Article pubs.acs.org/cm
Influence of Electrolyte Cations on Ni(Fe)OOH Catalyzed Oxygen Evolution Reaction Jeremie Zaffran,† Michaela Burke Stevens,‡ Christina D. M. Trang,‡ Michael Nagli,† Mahran Shehadeh,† Shannon W. Boettcher,*,‡ and Maytal Caspary Toroker*,† †
Department of Materials Science and Engineering, Technion - Israel Institute of Technology, Haifa 3200003, Israel Department of Chemistry and Biochemistry, University of Oregon, Eugene, Oregon 97403, United States
‡
S Supporting Information *
ABSTRACT: Iron-doped, nickel oxyhydroxide (Ni(Fe)OOH) is one of the best catalysts for the oxygen evolution reaction (OER) under alkaline conditions. Due to Ni(Fe)OOH’s layered structure, electrolyte species are able to easily intercalate between the octahedrally coordinated sheets. Electrolyte cations have long been considered inert spectator ions during electrocatalysis, but electrolytes that penetrate into the catalyst may play a major role in the reaction process. In a joint theoretical and experimental study, we report the role of electrolyte counterions (K+, Na+, Mg2+, and Ca2+) on Ni(Fe)OOH catalytic activity in alkaline media. We show that electrolytes containing alkali metal cations (Na+ and K+) yield dramatically lower overpotentials than those with alkaline earth cations (Mg2+ and Ca2+). K+ and Na+ lower the overpotential because they have an optimal acidity and size that allows them to not bind too strongly or alter the stability of reaction intermediates. These two features required for intercalated cation species provide insight into selecting appropriate electrolytes for layered catalyst materials, and enable understanding the role(s) of electrolytes in the OER mechanism.
■
INTRODUCTION Hydrogen production has been an active field for several years,1 largely due to the need for large-scale renewable energy storage. Hydrogen fuel can be obtained by splitting water in (photo)electrochemical cells,2,3 and catalyst materials are needed at the electrodes of such devices.4 The water oxidation occurring at the anode material has often been reported to be especially problematic due to the lack of active catalysts.5 This reaction is known as the oxygen evolution reaction (OER) and can be written as follows: 2H 2O → 4H+ + 4e− + O2
(1)
Among the various materials proposed to catalyze the OER, nickel oxyhydroxide (NiOOH) is one of the most efficient in alkaline conditions when doped with Fe.6,7 NiOOH has a layered structure with partially hydrogenated stacked nickel oxide layers.8,9 That crystal structure, known as β-NiOOH phase, has been considered by theoreticians in order to study substitutional doping effects, replacing in one specific layer Ni atoms by other metallic elements.10−13 However, water molecules and other electrolyte species existing in the media can intercalate between the catalyst layers under charging conditions, hence leading to the γ-NiOOH phase (see Figure 1; adapted from the information in refs 10 and 14−17). As seen in Figure 1, the Na+ cation is surrounded by oxygen ions belonging to neighboring water molecules and to the surface. Recent experimental studies reported an in-depth analysis of the role of intercalated anions19 and cations,20 from the electrolyte, on the efficiency of Ni(Fe)OOH catalysis. The most commonly used electrolyte cations, Na+ or K+, are © 2017 American Chemical Society
Figure 1. γ-NiOOH unit cell. Red: O; gray: Ni, white: H, yellow: Na. Created using VESTA visualization software.18
equivalent regarding the catalytic activity of NiOOH, while cations with significantly larger ionic radii (Cs+) decreased the NiOOH overpotential, apparently by promoting increased Ni− Received: February 7, 2017 Revised: May 17, 2017 Published: May 17, 2017 4761
DOI: 10.1021/acs.chemmater.7b00517 Chem. Mater. 2017, 29, 4761−4767
Article
Chemistry of Materials
Figure 2. Reaction mechanism for OER on pure and Fe-doped NiOOH. Red: O; gray: Ni, white: H, brownish: Fe. Norskov12 respectively). Our slab model contains five layers of metal cations, with a coverage of one monolayer (1 ML) and a vacuum of 20 Å. In order to preserve the network of interlayer water molecules, we always kept the same slab model for the different electrolytes. We adjusted the size of the box that was originally with Na+, in order to fit with the expansion or the contraction of the bulk observed with other cation electrolytes after geometry optimization. Only the interlayer space is significantly affected by cation intercalation as attested by the relaxed bulk geometries reported in the SI. Ni−O and Fe−O distances are not significantly affected by the cations. The differences for