New Insights in the Electrocatalytic Proton Reduction and Hydrogen

Oct 13, 2010 - Corresponding author, [email protected]. Present address: CEA, LITEN, DEHT, LCPEM, 17 rue des Martyrs, F-38054 Grenoble Cedex 9, ...
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J. Phys. Chem. A 2010, 114, 11861–11867

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New Insights in the Electrocatalytic Proton Reduction and Hydrogen Oxidation by Bioinspired Catalysts: A DFT Investigation Ali Kachmar,† Valentina Vetere,*,‡,§ Pascale Maldivi,‡ and Alejandro A. Franco† CEA, LITEN, DEHT, LCPEM (Laboratoire des Composants pour les Piles a` Combustible et Electrolyseurs, et de Mode´lisation), 17 rue des Martyrs, F-38054 Grenoble Cedex 9, France, CEA, INAC, SCIB, Laboratoire de Reconnaissance Ionique et Chimie de Coordination, Commissariat a` l′Energie Atomique et aux Energies AlternatiVes (CEA)sGrenoble, UJF, UMR_E 3 CEA-UJF, CNRS, FRE 3200, 17 rue des Martyrs, F-38054 Grenoble Cedex 9, France, and UniVersite´ de Toulouse, Laboratoire de Chimie et Physique Quantiques, UMR5626, 118 route de Narbonne, F-31062, Toulouse Cedex, France ReceiVed: July 29, 2010; ReVised Manuscript ReceiVed: September 9, 2010

In this paper, we present a DFT study of the proton reduction mechanism catalyzed by the complex [Ni(P2HN2H)2]2+, bioinspired from the hydrogenases. A detailed analysis of the reactive isomers is discussed together with the localizations of the transitions states and energy minima. The reactive catalytic species is a biprotonated Ni(0) complex that can show different conformations and that can be protonated on different sites. The energies of the different conformations and biprotonated species have been calculated and discussed. Energy barriers for two different reaction mechanisms have been identified in solvent and in gas phase. Frequency calculations have been performed to check the nature of the energy minima and for the calculations of entropic energetic terms and zero point energies. We show that only one conformation is mostly reactive. All the others species are nonreactive in their original form, and they have to pass through conformational barriers in order to transform in the reactive species. Introduction In recent years, in relation with the necessity to face energetic problems in a sustainable development context, hydrogen energy technologies, such as hydrogen-based fuel cells, have presented a growing interest. A highly promising method of hydrogen production for fuel cells is the polymer electrolyte membrane (PEM)-based electrolysis of water. PEM water electrolyzers (PEM-WE) offer a number of advantages for the electrolytic production of hydrogen, such as ecological safety, high gas purity, the possibility of producing compressed gases for direct pressurized storage without additional power inputs, etc.1-3 However, this technology still presents substantial technical challenges related to efficiency, cost (precious-metal-based catalysts are currently used to split water into H2, e.g., Pt, and O2, e.g., IrO2, RuO2, ...), and lifetime. These points constitute the main shortcomings toward the large-scale development and commercialization of this technology. Despite progress on reducing the amount of precious metals, typical loading still remains high and costly for widespread electrode commercialization. For this reason, precious-metalfree catalyst materials start showing a growing interest within the international research community. Among these materials, NiFe hydrogenases4-13 are unique bioenzymes that catalyze the hydrogen evolution reaction (HER) (2H+ + 2e- f H2) and hydrogen oxidation reaction (HOR) * Corresponding author, [email protected]. Present address: CEA, LITEN, DEHT, LCPEM, 17 rue des Martyrs, F-38054 Grenoble Cedex 9, France. † CEA, LITEN, DEHT, LCPEM (Laboratoire des Composants pour les Piles a` Combustible et Electrolyseurs, et de Mode´lisation). ‡ CEA, INAC, SCIB, Laboratoire de Reconnaissance Ionique et Chimie de Coordination, Commissariat a` l′Energie Atomique et aux Energies Alternatives (CEA)-Grenoble, UJF, UMR_E 3 CEA-UJF, CNRS, FRE 3200. § Universite´ de Toulouse, Laboratoire de Chimie et Physique Quantiques, UMR5626.

