http://pubs.acs.org/journal/aelccp
Strain Engineering Electrocatalysts for Selective CO2 Reduction
Ryan P. Jansonius,†,⊥ Lacey M. Reid,†,⊥ Carolyn N. Virca,†,‡ and Curtis P. Berlinguette*,†,‡,§ †
Department of Chemistry, The University of British Columbia, 2036 Main Mall, Vancouver, British Columbia V6T 1Z1, Canada Stewart Blusson Quantum Matter Institute, The University of British Columbia, 2355 East Mall, Vancouver, British Columbia V6T 1Z4, Canada § Department of Chemical & Biological Engineering, The University of British Columbia, 2360 East Mall, Vancouver, British Columbia V6T 1Z3, Canada
ACS Energy Lett. Downloaded from pubs.acs.org by UNIV AUTONOMA DE COAHUILA on 04/02/19. For personal use only.
‡
ABSTRACT: Lattice strain can enhance the activity and selectivity of electrochemical reactions by breaking the linear scaling relationship. Notwithstanding, the explicit use of strain to affect the CO2 reduction reaction (CO2RR) is rarely reported. In this Perspective, we highlight the opportunity to use strain to affect the activity and selectivity of CO2RR electrocatalysts. We summarize the existing challenges in isolating the influence of strain from convoluting factors (e.g., size, shape, electronic, and surfactant effects) that result from typical methods of inducing strain. We also propose ways to isolate strain effects using the application of mechanical strain to thin-film CO2RR catalysts. We designed this Perspective to motivate the use of joint empirical and computational studies to investigate CO2RR strain−activity−selectivity relationships.
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predicted from volcano plots that correlate catalytic activity with intermediate binding energy.6 The Sabatier principle holds true for CO2RR, but the binding energies of adsorbates to the metal catalyst also affect the selectivity of the reaction. The volcano plot for CO2RR as a function of metal−CO binding energy, for example, illustrates how the overall activity of a catalyst may be increased but at the expense of selectivity (Figure 1a). Gold shows the highest CO2RR activity, and selectively forms CO due to the weak Au−CO binding energy. Metals with weaker metal−CO binding energies (i.e., EM−CO > −0.5 eV) retain the selectivity for CO but with a lower overall activity. Copper, which is characterized by an intermediate CO binding energy of EM−CO ≈ −0.7 eV,2 displays a higher selectivity for hydrocarbons and oxygenates but with a lower overall activity than gold. In cases where there is stronger metal−CO binding (i.e., EM−CO < −1.7 eV), the formation of carbon products is inhibited because strong CO adsorption causes poisoning of the catalyst.2 As the binding energy of CO adsorbed (*CO) to a metal surface increases or decreases, the binding energies of *CHO, *COOH, and *CH2O change similarily, a consequence of these intermediates sharing a common binding mode to the metal through the carbon atom (Figure 1b). This linear scaling relationship exists for all transition metals. This situation
he electrochemical CO2 reduction reaction (CO2RR) is an electron transfer process that enables the conversion of CO2 into a myriad of reduced carbon products (e.g., carbon monoxide, formate, alcohols, and saturated and unsaturated hydrocarbons).1 A significant challenge in selectively forming these carbon products is that the reduction potentials for a range of CO2RR products all fall within a narrow ∼200 mV window.2 This challenge is further exacerbated by the parasitic hydrogen evolution reaction (HER) that occurs at similar potentials. New strategies are needed to enable the transformation of CO2 into a single, carbon-based product in order to realize scalable CO2 utilization schemes.
