH values lower to neutral conditions.
[00:05:51.220]Half the cus sites are occupied by OH and the other half are occupied by
[00:05:55.320]water. Finally, in acidic conditions,
[00:05:57.950]half the cus sites are covered by OH and the other half.
[00:06:01.180]Covered by oxygen.
[00:06:03.910]Now for the (011) facet, I found that high pH conditions,
[00:06:07.870]the surface cus sites are covered by water and neutral pH conditions.
[00:06:12.370]Half the cus sites are covered by OH and the other half are covered by water.
[00:06:16.660]Finally, in acidic conditions, the surface is fully.
[00:06:19.270]Oxidized.
[00:06:21.880]For the (100) facet,
[00:06:23.950]both alkaline and neutral conditions have the cus sites filled by water.
[00:06:28.040]In acidic conditions, the surface is again, completely oxidized.
[00:06:33.070]Next is the (110) facet. In alkaline conditions.
[00:06:36.460]The cus sites are completely occupied by water and the bridge sites are
[00:06:40.120]completely occupied by OH In neutral conditions.
[00:06:43.300]The cus sites are covered by OH and the bridge sites are also covered
[00:06:46.630]by OH. Lastly, in acidic conditions,
[00:06:50.500]the surface is again, completely oxidized. Lastly,
[00:06:54.910]the (101) facet was analyzed. In alkaline and neutral conditions.
[00:06:58.960]Half the cus sites are occupied by water and the other half are occupied by OH.
[00:07:03.940]Additionally, half the bridge sites are covered by OH. In acidic conditions.
[00:07:08.020]The surfaces is completely.
[00:07:09.460]Oxidized.
[00:07:11.860]The main conclusions from these phase diagrams is that each facet surface
[00:07:16.420]behaves differently. And there is no clear, uh,
[00:07:19.510]reason for Super Nernstian behavior just from the surface configurations.
[00:07:24.640]However, now that surface structures at various pH values are known.
[00:07:28.570]OER can be modeled and each intermediate step can be investigated.
[00:07:33.070]It should also be noted that these calculations are performed with vacuum above
[00:07:37.480]and no solvent. Incorporating a solvent will make calculations more
[00:07:42.010]costly in terms of running time,
[00:07:44.170]but important solvent effects might become apparent. Lastly,
[00:07:47.920]the amount of charge transfer involved with each hydrogen coupled electron
[00:07:52.000]transfer needs to be studied right now does assume that an integer electron
[00:07:57.490]is transferred for each hydrogen removed during OER,
[00:08:01.360]but this could be an invalid assumption that needs to be explored. Well,
[00:08:05.490]I'd like to thank Dr. Alexandrov and the other two students in his group,
[00:08:08.980]Alexandra and Iman,
[00:08:10.690]each of them provided lots of guidance and help with understanding and
[00:08:13.930]completing this research.
[00:08:15.670]I would also like to acknowledge that all 3d model figures are visualized
[00:08:21.250]using Vesta. And here are my references. Thank you.

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The pH Dependence of Oxygen Evolution Reaction

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