Figure 1. Ni(P2RN2R′)2]2+ (a) and [Ni(PNP)2]2+(b) catalyst.

(H2 f 2H+ + 2e-) with remarkable efficiency. This has led several groups to search for new bioinspired materials mimicking hydrogenases, based for example on Ni-Fe14,15 or Fe-Ru.14 Another kind of hydrogenase model is the mononuclear nickel based catalyst, studied by several groups,14,16-24 which provides very high efficiency and stability toward the HER, as evidenced by cyclic voltammetry experiments, and thus it is an interesting material candidate for H2 production. In the course of our studies on the improvement of PEMWE performance, we became interested in those materials catalysts that are very promising for the future development of low-cost and efficient hydrogen electrolyzers. In particular, our aim is the future development of multiscale simulations, able to describe the entire PEM-WE cells based on bioinspired catalyst. Among the nickel-based synthetic catalyst, our attention was drawn by the Ni(P2RN2R′)2]2+ (with R and R′ ) phenyl, benzyl, or cyclohexyl) complexes, in Figure 1a, which have been widely studied22-26 because of their high catalytic performance. Moreover, recently Artero et al. have reported nanostructured HER materials, where such nickel catalysts have been attached covalently on carbon nanotubes.25 In order to build a comprehensive modeling approach starting from the molecular level, it is necessary to describe the catalytic process mechanisms. But despite several experimental and theoretical efforts on the fundamental understanding of the underlying mechanisms behind the HER involving the [Ni(P2RN2R′)2]2+ catalysts,

10.1021/jp107104k  2010 American Chemical Society Published on Web 10/13/2010

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questions are still open about the reactive conformation of the catalyst for instance. Moreover, previous suggestions about the reaction intermediates are often contadictory.23,26 Some information about the mechanism has been proposed mostly by one group on the basis of experimental NMR studies at different temperatures.23 We know that the first step is the reduction of the Ni(II) complex to a Ni(0) biprotonated species by two e- and two protons, as in reaction 1. Following the Ni(0) biprotonated structure is an oxidation process to turn back to the Ni(II) species by losing a hydrogen molecule, as in reaction 2

NiII(P1RN2R′)2]2+ + 2e- + 2H+ f Ni0(P2RN2R′-H)2]2+ (1) Ni0(P2RN2R′-H)2]2+ f NiII(P1RN2R′)2]2+ + H2

(2) 2e- + 2H+ f H2 Although none has previously provided a discussion on the performance of the different conformations of the catalyst toward the HER/HOR reactions, the existence of different isomers for the Ni(0) biprotonated species for the [Ni(P2RN2R′)2]2+ catalysts have been showed23 by RMN studies. Moreover, the most stable isomer that shows a tetrahedral Ni(0) complex with a protonated amine on each cyclic ligand and a structure with C2V symmetry has been characterized. The same NMR study did not show structures with protonated Ni. So the authors suggested that also the transition state (TS) should be symmetric.23 In contrast, their search of transition state structure by DFT models on a similar system, i.e., the [Ni(PNP)2]2+,26 (see Figure 1b) has yielded only a disymmetric TS structure26 that shows the H2 molecule linked on one hand to the Ni(0) and on the other to an amine group. Although the experimental and theoretical studies have been performed on two different systems, [Ni(P2RN2R′)2]2+ and [Ni(PNP)2]2+, the same authors often used results of the first to discuss the second and vice versa. So the following questions are still open: (1) Does the [Ni(P2RN2R′)2]2+ have the same asymmetric mechanism that as been shown for the [Ni(PNP)2]2+ catalyst? (2) Which is the influence of a symmetric versus asymmetric pathway in the reaction performance? Those questions are important points to check, because the search of a symmetric versus asymmetric TS leads to two different mechanisms. In fact, in the case of an asymmetric TS, the heterolytic cleavage is achieved on strongly polarized H2 fragments (as H+H-). Instead, as suggested by Wilson et al.,23 the experimental evidence of a symmetric intermediate confirms a symmetric TS and H2 activation should be simultaneously achieved by the two amine nitrogens. The same proposed symmetric mechanism of the reverse H2 production implies a cooperative interaction of the dihydrogen ligand with both the metal center and nitrogen atoms. In our study, we restricted ourselves to the Ni(P2RN2R′)2]2+ catalyst, for which we dispose of abundant experimental results although the reaction mechanism is still a matter of debate. The final scope of our study is to be able to provide a complete rationalization of the reaction mechanism and estimations of the barriers. Then a multiscale theoretical study will be possible that will allow a simulation of the entire PEM-WE system.

