HEMATITE-BASED PHOTOANODES WITH MANGANESE, COBALT, AND NICKEL ADDITIVES
A photoanode, photochemical cell and methods of making are disclosed. The photoanode includes an electrode at least partially formed of hematite and having a surface doped with at least one of nickel, cobalt and manganese. The electrode surface may be doped with nickel. The surface may be doped with cobalt. The electrode surface may be doped with manganese. An aqueous solution may surround the photoanode and a cathode, the photoanode being configured to generate holes upon light absorption and the cathode being configured to emit electrons to the aqueous solution. A voltage source may be electrically coupled between the photoanode and the cathode.
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This application claims priority to earlier filed provisional application 61/554,287 filed on Nov. 1, 2011 which is incorporated herein in its entirety.
UNITED STATES GOVERNMENT RIGHTSThis invention was made with government support under Grant No. FA9550-10-1-0162 awarded by the U.S. Air Force Office of Scientific Research (AFOSR) and Grant No. DE-SC0002120 awarded by the Department of Energy. The government has certain rights in this invention.
FIELD OF INVENTIONThe present invention relates to photoanodes. More particularly, it relates to high efficiency photoanodes with reduced overpotential.
BACKGROUNDIn photocatalysis, sunlight is used to provide energy for endothermic chemical reactions and the energy is stored as chemical energy in the reaction products. For example, consider the water splitting reaction of H2O→H2+O2. Among many potential candidates for use as photoanodes, hematite (α-Fe2O3) stands out as being cheap, abundant, and non-toxic, with a close to optimum band gap. However, it has been shown experimentally that the use of hematite as a photoanode requires a large overpotential and exhibits low efficiency. It would be desirable to provide improved photoanode compositions and methods of making such photoanodes.
SUMMARY OF THE INVENTIONA photoanode, photochemical cell and methods of making are disclosed. The photoanode includes an electrode at least partially formed of hematite and having a surface doped with at least one of nickel, cobalt and manganese. The electrode surface may be doped with nickel. The electrode surface may be doped with cobalt. The electrode surface may be doped with manganese. The electrode may be bulk doped with at least one of cobalt and nickel to enhance light absorption. The electrode may be bulk doped with at least one of silicon and manganese to enhance band alignment and carrier transport. An aqueous solution may surround the photoanode and a cathode, the photoanode being configured to generate holes upon light absorption and the cathode being configured to emit electrons to the aqueous solution.
A photochemical cell is also disclosed, the photochemical cell is configured for electrolysis of water and includes a photoanode comprised of hematite and having a surface doped with at least one of nickel, cobalt and manganese configured for immersion in the water. The photochemical cell also includes a cathode electrically coupled to the photoanode, the cathode being configured for immersion in the water. The electrode surface may be doped with nickel. The electrode surface may be doped with cobalt. The electrode surface may be doped with manganese. The electrode may be bulk doped with at least one of cobalt and nickel to enhance light absorption. The electrode may be bulk doped with at least one of silicon and manganese to enhance band alignment and carrier transport. The photoanode may be configured to generate holes upon light absorption and the cathode may be configured to emit electrons to the aqueous solution. A voltage source may be electrically coupled between the photoanode and the cathode.
A method of making a photoanode is also disclosed. The method includes, providing an electrode at least partially formed of hematite and doping a surface of the electrode with at least one of nickel, cobalt and manganese. The electrode surface may be doped with nickel. The electrode surface may be doped with cobalt. The electrode surface may be doped with manganese. The electrode may be bulk doped with at least one of cobalt and nickel to enhance light absorption. The electrode may be bulk doped with at least one of silicon and manganese to enhance band alignment and carrier transport. An aqueous solution may surround the photoanode and a cathode, the photoanode being configured to generate holes upon light absorption and the cathode being configured to emit electrons to the aqueous solution. A voltage source may be electrically coupled between the photoanode and the cathode.
A method of making a photochemical cell configured for electrolysis of water is also disclosed. The method includes providing a photoanode comprised of hematite, doping a surface of the electrode with at least one of nickel, cobalt and manganese, and electrically coupling a cathode to the photoanode, the cathode being configured for immersion in the water. The electrode surface may be doped with nickel. The electrode surface may be doped with cobalt. The electrode may be bulk doped with at least one of cobalt and nickel to enhance light absorption. The electrode may be bulk doped with at least one of silicon and manganese to enhance band alignment and carrier transport. The electrode surface may be doped with manganese. An aqueous solution may surround the photoanode and a cathode, the photoanode being configured to generate holes upon light absorption and the cathode being configured to emit electrons to the aqueous solution. A voltage source may be electrically coupled between the photoanode and the cathode.
