Developing solid-state electrocatalysts based on design principles from nature: The oxidation of water and the reduction of CO2 to fuels

Investigators Thomas F. Jaramillo, Assistant Professor; Jens K. Nørskov, Professor; Kendra Kuhl, Graduate Student; Etosha Cave, Graduate Student; David Abram, Graduate Student; Jakob Kibsgaard, Post-doctoral Researcher; Sung-Hyeon Baeck, Visiting Professor (Inha Univ., South Korea). Lars C. Grabow, Research Associate; Nadia Luciw, Graduate Student (Technical University of Denmark) Abstract

The goal of this project is to combine experimental and theoretical methods in investigating and developing three types of catalysts: metal surfaces for CO2 reduction, metal sulfides for CO2 reduction, and metal oxides for the oxygen evolution reaction (OER). In the first 16 months of this project, we have: (1) Developed new instrumentation and methodology to conduct CO2-reduction studies. In particular, we developed a continuous flow electrochemical cell with on-stream head-space analysis by gas chromatography. We have also developed 1H-NMR and 13C-NMR methods to detect liquid products in the liquid electrolyte post-reaction. (2) Studied CO2 electro-reduction on 7 different metals: Cu, Ni, Pt, Fe, Au, Ag, and Zn, establishing their activity and selectivity for different gaseous and liquid products as a function of applied potential. (3) Calculated the free energy diagram for CO2 reduction on [4Fe-4S] and [Ni-3Fe-4S] cubane structures using density functional theory (DFT). In particular, we have shown that replacing a Fe atom with Ni increases the selectivity towards the desired product CO and simultaneously lowers the required overpotential to drive the reaction. (4) Supported individual [4Fe-4S] cubanes onto highly oriented pyrolytic graphite (HOPG), imaged them by STM, and have begun electrocatalytic studies on them for H2 evolution and for CO2 reduction. (5) Developed a computational model of the FeVco of Azotobacter vinelandii and studied its interaction with the first key intermediates of CO reduction (CO and HCO). (6) Synthesized and investigated specific crystalline phases of manganese oxides for the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR), namely -Mn2O3, -MnO2, -MnO2, -MnO2, -MnO2.

Introduction The overall objective of this project is to develop catalysts for key energy conversion

reactions, namely the electro-reduction of CO2 to chemical fuels (hydrocarbons and alcohols) and the electro-oxidation of water to O2 (a.k.a. the oxygen evolution reaction, OER). If coupled to renewable electricity sources such as wind or solar, such catalysts could potentially enable a CO2-neutral energy cycle in which CO2 is converted to fuels in a renewable and sustainable pathway. There are presently no efficient electrocatalysts for this process. In order to achieve our research goals of electrocatalyst development, we are combining experimental and theoretical methods in investigating three types of catalysts: (1) metal surfaces for CO2 reduction, (2) metal sulfides for CO2 reduction, and (3) metal oxides for the OER. Several of the investigated catalysts are inspired by nature and by studying the enzymatic process of CO2 reduction we expect to discover mechanistic details that will inform and guide the design of a bio-mimetic analogue.

Background In the past year, there have been a number of developments in the field of CO2

reduction and water oxidation. Particularly relevant results in the field of CO2 reduction come from laboratories at UC-Berkeley, the Technical University of Denmark (DTU), the University of Illinois, Urbana-Champaign (UIUC), Princeton University, and UC-Irvine. At UC-Berkeley, Prof. John Newman provided a detailed kinetic analysis of CO2 electro-reduction on Au and Ag surfaces.[1,2] They focused entirely on gas-phase products, however, without examining any liquid phase products formed, though any liquid products would be small relative to the major gas phase products of H2 and CO. The theoretical analysis performed in the group of Jens Nørskov at DTU shows that the potential determining catalytic step in CO2 reduction on Cu(211) is the protonation of CO to HCO as shown in Figure 1.[3] At UIUC, the research group of Paul Kenis has developed a microfluidic fuel cell-type platform for investigating CO2 reduction catalysts.[4] At Princeton University, Prof. Andy Bocarsly has continued in his efforts to understand the mechanistic pathway for methanol production involving pyridine and a metal electrode (e.g. Pd).[5,6] Prof. Markus Ribbe’s group at UC-Irvine has reported the discovery that vanadium nitrogenase from Azotobacter vinelandii can remarkably catalyze the (electro-) reduction of CO under ambient pressure and temperature.[7]

