Selective electrochemical generation of hydrogen peroxide from water oxidation
Venkatasubramanian Viswanathan, Heine A. Hansen, Jens K. Nørskov
SSelective electrochemical generation of hydrogenperoxide from water oxidation
Venkatasubramanian Viswanathan, ⇤ , † , § Heine A. Hansen, ‡ , § and Jens K.Nørskov ⇤ , ¶ , k Department of Mechanical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213,USA, Department of Energy Conversion and Storage, Technical University of Denmark,Kgs. Lyngby DK-2800, Denmark, and SUNCAT Center for Interface Science andCatalysis, Department of Chemical Engineering, Stanford University, Stanford, California,94305-3030, USA
E-mail: [email protected]; [email protected]
Water is a life-giving source, fundamental to human existence, yet, over a billion peoplelack access to clean drinking water. Present techniques for water treatment such as piped,treated water rely on time and resource intensive centralized solutions. In this work, wepropose a decentralized device concept that can utilize sunlight to split water into hydrogenand hydrogen peroxide. The hydrogen peroxide can oxidize organics while the hydrogen bub-bles out. In enabling this device, we require an electrocatalyst that can oxidize water while ⇤ To whom correspondence should be addressed † Department of Mechanical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, USA ‡ Department of Energy Conversion and Storage, Technical University of Denmark, Kgs. Lyngby DK-2800,Denmark ¶ SUNCAT Center for Interface Science and Catalysis, Department of Chemical Engineering, StanfordUniversity, Stanford, California, 94305-3030, USA § SUNCAT Center for Interface Science and Catalysis, Department of Chemical Engineering, StanfordUniversity, Stanford, California, 94305-3030, USA k SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, MenloPark, CA 94025-7015, USA a r X i v : . [ phy s i c s . c h e m - ph ] S e p uppressing the thermodynamically favored oxygen evolution and form hydrogen peroxide.Using density functional theory calculations, we show that the free energy of adsorbed OH ⇤ can be used as a descriptor to screen for selectivity trends between the 2e water oxidation toH O and the 4e oxidation to O . We show that materials that bind oxygen intermediatessu ciently weakly, such as SnO , can activate hydrogen peroxide evolution. We present arational design principle for the selectivity in electrochemical water oxidation and identifynew material candidates that could perform H O evolution selectively.The global energy consumption is projected to increase with the increased energy con-sumption being concentrated in areas that rank highest on the water scarcity index. A keychallenge in providing clean drinking water is to find a low-cost, energy-e cient approach totreatment and disinfection of water so that it is suitable for consumption. Providing piped,treated water requires time and resource intensive centralized facilities and an infrastructurethat does not exist in many places today. Conventional techniques for water disinfectiontypically involve the use of chlorine or ozone as the oxidant. However, chlorination gener-ates hazardous and carcinogenic compounds, while the use of ozone, though e cient andharmless, is expensive. Hydrogen peroxide is an attractive candidate for water treatmentas its degradation product is water and it has strong oxidative properties. H O is generated at an industrial scale through the oxidation of anthraquinone. Thisprocess is a multi-step method requiring significant energy input and generates substantialwaste and its transportation causes possible hazards. A direct e cient and economic routefor production of hydrogen peroxide could enable its wide-spread application in water treat-ment and other areas. The direct thermal catalytic synthesis from H and O on palladiumbased materials have been studied for many years. However, selectivity and productionrate of H O are far below the desired limit. An alternate synthetic route is through directelectrochemical reduction of oxygen and protons.
Experimental and theoretical studieshave shown it is possible to selectively activate hydrogen peroxide generation from oxygenreduction, however, this route requires electrocatalysts that are made out of expensive metals2 h o t o c a t a l y s t h + H O + e H H H H h ⌫ E g E g Figure 1: Schematic of a novel ‘dream’ device concept that can absorb photons from sunlightand use it to split water into hydrogen and hydrogen peroxide. Hydrogen peroxide willdecompose the organics in water and thus cleaning water and hydrogen will bubble out.such as gold, platinum and palladium.