Solar energy is a promising resource to help satisfy a fast growing global energy demand. Photoelectrochemical reactions can harvest solar energy by using sunlight to provide energy for endoergic chemical reactions, which in turn create reaction products that store chemical energy. The water splitting reaction, H2O→H2+O2 (E0=−1.23 V), produces H2 energy carriers, which can be used as a fuel itself or as a feedstock to produce liquid fuels. Water splitting requires not only energy input but also photoelectrocatalysts to accelerate the reaction. Hematite (α-Fe2O3, “α-” is omitted henceforth) has shown promise as a photocatalytic anode material. It has an indirect optical band gap of 1.9-2.2 eV which can absorb approximately 40% of the solar spectrum. It is cheap, abundant, nontoxic, and stable against corrosion. Its valence band and conduction band alignments permit water oxidation to produce oxygen, but it cannot form hydrogen without an applied voltage. Ti-doped hematite surfaces modified by exposure to a CoF3 solution can shift the conduction band position so that an external bias is not required to generate hydrogen from water. This offers the promise of full water splitting (as opposed to just oxidation).
Hematite has some shortcomings, however, including low conductivity, a small optical absorption coefficient and fast electron-hole recombination rates. Consequently, improving the efficiency of hematite as a photoanode is desirable. N-type doping with Ti, Si, Ge, Zr, etc., may increase carrier concentrations and hence conductivity. Nanostructuring may shorten hole diffusion pathways and reduce electron-hole recombination. Surface modifications with cocatalysts or doping may improve reaction kinetics and reduce overpotentials. By incorporating these strategies, a water oxidation photocurrent of >3 mA/cm2 was achieved using nanostructured hematite with an IrO2 cocatalyst at an applied potential of +1.23 V versus the reversible hydrogen electrode under standard solar illumination conditions.
Hematite has a corundum lattice structure, with lattice constants a=5.035 Å and c=13.747 Å. Below the Néel temperature (TN=963 K), Fe2O3 is antiferromagnetic with weak ferromagnetism. The high-spin d5 Fe3+ cations within one bilayer in the (0001) planes are ferromagnetically coupled to each other while antiferromagnetically coupled to the adjacent Fe bilayers. The two natural growth faces of hematite are the (0001) and the (0112) surfaces. Experimentally, both surfaces have been characterized under ultrahigh vacuum (UHV) and when in contact with water. Theoretically, the surface energies of the two surfaces under vacuum are similar, for example the (0001) surface is ˜0.1 J/m2 less stable. The water oxidation reaction on the (0001) surface was examined and provides some understanding the redox surface chemistry of hematite. The termination of the (0001) surface is less complicated with fewer reconstructions than other surfaces. The disclosure herein is also relevant to related surface chemistry on the hematite (0112) surface and hematite polycrystalline surfaces such as those present on actual photoanodes.
Under UHV conditions, the single-layer Fe-terminated (0001) surface, which is stoichiometric and nonpolar, has been suggested to be the most stable by X-ray photoelectron diffraction and scanning tunneling microscopy (STM) experiments. Coexistence of the single-layer Fe-terminated surface and the O-terminated surface was observed by STM and low-energy electron diffraction (LEED) under an oxygen pressure of 10−4-10−1 mbar. This coexistence was also predicted for certain oxygen partial pressures by full-potential linearized augmented plane wave density functional theory (DFT) calculations within the generalized gradient approximation (GGA) for exchange-correlation (XC). DFT-GGA calculations using projector augmented wave (PAW) potentials predicted that at a constant oxygen partial pressure of 0.2 bar, the most stable Fe2O3 (0001) surface below 500 K is the O-terminated surface. Because self-interaction errors (SIEs) inherent in standard Kohn-Sham DFT are large for localized Fe 3d electrons in Fe2O3, standard DFT incorrectly predicts narrow band gaps for bulk hematite and overestimates the interlayer spacings in both hematite bulk and surface structures. The DFT+U method, which includes exact intra-atomic exchange energy, has been proposed and successfully applied to strongly correlated materials to correct for the SIEs. Therefore, the above conclusions derived from DFT calculations may change if the more physically correct DFT+U theory is employed, as disclosed herein.