In the field of water oxidation, efforts are continuing at MIT in understanding their

Co-phosphate based electrode [8], though this catalyst operates at > 0.4 V overpotential, leaving much room for improvement. At UC-Berkeley, Prof. Alex Bell has been studying the OER on Au electrodes, revealing the presence of the –OOH intermediate for the first time – a breakthrough in understanding the reaction mechanism.[9]

Results Development of an electrochemical reactor for CO2 reduction studies

Critical to the study of CO2 electro-reduction catalysis is the development of instrumentation and a methodology such that current-voltage data for catalysts can be

Figure 1: Calculated free energy diagram for electro-catalytic CO2 reduction to CH4 on copper. The black path represents the free energy at U = 0 V. The free energy at the limiting potential of U = -0.74 V is shown in red.[7]

obtained along with product analysis for both liquids and gases, over a wide range of concentrations, for quantification of both major and minor products. For this purpose we have developed a continuous flow, compartmentalized compression cell for electrochemical studies, Figure 2. In order to maximize sensitivity of product detection, a fairly large area electrode ~ 4.5 cm2 is used in order produce a large quantity of reaction products, within a fairly small reactor volume ~ 10 mL such that the concentration of liquid products will be as high as possible. The anode and cathode are separated by an anion exchange membrane to mitigate CO2 reduction products from being oxidized at the anode. Experiments for each electrode are conducted over the course of one hour under a continuous flow of ~ 20 sccm CO2, with on-stream head-space analysis by gas chromatography (GC) every ~ 15 minutes. At the end of each experiment, the liquid electrolyte is preserved and studied by 1H-NMR and 13C-NMR in order to quantify the liquid products of reaction.

Studies of electrochemical reduction of CO2 on copper and on other transition metals

Copper is among the most interesting materials to study for CO2 reduction as it is known to produce hydrocarbons and alcohols, albeit at very high overpotentials (~ 1 V). Previous DFT studies have shown that this overpotential is largely determined by the protonation of CO to HCO.[3] Having developed our novel method of studying catalysts for CO2 electro-reduction, copper was a natural electrode material choice starting our investigation. Figure 3 exhibits copper’s current-voltage data as well as the current efficiency (product distribution data), both as functions of applied potential, in 0.5 M KHCO3 (aq.) saturated with CO2. As can be seen in Figure 3(a), the cathodic (negative) current density for copper increases as the applied potential becomes more negative. Note that the equilibrium potential for most CO2 reduction reaction (to CO, hydrocarbons, alcohols, etc.) are around 0.0 ± 0.2 V vs. RHE. The fact that an electrode potential

Figure 2: Experimental set-up for CO2 electro-reduction studies.

cathodic of -0.66 V vs. RHE is needed to draw > 1 mA/cm2 of current density is an indication of the sluggish kinetics of CO2 reduction on copper. At extremely cathodic potentials, -1.18 V vs. RHE, the current density reaches -11 mA/cm2 – a current density relevant for devices as a 10% efficient solar fuels device would produce approximately 10 mA/cm2 of current under AM1.5 conditions. The current-voltage data shown in Figure 3(a), however, does not reveal the products of reaction. For this we have used GC and NMR as described above to quantify the products, shown in Figures 3(b,c,d).