New strategies are needed to enable the transformation of CO2 into a single, carbon-based product in order to realize scalable CO2 utilization schemes. Heterogeneous electrocatalytic activity is dictated by the adsorption energies of reactants and intermediates to the catalyst surface.3,4 The Sabatier principle states that there is an optimal binding energy of a reaction intermediate where the preference for a particular reaction pathway is highest.5 In single-product catalytic reactions [e.g., HER, oxygen reduction reaction (ORR)], the catalytic activities of metals can be © XXXX American Chemical Society
Received: January 25, 2019 Accepted: March 21, 2019
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DOI: 10.1021/acsenergylett.9b00191 ACS Energy Lett. 2019, 4, 980−986
Perspective
Cite This: ACS Energy Lett. 2019, 4, 980−986
ACS Energy Letters
Perspective
opportunity to use lattice strain as a means to control CO2RR selectivity and break the linear scaling relationship.
This Perspective highlights the opportunity to use lattice strain as a means to control CO2RR selectivity and break the linear scaling relationship. What Is Lattice Strain? Lattice strain is defined as deviation from the equilibrium spacing between atoms in crystal lattice of a material. Strain (ε) is calculated as change in lattice parameter (astrained) normalized to unstrained bulk (abulk) (see eq 1).17 ε = ((astrained − abulk )/abulk ) × 100 (%)
the the the the (1)
Strain engineering has been widely used in materials science as a means of adjusting the mechanical, electronic, and optical properties of materials.18 For example, compressive strain modulates the optical properties of sodium metal, causing it to turn transparent at high compressive pressures.18 Semiconductor band gap energies can also be influenced by strain for engineering solar light harvesters to more efficiently absorb sunlight.19 Strain has also been proven to increase the activities of electrocatalysts; e.g., changing the lattice spacing of platinum affects ORR activity.20 Two theoretical models have posited how strain affects the electronic structure and, in turn, the binding energies of catalytic intermediates; namely, Norskov’s “d-band model” and Peterson’s “eigenstress model”.3,21 The d-band model postulates that strain−adsorbate relationships are derived from a shift in the center of the d-band as a function of interatomic spacing of the metal lattice.21 Strain-induced lattice expansion reduces orbital overlap in transition metal atoms such that the d-band narrows and shifts its center to a higher energy level.21 Higher-energy d-states promote a stronger interaction with adsorbates. A compressively strained copper surface, for example, is therefore predicted to reduce binding affinity for each of the *CO, *COOH, *CH2O, and *CHO intermediates, thereby causing an increase in overall CO2RR activity (Figure 1).2 A silver catalyst would conversely exhibit increased activity for CO2RR under tensile strain.2 d-Band theory predicts that the intrinsic activity of the catalyst could be modulated with strain; however, the effect on selectivity would be limited due to these linear scaling relationships. There exists empirical data supportive of the d-band model.6,14,15,22 The eigenstress model is a mechanics-based treatment of strain effects that provides a theoretical basis for how strain can break the linear scaling relations of intermediate binding energies.3 Eigenstress occurs where individual adsorbates induce strain at the catalyst surface; these interactions may either be additive or in opposition to fabrication-induced strains within the catalyst. The strain induced by the binding of the adsorbate is dependent on the chemical composition of the adsorbate as well as the orientation and position on the surface. As a result, the adsorption energies of the different species can increase or decrease independently due to differences in sizes and binding interactions with the strained surface. The eigenstress model predicts that the binding energies of N and NH2 at a Pt fcc (100) surface have directionally opposed responses to biaxial strain that result in a breaking of the linear
Figure 1. Binding energy affects CO2RR activity and selectivity. (a) CO2RR activity (indicated as current density) as a function of the calculated binding energy of CO to the metal surface (EM−CO). Adapted with permission from ref 2, Copyright 2014, American Chemical Society. (b) Linear scaling relationships with respect to EM−CO for each of the *CHO, *COOH, and *CH2O binding intermediates are nearly parallel. Consequently, changing the selectivity for a specific CO2RR product is inherently challenging. Adapted with permission from ref 7. Copyright 2012, American Chemical Society.