Water adsorption on the Fe2O3 (0001) surface has been experimentally characterized with various surface science techniques. In one study using X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy, Auger, and temperature programmed desorption (TPD), only ice condensation was observed on the stoichiometric surface at 175-220 K, while sputtered surfaces containing oxygen vacancies and concomitantly reduced Fe2+ ions chemisorb water strongly. Another study (on hydrated and hydroxylated hematite (0001) surfaces using XPS, scanning force microscopy (SFM), scanning electron microscopy (SEM), X-ray diffraction, and LEED) found adsorption and dissociation of H2O were restricted to the top monolayer of the surfaces under ambient conditions. Another study using XPS for water adsorption on hematite (0001) near ambient conditions observed hydroxylation of only the topmost surface layer before water molecule adsorption. Another study was directed at the hydrated hematite (0001) surface at room temperature with a nearly-water-saturated He atmosphere using crystal truncation rod (CTR) diffraction and DFT-GGA calculations. Spacings of the terminating Fe and O layers were measured by CTR. Identifications of the terminations were made by comparing the measured spacings to those predicted by DFT and taking into account the thermodynamic stabilities of different terminations predicted by DFT. It has been suggested that two different hydroxylated domains coexist: one domain corresponds to a full hydroxylation of the single-layer Fe-terminated surface ((HO)3—Fe—(HO)3—Fe—R, where “R” represents the remaining layers in the bulk), and another domain is a fully hydroxylated O-terminated surface ((HO)3—Fe—Fe—R) resulting from removal of a Fe(OH)3 species from the first structure. Since DFT-GGA overestimates the spacings of hematite as already discussed above, the identifications based on comparing experimental data to DFT-GGA values should be viewed with caution.
Theoretically, water adsorption and hydroxylation of hematite surfaces have been studied by both atomistic simulations and DFT. According to one classical potential simulation, the structure with Fe(OH)3 dissociating from the surface was 0.82 J/m2 more stable than the structure with Fe(OH)3 attached to the surface, implying desorption of Fe(OH)3. DFT-GGA studies of Fe-terminated or defective hematite (0001) surfaces predicted that defective surfaces with Fe-adatoms or vacancies are more reactive toward H2O. Theoretical studies detailing water oxidation mechanisms on hematite (0001) surfaces do not appear to be available.
Since the (HO)3—Fe—(HO)3—Fe—R domains might gradually evolve to (HO)3—Fe—Fe—R after Fe(OH)3 dissociates, this disclosure encompasses the surface chemistry of the hematite (0001) surface with the (HO)3—Fe—Fe—R model termination. This disclosure also encompasses doping effects on the surface chemistry. For cation substitutions, several first-row transition metals (Ti, Mn, Co, and Ni) are considered. These first-row transition metal cations have similar ionic radii to Fe, but they have different numbers of 3d electrons, giving rise to different stable oxidation states. Si doping is also tested because it is commonly used to increase electron conductivity for n-doped hematite. For anion substitutions, F doping is tested by substituting it in for a terminating O. Unlike the terminating O in the hydroxylated surfaces, the terminating F anions do not bond to any H atoms since they are monovalent and only interact with a surface Fe atom.
Water Oxidation Reaction
The following reaction mechanism scheme is used to identify fundamental aspects of water splitting reactions, in particular water oxidation.
H2O+*→*OH2 A:
*OH2→*OH+H++e− B:
*OH→*O+H++e− C:
H2O+*O→*OOH+H++e− D:
*OOH→O2+*+H++e− E:
The lone “*” represents a surface with one O vacancy site in the topmost layer. The symbols “*OH2”, “*OH”, “*O”, and “*OOH” represent the surface with the corresponding chemisorbed species residing in the O vacancy site. The mechanism involves four oxidation steps (steps B-E), each of which results in a proton ejected into the electrolyte that will eventually meet a transferring electron at the cathode.
Starting with step A, H2O first adsorbs onto the surface O vacancy site. The *OH2 species then undergoes two subsequent oxidation reactions to form *O. Another oxidation step allows *O to react with another water molecule to form the *OOH intermediate. In the last oxidation, O2 is released from the *OOH. The energy of H++e− is obtained implicitly by referencing it to the energy of H2 using the standard hydrogen electrode (SHE, 1/2H2→H++e−, at pH=0, p=1 atm, T=298 K). Applying an external bias φ on the proton-coupled electron transfer processes in reactions B-E is accounted for by including a −e·φ term in their reaction free energies. For example, when φ=0 at standard conditions, the free energy of reaction C is the same as the free energy of the reaction *OH→*O+1/2H2. Other electrochemical models explicitly simulate electrostatic responses to a constant field, or they generate an electric field via charged slabs and then compensate the extra charges with a uniform charge compensating background. In theoretical studies on water oxidation and oxygen reduction on Pt (111), these three models gave similar results. In this disclosure, the first and simplest model is adopted, which has also been applied to examine water oxidation on other metal oxide surfaces. Deviations from the standard pH=0 condition can be treated by adding a −kT ln10·pH correction, where k is the Boltzmann constant. Zero point energy (ZPE) and entropic contributions are also calculated or taken from standard tables (see Computational Details section). Enthalpy changes due to temperature increasing from 0 K→298 K are expected to be small and are normally neglected. Therefore, the reaction free energies are calculated as follows:
ΔGA=E(*OH2)−E(*)−EH
ΔGB=E(*OH)−E(*OH2)+1/2EH
ΔGC=E(*O)−E(*OH)+1/2EH
ΔGD=E(*OOH)−E(*O)−EH
ΔGE=E(*)−E(*OOH)+EO
EH
ΔG*O=E(*O)−E(*)−EH
ΔG*OH=E(*OH)−E(*)−EH
ΔG*OOH=E(*OOH)−E(*)−2EH
Lastly, the (average) H adsorption energy on the O-terminated surface is evaluated as:
Gad=[E(O-terminated slab+nH)−E(O-terminated slab)−(n/2)EH
Where n is the number of H atoms adsorbed onto the O-terminated surface.
