Membrane-Coated ElectrocatalystsâAn Alternative Approach To

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Membrane Coated Electrocatalysts—an Alternative Approach to Achieving Stable and Tunable Electrocatalysis Daniel V. Esposito ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03374 • Publication Date (Web): 28 Nov 2017 Downloaded from http://pubs.acs.org on December 1, 2017

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Article type: Viewpoint

Membrane Coated Electrocatalysts—an Alternative Approach to Achieving Stable and Tunable Electrocatalysis

Daniel V. Esposito

Columbia University in the City of New York Department of Chemical Engineering Lenfest Center for Sustainable Energy 500 W. 120th St., New York, NY 10027

Contact Information email: [email protected] phone: 212-854-2648

Keywords: Electrocatalysis, transport, encapsulation, selectivity, membrane, concentration overpotential, diffusion, multifunctional 1 ACS Paragon Plus Environment

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I. Introduction Electrocatalysis underlies many sustainable energy technologies, such as fuel cells and electrolyzers, by facilitating the efficient interconversion between electrical and chemical energy. Most of today’s commercial electrocatalytic technologies involve relatively simple electrochemical reactions, such water electrolysis, for which reaction selectivity is not typically an issue. If electrocatalytic technologies are going to have more far-reaching impacts across the energy and chemical industries, electrocatalysts capable of achieving high selectivity in complex electrochemical reactions must be developed.1 Simultaneously, these electrocatalysts must have high activity, excellent durability, and sufficiently low material costs that will enable new electrochemical technologies to be successful in the marketplace. To date, significant progress has been made towards designing better electrocatalysts through an approach that is based on understanding the energetics that underpin the interactions between reactive species and the electrocatalyst surface.1–3 Density functional theory (DFT) has been an especially valuable tool for elucidating trends in catalytic activities that can be used to guide the design of active electrocatalysts.3–5 Typically, DFT is used to calculate the binding energy (BE) of reactive intermediates with different adsorption sites on the catalyst surface. Since surface bond energies correlate with the activation energies of elementary reaction steps, and the BEs of different reactants often correlate with each other, one or two BEs can often serve as “descriptor” parameters to predict catalytic activity.3 This descriptor-based approach to electrocatalyst design has proven highly effective for some reaction systems, but the view that catalyst selectivity and reactivity can be solely described by the BEs of reactive intermediates can break down for more complex catalyst architectures. For example, transport phenomena, which may be independent from a reactant species BE, can have very large effects on reaction 2 ACS Paragon Plus Environment

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selectivity for mesoporous or three-dimensional (3D) catalysts such as zeolites, metal organic frameworks (MOFs),6 and enzymes. Mesoporous zeolite catalysts are well-known for their ability to leverage pore sizes of molecular dimensions that enable selective diffusion, and therefore selective catalysis.7–9 In enzyme catalysis, transport-mediated “channeling effects” are important for shuttling reactants, intermediates, and products between disparate reactive sites.10,11 The application of transport-induced reaction selectivity has also been realized for electrocatalysts based on porous metal electrodes, showing impressive selectivity gains for CO2 and O2 reduction reactions based on differences in mass transport rates through the porous electrocatalyst.12,13 The idea of controlling transport phenomena to control reaction pathways is a promising approach to achieving advanced electrocatalytic functionality, but transport is rarely taken into account in the computational design of electrocatalytic materials. This Viewpoint article discusses an emerging class of electrocatalysts that can leverage transport phenomena to control catalytic pathways while providing unique active sites and inherent advantages for electrocatalyst stability. These electrocatalysts possess an architecture that is distinct from the aforementioned porous electrocatalysts in that the active electrocatalyst component is not porous or permeable. Rather, this article focuses on electrocatalysts for which an ultrathin, permeable overlayer fully or partially encapsulates the active catalyst. Several examples of overlayer-encapsulated electrocatalysts reported in literature are illustrated in Figure 1. This overlayer can impart significant stability benefits on the electrocatalyst by mitigating nanoparticle coalescence and dissolution, while offering control of reaction pathways through a variety of mechanisms. For any of these benefits to be realized, an essential property of the overlayer is that it be permeable to the electroactive species of interest. Permeability of the encapsulating layer often relies on the presence of microporosity (pore sizes < 2 nm14) and/or 3 ACS Paragon Plus Environment

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interstitial spaces having dimensions similar to those of the electroactive species of interest. Through physical and/or chemical interactions between a reactant molecule (i.e. the permeant) and the overlayer material, large differences in diffusivities for various species may be achieved. The encapsulating layers can thereby enable selective transport of reactants and products between the bulk electrolyte and buried active catalyst, for which reason this class of electrocatalysts can be collectively referred to as membrane coated electrocatalysts (MCECs).