Our core idea is to identify a synthetic route for hydrogen peroxide that uses water as itsonly reactant and uses sustainable electricity preferably from sunlight. In Figure 1, we showthe schematic of a decentralized water treatment device. This device employs a materialthat can absorb photons and generate electrons and holes with appropriate energy such thatthe electrons can reduce protons to hydrogen and holes can oxidize water to H O . Toenable this device, two material challenges need to be overcome. The first is we require aphoton absorber whose band positions are suitably aligned such that it can catalyze hydrogenevolution and hydrogen peroxide evolution. The second, more formidable requirement is ofan electrocatalyst that can catalyze H O evolution and suppress the thermodynamicallyfavored O evolution. The second electrocatalyst requirement forms the focus of our presentwork.In this work, using thermodynamic analysis based on density functional theory calcula-tions, we demonstrate the existence of material candidates that can activate H O evolution3hrough the oxidation of water. We show that the free energy of adsorbed OH ⇤ can be usedas a descriptor, to a first approximation, for determining trends in 2e vs 4e oxidation ofH O. We identify materials that are good candidate materials for H O generation. Amongthese, we identify SnO and TiO as candidate materials with good selectivity and ouranalysis provides a quantitative foundation for the identification of more e cient, selectiveelectrocatalyst materials. This analysis provides a necessary, but not su cient criterion for agood selective electrocatalyst. Undoubtedly, kinetic barriers are important for determiningselectivity, however, such a thermodynamic analysis has proved successful in determiningselectivity trends for oxygen reduction. Results
An ideal electrocatalyst for the 4 electron oxygen evolution reaction should be capable offacilitating oxidation of H O just above the equilibrium potential of 1.23 V. As a mini-mum requirement, the four charge transfer steps should have reaction free energies of thesame magnitude equal to the equilibrium potential of 1.23 eV. We consider the associativemechanism shown below: O(l) + ⇤ ! OH ⇤ + H O(l) + H + + e , (1a)OH ⇤ + H O(l) + H + + e ! O ⇤ + H O(l) + 2H + + 2 e , (1b)O ⇤ + H O(l) + 2H + + 2 e ! OOH ⇤ + 3H + + 3 e , (1c)OOH ⇤ + 3H + + 3 e ! O (g) + 4H + + 4 e + ⇤ . (1d)However, an ideal electrocatalyst for the two electron oxidation of water to hydrogen peroxideshould facilitate the oxidation just above the equilibrium potential of 1.77 V. This impliesthat each of the two charge transfer steps must have a reaction free energy of 1.77 eV. We4onsider a similar associative mechanism for H O production,2H O(l) + ⇤ ! OH ⇤ + H O(l) + H + + e , (2a)OH ⇤ + H O(l) + H + + e ! H O (l) + 2H + + 2 e . (2b)Figure 2: Free energy diagram of water oxidation plotted at U = 1.77 V, versus the reversiblehydrogen electrode on SnO (110) and RuO (110). On SnO (111), the limiting step for 2e oxidation is the activation of H O as OH ⇤ while that for the 4e oxidation is the oxidationof OH ⇤ to O ⇤ . On RuO (111), the limiting step for 2e oxidation is the formation of H O from OH ⇤ while that for the 4e oxidation is the oxidation of O ⇤ to OOH ⇤ .In Figure 2, we show the calculated free energy diagram for the 2e and 4e oxidationof water on rutile type SnO and RuO at U = 1.77 V, versus the reversible hydrogenelectrode. Using the free energy diagrams, an important parameter, the thermodynamiclimiting potential, U L , can be extracted and this is defined as the lowest potential at whichall of the reaction steps are downhill in free energy. This approach has been successfully usedto rationalize trends in hydrogen evolution, oxygen reduction and oxygen evolution. The potential determining step for the 2e oxidation on SnO is the activation of H O asOH ⇤ while that for the 4e oxidation is the oxidation of OH ⇤ to O ⇤ . It is to be noted that the5alculated limiting potential for the 2e oxidation is lower than that for the 4e oxidationand we would expect this to show selectivity towards hydrogen peroxide generation. Incontrast, the potential determining