implies that the binding energy of a single adsorbate cannot be modified independently of the binding energy of other adsorbates.5 Consequently, there are limits to how much one can control selectivity by varying transition metal composition. Theory suggests that lattice strain can break linear scaling relationships to enable selective CO2RR catalysis.3 When lattice strain couples with strain induced by the interaction of the adsorbate with surface sites, an increase or decrease in the adsorbate binding energy occurs.3 The net change in binding energy depends on whether or not the fabrication-induced strain and adsorbate-induced strain have the same magnitude and mathematical sign (corresponding to compressive or tensile strain). Adsorbate-induced strain is influenced by several factors, including the chemical composition of the adsorbate, the exposed crystal facet, and the specific binding site. On this basis, strain offers an experimental lever to influence the activity and potentially the selectivity of CO2RR electrocatalysts.3,8 CO2RR electrocatalyst design strategies have focused primarily on elemental composition9 and mesoscopic structure.10−12 Mixed-metal CO2RR catalysts have demonstrated selectivities that differ from their component metals because of the modification of the electronic structure or morphology of the surface,13 while geometric effects (e.g., shape11 or size10) have been correlated to changes in both catalytic activity and selectivity. However, the explicit role of strain in these materials is typically not contemplated. To date, reports of strain-engineered electrocatalysts have largely been confined to ORR6,14,15 and HER.16 This Perspective highlights the 981
DOI: 10.1021/acsenergylett.9b00191 ACS Energy Lett. 2019, 4, 980−986
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scaling relationship.3 An important outcome of this model, and how it could impact CO2RR electrocatalysis, is the indication that the binding energies of *CO, *CHO, *CH2O, and *COOH may potentially be modulated independently by affecting the lattice strain in metal catalysts. Strained catalysts can be fabricated using misfit, shape, or defect strain. Misfit strain is achieved through the growth of a heteroepitaxial thin film over a substrate metal film or nanoparticle. Successful growth of a heteroepitaxial bilayer thin film or core−shell nanoparticle yields a material where the mismatch in lattice parameters between two metals induces homogeneous lattice strain in the outermost metal layers.17 Strain can be estimated in misfit-strained systems by calculating the percent difference between the bulk lattice parameters of the core and shell materials. Strasser and coworkers demonstrated how misfit strain can affect catalytic activity by showing how strain in the outer platinum layers of Pt−Cu core−shell nanoparticles promoted a significant increase in ORR activity by weakening the surface binding affinity for adsorbed *O and *OH intermediates.22 Misfit strain in heteroepitaxial bilayer materials can also be adjusted by changing the composition of the nanoparticle core or underlying substrate (Figure 2a,b).6,15,22 Shape strain arises in different geometries of nanoparticles through the formation of twin boundaries in the nanocrystal, which results in an inhomogeneous strain distribution across the face and edge
regions of the nanoparticle (Figure 2c).23,24 Defect strain manifests at grain boundaries25 or vacancies (Figure 2d).26 Lattice Strain Ef fects on ORR Electrocatalysis. Strain−activity trends for ORR electrocatalysis show that imparting strain through lattice mismatch, alloying, or synthesis of nanomaterials leads to notable increases in ORR activities.6,17,27 For example, compressive strain applied to a platinum thin film by a NiTi shape memory alloy substrate yields a 52% enhancement in the kinetic rate constant of ORR, while strain in Pd− Pt core−shell icosahedral nanomaterials is responsible for a 29fold increase in activity.28,27 Chorkendorff and co-workers provided one of the most defining studies on how strain affects electrocatalysis by systematically imposing compressive strain on platinum through alloying with a series of lanthanides.4 Not only did they observe up to a 6-fold enhancement in activity over pure platinum, this experimental design enabled the generation of a high-resolution volcano plot of platinum activity as a function of lattice spacing.4 It is intriguing to consider how this demonstration of lattice strain impacting ORR activity can be translated to control CO2RR activity and selectivity. How Strain Has Been Used to Affect CO2RR Activity and Selectivity. There are few reports where strain is explicitly used to affect the activity and selectivity of CO2RR electrocatalysts.24,25,29−34 For example, defect strain found at grain boundaries of oxide-derived copper catalysts was found to benefit the conversion of CO2 to hydrocarbons and oxygenates.34 This type of strain was also invoked to explain the increase of the rate of converting CO2 to CO 3-fold for polycrystalline gold (Figure 3a).25 Shape strain was reported to yield a 1.7-fold enhancement of CO2 to