Computational Models
The VASP program (version 5.2) was used for all calculations. The Perdew, Burke, and Ernzerhof (PBE) GGA exchange-correlation (XC) functional was employed. All-electron frozen core PAW potentials were used for the ion-electron terms (“ion” refers to a nucleus screened by its core electrons). Spin-polarized DFT+U theory was used to properly describe the antiferromagnetism of hematite and to correct the DFT SIEs for strongly correlated electrons in the first-row transition metal ions. The efficacy of this theory is evident for our purposes, as it has been shown to predict, e.g., more accurate redox potentials and oxidation energies of transition metal oxides than standard DFT. The rotationally-invariant DFT+U formalism was used as proposed by Dudarev et al. and implemented by Bengone et al. DFT+U predictions can depend on the value used for U−J, the one parameter in the theory, which represents the spherically averaged intra-atomic Coulomb minus exchange energy of the localized (here d) electrons that suffer the most from SIE. Rather than fit this parameter to match experiment as is frequently done, a fully ab initio value of U−J=4.3 eV was used for Fe, as derived for Fe3+ in Fe2O3 using a size-converged electrostatically-embedded cluster model within unrestricted Hartree-Fock theory. Our previous work using the ab initio derived U−J value (4.3 eV) for hematite showed that DFT+U predicts ground state structures and ground and excited state electronic properties of bulk hematite in very good agreement with experiments, in contrast to DFT alone. The DFT+U method using U−J=4.3 eV was validated in studies of the two lowest-energy surfaces of hematite. In this disclosure, two scenarios of U−J values were tested for the other first-row transition metals as dopants. In one case, the same U−J=4.3 eV was used for all the transition metals, so as to not artificially bias the 3d electron occupations among different transition metal cations. In the other case, the U−J was derived from experiments or from ab initio calculations (5.0 eV for Ti4+, 3.5 eV for Mn2+, 4.0 eV for Co3+, and 3.8 eV for Ni2+). Oxygen- and hydrogen-containing species were treated with standard DFT-PBE, since they do not exhibit much SIE; DFT-PBE describes molecular thermochemistry moderately accurately, with a mean unsigned error of 0.17 eV for alkyl bond dissociation energies (ABDE4 database).
The hexagonal unit cell of pure hematite was fully optimized at the PBE+4.3 (shorthand notation for using the PBE XC functional and U−J=4.3 eV) level, leading to predicted lattice vectors a=5.10 Å and c=13.92 Å, both of which are within 1% of the experimental values of a=5.04 Å and c=13.75 Å. (1×1) and (2×2) slab models of four stoichiometric units (˜9.3 Å thick, see
Zero point energy (ZPE) and entropic contributions are presented for individual species in Table 1 and for individual reactions or binding energies in Table 2. The ZPE corrections were obtained from vibrational frequencies derived from Hessians calculated from finite differences of analytic gradients on single molecules in vacuum or adsorbates on (1×1) pure hematite slab models. The ZPE corrections calculated by Valdés et al. using DFT-GGA for related reactive species on the rutile TiO2 (110) surface are included in Table 1 for comparison. The ZPE corrections calculated for reactive species on the Fe2O3 (0001) surface are very close to those for the TiO2 (110) surface, with differences ≦0.07 eV. This similarity suggests that the vibrational frequencies of O—O and O—H bonds do not change significantly for different metal oxide substrates.
The entropic contributions for gaseous molecules are taken from standard thermodynamics tables. The entropic contributions to the total energy of a water molecule in solution is evaluated as the entropic contribution to the energy of a gas phase water molecule minus the condensation energy from gas to liquid for water at 298.15 K. This scheme enables us to physically model the absolute energy of a water molecule in liquid phase including solvation and standard state corrections. Entropic contributions from absorbed species on the surfaces are small, so they are usually omitted, as disclosed herein. For H adsorption, ΔZPE values in Equation (9) for different coverages of H are calculated to be close to 0.18 eV with deviations <0.01 eV. Since the entropic contributions from slabs with H adsorption are omitted, the −TΔS term in Equation (9) is 0.20 eV (one half of that for the H2 molecule, see Table 1). Therefore, the ΔZPE−TΔS term in Equation (9) is set to 0.38 eV.
Table 1 shows the entropic energy contributions (T=298.15 K) and zero-point energy (ZPE) corrections for gaseous and adsorbed molecules and reactive species on hematite (0001).