Figure 1. Membrane coated electrocatalysts (MCECs). a.) Cross-sectional TEM image of a SiOx-encapsulated Pt nanoparticle electrocatalyst recently developed for water electrolysis.15 Also shown are oxide-encapsulated Rh and Pt nanoparticles encapsulated by b.) chromium oxide16 and c.) silica,17 respectively. d.) Schematic of model MCEC based on a thin film electrocatalyst with continuous overlayer having a uniform thickness, to. Inset of Figure 1d illustrates the electrochemical reduction of an oxidant species, O, to form a reductant species, R, at the buried interface between the overlayer and electrocatalyst. Figure 1a is reproduced from ref 15. Copyright 2016 American Chemical Society. Figure 1b is reproduced with permission from ref 16. Copyright 2006 Wiley-VCH Verlag GmbH & Co. Figure 1c is reproduced from ref 17. Copyright 2014 American Chemical Society.

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One example of a MCEC that was recently demonstrated by our lab15 is shown in Figure 1a, which contains a TEM image of a Pt nanoparticle encapsulated by a thin silicon oxide (SiOx) overlayer. The SiOx overlayer was made by a room temperature ultraviolet (UV)-Ozone process,18–20 which is known to produce permeable metal oxide (MO) layers suitable for gas separation membranes.21 Catalyst structures similar to that shown in Figure 1a have been commonly seen in the high temperature (thermal) catalysis field, where catalytic nanoparticles may become encapsulated by ultrathin metal oxide (MO) layers when exposed to high temperatures.22–25 This spontaneous encapsulation of metal catalysts by oxides is known as “strong metal support interaction” (SMSI), although similar structures have also been made intentionally by processes such as sol gel synthesis26 and molecular layer deposition.27,28 Regardless of origin, MO-encapsulation has been known to be effective in the thermal catalysis field for suppressing nanoparticle agglomeration 26,27 and promoting unique catalytic properties for hydrogenation reactions, isomerizations, ring-opening reactions, and more.29,30 In comparison to thermal catalysis, there are significantly fewer reports of overlayer-encapsulated catalysts being applied for low-temperature electrochemistry. Despite this disparity in research activity, the use of overlayer-encapsulation for low-temperature electrocatalysis may present new opportunities and advantages thanks to i.) an expanded library of catalytic and porous materials that can be used at low temperatures and ii.) the ability to use overlayers to alter reaction pathways through their interactions with charged species, which are ubiquitous in electrocatalysis. In both thermal- and electro-catalysis, research on MO-encapsulated catalysts has focused on supported nanoparticles due to their relevance to real-world applications where it is desirable to maximize the catalyst surface area for a given catalyst loading. However, it is very 5 ACS Paragon Plus Environment

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difficult to elucidate the fundamental kinetics and transport phenomena occurring in nanoparticle-based MCECs due to poorly defined and/or non-uniform structures. For example, overlayer coatings such as those seen in Figure 1 tend to be non-uniform, making it very difficult to deconvolve electrochemical current associated with species transport through the overlayer from that associated with reaction(s) taking place through pinholes and/or on uncoated portions of the nanoparticles. For this reason, fundamental studies on model planar MCECs, such as that shown in Figure 1d, can be of great value to quantifying key kinetic and transport parameters that underlie MCEC operation.31–35 Motivated by the goal of creating a framework for the rational design of MCECs, this article provides an overview of the key operating principles of MCECs while highlighting select demonstrations of these principles. Open questions relating to the fundamental chemistry and physics of MCECs are also discussed, and key barriers identified that must be overcome if MCECs are going to find broad application as a tunable architecture for a wide range of materials and applications. II. Mass Transfer Losses in MCECs Before discussing advantages of the MCEC design, it is important to first address a disadvantage that is inherent to many MCECs: mass transfer losses. Mass transfer losses are common in many electrochemical systems, but can be particularly important for MCECs when the rate of reaction at the buried interface approaches the maximum rate of diffusion of the reactant species through the permeable overlayer. Even for thin, nanoscale overlayers, diffusion can easily limit reaction rates. This is because the effective diffusion coefficients (De) of molecules within porous solid


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materials are often many of orders of magnitude lower than the diffusion coefficients (D) of the same species in a bulk liquid or gas.36 Being able to predict and quantify mass transfer losses in MCEC overlayers is of great importance because the resulting efficiency losses could offset the benefits of the MCEC design. Here, we consider transport of electroactive oxidant (O) and reductant (R) species through a continuous, pinhole-free overlayer that encapsulates the active catalyst material in a 1D geometry such as that shown in Figure 1d. In general, mass transfer of an electroactive species, j, is described by the Nernst-Planck Equation, shown here for 1D transport:37