CO conversion at a palladium nanoparticle surface (Figure 3b).24 Misfit strain has also been demonstrated for CO2RR with copper monolayers grown on platinum, gold, and other metal nanoparticles or films.29−33 A copper shell about a platinum core, for example, is under tensile strain because the copper lattice parameter is ∼8% smaller than that of platinum. Reducing the number of copper layers increases the tensile strain of the shell, and, in turn, yields a higher selectivity for C2H4 formation.29 As the number of copper layers is increased, strain is relaxed and the product selectivity shifts toward H2 and methane to more closely resemble the selectivity of bulk copper.29 These straininduced changes in selectivity are observed for both nanoparticles and films.29,30,32 Strain engineering can benefit CO2RR electrocatalysis indirectly by suppressing the competing HER. This scenario was clearly demonstrated for tensile-strained icosahedral nanoparticles, where the partial current for H2 evolution was reduced by nearly half despite an increase in total current.24 For copper catalysts, the ΔGads of the key *H intermediate is calculated to positively shift under tensile strain, thereby decreasing HER activity.33 This claim is corroborated by experiments showing a 30% decrease in HER activity for a tensile-strained copper shell about a gold core.29 What Are the Challenges of Studying Strain for CO2RR? An empirical determination of strain−activity−selectivity relationships for CO2RR requires a wide scope of incrementally strained catalysts prepared by a single fabrication method. Achieving this goal is not trivial.17 The size and shape of nanoparticle catalysts are difficult to control because of differing surface-area-to-volume ratios as the nanoparticle shape is adjusted, which is further compounded by mass transport kinetics and electric field concentrations around edge
Figure 2. Elastic strain in nanomaterials. Illustration of latticemismatched epitaxy leading to (a) tensile or (b) compressive strain in bimetallic thin films and nanoparticles. (c) Certain nanoparticle geometries result in lattice strain arising from a higher density of twin boundaries. (d) The intersection of grains on polycrystalline metal surfaces causes strain due to differing orientation of the lattice at the grain boundary. 982
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thickness must be identified in order to balance these two competing effects. The quantification of lattice strain is also challenging. Strain affects materials in three dimensions, yet strain is most often characterized in one or two dimensions.17 The atomic resolution provided by transmission electron microscopy (TEM) techniques enables the lattice spacing of a metal surface to be accurately measured, presenting the opportunity to quantify strain in accordance with eq 1.15,16 However, this methodology neglects how biaxial strain affects the lattice spacing between atomic layers (i.e., the Poisson effect),38 and requires more sophisticated high-resolution electron backscatter diffraction mapping to define three-dimensional strain.25 Another limitation of TEM is that only the atoms at the surface of a material are sampled.38 For nanoparticulate samples, this issue can be addressed indirectly by constructing 3D models from 2D TEM images.16 For thin epitaxial overlayers, strain is typically inferred (rather than quantified) by comparing the lattice spacing of the bulk substrate material and the thin overlayer.29−32 Resolving strain effects in latticemismatched catalyst materials will likely need to emulate the large sample size of Chorkendorff and co-workers’ evaluation of ORR electrocatalysts. Ligand, geometric, and fabrication artifacts that affect the electronic structure of the catalyst dband must be controlled during catalyst preparation.6,39 Alternative Strategies to Test How Strain Impacts CO2RR Electrocatalysis. We are intrigued by the possibility of using mechanically applied strain to develop strain−selectivity− activity relationships for CO2RR electrocatalysts. Mechanically applied strain can be imparted on a thin electrocatalytic film adhered to a flexible substrate by extrinsically stretching or compressing the underlying substrate (Figure 4).40−45 (This
Figure 3. Grain boundaries and shape strain enhance CO2 reduction catalysis. (a) The grain orientation of a cross section of polycrystalline gold wire was mapped by electron backscatter diffraction. A scanning electrochemical cell was rastered across the 17° grain boundary (depicted as a dashed line), and a sharp increase in current for CO2 reduction was measured under 1 atm of CO2 at −0.99 V (Ag/AgCl). From ref 25. Reprinted with permission from AAAS. (b) Surface strain maps of palladium octahedral and icosahedral nanoparticles. Corresponding plots of the projected density of states in the Pd(111) d-band as a function of binding energy for three different amounts of surface strain. Strain is shown to cause a d-band shift, which is purported to affect the CO2RR selectivity at the nanoparticle surface. Reproduced with permission from ref 24. Copyright 2017, John Wiley and Sons.