Table 2 shows the entropic energy contributions (T=298.15 K) and ZPE corrections for reaction steps A-E and adsorbate binding energies, ΔGadsorbate. Values in parentheses for reactions A and E have a water molecule from the water layer bound to the O vacancy site in the otherwise clean “*” species, effectively making this intermediate an *OH2 species (see bottom of
Results
A) H adsorption on the O-terminated Fe2O3 (0001) surface
Equilibrium structures were obtained for the fully hydroxylated Fe2O3 (0001) surface via H adsorption on the O-terminated Fe2O3 (0001) surface. The H atoms adsorbed onto this surface form terminating OH— groups that can adopt different orientations in which the H lies nearly within the O planes (e.g.,
B) Water oxidation reactions on the fully hydroxylated Fe2O3 (0001) surface
The minimum energy structures obtained by optimizing O-terminated slabs with different H coverages (as described above) are used as *O, *OH, and *OH2 intermediates. The initial structures for *OOH and “*” were built manually starting from the optimized slabs at 2/3 ML H-coverage and then fully relaxed to find minima. The reaction pathway intermediates and the Bader charges for their surface ions are displayed in
The cumulative reaction free energies (ΔG298) for the proposed reactions steps are plotted in the center of
Solvent effects are considered by adding a water overlayer (consisting of three water molecules for the (1×1) slab) on both sides of the slab. Earlier studies by Rossmeisl, et al. considered multiple layers of water to simulate the interface between an electrolyte and an electrified Pt (111) surface, and they found a single water layer capably described the variation of potential through the interface, suggesting that for flat surfaces such as the basal plane of hematite, a monolayer of water may be sufficient to capture both electrostatics and H bonding interactions. The initial geometries of the water layer are chosen manually, and they were built to maximize hydrogen bonding between the water layer and the surface O(H) species to assess the maximum perturbation caused by the water layer. The final optimized geometries are shown in
The cumulative ΔG298 for reactions with a monolayer of water on top is also plotted in
The ΔG298s for the individual water oxidation reaction steps on the (1×1) and (2×2) slabs are provided in Table 3 below. The (1×1) slab (with 1/3 ML reactive sites) and the (2×2) slab (with 1/12 ML reactive sites) provide a measure of the reaction energy dependence on surface reactive site concentrations. The ΔG298s for the (2×2) slab under vacuum are comparable to those for the (1×1) slab under vacuum, with differences ≦0.26 eV. The overall reaction potential for the (2×2) slab under vacuum is 0.2 V higher than the one for the (1×1) slab under vacuum. The similarity suggests that the reaction on the surface is fairly localized and exhibits only a small coverage dependence.
Table 3 shows the free energies of reactions (ΔG298, in eV) without external bias (φ=0) and reaction potentials (φrx, in V) for water oxidation on the fully hydroxylated, pure Fe2O3 (0001) surface. The numbers in parentheses are reaction energies omitting ZPE and entropic corrections. The most positive reaction energies within each column (equal to φrx) are in bold italic. The coverages refer to the concentration of oxygen vacancies at each surface.
C) Water oxidation reactions on the fully hydroxylated Fe2O3 (0001) surface with dopants
Table 4 (below) presents the reaction free energies and reaction potentials for water oxidation on the doped, fully hydroxylated hematite (0001) surfaces under vacuum and explicitly solvated conditions. The cation or anion dopants are bonded directly or positioned adjacent to the reaction site (see
Table 4 also shows results from the (2×2) slab model under vacuum, employing different U−J values appropriate for different dopants. These data correspond to lowering the dopant concentration from 1/2 in the Fe bilayer of the (1×1) slab to 1/8 in the Fe bilayer of the (2×2) slab, as well as lowering the concentration of reactive sites from 1/3 to 1/12 ML. The predictions exhibit only a small dependence on the concentrations of dopants or reactive sites, with a largest difference of 0.26 eV for ΔG298 and 0.12 V for φrx. Therefore, similar to what is found for pure hematite in the previous section, the reaction is quite spatially localized and most sensitive to changes at adjacent sites.
The dopant effects resulting from the model employing appropriately different U−J values was also analyzed. Including one layer of water has a similar effect on doped surfaces as with pure hematite (Table 4). The φrx changes less than 0.1 V in all cases except for Si and F, which increase by 0.15 V and 0.28 V, respectively. To test the dependence of φrx on dopant concentrations, results for Ti, Mn, Co, Ni, and Si doping in the (2×2) slab models are also presented in Table 4. Calculations on F doping did not consistently converge properly for the larger (2×2) supercell, and therefore they are not reported.