Figure 2. Orientation of the chair (a) and boat (b) conformation for the six-member ring.

So here we present the first detailed and complete study of the H+/H2 reactions catalyzed by [Ni(P2N2)2]2+ (where R and R′ of Figure 1a have been substituted by H) by a DFT analysis. The Ni and the four P atoms are located in a plane, and the four six-membered rings can exist in different conformations, boat-type or chair-type (see Figure 2). In particular we were interested in the study of reaction 1 and 2 for different conformations of reactants. Starting from the two most stable conformations of the catalyst, the reaction profiles were derived and the energy barriers were estimated. Harmonic frequency calculations were also performed to confirm the nature of the obtained minima and transition state structures. More efforts have been devoted to reaction 2 which is the ratedetermining step. In order to work on a consistent chemical model close to the experimental system, we included acetonitrile solvation both as a continuum model and as a discrete solvent molecule able to coordinate to the metal ion, a phenomenon that has been observed experimentally.23 Computational Details All calculations were carried out by using density functional theory (DFT) with the Perdew-Burke-Ernzerhof (PBE)27 exchange-correlation functional as implemented in the ADF code.28-30 The basis set for hydrogen is triple-ζ plus one p-type orbital. For nonmetal atoms, the frozen core is 1s for C and N and 1s2s2p for P and Ni and is described by a single Slater function, whereas the valence set is triple-ζ and supplemented with a d-type polarization function. All minima and TS have been characterized by the mean of analytical frequency calculations. The enthalpic energy barriers, ∆H, have been estimated using the ADF approach, based on the calculation of the total bonding energy (TBE) from initial atomic fragments.31 Moreover to make an energetic balance between two chemical species as in reactions 1 and 2, it is necessary to have the same atoms on each side. Thus in order to describe the reaction barriers of reaction 2 with an acetonitrile molecule directly coordinated to the product, we need to add the energy of an isolated acetonitrile molecule to the reactant and TS balance. In this case the reaction 2 becomes

Ni0(P2N2-H)2]2+ + CH3CN f NiII(P2N2)2CH3CN]2+ + H2

(3)

Entropic terms and zero point energy correction (ZPE) have been computed in order to obtain free energy barrier values, ∆G. Because of the size of the system and the necessity of computing frequencies calculations for each minimum, we restricted our approach by using a solvatation model, COSMO, that uses a continuum medium and a dielectric constant. So that no explicit solvent molecules have been added to the systems. This is an important approximation for solvation, because the presence of explicit solvent molecules can have important effects on energy barriers. Further studies are under development and will be discussed in a future publication.

Reduction and Oxidation by Bioinspired Catalysts

Figure 3. Definition of nomenclature for the isomers.

Figure 4. The [Ni(P2N2)2]2+ system in three conformations, the c-b/ c-c (a), the c-b/b-c (b), and the c-b/c-b (c), visualized perpendicular to the Ni P4 plane.