step for 2e oxidation on RuO is the formation of H O from OH ⇤ while the potential determining step for 4e oxidation is the oxidation of O ⇤ toOOH ⇤ . For RuO (110), the activation to OH ⇤ is facile at 1.77 V, however, because thefurther oxidation of OH ⇤ to O ⇤ is strongly exothermic, selectivity for H O is expected tobe low on RuO . Materials with an OH ⇤ binding energy between that on SnO and RuO are expected to have improved activity for the 2e oxidation provided selectivity for the 4e oxidation can be suppressed.The trends for oxygen electrochemistry is determined by the binding of three key in-termediates, O ⇤ , OH ⇤ and OOH ⇤ . However, it has been demonstrated that the bindingof these intermediates on oxide materials are correlated. This enables the activity, givenby the limiting potential, U L , to be plotted as a function of a single descriptor, to a firstapproximation. These plots allow for the quantitative determination of the descriptor valuesthat yield optimal catalyst activity. Generalizing this analysis, for the 4e oxidation of water, in the case of materials thatbind oxygen intermediates too strongly, we have step 1c associated with the oxidation ofadsorbed O ⇤ being the limiting step. Therefore, the free energy di↵erence of the limitingstep is given by, G = G OOH ⇤ G O ⇤ . (3)In the case of the materials that bind oxygen intermediates too weakly, we have step 1bassociated with the oxidation of adsorbed OH ⇤ being the limiting step for the 4e oxidationof water. Therefore, the free energy di↵erence of the limiting step is given by, G = G O ⇤ G OH ⇤ (4)For the 2e oxidation of water, the activity of materials that bind oxygen intermediates6oo strongly, reaction 2b associated with the oxidation of OH ⇤ to H O is the limiting step.The free energy of the limiting step is given by, G = G H O (l) G OH ⇤ . (5)The activity of materials on the weak binding leg of the volcano is limited by 2a associatedwith the activation of water as OH ⇤ and the free energy di↵erence of the limiting step isgiven by, G = G OH ⇤ G H O(l) (6)In this work, we have chosen free energy of OH ⇤ , G OH ⇤ , as the descriptor and this choiceis made as the activity for hydrogen peroxide evolution is directly determined by G OH ⇤ .In Figure 3, we show the calculated thermodynamic limiting potentials, U L , as a function ofthe free energy of OH ⇤ , G OH ⇤ .The calculated activity volcano shows that the optimal catalyst for peroxide evolutionshould exhibit a free energy of OH ⇤ adsorption of 1.77 eV. We identify RuO , PtO as mate-rials that bind oxygen too strongly for peroxide evolution. PtO calculations are performedon a rutile structure. According to our analysis, we also identify SnO and TiO as materialsthat bind oxygen too weakly and have slightly lower potentials for peroxide evolution thanoxygen evolution. It is worth highlighting that kinetic barriers are also important for deter-mining selectivity and the activity volcano analysis presented here provides a necessary, butnot su cient criterion for a good selective electrocatalyst. Discussion
The only direct experimental demonstration for selectively making H O over O throughelectrochemical oxidation of water has been the recent work of MacFarlane and co-workerswho employed MnO x electrode with an ionic liquid based electrolyte to tune the thermo-7igure 3: Activity volcano for the 2e and 4e oxidation of water. The scaling relationsused to construct the volcano are G OOH = G OH + 3 . O = 2 G OH + 0 . dynamics and showed H O formation. Based on our analysis, we find that the limitingpotentials for hydrogen peroxide evolution and oxygen evolution on MnO are quite closeand small changes in surface energetics due to solvent e↵ects (ionic liquid) could a↵ect theselectivity. There has been no direct experimental demonstration of electrochemical H O generation in a purely aqueous system. It is worth pointing out that photogeneration ofH O has been demonstrated on TiO and ZnO. It has also been demonstrated that electrodes that have high overpotentials for oxygenevolution exhibit enhanced performance towards the decomposition of organics although theexact details of the mechanism are still unclear.