a given reaction intermediate between two electrolytes are lower than 0.01 Å (for Ca2+ or K+). After changing the lattice constant of the slab for each electrolyte, we performed relaxation for the intercalated electrolyte ion positions. We used the same geometric configuration for each electrolyte and for each reaction intermediate in order to isolate the effect of the electrolyte. We fixed the convergence criterion of geometry optimization to 0.05 eV/Å for ionic relaxation, and to 10−4 eV for electronic iterations. When the catalyst is undoped, two Ni atoms are exposed at the surface. However, when it is Fe-doped, then one Ni atom and one Fe atom are exposed, corresponding to a doping ratio of 10% (vs other metal cation centers). Such a concentration of Fe is often reported by experimentalists and is near-optimal for OER activity.12,28,29 Various mechanisms have been proposed for OER on NiOOH surfaces.10−12,30 In the following, we focused on a reaction path proposed by Selloni and co-workers for pure NiOOH for the purpose of comparison to previous literature.10 On the basis of a thorough configurational search, we were able to find some specific intermediates leading to an overpotential lower than values usually reported in the literature10,12 for NiOOH with Na species (Figure 2). Thus, the solvent effect is already incorporated in the model by the presence of an additional spectator water molecule coadsorbed near the active site. Such a microsolvation model was already successfully used in a joint experimental and computational work related to heterogeneous catalysis.31,32 We computed in each situation the overpotential η as presented in eq 1. It is obtained as the difference between the highest reaction energy maxi{ΔGi} related to one specific step i of the OER mechanism, and the DFT calculated oxidizing potential of water E°ox/water (i.e., 1.14 V, compared to the experimental value, 1.23 V vs RHE). In the following, the highest energy step maxi{ΔGi} will be also denoted “PDS”, for “Potential Determining Step”. Reaction energies are corrected by the zero-point-energy (ΔZPE) and the entropic contribution (TΔS) taken from Table 1 in ref 10, before applying eq 1.
O bond lengths.20 Among the anion electrolytes, in carbonatefree electrolyte, the most basic ones yielded the lowest overpotentials.19 However, it remains unclear exactly how electrolyte species affect the catalyst activity in Fe-doped and undoped γ-NiOOH (denoted simply as “Ni(Fe)OOH” in the following). Improved understanding of the effects of intercalated species on the activity would provide additional insight into the operational mechanism of these complicated, but highly active, catalyst structures. In a joint experimental and theoretical study, we compare catalyst activity of Ni(Fe)OOH with mono- and divalent cation species. Experimentally, we carefully measured the electrochemical activity of Ni(Fe)OOH with several cations, namely, Na+, K+, Ca2+, and Mg2+. We then theoretically investigated electronic, energetic, and structural change induced by the electrolyte. According to our calculations, monovalent electrolytes perform better than divalent ones, in general agreement with the experiments. We find that the cation acidity and its size are among the main parameters influencing the surface reactivity. Indeed, the cations appear to impact the relative stability of key reaction intermediates, and as a result, the overpotential value is modulated.
■
METHODS AND MATERIALS
Computational. The theoretical results of the current study were obtained with the VASP code in the spin-polarized density functional theory (DFT) framework.21−23 Due to the importance of the Hubbard correction to deal with metal oxide and hydroxide materials,18 we worked with the method of DFT+U in the Dudarev et al. formalism24 with the Perdew−Burke−Ernzerhof (PBE) exchange-correlation (XC) functional,25 and a (U-J) value of 6 eV for Ni and 3.5 eV for Fe, as previously chosen for this material in the literature.10−12 Indeed, several papers report GGAs efficiently for modeling water oxidation activity on NiOOH surface.10−12 The projected augmented wave (PAW) formalism26 was used to model the electron−ion interaction, with an energy cutoff of 600 eV. We used a k-grid of 2 × 3 × 1 in the Monkhorst−Pack scheme.27 All of these numerical parameters, and also the thickness and the vacuum of the slab model, enabled us to reach an accuracy of 0.10 eV regarding reaction energies according to conventional protocols of determining convergence accuracy threshold.11 We started by optimizing individually the bulk unit cells with four different electrolytes (see List of bulk structures section 7, SI). The unit cell is overall neutral and stoichiometric, where the electrolyte cations turn out to be positively charged after converging the electron density. Then, we prepared a slab with the Na+ electrolyte by cleaving the bulk at the (101) facet, since this facet was proposed to be catalytically active in previous literature (this facet is analogous to the (01̅15) or (011̅2) facet of the beta-phase used by Selloni10 and