For simplicity of the discussion, in Figure 3, we define the following nomenclature for the different isomers: 1-2/3-4 so that structure on the left is the so-called “boat-chair/boat-chair” (hereafter “b-c/b-c”)and on the right “boat-chair/boat-boat” (hereafter “b-c/b-b”). Results and Discussion The first part of our study concerns the reactants and products of reaction 1. In particular, we present a conformational study of the reactant, the Ni(II)(P2N2)2]2+ complex, and of the product, the biprotonated Ni(0) structure, were two protons are added on two nitrogen atoms i.e. the Ni(0)(P2N2-H)2]2+ complex, or to a Ni atom and on a single nitrogen atom, i.e. the Ni(0)H(P2N2)(P2N2-H)]2+ complex Then the reaction mechanism of reaction 2 will be discussed together with an analysis of TS and intermediate structure. 1. Reactant of Reaction 1: NiII(P2N2)2]2+. Previous theoretical and experimental studies14,23,26 have discussed structures (reactant, TS or intermediate) with mainly two conformations, the chair-boat/chair-boat (c-b/c-b) and the chair-boat/ boat-boat (c-b/b-b), but without discussing the reason for their choice. Nevertheless, the selection of a particular structure is not innocent for the description of the catalytic mechanism. For that reason, we decided to study and compare the energy of different conformations, although we are not interested in a systematic study of all possible conformations, but just in the determination of the conformations that can be reactive toward the HER. In the case of the NiII(P2N2)2]2+, we can obtain seven different conformations: b-b/b-b, c-b/b-b, c-c/b-b, c-b/c-b, c-b/ b-c, c-b/c-c, and c-c/c-c. The last structure, c-c/c-c, has been excluded from our study because it will yield products (of reaction 1) where the “reactive protons” are too far in space. The other six structures have been optimized in acetonitrile as solvent. A list of all the structures and energies is given in the Table SI in the Supporting Information. All the optimized structures show a planar Ni/P surrounding. Our calculations have shown that three conformations are almost at the same energy which is the lowest: the c-b/c-c (Figure 4a), the c-b/b-c (Figure 4b), and the c-b/c-b (Figure 4c). At 3.37 kcal/mol higher in energy we have obtained a c-b/ b-b conformation, followed by the b-b/b-b at 6.51 kcal/mol and the b-b/c-c at 9.12 kcal/mol. Because of the weak energy difference between the structures, at room temperature an equilibrium distribution is thus obtained. Nevertheless, the question is now about the reactivity of such different conformation toward the HER reaction. 2. Product of Reaction 1 and Reactant Reaction 2: Ni0(P2N2-H)2]2+ and Ni0-H(P2N2)(P2N2-H)]2+. We have done a similar study on the biprotonated Ni(0) structures (i.e.,

J. Phys. Chem. A, Vol. 114, No. 43, 2010 11863 product of reaction 1 and reactant of reaction 2). About this reaction, previous studies14,23,26 all confirm that a first protonation is on a N atom, but for the second H+ addition, two contradictory mechanisms have been proposed, protonation on the Ni center and protonation on a second N atom.14,23 As the choice between the two mechanisms is unclear, we decided to study both. First we will discuss the hypothesis of the protonation on two N atoms ((Ni0(P2N2-H)2]2+). It should be noticed that starting from the optimized structure of the Ni(II) reactant, the addition of two electrons and two protons gives rise to the 2Nbiprotonated Ni(0) complexes showing a tetrahedral NiP4 geometry. So that several conformations that were different for the Ni(II) system yield the same biprotonated Ni(0) structure. For example structure issuing from the Ni(II) c-b/c-b and c-b/ b-c will give the same Ni(0) structure hereafter called cb/cb. Moreover, being interested only in conformations that can have a catalytic power in reaction 2, we have chosen to study only conformations were the two protonations occur both on N, in boat conformation, each on a different diphosphacyclooctane (P2N2) ligand. In fact, protonations on N in chair conformations will give protons too far in space. Moreover, protonation on two N in boat conformation but on the same (P2N2) ligand will give protons in opposite positions with respect to the Ni center, so again too far. Using such criteria we have selected the first three structures shown in Table 1 for the biprotonated Ni(0) structure. We checked that all optimized structures were true minima by calculating the harmonic frequencies, which were all real. As we can see in Table 1, the most stable structure is the cb/cb conformation and the two protons are on the 2 N in boat conformations on different ligands. The two protons are oriented in the direction of Ni with distances of around 2.5 Å to the Ni center. Previous NMR study23 suggested that such isomers should be the most stable and they isolated it at -70 °C. At 4.15 kcal/mol higher in energy we obtained the cb/bb conformation, were protons are located on two nitrogen atoms, in a boat conformation, one on the left the other on the right sides of the structure. As before, the protons are oriented in the direction of Ni that retains a tetrahedral geometry. The biprotonated structure i.e., the bb/bb conformation is 7.2 kcal/mol higher in energy. Finally, the fourth structure proposed in Table 1, corresponds to a Ni(0) 2N-biprotonated structure arising from the very stable c-b/c-c conformation. Although such structure shows two protons far in space, being one proton on a N in a chair conformation, we studied it because previous NMR studies23 have suggested that it could have an important weight in the isomer mixture for temperatures above -0 °C. In particular the NMR studies have suggested the presence of one proton linking on the same side two N in the chair conformation. Indeed, the computed energy difference with respect to the most stable structure (issued from the cb/cb conformation) is weak, being 3.7 kcal/mol. Such an isomer is thus a stable product. Nevertheless, due to the large distance between the two protons, we do not expect this isomer to be reactive toward the catalytic reaction. Yet such a conformation could be transformed in the reactive cb/cb structure passing through a rotational barrier of around 11.4 kcal/mol (see Figure 5) to transform a chair to a boat conformation. In particular, the estimation of such a barrier has been performed considering that in the c/b conversion the maximum corresponds to a planar P-C-N-C dihedral angle of the cycle.