Dopants retain similar charges in the (1×1) and (2×2) slabs in all cases except Ti. The latter can have either a +4 or a +3 charge when near the *OH and *OOH species at the lower coverage afforded by the (2×2) slab. When Ti has a +4 charge, a nearby Fe3+ cation is reduced to Fe2+ (just as for Ti doping in the (1×1) slab, vide infra). When Ti has a +3 charge, all the Fe cations have +3 charges. The *OH (or *OOH) with a nearby Ti3+ is higher in energy by 0.21 (or 0.08) eV than that with a nearby Ti4+, resulting in a 0.08 V difference in φrx between the Ti4+/Fe2+ and Ti3+/Fe3+ scenarios. Since the energy difference between these two cases depends on the U−J values used for Fe and Ti. The results for both (2×2) slab scenarios are shown in Table 4. Overall, predictions from (2×2) slabs are very similar to those of the (1×1) slabs, with largest differences being 0.26 eV for individual ΔG298 values and only 0.12 V for φrx. These small differences suggest that the water oxidation reaction on hematite surface depends primarily on the local chemical environment and does not drastically change with the dopant concentration. This conclusion should also apply to F doping.
Table 4 shows the free energies of reactions (ΔG298, in eV) without external bias (φ=0) and reaction potentials (φrx, in V) for the water oxidation reaction on the doped fully hydroxylated (1×1) and (2×2) Fe2O3 (0001) surfaces. The most positive reaction energies within each column (equal to φrx) are in bold italic. Ti doping can occur in two different ways with the lower coverage (2×2) slab: values outside of parentheses are for the Ti4+/Fe2+ scenario that exists also for the higher coverage (1×1) slab, and values within parentheses are for the Ti3+/Fe3+ scenario that only occurs at the lower coverage.
Table 5 below reports the Bader charges and magnetic moments of dopants placed as nearest neighbors to the reactive O vacancy sites in the (1×1) slabs to discern their maximum effect. The charges and magnetic moments of Fe in the pure hematite surface are nearly identical to those of F doping and have a maximum difference of 0.1 in charge or 0.1μB in magnetic moment. The changes in charges of cation dopants for different surface reaction intermediates follow similar trends to what was found above for the pure hematite surface, but they are smaller in magnitude for Ti, Co, and Ni. The charges on the dopants in the * and *OH2 species are smaller than the charges on the dopants with the other three reactive species present. Si dopants are an exception to this, however. Here, the charge on Si remains +3.1 throughout the catalytic reaction cycle, while O anions become more negatively charged compared to O anions in pure or other cation-doped hematite surfaces due to Si being electropositive.
The oxidation states and electronic configurations of the dopants were analyzed by combining information extracted from Bader charges, magnetic moments, and projected densities of states (PDOS). Since the fully hydroxylated surface (*OH) is the most bulk-like, the PDOS of the pure and doped hematite slabs containing the *OH species (
Table 5 shows the Bader charges (q) and magnetic moments (a, in absolute values) of the dopants in the fully hydroxylated (1×1) Fe2O3 (0001) surfaces under vacuum. For F doping, the charges and magnetic moments of the Fe in the cation substitution sites are given.
The effects of using different dopants were also analyzed. The cumulative ΔG298s for the case under vacuum with different U−J values for different dopants are plotted in
Scaling relationships among binding energies of *OH, *O, and *OOH have been proposed from studying various transition metal oxide surfaces. For example, the differences in binding energies between *OH and *OOH are constant for a series of transition metal oxides.
Where 3.464 is the intercept from the linear fitting of the orange curve in
To further understand why different dopants affect ΔG*OΔG*OH values, we analyze various properties of the reactive species. In
ΔG*OOH−ΔG*OH=(ΔF*OOH−ΔG*O)+(ΔG*O−ΔG*OH)=ΔGD+ΔGC=3.464
(see equation within
On the other hand, when the dopants are less positively charged than Fe, the O anions are less negatively charged. This favors *OOH formation but not *O formation. Therefore, it is believed that the most energetically favorable pathway requires moderate propensity for hole localization on the active O anions. The balanced bonding from Ni doping therefore gives the smallest reaction potential.
D) Perspectives on Photoelectrocatalysis—Model
Developing efficient photoelectrocatalysts requires optimizing various properties (e.g., band gaps, band edge character and alignments, electron/hole conductivity and lifetime, and reaction thermodynamics and kinetics). The current study focuses on reaction thermodynamics only. Water oxidation reaction steps are simulated here at the periodic DFT+U level by referencing to the SHE to avoid explicit modeling of proton release into water and electron injection into the semiconductor. Referencing to the SHE greatly simplifies the computation and has been successfully demonstrated in previous electrochemical modeling. The model we adopt also does not account for photoexcited holes within hematite. However, Valdéz and Kroes reported a DFT study on TiO2 showing that calculations using neutral clusters as reactive catalysts give very similar results as calculations on positively charged clusters with one hole. They also showed that these cluster calculations gave similar results as periodic DFT calculations. Physically, localized holes at the hematite surface should enhance water oxidation, thus charge neutral models should provide a theoretical upper bound for overpotential estimates based on thermodynamics. On the other hand, since kinetic barriers were not evaluated here, the estimated overpotentials based on thermodynamics are lower bound estimates for the measured overpotentials. These two counter factors compete, resulting in error cancellation to some degree. Experimentally, the overpotential for the hematite photoanode has most recently been estimated at 0.5-0.6 V. The overpotential calculated herein of 0.77 V for liquid phase reaction on a pure (1×1) hematite slab (denoting 1/3 ML reactive sites) is just slightly above that experimental range, showing this model's approximate predictive capacity. Lower reactive site (and dopant) coverages give slightly higher overpotentials, but these may be less representative of a typical hematite/water interface that likely contains significant concentrations of defects (vacancies, grain boundaries, etc.) that will promote reactive site formation.