(1)

where Jj is the flux of species j, De,j is its effective diffusion coefficient within the overlayer, Cj is concentration of j within the overlayer, x is the distance from the overlayer/catalyst interface, zj is the species charge, F is the Faraday constant, Rg is the gas constant, T is temperature, v(x) is bulk fluid velocity, and

(x) is the potential. In order from left to right, the three terms in

Equation (1) correspond to transport by diffusion, migration, and convection. Since v=0 inside a pinhole-free overlayer, the convection term can be ignored. If it is further assumed that the MCEC is operated at small overpotentials and/or in a sufficiently large concentration of supporting electrolyte, it is reasonable to neglect electromigration effects such that transport occurs predominately by diffusion. In this case, integration of the steady state continuity equation leads to a linear concentration profile of the reactant species within the overlayer. Figure 2a illustrates hypothetical steady state concentration profiles for electroactive O and R species diffusing through a permeable overlayer during electrochemical reduction of O. The current density, i, may then be described by a Fick’s law expression for the diffusive flux of the 7 ACS Paragon Plus Environment

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reactant species, O, where the concentration gradient is given by the finite difference between the concentration of O at x=0 (the catalyst surface) and x=to (the overlayer/electrolyte interface):

(2)

where n is the electron transfer number, F is the Faraday constant, and Equation (2) follows the convention that reduction current is negative. If the MCEC is operated under mass transfer limiting conditions such that CO(x=0) ≈0 M, Equation (2) simplifies to give an expression for the mass transfer limited current density associated with the reduction of O at the cathode, il,c: (3) where SO is the partition coefficient for O and CO,b is the concentration of O in the bulk electrolyte. The partition coefficient of species j, Sj, is defined here as the equilibrium ratio of the concentration of a species within the surface of the overlayer to its concentration in the bulk electrolyte. The product of Sj and De,j is commonly referred to as the permeability coefficient, Pj. Equation (3) is very similar to that used to describe the transport of electroactive species across diffusion boundary layers in the bulk electrolyte,38 and an analogous expression can be written to describe the mass transfer limiting current density for the oxidation of R, il,a. Figure 2b shows the inverse relationship between il,a and to and highlights the strong influence of the permeability coefficient on il for a given overlayer thickness.


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Figure 2. Mass transfer losses in MCEC overlayers. a.) Schematic side-view of steady state concentration profiles of oxidant (O) and reductant (R) species within the permeable overlayer of an MCEC for the case where all mass transfer occurs by diffusion. b.) Modeled mass transfer limiting current densities (il) as a function of overlayer thickness (to) for several values for the species permeability coefficients (Pi) and a reactant concentration of Cj,b=0.5 M. c.) Modeled concentration overpotential (ηconc) losses as a function of overlayer thickness (to) for a constant current density (i = 3 mA cm-2) and four different values of Pj. (d.) Modeled ηconc versus to curves for a constant Pj=1x10-10 cm2 s-1 and five different values of i. For c.) and d.), it was assumed that PO=PR , SO=SR, CO,b=0.5 M, and CR,b= 0.1 M. All calculations were performed for n=1, T=298 K, and assuming that mass transfer only occurs through diffusion.

Even if the desired operating current density of the MCEC doesn’t exceed il, MCECs can still incur significant concentration overpotential losses (ηconc) due to depletion of the reactant concentration at the buried interface compared to that in the bulk electrolyte. An expression for 9 ACS Paragon Plus Environment

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ηconc can be derived from the Nernst Equation, into which expressions for the concentrations of O and R at the buried interface from Equation (2) may be inserted. As derived in the Supporting Information, the final expression for ηconc for a generic redox reaction O+ n·e- ⬌R is given by:

.

(4)

Within Equation (4), the dimensionless ratio (i/il) resembles the Thiele modulus, a parameter defined as the ratio of the reaction rate to the (maximum) diffusion rate.39 It

should

also

be

noted that i in Equation (4) is an average local current density obtained by normalizing the current by the electrochemically active surface area (ECSA) of the electrocatalyst, not the geometric area of the electrode. For an electrocatalyst loading of 1 mg per cm2 of electrode, and an ECSA of 50 m2 g-1 of catalyst, the local current density at the catalyst surface is ≈ 3 mA per cm2 of ECSA for a device operating at 1.5 A per cm2 of geometric electrode area. Equation (4) can be used to establish general guidelines for choosing overlayer thicknesses that won’t lead to prohibitively large ηconc for given operating parameters. Figure 2c shows how ηconc varies with to for several values of Pj and with n=1, CO,b= 0.5 M, CR,b= 0.1 M, PO=PR, SO=SR, and i = -3 mA cm-2 held as constants. For these conditions, it is seen that ηconc is negligible for to < 20 nm so long as Pj > 5x10-10 cm2 s-1, but that to < 5 nm should be utilized for cases where Pj < 5x10-11 cm2 s-1. Alternately, Equation (4) can be used to predict the effect of operating current density on ηconc for a constant Pj, as has been done in Figure 2d. If the design objective is to design an MCEC such that ηconc remains less than 100 mV, this figure shows that to should be kept below ≈14 nm, ≈ 8 nm, or ≈4 nm, for operating current densities of 3, 5, or 10 mA cm-2, respectively.