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TABLE 1: Selected 2N-Biprotonated Structuresa

a The name of the conformation is given, together with the absolute energy in eV and the energy difference, ∆G in kcal.mol-1, with respect to the most stable structure and first frequency in cm-1.

Figure 5. Rotational barrier for the inversion of one six-membered ring from chair to boat. The barrier has been estimated considering the TS structure which has a planar P-C-N-C dihedral angle.

A second possibility in the mechanism is to have a Ni(0) biprotonated structure on one N and on the Ni atom: Ni(0)-H(P2N2)(P2N2-H)]2+. We underline that experimental NMR studies23 did not show structures with protonated Ni. In contrast, the same authors,26 in the search of transition state structure by DFT models have proposed for the similar [Ni(PNP)2]2+ complex a distorted TS structure, with one

imaginary frequency that was not quantified and that shows the H2 molecule linked both to the Ni(0) and to an amine group. Nevertheless, if the TS was asymmetric, it should be related to an asymmetric intermediate that shows a protonated Ni center. For that reason we have developed a search of stable minima on such asymmetric structures. A stable minimum with all real frequencies has been obtained for the structure depicted in Figure

Reduction and Oxidation by Bioinspired Catalysts

Figure 6. Ni, N-biprotonated stable minima in the cb/cb conformation.

6, i.e., the cb/cb conformation. The optimized geometry shows a Ni-H bond length of 1.51 Å and N(protonated)-H of 1.06 Å. Moreover, the two protons show a H-H distance of 1.78 Å. For all the other conformations, no stable minimum has been observed. Moreover, the energy is 10 kcal/mol higher with respect to the corresponding N-N biprotonated product. Thus we can conclude that the reaction mechanism most probably starts from 2N-biprotonated Ni(0) structures, since the mixed N, Ni protonated structure is higher in energy. This is consistent with previous experimental studies23 that have not revealed any Ni-H bond signal in their NMR analysis. 3. Proposed Mechanism. The step from the biprotonated Ni(0) structure to the H2 evolving system (reaction 3) has then been further studied and the proposed mechanism is depicted in Figure 7 together with free energy barriers. First of all we have begun the study of the catalytic reaction starting from the most stable N,N biprotonated cb/cb conformation. The first step goes from the tetrahedral Ni(0) biprotonated structure to a transition state 14.52 kcal/mol higher. Two

J. Phys. Chem. A, Vol. 114, No. 43, 2010 11865 hydrogen atoms are coordinated to the Ni center in the perpendicular plane to the N-N-Ni one. The optimized structure, energies, frequencies, and selected geometrical parameters are collected in Table 2. The H-H distance is 0.78 Å, the Ni-H distance is 1.84 Å and the N-H distance is 3 Å. It seems that protons have completely migrated from the two N atoms to the Ni and that the H-H molecule is mostly formed. The high negative first frequency is in the transition state theory, related to the lifetime of the TS.32,33 So for such a structure, were we found a frequency of -406 cm-1, the lifetime of the TS should be high. Following a rotation of around 90° of the H-H molecule, we have obtained an intermediate structure at 2.88 kcal/mol lower energy. The H-H distance is now 0.79 Å and the Ni-H distance is 1.87 Å. In such an intermediate structure the two H are in the direction of the N atoms and they show a H-N distance of ca. 2.48 Å. Such a structure is in agreement with previous theoretical and experimental studies of Dubois et al.;23 nevertheless, the authors were unable to find the corresponding TS structure that we have previously proposed. We have tried to link such a structure via an IRC analysis; nevertheless we could not achieve to a convergence of the pathway. The characterization of the symmetric TS and intermediate is strongly interesting. First of all, from a fundamental point of view, it confirms a symmetric activation mechanism for H2 without asymmetric polarization to form protonic and hydridic species. Second, it suggests that if other conformations of the reactants are close in energy to the most stable b-c/b-c

