Article pubs.acs.org/JPCC
Role of Water and Adsorbed Hydroxyls on Ethanol Electrochemistry on Pd: New Mechanism, Active Centers, and Energetics for Direct Ethanol Fuel Cell Running in Alkaline Medium Tian Sheng, Wen-Feng Lin,* Christopher Hardacre, and P. Hu* School of Chemistry and Chemical Engineering, The Queen’s University of Belfast, Belfast BT9 5AG, United Kingdom S Supporting Information *
ABSTRACT: First principles calculations with molecular dynamics are utilized to simulate a simplified electrical double layer formed in the active electric potential region during the electrocatalytic oxidation of ethanol on Pd electrodes running in an alkaline electrolyte. Our simulations provide an atomic level insight into how ethanol oxidation occurs in fuel cells: New mechanisms in the presence of the simplified electrical double layer are found to be different from the traditional ones; through concerted-like dehydrogenation paths, both acetaldehyde and acetate are produced in such a way as to avoid a variety of intermediates, which is consistent with the experimental data obtained from in situ FTIR spectroscopy. Our work shows that adsorbed OH on the Pd electrode rather than Pd atoms is the active center for the reactions; the dissociation of the C−H bond is facilitated by the adsorption of an OH− anion on the surface, resulting in the formation of water. Our calculations demonstrate that water dissociation rather than H desorption is the main channel through which electrical current is generated on the Pd electrode. The effects of the inner Helmholtz layer and the outer Helmholtz layer are decoupled, with only the inner Helmholtz layer being found to have a significant impact on the mechanistics of the reaction. Our results provide atomic level insight into the significance of the simplified electrical double layer in electrocatalysis, which may be of general importance.
1. INTRODUCTION It is well-known that the electrical double layer plays the most important role in electrochemistry. For an electrode with an external electric potential, the excess charge is regulated on the surface, resulting in a specific interfacial region, i.e., electrical double layer. The specific interfacial region describes the interaction between a charged surface and an electrolyte solution, and the variation of electric potential near the surface in contact with solutions or solid-state fast ion conductors. In fact, it is not only fundamental to the electrochemical behavior of electrodes but also to all heterogeneous fluid-based systems, such as colloidal particles, porous materials and semiconductors.1−5 Even though the general structure of the specific interfacial region is well accepted, the understanding of how it affects the electrochemistry of a given catalytic system at molecular or atomic level is very poor. There is a good reason for this: it is extremely difficult to investigate reactions occurring at the electrode in the presence of an electrical double layer experimentally at the atomic level, and it is equally challenging to study the chemistry theoretically due to the complexity of such systems. However, there is no doubt that obtaining insight into the specific interfacial region is not only scientifically one of the most important goals in chemistry but also of paramount importance technologically, as many applications can be affected by the electrical double layer and © 2014 American Chemical Society
the improvement of understanding may have a considerable impact on these applications. In recent years, with the improvement of density functional theory (DFT) calculations, simulations and some theoretical studies have advanced our understanding of the electronic or geometrical structure of the electrical double layer, such as the water structures, potential of zero charge, and electric capacitance.6−19 Currently, some first principles simulations of the electrical double layer have been explored by several groups. In the description of the ions in the diffuse layer region and the solvated effects, a primitive model was usually employed on the basis of the modified-Poisson−Boltzmann theory, in which the ions were represented by charged hard spheres and a periodic dielectric continuum-solvation model was implemented.7−9 In another approach, explicit water molecules were placed directly into the unit cell, and the addition of protons or fractional charges was utilized to adjust the work function.10−15 In spite of this, the significance of electrical double layer has still rarely been studied in electrocatalysis. One main reason for this is that the simulation of electrical double layer with explicit water molecules requires Received: August 9, 2013 Revised: February 11, 2014 Published: February 17, 2014 5762
dx.doi.org/10.1021/jp407978h | J. Phys. Chem. C 2014, 118, 5762−5772
The Journal of Physical Chemistry C
Article
Pd(111) slabs modeled by p(3 × 6) unit cell with the height of 30 Å. The four-layer slabs containing 72 Pd atoms and the bottom two layers were fixed in the slab. A 4 × 2 × 1 Monkhorst−Pack k-point sampling was used. All the transition states were localized with constrained minimization approach, and the convergence of forces was set to 0.05 eV/ Å using a modified VASP.50−52 For the calculation of the OH− adsorption potential, OH− → OHads + e−, a thermodynamic method has been reported in the literature.53 To determine the structure of the simplified electrical double layer, ab initio molecular dynamics (AIMD) simulations were used at a constant temperature of 350 K (0.5 fs per step). In the simulation of the initial structures of electrical double layer, Pd surface with adsorbates were optimized first, and then 24 water molecules were added for AIMD with a vacuum layer of 7 Å above the water layer. The running time of molecular dynamics simulations is extended to 20 ps in the presence of water until there is no significant change of the local water structure between adsorbates and water molecules. In all the molecular dynamics calculations, the Pd surface and adsorbed species were fixed. For each state including intermediates and transition states, at least five structures (typically six structures) from molecular dynamics simulations were optimized, and the most stable one is used.54 It is noticed that the variability of water configurations would result in the uncertainty that the most stable one is successfully obtained, but in current scope too many samples is timeconsuming.