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The preceding analyses is very useful for probing relationships between mass transfer losses, MCEC properties, and operating conditions, but significant variations in parameter space are likely to be encountered across different types of materials and chemistries. Additionally, an accurate description of ηconc in many real-world MCECs will likely require more complicated models. Particular attention should be given to migration effects, whereby ionic species move in response to electric fields that can develop within the permeable overlayer.40 Migration can be especially important in systems where at least one of the electroactive species of interest is an ion that is responsible for carrying a significant fraction of ionic current. Operation in a supporting electrolyte can greatly reduce the contribution of migration to overall species flux, but this may not be true for MCECs if the supporting electrolyte ions have low De,j or Sj within the MCEC overlayer. Other possible complications include overlayers with depth-dependent De,j or Sj,31 interactions between ions and/or the overlayer that require the use of concentrated solution theory,40 overlayers containing pinholes or cracks,33,34 and mass transfer losses in the bulk electrolyte.31 It is expected that a combination of molecular modeling tools, electroanalytical measurements, and detailed physical characterization of overlayers will be essential for developing more accurate transport models for MCECs. With inspiration from studies of semipermeable polymeric34 and silicon oxide layers,35 scanning electrochemical microscopy (SECM) is one electroanalytical tool that is expected to be especially valuable in its ability to quantify permeabilities through ultrathin overlayers and image spatial heterogeneities in overlayer properties due to cracks or pinholes. Even if the physics of mass transfer processes are thoroughly understood, finding an optimum overlayer thickness may become complicated by trade-offs between MCEC performance and stability. Although decreasing overlayer thickness is beneficial for minimizing 11 ACS Paragon Plus Environment

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concentration overpotential losses, it may be desirable to increase overlayer thickness for other functions. For example, it can be expected that thicker overlayers will be more effective at mitigating electrocatalyst degradation, especially if the overlayer itself is expected to undergo some amount of dissolution over time. Thus, many MCECs are expected to exhibit an optimum overlayer thickness that maintains stability benefits without incurring significant concentration overpotentials at the target operating current density. III. Stability Benefits of the MCEC Architecture To date, one of the most frequently reported benefits of encapsulating electrocatalysts with thin overlayers is the ability to improve electrocatalyst stability.15,17,33,41–43 Understanding how overlayers improve electrocatalyst stability is complicated by the fact that degradation mechanisms are often strongly dependent on operating conditions (temperature, applied potential, current density), choice of electrolyte, type of support material, and the presence of impurities.44–46 Four of the most commonly considered electrocatalyst degradation mechanisms are shown in Figure 3: a.) dissolution or oxidation of the catalyst, b.) coalescence of catalyst nanoparticles, c.) physical detachment of the catalyst from the support, and d.) poisoning effects whereby an impurity species deposits onto the surface of the catalyst and decreases its catalytic activity. As illustrated in Figure 3e, the MCEC architecture has potential to prevent all four of these degradation mechanisms. By encapsulating both the nanoparticle and substrate/support, the overlayer can serve as a “glue” that helps to anchor nanoparticles to the support surface, thereby mitigating detachment and coalescence. In its role as membrane, the overlayer can permit transport of the desired redox species while selectively blocking the influx of contaminants and suppressing dissolution of the underlying catalyst.


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Figure 3. Stability benefits of the MCEC architecture. Depicted are common mechanisms of nanoparticle electrocatalyst degradation, including a.) electrochemical dissolution due to oxidation, b.) coalescence or agglomeration of smaller particles into larger ones, c.) physical detachment from the support due to poor adhesion, and d.) poisoning whereby an impurity or poison species (P) adsorbs onto the active electrocatalyst and significantly reduces its activity towards the desired reaction (O + e-→R). e.) Side-view of a nanoparticle MCEC that is resistant to all four modes of degradation illustrated in a.)-d.).