Figure 4. Mechanically applied strain as an analytical tool for studying strain effects for CO2RR. A generalized schematic of the components of a mechanical strain analytical study are presented and inspired by refs 40−43. The electrochemical cell must accommodate a standard three-electrode configuration in addition to an actuator that can apply and measure discrete amounts of force to a supported thin-film catalyst.
type of strain is measured as the change in normalized length of the substrate in response to the external force.) Peterson, Kumar, and co-workers mechanically modulated lattice strain for flexible thin films of palladium and platinum electrodes inserted in an electrochemical cell fitted to a tensile testing apparatus.40−43 This setup was capable of testing how mechanically applied strain from −2% to 2% affected ORR and HER activity.40−43 Piezo actuators have been used to apply strain on thin platinum and gold films on polyimide plastic substrates to show a strain dependence on ORR activity using cyclical strain modulation.44
sites.34 Surfactants used to precipitate and stabilize nanoparticle materials during synthesis can also mask strain effects by influencing binding to the catalyst.17,35 For thin-film electrocatalysts, the ligand effect from the underlying metal substrate is predicted to dominate over any strain effects within the first two to three epitaxial monolayers,33,36 and strain relief occurs for the outermost layers of thicker films to restore the equilibrium lattice spacing of the bulk.37 The appropriate film 983
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relationships to resolve the role of strain. These challenges include decoupling strain from other electronic effects in lattice-mismatched epitaxial thin-film architectures and avoiding surfactant effects on catalyst activity in nanoparticle systems. A joint empirical and computational approach to interrogate the effects of mechanical strain on CO2RR could enable the discovery of more active and selective electrocatalysts. We also highlight the opportunity that mechanically applied strain may offer in terms of testing how strain impacts activity and selectivity. Establishing how strain affects CO2RR electrocatalysis is critical to developing an electrolysis system capable of product-selective CO2 reduction.