Figure 3(b) reveals the major products of reaction. H2, CO, and formate are the

dominant reaction products at lower potentials (anodic of -0.95 V vs. RHE), while methane (CH4) and ethylene (C2H4) dominate at more cathodic potentials. Clearly, reaction pathways on copper change as a function of potential, consistent with DFT calculations.[3] Figure 3(c) shows the minor products of reaction in the same experiment (all < 10%), with ethanol and n-propanol being produced in modest quantities. Figure 3(d) zooms in on the products with current efficiencies < 1%. Though these products are created in small quantities, the fact that they are created at all is very interesting – molecules such as allyl alcohol, acetone, propionaldehyde, acetaldehyde, and methanol. Monitoring the trace products is important since now we know that they are being produced, we can then aim to identify what are the active sites producing them.

Figure 3: CO2 electro-reduction on copper. (a) Current-voltage data, (b-d) current efficiency as a function of applied potential.

Figure 4 shows a pie chart with the product distribution measured at -1.18 V vs. RHE.

Approximately 54% of the current is going towards hydrocarbons (methane and ethylene). Over 11% of the current is going to alcohols, predominantly ethanol and n-propanol (though interestingly, very little methanol). The fact that 65% of the current is going to hydrocarbons and alcohols is excellent news, however an overpotential of ~ 1 V is unacceptable for electrochemical fuel synthesis. There is a need to understand how copper works, as well as other metals in order to develop design principles for new catalysts. To this end, we have also studied Au, Ag, Pt, Zn, Fe, and Ni in the exact same manner as described for Cu. Figure 5 shows the current-voltage data for these metals (along with Cu for comparison). Just as this figure elucidates significant differences in each metal’s current-voltage data, there are wide ranging differences in the product distribution. This is still a work in progress that we expect to report in full next year. However, it should be noted that many of these metals produce hydrocarbons and/or alcohols, though not necessarily at the same high concentrations as copper, and oftentimes at different potentials.

Figure 5: Current-voltage data for 7 transition metals: Cu, Au, Pt, Ag, Zn, Fe, & Ni.

Figure 4: Product distribution of copper at -1.18 V vs. RHE.

Theoretical Investigations of Enzyme catalysis for CO2 and CO reduction Ni containing carbon monoxide dehydrogenase (Ni-CODH) has been shown to

reversibly reduce CO2 to CO and is a promising starting material for the design of efficient CO2 reduction electrocatalysts.[10] The active center of this enzyme is a [Ni-4Fe-5S] cluster, which resembles a four metal [Ni-3Fe-4S] cubane-like structure with an additional binuclear site consisting of a Fe atom and a S atom. To mimic the active site of CODH, we used the [4Fe-4S] and the [Ni-3Fe-4S] cubane structures with Cl atoms bound to each of the metal atoms in the structure to represent the bonds to the amino acid residues in the biological enzyme. In addition, two ammonium ions were included in the model set-up to obtain charge neutrality.

The first protonation of CO2 can occur either on the O atom leading to a COOH intermediate, or on the C atom leading to formate (OCHO). The optimized binding geometries for both intermediates on the [4Fe-4S] and the [Ni-3Fe-4S] cubanes are shown in Figure 6. Formate binds monodendate to a Fe atom and is the undesired reaction intermediate, because its further protonation yields formic acid. The desired electrocatalytic reduction of CO2 to CO proceeds through the COOH intermediate, which binds to a S atom on the cluster.

We used density functional theory (DFT) to investigate both CO2 reduction pathways

on the cubane model structures and show the resulting free energy diagram in Figure 7. The free energy was calculated at pH = 0 and no applied voltage (U = 0 V) and includes zero-point energy corrections, heat capacity contributions, entropy terms and an approximate hydrogen-bond stabilization of COOH by the surrounding solvent (-0.25 eV). On the Fe-only cluster ([4Fe-4S]) the undesired OCHO intermediate is -0.22 eV more stable than the COOH intermediate. Upon substitution of a Fe atom with Ni we predict a switch of the stabilities of COOH and OCHO. Ni simultaneously stabilizes the COOH intermediate and destabilizes OCHO, such that COOH is -0.33 eV more stable

Figure 6: Adsorption geometries of COOH (top row) and OCHO (bottom row) on [4Fe-4S] (left column) and [Ni-3Fe-4S] (right column) structures. (a) *Fe-COOH (b) *Ni-COOH (c) *Fe-OCHO (d) *Ni-OCHO

than OCHO. The opposite effect of Ni substitution on the binding strength of COOH and OCHO may be related to the differences in binding sites of these two intermediates. More work is necessary to fully understand this system, but the present results clearly demonstrate that the catalytic properties of cubane structures can be tailored in a targeted fashion.