The ability of the MCEC structure to suppress nanoparticle coalescence was demonstrated by Takenaka et al. when they coated Pt oxygen reduction reaction (ORR) electrocatalysts with ultrathin silica layers.17,43,47 These studies showed that the silica coating was very effective at reducing Pt or Pd particle agglomeration while significantly enhancing the stability of electrode performance during long term stability tests. A more recent demonstration of this benefit was reported by our group, which found that ultrathin layers of silicon oxide (SiOx) were highly effective at stabilizing Pt nanoparticle co-catalysts on the surface of p-Si photocathodes during photo-driven hydrogen evolution.15 In that study, a room temperature UVOzone synthesis process was used to deposit 2-10 nm thick SiOx overlayers on top of the electrodeposited Pt nanoparticles (Figure 1a). The stability was investigated using 12 hour chronopotentiometry measurements, revealing that the HER performance of a control sample lacking the SiOx overlayer degraded substantially over the 12 hours while the SiOx-coated sample containing the identical Pt loading maintained a constant overpotential. In post mortem 13 ACS Paragon Plus Environment

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analysis of the sample lacking the SiOx overlayer, SEM images revealed a highly heterogeneous surface, a significant decline in the number density of small nanoparticles ( < 40 nm) and concomitant increase in large nanoparticles ( > 50 nm). By contrast, SEM images of the SiOx coated sample revealed a highly homogeneous dispersion of catalyst particles and high number density of small nanoparticles. These results suggest that the MCEC design could be a promising approach to stabilize ultra-small ( < 2 nm) nanoparticle electrocatalysts. Such small particles are known to exhibit unique catalytic properties48,49 and are attractive for minimizing catalyst loadings, but their small size also makes them more prone to agglomeration and dissolution.45,50,51 Takenaka et al. have also shown that ultrathin silica coatings can help to reduce the rate of dissolution of ORR electrocatalysts such as Pd.43,47 These results are particularly exciting because they suggest that the MCEC overlayer might be able to protect active electrocatalyst materials that are not normally stable for a given application. Similarly, the overlayer may simultaneously protect underlying support materials, such as carbon, that may be susceptible to oxidation or dissolution. In preventing or mitigating dissolution of the underlying active catalyst and/or support, the thin overlayers can be thought of as non-native passive oxides, mimicking the anti-corrosion properties of naturally occurring native oxides. However, there are relatively few demonstrations of this benefit to date, and a molecular-level understanding of how SiO2 overlayers prevent dissolution is lacking. From a practical standpoint, it will also be important to evaluate the long-term durability of ultrathin non-native overlayers. Unlike native oxides that passivate their parent metals, non-native overlayers will not be automatically replenished by the underlying material if they are gradually lost to the solution over time.


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Even if a catalytic nanoparticle is inherently resistant to dissolution and agglomeration, it is very rarely resistant to degraded performance that can result from the presence of a poison or impurity in the electrolyte. Such poisoning effects are well-known to occur when a metal cation impurity such as Cu2+ or Fe3+ deposits onto the surface of the electrocatalyst,52–54 but anion impurities such as sulfur and organic compounds have also been known to have deleterious effects on catalytic activities.51,55–57 Water that is fed into electrolysis applications must therefore be thoroughly purified, which can add substantial operating and capital costs to the system.58 As recently demonstrated by our lab for SiOx-encapsulated Pt thin films employed for the hydrogen evolution reaction (HER),33 the MCEC design can mitigate poisoning by selectively enabling transport of the desired species (H+) while blocking impurities such as Cu2+ from reaching the catalytically active buried interface. While bare Pt control samples were found to require ≈0.5 V additional overpotential in the presence of Cu2+ due to the poisoning effect, SiOx-encapsulated MCEC electrodes were found to be nearly unaffected. Besides decreasing water purification requirements, such impurity tolerance could enable electrochemical devices to be constructed from lower cost materials that might normally leach impurities into the electrolyte. IV. Controlling Reaction Kinetics with the MCEC Architecture There are many possible mechanisms by which a permeable overlayer can influence reaction pathways. Several of these mechanisms are illustrated in Figure 4 for a MO-encapsulated planar metal (M) film. In the first two panels (Fig. 4a, 4b), the permeable MO overlayer serves as a nanoscale membrane that selectively controls the transport of electroactive species between the bulk fluid and the buried MO/M interface. The relative rates of transport of electroactive species can be dependent on their size (Fig. 4a), shape, and/or charge (Fig. 4b) relative to the charge, pore shapes, and/or pore sizes within the permeable overlayer. Size- and shape-selective 15 ACS Paragon Plus Environment

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transport in mesoporous systems are well-known concepts that underlie zeolite-7–9 and MOF-6 based catalysis and membranes.59,60 In electrocatalysis, the charge of electroactive species can also affect transport rates due to electrostatic interactions with their surroundings and/or migration in the presence of electric fields. Although not illustrated in Figure 4, differences in chemical interactions between reactants and MCEC overlayers can also impart selective transport by increasing the partition coefficient (Sj) of one species relative to the other.