■
η = max i{ΔGi}/e − E°ox/water
(2)
EXPERIMENTAL SECTION
Film Preparation. Ni (oxy)hydroxide and Ni(Fe) (oxy)hydroxide metal films were cathodically deposited onto Au (∼100 nm) (with a Ti (∼25 nm) adhesion layer) coated 5 MHz quartz crystal microbalance (QCM) crystals (Stanford Research Systems). Deposition solutions 4762
DOI: 10.1021/acs.chemmater.7b00517 Chem. Mater. 2017, 29, 4761−4767
Article
Chemistry of Materials were prepared from Ni(NO3)2·6H2O (Sigma-Aldrich, 99.999%) and FeCl2·4H2O (Sigma-Aldrich, 98%) in 10:0, 8:2, and 9.9:0.1 (mol:mol, total 0.1 M total metal) ratios with nanopure 18.2 MΩ cm water as the solvent for the NiOxHy and Ni(Fe)OxHy films, respectively. When discussing experimental work, we use the general (oxy)hydroxide notation, MOxHy (where M is a metal cation), as to not specify a specific oxidation state or proton stoichiometry. However, a specific stoichiometry is used for computational work. Film Ni:Fe ratios were determined by X-ray photoelectron spectroscopy (XPS). To prevent the oxidation of FeCl2 into insoluble FeOOH, the solution was covered in parafilm and purged with N2 for 10 min prior to and between depositions.29,33 Prior to film deposition, the QCM crystals were cycled in 1 M aq. H2SO4 (Sigma-Aldrich) (2.5 to −2.5 V at 200 mV s−1) and cycled twice in 0.1 M electrolyte (0.0−0.850 V Hg|HgO at 10 mV s−1) to ensure a clean electrode surface.34 Electrodeposition. The deposition cell consisted of a twoelectrode configuration using carbon cloth as the counter electrode. All films were deposited at a constant cathodic current density of −2 mA cm−2 from unstirred 0.1 M solutions. Following deposition, films were briefly submerged into nanopure water to remove residual metal ions. Film loading was determined from the difference in measured QCM resonance frequency in nanopure water before and after deposition. Depositions at −2 mA cm−2 of K+ ≈ Na+ ≈ Li + and NiOxHy in Fe saturated solution: K+ ≈ Na+ >
■
RESULTS AND DISCUSSION We show using theory that Mg2+ and Ca2+ inhibit OER activity on the NiOOH surface comparatively to Na+ and K+, and this is 4763
DOI: 10.1021/acs.chemmater.7b00517 Chem. Mater. 2017, 29, 4761−4767
Article
Chemistry of Materials
Figure 4. (A) NiOxHy, and (B) Niz−1FezOxHy at approximately z = 0.1 (top) and z = 0.5 (bottom) in 0.1 M KOH before and after the addition of 1 mM Ca(OH)2.
spiked Ca(OH)2 or Mg(OH)2 KOH solution compared to the films run in the pristine KOH. Comparison of Experimental and Theoretical Results. The predicted tendency for alkaline earth metals to lower the NiOOH catalytic activity is consistent with experimental observations presented in the current paper for Ca2+. Regarding Mg2+ electrolyte, experiment reports an overpotential value lower than Ca2+ for the undoped surface, and similar to pure K+ in the Fe-doped situation. This difference between theoretical and experimental results is explained by the very low solubility of Mg2+ ions in aqueous base and thus the inability of Mg2+ to significantly incorporate in NiOOH. However, in our theoretical model, we considered Mg2+ as an interlayer species as we did for Na+, K+, or Ca2+. In the next sections, we explain the fundamental trends in overpotential using the theoretical model. On the Electrolyte Acidity. Alkaline and alkaline earth elements lead, respectively, to mono- and divalent cations, both being Lewis acids. As we will demonstrate, electrolytes with a weak acidity appear to lead to lower OER overpotentials when incorporated into the Ni(Fe)OOH structure. Within the layered NiOOH structure, each electrolyte cation is coordinated to O atoms belonging either to water molecules or to inner layers of the material, hence giving effective “molecular clusters” embedded in the interlayer space (Figure 5). The distance between electrolyte cation and surrounding O (denoted “X−O distance”) is dependent on the electrolyte
Figure 3. Overpotential of NiOxHy (bottom) and Niz−1FezOxHy (top, z = 0.1, patterned, and z = 0.5, solid) films at a TOFtm of 0.1 s−1 in 0.1 M KOH, 0.1 M NaOH, 1 mM Ca(OH)2 in 0.1 M KOH, and 1 mM Mg(OH)2 in 0.1 M KOH at steady state. All NiOxHy films were analyzed in Fe-free conditions.