Considerably more challenging than the reduction of CO2 to CO is the reduction of

CO to hydrocarbons. However, a recent report suggests that the cofactor of vanadium modified nitrogenase (FeVco) is capable of reducing CO to C2H4, C2H6, and C3H8, while the cofactor or the more common molybdenum nitrogenase (FeMoco) does not produce any hydrocarbons.[7] The detailed picture of how FeVco catalyzes this reaction is unknown, but as a starting point we focus on CO and HCO, which are known to determine overpotential on Cu.[3] Having the ability to tune the stabilities of adsorbed CO and HCO independently would be a big leap forward in the design of an efficient CO (photo-) electro-reduction catalyst.

Building upon previous work on FeMoco [11] we studied the interactions of CO with the resting state of FeVco and its protonation to HCO. The structure of FeVco is not as well known as for FeMoco, but EPR and EXAFS experiments suggest that the spin and geometric structure of FeVco is similar to FeMoco.[12] Indeed, our DFT results indicate that substitution of Mo with V in FeMoco results in a structure that agrees well with the available experimental data.

Figure 7: Free energy diagram at pH = 0 and U = 0 V for the reduction of CO2 to CO (red path) and HCOOH (blue path) on the [4Fe-4S] and [Ni-3Fe4S] clusters.

In Figure 8 we show the adsorption geometries of CO and HCO on FeVco. Note that

CO binds to a Fe site, while HCO binds to a S atom. The same binding preference of CO and HCO is also observed on the [4Fe-4S] and [Ni-3Fe-S] cubane structures. The similarity of this binding scenario with COOH and OCHO binding described earlier suggests that targeted modifications will allow changing the stability of CO and HCO independently. For the cubanes and FeVco the binding energies of CO and HCO are tabulated in Table 1. The data shows that the binding energy of CO varies little, but there is a significant difference in HCO binding on FeVco compared to the cubanes. While these results are very promising, further investigations are necessary to explain the origin of the observed differences.

Table I: Binding energies of CO and HCO taken with respect to CO and H2 in the gas-phase.

Structure ECO / eV EHCO / eV [4Fe-4S] -0.76 -0.15 [Ni-3Fe-4S] -0.60 -0.12 FeVco -0.71 -0.50

Initial investigations of [4Fe-4S] cubane molecules on highly oriented pyrolitic graphite (HOPG)

For the reasons described above, transition metal sulfides are a very interesting class of materials for CO2 reduction. In parallel with the theoretical investigations, we have begun experimental investigations of sulfide materials. Figure 5 shows our initial results in working on [4Fe-4S] cubane molecules, which we have already begun to examine for CO2 electro-reduction. These compounds were generously donated to our laboratory by Prof. Ed Solomon (Dept. of Chemistry, Stanford University). We subsequently developed methodologies to support these molecules onto highly oriented pyrolytic graphitie

Figure 8: Computational model of FeVco with the preferred adsorption geometries of (a) CO and (b) HCO.

(HOPG) surfaces, and then imaged them by scanning tunneling microscopy (STM). Figure 9(a) shows a pristine HOPG. Figure 9(b) shows physisorbed [4Fe-4S] cubane molecules that interact very weakly with the substrate, leading to a self-assembled monoloayer (SAM) consisting of well-ordered stripes. Unfortunately this SAM is not stable under reaction conditions, so in an effort to chemisorb the cubanes to the surface, we developed a methodology to form atomic pits on the HOPG through an oxidative electrochemical process. The STM image of this particular cubane preparation is shown in Figure 9(c), where the cubanes are anchored much more strongly than those shown in Figure 9(b). We have already begun electrochemical characterization of these molecules. We look forward to sharing our results in the next reporting period.