Figure 4. Mechanisms by which encapsulation of an active metallic (M) electrocatalyst with an overlayer can affect reaction pathways and catalytic activity, including a.) size-selective electrocatalysis, b.) charge-selective electrocatalysis, c.) overlayer-assisted electrocatalysis, and d.) confinement effects. In a.) the spheres around a desired oxidant species, O1, and a competing oxidant species, Oc, represent the hydration spheres of those molecules. In d.), the orange atom belongs to a reactant species located at the buried interface between the overlayer and the active catalyst material (M), and the yellow area represents the void space occupied by that reactant.

Besides the previously cited study,33 two related examples of selective transport in MCECs have been reported for Cr2O3 or MoOx overlayers used to encapsulate Rh16 and Pt32 16 ACS Paragon Plus Environment

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HER electrocatalysts, respectively. In both studies, the metal nanoparticles were serving as cocatalysts deposited onto semiconductor photocatalysts for overall water splitting, and it was shown that their encapsulation by ultrathin MO overlayers was highly effective at suppressing the undesirable water formation back reaction. In the latter study by Garcia-Esparza et al.,32 rotating disc electrode (RDE) studies of planar MoOx-encapsulated Pt electrodes showed that the MoOx nanomembranes exhibited poor permeability for both H2 and O2. It was further suggested that the hydrated MoOx overlayers promoted selective transport of H2O/H+ reactants thanks to favorable chemical interactions between the MoOx and H+ (i.e. large SH+), while suppressing transport of less soluble O2 species. MO-overlayers can also affect reaction pathways through their direct participation in one or more elementary reaction steps (Fig. 4c,d). In thermal catalysis, it is well-known that some oxide materials can themselves be catalytic for certain reactions, with one example being solid acid-catalysts (Fig. 4d)61,62 and another being Mars-van Krevelen-type mechanisms where oxygen atoms in the MO lattice partake in reactions.63 Hydroxyl groups on or within MO materials may also affect reaction pathways (Fig 4c), with one of the most commonly cited examples being the so-called bifunctional effect in alcohol oxidation electrocatalysis whereby hydroxyl-containing MO support materials such as TiO2 or Nb2O5 help to facilitate the removal of CO-intermediates from the surface of the metallic catalyst such as Pt.64–66 The triple phase “boundary sites” at the edge of the MO and M components have also been known to exhibit unique catalytic properties in thermal catalysis,29,67,68 and recent theory papers have suggested that interface/boundary sites between two different classes of materials could be a promising approach to breaking scaling relations.69,70 Because many MO materials are semiconductors capable of creating electron/hole pairs under illumination,71,72 their use as permeable overlayers 17 ACS Paragon Plus Environment

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may also create interesting possibilities for photo-active MCECs capable of coupled electrocatalysis- photocatalysis reaction mechanisms. Finally, permeable overlayers may alter catalytic activity and selectivity by modifying energy barriers for reactions occurring at the buried overlayer/metal interface. A hypothetical reactant species located at the buried interface of an MCEC is shown in Figure 4d. The catalytic properties of active sites in buried interfaces are rarely discussed in literature, although these sites share some similarities to so-called “confined catalysts” in which reactions take place inside nanoscopic voids within 2D layered materials,73 zeolites,74 or nanotubes.74 Within these confined environments, catalyst and/or support atoms may interact with reactive intermediates in 3dimensional (3D) space in ways that are not possible with conventional 2D catalysts, requiring additional degree(s) of freedom to describe activity trends. By this means, 3D catalytic environments provide a promising approach to break energetic scaling relations between reactive intermediates that limit the maximum activity and allowable reaction pathways of conventional catalysts.73 At the buried interface of an MCEC, MO atoms may alter reaction energetics and pathways by modifying the chemical, physical, and electronic environments of reactant molecules compared to those encountered at the surface of a bare metal exposed to the bulk electrolyte. Specific mechanisms may include, but are not limited to the following: i.) steric confinement that restricts molecular orientations, ii.) multidentate ligand effects whereby a reactant molecule is simultaneously coordinated by MO and M atoms, and iii.) by MO-induced disruption of the electrochemical double layer (ECDL), which is known to play an important role in electrocatalysis.75–77 Indeed, the Debye length associated with the ECDL may be greatly altered in MCECs due to large differences in species concentrations and permittivity within permeable overlayers compared to a bulk aqueous electrolyte. These changes to the Debye length 18 ACS Paragon Plus Environment