Photoelectrocatalytic water oxidation on hematite starts with light absorption in the near surface region of hematite. The resulting electrons and holes in the hematite anode are then separated: electrons flow to the external circuit, while holes migrate to the surface and react with water. Previously, theoretical calculations on optical excitations of pure hematite using an electrostatically embedded cluster showed that a charge transfer excitation from O to Fe is much higher in energy than the Fe d-d transition. Since hole localization on O is necessary in the water oxidation reaction (O evolves from −2 to 0 charge), the unfavorable ligand to metal charge transfer (LMCT) may limit the hole concentration on O in undoped hematite photoanodes, which in turn reduces their efficiencies.
Doping can enhance the efficiency of photoelectrocatalysis on hematite through different means. In the light absorption process, introducing other cation elements with lower-lying LMCT excitation states in their oxide phases might increase photogenerated hole concentrations on O anions. Since mid-to-late first-row transition metal oxides are of mixed Mott-Hubbard and charge-transfer character, additional bulk doping of hematite with Co or Ni might promote O hole concentration via LMCT between Co/Ni and O centers. Moreover, surface modifications with different dopants will affect the VBM level of the hematite slabs. Specifically, Ti, Mn, or Si doping shifts the VBM to be less negative while F doping shifts the VBM to be more negative (
Lastly, as disclosed herein dopants adjacent to reaction sites change reaction thermodynamics. The presence of dopants can modulate the bonding strengths between the surface and the intermediate adsorbed species in the water oxidation reaction. According to the Sabatier principle, interactions between the catalysts and the adsorbates should be intermediate: neither too weak to adsorb the reactants nor too strong as to inhibit product leaving the catalyst. Among a series of dopants, Co and Ni are predicted as the most effective additives to reduce overpotentials because their less positive charge compared to Fe provides optimal binding strengths to the O, OH, and OOH adsorbates.
A review of measurements of photoelectrochemical properties of Ni-doped hematite by Liu, et al. reveals that Ni-doping leads to higher photocurrent densities for water oxidation compared to pure hematite samples. The improved performance of the Ni-doped hematite surface was attributed to increased conductivity and higher charge separation efficiency, but our work suggests additionally that the reaction thermodynamics is improved. An observed reduction of 0.05 V in the onset potential for Ni-doped samples further validates the disclosed effectiveness of Ni-doped hematite.
Photochemical Cell Structure
Disclosed herein are ab initio DFT+U calculations to characterize the thermodynamics of water oxidation on the hematite (0001) surface. Our previous work demonstrated that extensions beyond standard DFT (i.e. ab initio DFT+U) must be employed to obtain accurate structures and electronic properties of hematite. In this disclosure, reaction potentials are calculated for water oxidation in both gas and liquid phases on a (1×1) hydroxylated hematite slab (1/3 ML reactive sites) and in the gas phase for a (2×2) slab (1/12 ML reactive sites). Since actual hematite electrode surfaces under hydrating conditions undoubtedly have a complex structure with polycrystalline facets, vacancies, and hydroxylation, it is believed that predictions based on the explicitly solvated, hydroxylated (1×1) slab that contains a higher concentration of reactive sites to be more representative of actual electrochemical conditions. This model gives a reaction potential φrx, defined as the minimum potential that makes ΔG≦0 for all individual electrochemical steps, of 1.88 V, corresponding to an overpotential of 0.77 V. This calculated overpotential is in reasonable agreement with measured overpotentials of 0.5-0.6 V for hematite photoanodes.