Studies of highly crystalline manganese oxide powders for the oxygen evolution reaction (OER)

In order to reduce CO2 electrochemically, one needs a source of protons and electrons. Water is an excellent candidate to supply these reactants as water is inexpensive and abundant. However, oxidizing water is a challenging task. We aim to develop catalysts that can do just that. Inspired by a nano-structured manganese oxide developed in a separate project, in our GCEP work one of our first aims is to study well-

Figure 9: Synthesis of [4Fe-4S] cubane molecules onto graphitic surfaces. (a) pristine graphite, (b) physisorbed [4Fe-4S] cubane molecules, (c) chemisorbed [4Fe-4S] cubane molecules after a light electrochemical oxidation of the graphite prior to deposition.

defined manganese oxides for the OER. We have synthesized 5 highly crystalline powders of manganese oxides for this purpose, using a variety of synthetic methods: -Mn2O3, -MnO2, -MnO2, -MnO2, and -MnO2. The x-ray diffraction patterns shown in Figure 6(a) reveal the highly crystalline nature of the prepared materials. These powders were then supported onto electrodes for study of the OER as well as the oxygen reduction reaction (OER) in 0.1 M KOH, Figure 10(b). We see a large variation in activities, which is expected given the significant changes in crystal structure of the materials. There is much work to be done in understanding the origins of these differences, producing this knowledge is an on-going effort that will help in the design of future metal oxide OER catalysts.

Progress

The ultimate goal of this project is to develop improved materials for two important energy conversion reactions, the CO2 reduction reaction and the oxygen evolution reaction (OER). We have come a long way in the first 16 months of this project, having studied the chemistry of a large number of metal electrode surfaces for CO2 reduction, as well as metal sulfide materials. We have also investigated a number of metal oxides for the OER. We have progressed substantially in gaining deep knowledge of these materials and their surface chemistry. This will be crucial in developing new systems that can reduce CO2 emissions substantially by converting CO2 into useful chemicals in a low temperature process that can be coupled to renewable electricity such as wind or solar.

Figure 10: Well-defined, crystalline manganese oxide powders for OER and ORR catalysis. (a) XRD patterns revealing well-defined crystals, (b) electrochemical measurements of OER and ORR activity in O2-saturated 0.1 M KOH.

Future Plans We will continue studying transition metal surfaces in order to gain a deep

understanding of how CO2 reduction activity and selectivity couple to the electronic and surface structure of materials. Guided by our DFT results we continue our study of this reaction on metal sulfide materials, from molecular cubane catalysts to bulk sulfide surfaces as well as nano-structured metal sulfides. The structural model of the resting state of FeVco will be verified or improved by comparison with spectroscopic methods (XAS, XES at SSRL and LCLS). In addition to HCO, other possible CO reduction intermediates will be studied and characterized until a complete free energy diagram for CO electro-reduction is available. Finally, we aim to explain the differences in catalytic behavior between FeVco and FeMoco and suggest a recipe for the targeted modification of enzymes and their metal sulfide analogues for CO2 reduction. We will also continue our investigation of manganese oxide based OER catalysts. In particular, we will focus on mixed-metal oxide systems where the electronic and geometric factors governing catalysis can be tuned.

Publications 1. T.F. Jaramillo and J.K. Nørskov, Developing solid-state electrocatalysts based on design

principles from nature: The oxidation of water and the reduction of CO2 to fuels, Global Climate Energy Project, Sixth Annual Research Symposium, Creating a Sustainable Energy System for the 21st Century & Beyond, September 2010, Stanford University.

2. K.P. Kuhl and T.F. Jaramillo, Electrochemical reduction of carbon dioxide on transition metal surfaces, 2011 Gordon Research Conference on Solar Fuels, January 2011, Ventura, CA.