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may have important implications for the reaction energetics at the buried interface, as well as the transport of electroactive ions. All of these influences may alter the BE of a reactant molecule and thereby influence the energy barriers of competing reaction pathways. However, efforts to quantitatively understand how reaction energetics are altered at buried interfaces and deconvolute these affects from the other mechanisms shown in Figure 4 are in their nascent stages. V. Towards the Rational Design of MCECs The large number of mechanisms by which MCECs may alter reaction pathways suggests that there are many degrees of freedom involved with the design of these composite electrocatalysts. Figure 5 shows a side-view schematic of a model MCEC, with key structural parameters and properties listed around the periphery. By varying the choice of material(s), the synthesis method(s), and synthesis conditions, many of these parameters can be modified in ways that alter local reactant concentrations and/or reaction energetics as described in the previous section. Furthermore, the properties and characteristics of MCECs may also be influenced by the operating conditions of the electrochemical reactor. Thus, there is a very large number of “control knobs” that may be adjusted during the synthesis and/or operation of MCECs to tune the properties of these multifunctional electrocatalysts.


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Figure 5. Towards rational design of MCECs. Schematic side-view of a model MCEC surface and lists of key electrocatalyst properties, characteristics, operating conditions, and performance metrics. This schematic illustrates a “forward design” approach that proceeds in a clockwise progression from materials synthesis up to evaluation of the electrocatalyst performance. Developing a fundamental understanding of the relationships between parameters included in this schematic will be key to enabling an “inverse design” approach for rapid development of MCECs for a wide variety of electrochemical reactions.

The potential tunability of MCECs makes them especially attractive for complex electrochemical reactions that are characterized by multiple important elementary reaction steps. However, the large number of control knobs makes the task of rationally designing MCECs a daunting one. To date, existing demonstrations of MCECs have been largely based on empirical approaches to catalyst design in which the electrocatalyst is synthesized, its properties and performance analyzed, and the process repeated based on an improved understanding of the processing-structure-property-performance relationships. This “forward design” approach, represented by the clockwise progression of tasks in Figure 5, can be very inefficient for such a large design space. In order to accelerate the MCEC design process, it will be essential that a combined experimental and theory approach be employed that takes a holistic view of 20 ACS Paragon Plus Environment

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electrocatalyst design, placing equal emphasis on understanding reaction energetics, transport behavior, and degradation phenomena. Experimentally, the use of advanced characterization tools capable of measuring the atomic-structure—pore sizes, porosity, water content, internal surface chemistry, etc.—of MCEC overlayers is expected to be of great value for elucidating structure-property relations. Such measurements can be challenging due to the small amount of material involved, difficulties in “seeing” sub- 2 nm pores, and complexities associated with characterizing the nanostructure of amorphous overlayers. Likewise, spectroscopic and electroanalytical measurements for characterizing reactive intermediates at the buried interface will not be trivial, but are expected to provide deep insights about reaction energetics and mechanisms at these unique active sites. For example, in situ vibrational spectroscopies such as sum frequency generation (SFG),78 infrared (IR) spectroscopies,79 and surface enhanced Raman spectroscopy (SERS)80–82 have proven highly valuable in their ability to probe the identity, orientation, and energetics of adsorbates in electrochemical systems.83 SERS81,84 and SFG85,86 are especially attractive for studying MCECs thanks to their inherently large sensitivity to interfaces and ability to probe the structure of ECDLs. X-ray based scattering and spectroscopy techniques can also be expected to provide complementary insights about the molecular nature of buried interfaces87 and ECDLs77 in MCECs. Coupled with electroanalytical techniques, in situ optical and X-ray spectroscopies have the potential to greatly enhance our understanding of electrocatalysis at the overlayer/catalyst buried interface. More detailed characterization of MCEC structure and properties is also desirable to provide input parameters to first principles and continuum models. Similarly, measurements of well-defined model MCEC electrodes such as planar electrodes31–35 are highly attractive for 21 ACS Paragon Plus Environment

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validating these models. It can be anticipated that multi-physics, multi-scale modeling will be extremely important for elucidating reaction mechanisms and describing the structure-propertyperformance relationships that underlie the reaction energetics and transport properties of MCECs. Once established, these relationships can serve as the basis for i.) determining the performance limitations of MCECS and ii.) developing design rules used to guide the selection and assembly of various overlayer and catalyst materials for a given electrochemical reaction pathway. Such a unified modeling framework would be extremely powerful because it would enable a more predictive “inverse design” approach88 that would reverse the direction of the block arrows in Figure 5. Under such an approach, macroscopic performance metrics (e.g. desired product species, activity targets, stability requirements) are specified for an electrochemical reaction of interest and fed into simulation tools that would “discover” the combinations of structures, compositions, and operating conditions that could enable those metrics to be achieved. With optimal structures and compositions identified, the focus would then shift to identifying the best precursors and synthesis strategies to achieve them. Significant experimental and computational challenges remain, but if this vision for the rational design of MCECs can be achieved, a tunable electrocatalyt architecture will have been developed that has great potential to achieve high stability, activity, and selectivity for a wide range of electrocatalytic reactions. Associated Content Supporting Information