Cation doping (Ti, Mn, Co, Ni, Si) was introduced by direct cation substitutions, and F doping was introduced by substituting an OH group on the fully hydroxylated surface. Including ZPE and entropic corrections shifts reaction energy levels, qualitatively changing the predictions of which dopants act to reduce the overpotential. By accounting for ZPE and entropic corrections and by using ab initio U−J values for the dopants, Co or Ni doping reduces the φrx of pure hematite by up to 0.15 V. In contrast, Ti, Mn, Si, and F doping increased the φrx beyond that of pure hematite, suggesting Co and Ni additions are candidates to improve the catalytic activity of pure hematite. The doping effects were analyzed by comparing charges of the dopants and active O anions as well as the binding energies of O, OH, and OOH adsorbates. Specifically, optimal binding of O, OH, and OOH reactive species is the key to reduce the reaction potential. Co or Ni, both with charges less positive than Fe, produce intermediately-charged O anions that best balance the binding strengths among O, OH, and OOH, yielding the smallest reaction potential.
Although features and elements are described above in particular combinations, each feature or element may be used alone without the other features and elements or in various combinations with or without other features and elements.
Claims
1. A photoanode comprising an electrode at least partially formed of hematite and having a surface doped with at least one of nickel, cobalt and manganese, the electrode having a (0001) surface Fe bilayer doped with a concentration in the 1/8 to 1/2 range.
2. The photoanode of claim 1, wherein the electrode surface is doped with nickel.
3. The photoanode of claim 1, wherein the electrode surface is doped with cobalt.
4. The photoanode of claim 1, wherein the electrode surface is doped with manganese.
5. The photoanode of claim 1, wherein the electrode is bulk doped with at least one of cobalt and nickel to enhance light absorption.
6. The photoanode of claim 1, wherein the electrode is bulk doped with at least one of silicon and manganese to enhance band alignment and carrier transport.
7. The photoanode of claim 1, further comprising an aqueous solution surrounding the photoanode and a cathode, the photoanode being configured to generate holes upon light absorption and the cathode being configured to emit electrons to the aqueous solution.
8. The photoanode of claim 7, further comprising a voltage source electrically coupled between the photoanode and the cathode.
9. A photochemical cell configured for electrolysis of water, the photochemical cell comprising:
- a photoanode comprised of hematite and having a surface doped with at least one of nickel, cobalt and manganese configured for immersion in the water; and
- a cathode electrically coupled to the photoanode, the cathode being configured for immersion in the water.
10. The photochemical cell of claim 9, wherein the electrode surface is doped with nickel.
11. The photochemical cell of claim 9, wherein the electrode surface is doped with cobalt.
12. The photochemical cell of claim 9, wherein the electrode surface is doped with manganese.
13. The photochemical cell of claim 9, wherein the electrode is bulk doped with at least one of cobalt and nickel to enhance light absorption.
14. The photochemical cell of claim 9, wherein the electrode is bulk doped with at least one of silicon and manganese to enhance band alignment and carrier transport.
15. The photochemical cell of claim 9, wherein the photoanode is configured to generate holes upon light absorption and the cathode is configured to emit electrons to the water.
16. The photochemical cell of claim 15, further comprising a voltage source electrically coupled between the photoanode and the cathode.
17. A method of making a photoanode, the method comprising:
- providing an electrode at least partially formed of hematite; and
- doping a surface of the electrode with at least one of nickel, cobalt and manganese.
18. The method of claim 17, wherein the electrode surface is doped with nickel.
19. The method of claim 17, wherein the electrode surface is doped with cobalt.
20. The method of claim 17, wherein the electrode surface is doped with manganese.
21. The method of claim 17, wherein the electrode is bulk doped with at least one of cobalt and nickel to enhance light absorption.
22. The method of claim 17, wherein the electrode is bulk doped with at least one of silicon and manganese to enhance band alignment and carrier transport.
23. The method of claim 17, further comprising surrounding the photoanode and a cathode with an aqueous solution, the photoanode being configured to generate holes upon light absorption and the cathode being configured to emit electrons to the aqueous solution.
24. The photoanode of claim 23, further comprising electrically coupling a voltage source between the photoanode and the cathode.
25. A method of making a photochemical cell configured for electrolysis of water, the method comprising:
- providing a photoanode comprised of hematite;
- doping a surface of the electrode with at least one of nickel, cobalt and manganese; and
- electrically coupling a cathode to the photoanode, the cathode being configured for immersion in the water.
26. The method of claim 25, wherein the electrode surface is doped with nickel.
27. The method of claim 25, wherein the electrode surface is doped with cobalt.
28. The method of claim 25, wherein the electrode surface is doped with manganese.
29. The method of claim 25, wherein the electrode is bulk doped with at least one of cobalt and nickel to enhance light absorption.
30. The method of claim 25, wherein the electrode is bulk doped with at least one of silicon and manganese to enhance band alignment and carrier transport.
Type: Application
Filed: Nov 1, 2012
Publication Date: Dec 8, 2016
Applicant: THE TRUSTEES OF PRINCETON UNIVERSITY (Princeton, NJ)
Inventors: Emily Ann Carter (Belle Mead, NJ), Peilin Liao (Princeton, NJ), John Andrew Keith (Plainsboro, NJ)
Application Number: 13/666,662