the use of time-consuming molecular dynamics calculations.17 Here, we present a study on how a simplified electrical double layer may affect the reaction mechanism, kinetics, and thermodynamics for ethanol oxidation at the atomic level using first principles calculations. The electro-oxidation of ethanol is chosen in this work for the following reasons: It is the core reaction in the direct ethanol fuel cell (DEFC) which can convert the chemical energy of alcohols to electric energy directly and continuously.20−23 The equilibrium potential for ethanol electrooxidation reaction (EOR) is ca. 0.19 V versus RHE in alkaline solution (to form CO32‑/HCO3−) as the anode reaction. On the cathode, oxygen reduction reaction (ORR) has a reversible aqueous potential of ca. 1.23 V versus RHE. Such a direct ethanol fuel cell can provide a theoretical single cell voltage above 1 V (see below): anode: C2H5OH + 5H 2O → 2CO32 − + 16H+ + 12e− (1)
U = 0.19 V cathode: O2 + 4H+ + 4e− → 2H 2O
U = 1.23 V
(2)
Several theoretical studies have been carried out toward the establishment of mechanisms for ethanol electro-oxidation, and a range of metal surfaces have been examined by first principles calculations in the literature.24−35 However, the results were mostly obtained at the gas/solid interfaces, which is not convincing enough because the electrocatalytic ethanol decomposition which occurs at the electrolyte/electrode interface is more complicated than those of the gas/solid interfaces. To date, some DFT calculations in the aqueous phase have been carried out, which is of importance in the understanding of aqueous heterogeneous catalysis.35−40 Despite this progress, the following fundamental questions remain to be answered. Does the IHP affect the reaction mechanisms on electrodes compared to the results from gas/ solid interfaces? Does the active site remain to be the same with/without the IHP? What are the roles that the IHP and OHP play in the electrochemistry, respectively? In order to address these questions, in this work we have investigated the ethanol oxidation on Pd electrode in the presence of a simplified double layer within a DFT framework. We reveal the significant roles of water and adsorbed OH in the electrooxidation of ethanol on the Pd electrode: The OHads in the electrical double layer can dramatically change the reaction mechanism and the active site. The effects of IHP and OHP on the electrochemistry are also analyzed in this study. The paper is organized as follows. The calculation details are mentioned in section 2. The calculated results, including the mechanisms in the generation of acetaldehyde, acetic acid, and protons, are shown in section 3. Some analyses and discussions on the significance of electrical double layer are presented in section 4. Finally, the conclusions are summarized.
3. RESULTS 3.1. Simplified Model of Electrical Double Layer under Alkaline Condition. It is known that a typical electrical double layer in alkaline condition contains the reactants, anions, and cations, such as Oads, OHads (OH−), K+ (or Na+), and SO42‑ (or ClO4−), and the structure of electrical double layer is extremely complicated. Hence, a simplified model is required for two main reasons. First, to obtain the exact structure of electrical double layer is beyond the scope of current simulations and some approximations must be taken in order to simulate the electrical double layer feasibly. Second and more importantly, simplified models may provide some key insight into the electrocatalysis as long as the simplified models possess the main features of the electrical double layer. It may even give clearer trends and understanding than more complicated electrical double layer models because of its simplicity. In particular, we may learn a great deal of electrocatalysis chemistry when the simulation with a simplified model of electrical double layer is compared to that without it. In the present work, a simplified model is introduced and described in details below. Many studies have shown that, on Pd electrode, the applied potential of 0.6−0.8 V versus RHE, where OH− anion adsorbs on the surface to compensate the positively charged electrode, is the most active potential region for the ethanol electrooxidation.23,58,63,64 Above 1.1 V, the predominant surface species is O, leading to the active sites being blocked, and therefore, the adsorbed oxygen is not included in our model. Because we are simulating the Pd electrode performance under the alkaline, anions of Br−, SO42‑, ClO4− are negligible, leading to another simplification of electrical double layer model. Meanwhile, the adsorption of cations Na+ or K+ on the metal surface at positive potential region should be limited, and they are more likely to exist in the outer Helmholtz plane (OHP) or
2. CALCULATION METHODS All the electronic structure calculations were performed using the Vienna Ab-Initio Simulation Package (VASP), using Perdew−Burke−Ernzerh (PBE) generalized gradient approximation (GGA) exchange-correlation functional. The projectoraugmented-wave (PAW) pseudopotentials were utilized to describe the core electron interaction.41−49 The cutoff energy was set to 400 eV having been tested to be accurate enough for energy calculations. All the reactions were carried out on 5763
dx.doi.org/10.1021/jp407978h | J. Phys. Chem. C 2014, 118, 5762−5772
The Journal of Physical Chemistry C
Article
Figure 1. Comparison of the realistic electrical double layer and the simplified double layer at the electrolyte/electrode interface used in the present work: green, Pd; gray, C; red, O; white, H; purple, Na. this notation is used throughout the paper.
diffuse layer in their hydrate forms (see the Discussion section). Thus, it is expected that they have little effect on the electrocatalysis, resulting in a further simplification. Taking all of these into consideration, we establish a simplified electrical double layer model, in which the inner Helmholtz plane (IHP) contains OHads, H2O, and reactants (i.e., ethanol) and the OHP are represented with some water molecules. The comparison of the realistic model and the simplified model used in the present work is presented in Figure 1. It is well-known that the dynamic electrical double layer structures are extremely difficult to investigate experimentally. Herein, we attempt to build a simplified model based on molecular dynamics simulations within a DFT framework. We started with a stable structure with the OHads coverage of 0.33 ML which was previously reported as the initial structure, in which water layers containing reactants were also included. Simulations of molecular dynamics as long as 20 ps were performed for the initial structure at constant temperature of 350 K. The results obtained showed that, after 5 ps, the structure was already stable, and the total energy was not changed significantly (