The Supporting Information is available free of charge on the ACS Publications website and includes the following content: Derivation of the Concentration Overpotential for a 1D Permeable Overlayer


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Acknowledgements The author acknowledges Columbia University (start-up funds) and the National Science Foundation (NSF) Center for Precision Assembly of Superstratic and Superatomic Solids for funding (DMR-1420634). He also acknowledges Natalie Labrador and Alan West for helpful discussions and suggestions that influenced the content of this article.

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ACS Catalysis

Figure 1. Membrane coated electrocatalysts (MCECs). a.) Cross-sectional TEM image of a SiOx-encapsulated Pt nanoparticle electrocatalyst recently developed for water electrolysis.15 Also shown are oxideencapsulated Rh and Pt nanoparticles encapsulated by b.) chromium oxide16 and c.) silica,17 respectively. d.) Schematic of model MCEC based on a thin film electrocatalyst with continuous overlayer having a uniform thickness, to. Inset of Figure 1d illustrates the electrochemical reduction of an oxidant species, O, to form a reductant species, R, at the buried interface between the overlayer and electrocatalyst. Figure 1a is reproduced from ref 15. Copyright 2016 American Chemical Society. Figure 1b is reproduced with permission from ref 16. Copyright 2006 Wiley-VCH Verlag GmbH & Co. Figure 1c is reproduced from ref 17. Copyright 2014 American Chemical Society. 107x64mm (300 x 300 DPI)

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Figure 2. Mass transfer losses in MCEC overlayers. a.) Schematic side-view of steady state concentration profiles of oxidant (O) and reductant (R) species within the permeable overlayer of an MCEC for the case where all mass transfer occurs by diffusion. b.) Modeled mass transfer limiting current densities (il) as a function of overlayer thickness (to) for several values for the species permeability coefficients (Pi) and a reactant concentration of Cj,b=0.5 M. c.) Modeled concentration overpotential (ηconc) losses as a function of overlayer thickness (to) for a constant current density (i = 3 mA cm-2) and four different values of Pj. (d.) Modeled ηconc versus to curves for a constant Pj=1x10-10 cm2 s-1 and five different values of i. For c.) and d.), it was assumed that PO=PR , SO=SR, CO,b=0.5 M, and CR,b= 0.1 M. All calculations were performed for n=1, T=298 K, and assuming that mass transfer only occurs through diffusion. 83x82mm (300 x 300 DPI)


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ACS Catalysis

Figure 3. Stability benefits of the MCEC architecture. Depicted are common mechanisms of nanoparticle electrocatalyst degradation, including a.) electrochemical dissolution due to oxidation, b.) coalescence or agglomeration of smaller particles into larger ones, c.) physical detachment from the support due to poor adhesion, and d.) poisoning whereby an impurity or poison species (P) adsorbs onto the active electrocatalyst and significantly reduces its activity towards the desired reaction (O + e-→R). e.) Side-view of a nanoparticle MCEC that is resistant to all four modes of degradation illustrated in a.)-d.). 66x25mm (300 x 300 DPI)


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Figure 4. Mechanisms by which encapsulation of an active metallic (M) electrocatalyst with an overlayer can affect reaction pathways and catalytic activity, including a.) size-selective electrocatalysis, b.) chargeselective electrocatalysis, c.) overlayer-assisted electrocatalysis, and d.) confinement effects. In a.) the spheres around a desired oxidant species, O1, and a competing oxidant species, Oc, represent the hydration spheres of those molecules. In d.), the orange atom belongs to a reactant species located at the buried interface between the overlayer and the active catalyst material (M), and the yellow area represents the void space occupied by that reactant. 71x60mm (300 x 300 DPI)


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ACS Catalysis

Figure 5. Towards rational design of MCECs. Schematic side-view of a model MCEC surface and lists of key electrocatalyst properties, characteristics, operating conditions, and performance metrics. This schematic illustrates a “forward design” approach that proceeds in a clockwise progression from materials synthesis up to evaluation of the electrocatalyst performance. Developing a fundamental understanding of the relationships between parameters included in this schematic will be key to enabling an “inverse design” approach for rapid development of MCECs for a wide variety of electrochemical reactions. 85x41mm (300 x 300 DPI)


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