For instance, it has been shown thatSnO which is poor at catalyzing oxygen evolution exhibits a higher e ciency for organicdegradation compared to Pt. We suggest that generation of hydrogen peroxide is a pre-cursor to the decomposition of organics and based on this assumption, our analysis attributes8he enhanced e ciency for the decomposition of organics to the weaker binding of oxygenintermediates on SnO compared to Pt. As a result, we would expect little decompositionof organics on PtO and significant decomposition on SnO .Our descriptor based approach can be used to identify possible candidate materials thatcould be e↵ective at selectively catalyzing peroxide evolution. An avenue could be the dopingof metal ions on cheap material such as TiO , which has been pursued for oxygen evolution. We have searched for suitable doped TiO candidates for selective peroxide evolution. Basedon our analysis, we identify TiO doped with Ru or Ir are interesting candidate materials thatcould exhibit enhanced activity for peroxide evolution, as discussed in the SI. In addition toselectivity, stability is a stringent requirement for water oxidation electrocatalysts and weexpect the stability requirement to be play a crucial role in the selection of an electrocatalyst.Finally, the provided device requires a photon absorber whose band positions are suitablyaligned for hydrogen peroxide generation. This poses a requirement for a photon absorberthat the valence band maximum is greater than 1.77 V vs normal hydrogen electode (NHE)and the conduction band minimum is less than 0 V vs NHE. There are many materialcandidates that satisfy this criterion, including SnO . Hence, identifying a suitable photonabsorber is less challenging than finding a selective electrocatalyst. This analysis suggeststhat it is possible to identify a single material that can carry out the catalysis as well as thephoton absorption.We have outlined a quantitative framework for determining selectivity in electrochemicalwater oxidation. We show that it is possible to selectively catalyze the 2e oxidation toH O over the thermodynamically favored oxygen evolution. This can be accomplished undercertain range of potentials by choosing catalysts that are ine cient at carrying out oxygenevolution. We show SnO and TiO as materials that exhibit suitable bonding characteristicsfor peroxide evolution and identify doped TiO candidate materials that could carry out thisprocess more e ciently. This shows that it is possible to selectively form fuels or chemicalsthat involve a smaller number of proton-coupled electron transfer over its thermodynamically9avored competing reaction that involves a larger number of proton-electron transfer. Weexpect this core idea to be broadly useful given the ubiquity of adsorbate scaling relationsand we expect it to be particularly useful for nitrogen and carbon electrochemistry. Methods
Free energy diagrams:
The free energy diagram is calculated based on density functionaltheory calculations which accounts for zero point energy and entropic corrections and adetailed discussion is presented in the SI. The e↵ect of potential, U, is included by shiftingthe free energy of an electron by -eU and the free energy at a potential U, can thus becalculated using the relation, G = G eU where U is the potential relative to thereversible hydrogen electrode and G is the calculated reaction free energy under standardconditions. Computational details:
A detailed description of the computational details is givenin the SI.
Acknowledgements
The authors acknowledge support from the Department of Energy, Basic Energy Sciencesthrough the SUNCAT Center for Interface Science and Catalysis. V.V. acknowledges helpfuldiscussions with Ramya Yeluri.
Author contributions:
V.V. and H.A.H conceived the idea and carried out the theoreticalcalculations. All authors discussed the results and co-wrote the manuscript.
Additional InformationSupplementary Information accompanies this paper.
Competing financial interests:
The authors declare no competing financial interests.
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E-mail: [email protected]; [email protected] ⇤ To whom correspondence should be addressed † Department of Mechanical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, USA ‡ Department of Energy Conversion and Storage, Technical University of Denmark, Kgs. Lyngby DK-2800,Denmark ¶ SUNCAT Center for Interface Science and Catalysis, Department of Chemical Engineering, StanfordUniversity, Stanford, California, 94305-3030, USA § SUNCAT Center for Interface Science and Catalysis, Department of Chemical Engineering, StanfordUniversity, Stanford, California, 94305-3030, USA k SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, MenloPark, CA 94025-7015, USA omputational Details Formation energies of OH ⇤ and O ⇤ on SnO (110) and OH ⇤ , O ⇤ and OOH ⇤ MnO (110) areobtained from