The clarity with which correlations have been elucidated between mechanical strain and activity for ORR and HER catalysts suggests that this strain modulation method could be translated effectively to CO 2RR.40−44 The mechanical application of strain offers many advantages over conventional epitaxial methods, including the ability to test a larger scope of samples by virtue of vastly easier sample preparations, accessibility to intermediate strain values, and in situ modulation of strain. Notwithstanding, challenges do arise due to strain relief yielding a lesser amount of lattice strain on the electrocatalyst film than what can be accessed with conventional misfit- or shape-strained catalysts.6,42,46 These differences arise because strain applied to a thin film through the substrate does not propagate quantitatively to the surface due to surface roughness, grain boundary slip, defect formation, and strain relaxation.37,47,48 Strain is also relieved through delamination of the thin film from the substrate or fracturing when applied strains exceed a material-dependent critical strain of approximately 1−3%.46,47 There are several ways to potentially address these experimental issues. For example, nanoscale grain sizes may extend the elastic region of metal deformation according to the Hall−Petch effect.49 Amorphous phases may also increase the critical strain of a metal by decreasing the opportunity for slip dislocation through grains that are under strain.44 We note that there is no analytical study available that reports on how mechanically applied strain affects CO2RR. One possible solution is to examine the d vs sin2 ψ relationship using x-ray diffraction (XRD), a technique that has been used to characterize strain ex situ for flexible electronic applications.46,50 Lattice strain can also be measured indirectly by quantifying the shift in the d-band using in situ x-ray photoelectron spectroscopy (XPS).42 Computational Support of Empirical Studies of Strain Ef fects on Catalysis. In order to define the extent at which strain affects CO2RR, empirical studies will need to be complemented by computationally tracking strain-derived changes to intermediate and transition state binding energies.24 The relative rates of HER and CO2RR can now be predicted (as well as the major CO2RR reaction product) using a two-parameter descriptor that analyzes the binding affinity of H-adatoms and CO molecules,51 but methods to predict the distribution of carbon products are still at an early stage of development. In order to computationally develop how strain affects the reaction products formed, there must be careful consideration of functionals,52,53 kinetics,54 solvent molecules,55−57 and changes in interfacial charge densities and surface potentials.58 A purely thermodynamic approach is insufficient to model selectivity because activity can be limited by the scaling relationships between transition states, not just reaction intermediates.54 Solvent adsorption up to two layers from the surface interacts with adsorbate binding, causing changes in the surface potential, and can be used to model the electrochemical double layer.57 The importance of appropriate functionals33,51,52 is made apparent by the work of Nørskov and coworkers,52 who report a substantial decrease in the highest free-energy barrier for CO2 hydrogenation to methanol on a stepped copper surfaced by 0.4 eV using the BEEF-vdW functional, which correlates with the experimentally observed selectivity. Lattice strain offers the opportunity to break the linear scaling relationship for CO2RR electrocatalysts, but there are significant challenges in developing empirical strain−activity
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Ryan P. Jansonius: 0000-0002-4014-2068 Curtis P. Berlinguette: 0000-0001-6875-849X Author Contributions ⊥
R.P.J. and L.M.R. contributed equally to this work. L.M.R., R.P.J., and C.P.B. devised the concept. All authors contributed to the construction of the manuscript. C.P.B. supervised the project. Notes
The authors declare no competing financial interest. Biographies Ryan Jansonius is a Ph.D. candidate in the Berlinguette group at the University of British Columbia. He received his B.Sc. (Hons) degree from the University of Calgary in 2016. His current research interests are related to renewable energy storage using electrosynthesis. Lacey Reid received her B.Sc. from Acadia University in 2010 and her Ph.D. in Chemistry from Queen’s University in 2017. Her postdoctoral research in the Berlinguette group at the University of British Columbia focused on the fabrication of thin films for clean energy and CO2 electroreduction. Carolyn Virca is a Stewart Blusson Quantum Matter Institute Postdoctoral Fellow at the University of British Columbia. She received her Ph.D. in Chemistry from Portland State University in 2018 and her B.S. degree in Chemistry from the University of Portland in 2013. Her postdoctoral work in the Berlinguette group focused on using computational chemistry to understand energy conversion processes. Curtis Berlinguette is a Canada Research Chair in Energy Conversion and Professor of Chemistry and Chemical & Biological Engineering at the University of British Columbia. His research program is dedicated to accelerating the discovery and deployment of clean energy technologies, including advanced solar cells, energy storage, and CO2 utilization technologies.
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ACKNOWLEDGMENTS
The authors are grateful to the Canadian Natural Science and Engineering Research Council (RGPIN337345-13), Canadian Foundation for Innovation (229288), Canadian Institute for Advanced Research (BSE-BERL-162173), and Canada Research Chairs for financial support. 984
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