Cs+ > Li+) and are now extended to encompass divalent cations and codeposited Ni(Fe)OxHy films with 10% and 50% Fe.20 The experimental results are in general agreement with our theoretical trends that predict similar activity for the monovalent cations K+ ≈ Na+ and lowest activity for Ca+. The Mg(OH)2 electrolyte is an anomaly since it is not appreciably soluble, as discussed below, and thus did not influence catalysis. Due to the low molar solubility of Ca(OH)2 and Mg(OH)2 in water, the effect of Ca(OH)2 and Mg(OH)2 was tested by spiking a solution of 0.1 M KOH with 1 mM of Ca(NO3)2 and Mg(NO)2 nitrate salts, respectively. In the case of Mg(OH)2, there is no indication that the sparse Mg2+ in solution interacted with the catalyst films. In the case of Ca(OH)2, however, for both the NiOxHy and the Ni(Fe)OxHy films, there was a significant decrease in activity and a change in the Tafel kinetic parameters (Figure 4A,B). XPS analysis indicated that there is between 2% and 8% calcium remaining on the surface of the Ni and Ni(Fe)OxHy films even after all the potassium has been washed away (Figure S5). The broad Ca 2p peaks are difficult to deconvolute and likely represent a mixture of CaCO3 and Ca(OH)2. The Fe-rich Ni0.5Fe0.5OxHy film was most visibly affected by the Ca2+ in solution and an ∼100 mV cathodic shift in cathodic peak potential, a broadening of the redox wave, and an ∼60 mV anodic shift in the OER onset (Figure 4B), as well as a 20 mV dec−1 increase in Tafel slope (Figure S6) is observed. Despite the peak broadening, the cathodic peak volume remains nominally the same. For NiOxHy, there was only a minimal shift in the redox wave, ∼40 mV (based on the cathodic peak), no significant change in redox features, an ∼10 mV dec−1 change in the Tafel slope, and an ∼40 mV anodic shift of the OER onset. SEM images (Figure S7) show no difference in morphology between films run in
Figure 5. Bond distances (in Å) for molecular clusters formed by electrolyte species and surrounding O atoms that exist in the interlayer space of NiOOH. We represented here the upper cluster of the slab model for intermediate A. Red, white, yellow, orange, purple, blue spheres represent O, H, Na, Mg, K, Ca ions, respectively. 4764
DOI: 10.1021/acs.chemmater.7b00517 Chem. Mater. 2017, 29, 4761−4767
Article
Chemistry of Materials nature. We can see in particular for the topmost cluster that the highest X−O distance is obtained for K+, 2.85 Å on average, and the shortest bonds are observed for Mg2+, 1.98 Å on average. Na+ and Ca2+ are intermediate with an average bond length of ∼2.40 Å for both of them. Its large ionic radius and its high oxophilicity comparing to K+ allow Ca2+ to connect five oxygen ions (belonging to the solid surface and adjunct water molecules) rather than four. As a result, the X−O distance tends to shorten from the left to the right in one row of the periodic table, and from the bottom to the top in one given column. This evolution can be correlated to electronegativity, and thus an alkaline earth cation globally binds stronger with water than alkali cations. This trend is also confirmed by Bader charge analysis showing that the X−O bond is significantly more ionic for divalent species than for monovalent electrolytes. Indeed, as reported in the SI, when bonded to Mg2+ or Ca2+, the average charge located on O atoms forming the topmost molecular cluster is of −1.26 and −1.13 e, respectively. In contrast, when connected to the Na+ or K+ center, the average O charge is of −1.17 and −1.10 e, respectively. In alkaline earth based clusters, water molecules are thus tightly attached to the central cation. However, with alkali electrolytes, X−O bonds are much “looser”, potentially allowing interlayer water molecules close to the surface to interact with adsorbed intermediate species and thus facilitating the OER process. Similar observations were made for modeling other electrolyte clusters in the slab model, and also for other reaction intermediates in both Fe-doped and undoped NiOOH. Since cations are Lewis acids, we conclude that electrolytes with a strong acidity such as Ca2+ are not efficient for OER. During the water oxidation process, the OH bond scission should be assisted by nucleophilic species such as water or oxygen based compounds.32 Due to the high affinity of alkaline earth cation electrolytes for O atoms, the electrolytes can strongly bind to water and other OER intermediates, hence suppressing effective interaction with surface intermediates. This could explain why electrolytes presenting a weak acidity are preferable for OER. Our conclusion is consistent with previous experimental study focusing on anion electrolytes, which demonstrated that more-basic anions (such as carbonate or phosphate) species correlate with decreased OER overpotentials compared to more acidic ones (such as chloride).19 On the Electrolyte Size. In addition to the acidity, the cation size may directly impact the overpotential value. We focus the analysis here on the Fe-containing catalysts, but similar consideration applies to the pure Ni (oxy)hydroxides. As it can be seen on the reaction cumulative free energies presented in Figure 6, for Mg2+ and Ca2+ electrolytes, the PDS is step 2 (see Table S4 in the SI). Regarding Mg2+, it is apparent that the high energy of this step is due to the relative instability of the C intermediate comparatively to other electrolytes; the cumulative free energy is highest for Mg2+ in intermediate C. However, the high overpotential observed for Ca2+ is due to an overstabilization of the B intermediate, as the cumulative free energy appears lowest in intermediate B in the graph in Figure 6. The relative stability of those reaction intermediates is confirmed by adsorption energies reported in the SI. Adsorption energy of the C intermediate on the surface is significantly higher with Mg2+ electrolyte (+3.99 eV) compared to other cations (