Figure 7. Proposed mechanism for HER by the [Ni(P2N2)2]2+ complex. Energy barriers are expressed in kcal/mol. The isolated CH3CN was not shown on each step for the sake of simplicity.

TABLE 2: Energies in Acetonitrile Solvent, First Frequencies, and Selected Bond Distances for the TS and the Intermediate

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conformation, and strongly present in solution at room temperature, in reality they are not reactive in such form because using the transition state theory they cannot be bonded to such symmetric TS. So for instance, in the case of the c-b/b-b conformation, the first step is the conversion to c-b/b-c through a rotational barrier of around 10 kcal/mol. In the case of other conformations, the barrier is higher because more than one cycle has to rotate. Such asymmetric conformation could be reactive only if a asymmetric TS could be found. By comparison with the one proposed asymmetric TS for the [Ni(PNP)2]2+ structure26 which shows a protonated Ni and N, we have tried to identify such an asymmetric TS without success. It should be underlined that for such a structure with a protonated Ni and N, the use of low criteria in energy and gradient convergence often gives rise to structures which show a very weak imaginary first frequency (around -10 cm-1) that could lead to false interpretation of TS whereas the tightening of the criteria shows that they are indeed stable minima. In the case of the asymmetric TS for [Ni(PNP)2]2+, the imaginary frequency has not been described by the authors,26 so we cannot conclude if such a TS is the result of a convergence problem or if it is the proof of a completely different mechanism with respect to the Ni(P2N2)2]2+ catalyst as proposed in our study. Although we have mostly concentrated our study on the HER mechanism, some of our results can be interesting also in the discussion of the reverse HOR (hydrogen oxidation reaction) mechanism. In fact, the analysis of energy barriers in Figure 7 shows that the main problem is to replace the acetonitrile solvent, coordinated to the catalyst, with the dihydrogene molecule. Indeed, the energy gap between the Ni(II) catalyst coordinated to acetonitrile and the same catalyst coordinated to H2 is 17.24 kcal/mol. Then after the coordination of the H2 molecule, the reaction barrier to the transition state is very weak (2.88 kcal/mol). We also tried to use a different solvent, as for instance water. In this case, we determined that the corresponding gap between the Ni(II) structure coordinated to water and to dihydrogen was decreased to 12.9 kcal/mol. As for the HER reaction, the HOR mechanism needs a symmetric structure of the catalyst to coordinate the H2 molecule to yield what we have previously called the “intermediate” structure for HER. Conclusion In this paper we have presented a detailed analysis, based on DFT calculations, of the [Ni(P2N2)2]2+ reactivity properties toward the HER. The most interesting point of this study is the discovery of a symmetric TS that is completely different from the asymmetric one proposed for the [Ni(PNP)2]2+mechanism.26 We have so proposed a symmetric pathway (symmetric reactant, TS, and intermediate, etc.) for the HER reaction that is fully coherent to the experimental proposed symmetric intermediate.23 Such results are interesting for further experimental developments in order to increase the performance of the catalyst. In fact, only symmetric conformation as the most stable c-b/c-b is reactive toward the HER. All the other conformations of the biprotonated reactant need to pass through a conformational change in order to be reactive. In order to increase the performance, we can suggest that experimental efforts have to be devoted to the chemical stabilization of the reactive conformation. Moreover, the estimation of the energy barriers opens the possibility to develop multiscale kinetic simulations on the entire PEM-WE system allowing the prediction of experimental observables such as cyclic voltammetry, polarization curves, electrochemical impedance spectroscopy, etc. This kind of simulation has been already started on the basis of the

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