Density Functional theory calculations by Man et al. Additional calculationsare performed on SnO (110) to obtain the formation energy of OH ⇤ . It is found that OOH ⇤ spontaneously transfers a hydrogen atom to a nearby bridging oxygen atom. The free energyof OOH ⇤ on SnO (110) is therefore approximated from the scaling relation between OH ⇤ and OOH ⇤ and the formation energy of OH ⇤ . The computational details used are similar to those in ref. 1 and given below for com-pleteness.Density functional theory calculations are performed with the DACAPO DFT code. Ioniccores are described using Vanderbilt ultrasoft pseudo potentials and Kohn-Sham states areexpanded in plane waves with an energy cuto↵ of 350 eV, while the electron density isexpanded in plane waves with an energy cuto↵ corresponding to 500 eV. Occupation ofone-electron states follows a Fermi-Dirac distribution with k B T = 0 . k B T = 0 eV. E↵ects of exchange and correlation are described using theRPBE functional. The SnO (110) surface is modeled using a slab with a (1 ⇥
2) surface supercell cellconsisting of 4 trilayers. The geometry of the bottom two trilayers is fixed in the bulkposition, and adsorbates are added to the topside of the slab. Slabs are separated by 15 ˚Aof vacuum and the electrostatic dipole interaction between periodically repeated slabs hasbeen removed. The first Brilluin zone is sampled using 4 ⇥ ⇥ Adsorbates and the two topmost trilayers are optimized until the maximum forcecomponent is below 0.05 eV/˚A. 2 ree Energy Diagrams
Potential dependent free energies are calculated using the computational hydrogen electrodereference. Consider the initial step in water oxidation, written here in acidH O(l) + ⇤ ! OH ⇤ + H + + e . (1)At 0 V vs an reversible hydrogen electrode (RHE) the reaction H (g) ! H + + e (2)is in equilibrium, so the chemical potential of a proton and an electron is equal to thechemical potential of H (g) µ H (g) = µ H + + µ e . (3)Therefore the reaction free energy of eq. 1 at 0 V versus RHE, G , can be calculated fromthe equivalent gas phase reactionH O(l) + ⇤ ! OH ⇤ + H (g) (4) G = G (OH ⇤ ) G ( ⇤ ) µ H O(l) + 12 µ H (g) . (5)At an arbitrary potential, U versus RHE, the chemical potential of an electron is shifted by eU and correspondingly the reaction free energy of eq. 1 is given by G = G eU. (6)The approach is easily generalized to include formation of O ⇤ and OOH ⇤ adsorbates.The reaction free energy of eq. 4 is calculated from DFT simulations by adding con-tributions from entropy and vibrational zero-point energies (ZPE) to the reaction energies3btained from DFT. G = E DF T + E ZPE T S. (7)For adsorbates, the zero point energy is calculated from vibrational frequencies calculatedon RuO (110) and taken from ref. 1, while the entropy of adsorbed species is assumed tobe negligible. For molecules, the ZPE is obtained from DFT calculated frequencies, whilethe entropy is taken from experiment. The chemical potential of H O(l) is calculated asthe chemical potential of H O(g) at 0.035 bar, which is the vapor pressure of H O at roomtemperature. The contributions from ZPE and entropy are listed in S1.
Table S1: Zero point energies and entropic contributions to adsorbates and wateroxidation reactions. Energies are in eV.
T S T S E
ZP E E ZP E E ZP E T S H O(l) 0.67 0.56H (g) 0.41 0.27OH ⇤ ⇤ ⇤ ⇤ + H (g) 0.20 -0.47 0.50 -0.06 0.41O ⇤ + H (g) 0.41 -0.27 0.34 -0.22 0.05OOH ⇤ + H (g) 0.61 -0.73 0.83 -0.29 0.45c Screening for new candidate materials
Based on the activity volcano, we can screen for new candidate materials that could posseshigher selectivity than SnO . One strategy is to use a cheap material such as TiO andusing doping to tune the adsorption energy. Our analysis suggests that a strengthening ofthe OH ⇤ binding energy by about ⇠ could lead to substantiallyimproved selectivity and electrocatalytic activity for H O evolution.We screened the calculated adsorption energies on doped rutile TiO (110) surfaces basedon a substitutional model with 6.25% transition-metal impurities relative to the host Ti4toms in the slab, that is, M-Ti O . The analysis considered transition metals M=V, Nb,Ta, Cr, Mo, W, Mn, Fe, Ru, Ir, and Ni as dopants at four di↵erent substitutional sites.Among the considered cases, our analysis suggests the most promising candidates are Ir orRu-doped TiO as shown in Figure S1.Figure S1: Activity volcano for the 2e and 4e oxidation of water with the identified dopedTiO candidates. The adsorption site for the doped TiO candidates, Ir and Ru, is on topof a 5-fold coordinated Ti site while the Fe-doped case is on top of a 6-fold coordinated Tisite. References (1) Man, I. C.; Su, H.-Y.; Calle-Vallejo, F.; Hansen, H. A.; Mart´ınez, J. I.; Inoglu, N. G.;Kitchin, J.; Jaramillo, T. F.; Nø rskov, J. K.; Rossmeisl, J. Universality in OxygenEvolution Electrocatalysis on Oxide Surfaces.
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