3. T.F. Jaramillo, K.P. Kuhl, and E. Cave, Electrocatalytic conversion of CO2 to fuels on metal surfaces, 2011 Materials Research Society Spring Meeting & Exhibit, April 2011, San Francisco CA.

References

1. Delacourt, C.; Ridgway P.L.; Newman, J. Mathematical Modeling of CO2 Reduction to CO in Aqueous Electrolytes I. Kinetic Study on Planar Silver and Gold Electrodes. J. Electrochem. Soc. 157, B1902-B1910, 2010.

2. Delacourt, C. and Newman, J. Mathematical Modeling of CO2 Reduction to CO in Aqueous Electrolytes II. Study of an Electrolysis Cell Making Syngas (CO + H2) from CO2 and H2O Reduction at Room Temperature. J. Electrochem. Soc. 157, B1911-B1926, 2010.

3. Peterson, A. A.; Abild-Pedersen, F.; Studt, F.; Rossmeisl, J.; Nørskov, J. K. How copper catalyzes the electroreduction of carbon dioxide into hydrocarbon fuels. Energy Environ. Sci. 3, 1311-1315, 2010.

4. Whipple D.T.; Finke, E.C.; Kenis P.J.A. Microfluidic Reactor for the Electrochemical Reduction of Carbon Dioxide: The Effect of pH. Electrochem. Sol. State Lett. 13, D109-111, 2010

5. Morris, A.J.; McGibbon, R.T.; Bocarsly, A.B. Electrocatalytic Carbon Dioxide Activation: The Rate-Determining Step of Pyridinium-Catalyzed CO2 Reduction. CHEMSUSCHEM 4 191-196, 2011.

6. Cole, E.B.; Lakkaraju, P.S.; Rampulla, D.M.; Morris, A.J.; Abelev, E.; Bocarsly, A.B. Using a One-Electron Shuttle for the Multielectron Reduction of CO2 to Methanol: Kinetic, Mechanistic, and Structural Insights. J. Amer. Chem. Soc. 132 11539-11551, 2010.

7. Lee, C. C.; Hu, Y.; Ribbe, M. W. Vanadium Nitrogenase Reduces CO, Science 329, 642, 2010. 8. Surendranath, Y.; Kanan, M.W.; Nocera, D.G. Mechanistic Studies of the Oxygen Evolution

Reaction by a Cobalt-Phosphate Catalyst at Neutral pH. J. Amer. Chem. Soc. 132, 16501-16509, 2010.

9. Yeo, B.S.; Klaus, S.L.; Ross, P.N.; Mathies R.A.; Bell A.T. Identification of Hydroperoxy Species as Reaction Intermediates in the Electrochemical Evolution of Oxygen on Gold. CHEMPHYSCHEM, 11, 1854-1857, 2010.

10. Parkin, A.; Serevalli, J; Vincent, K. A.; Ragsdale, S. W.; Armstrong, F. A. Rapid and Efficient Electrocatalytic CO2/CO Interconversions by Carboxydethermus hydrogenoformans CO Dehydrogenase I on an Electrode. J. Am. Chem. Soc. 129, 10328-10329, 2007.

11. Hinnemann, B.; Nørskov, J. K. Catalysis by Enzymes: The Biological Ammonia Synthesis. Top. Catal. 37, 55-70, 2006.

12. Fay, A. W.; Blank, M. A.; Lee, C. C.; Hu, Y.; Hodgson, K. O.; Hedman, B.; Ribbe, M. W. Characterization of Isolated Nitrogenase FeVco. J. Am. Chem. Soc. 132, 12612-12618, 2010.

Contacts Thomas F. Jaramillo: jaramillo@stanford.edu Jens K. Nørskov: norskov@stanford.edu Kendra Kuhl: kpkuhl@stanford.edu David Abram: dabram@stanford.edu Jakob Kibsgaard: kibs@phys.au.dk (no longer at Stanford) Sung-Hyeon Baeck: shbaeck@stanford.edu Lars C. Grabow: grabow@stanford.edu Nadia Luciw: nadia.luciw@fysik.dtu.dk