Nanoporous Metals as Electrocatalysts: State-of-the-Art, Opportunities

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Nanoporous Metals as Electrocatalysts: Stateof-the-Art, Opportunities, and Challenges Wesley W Luc, and Feng Jiao ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b01803 • Publication Date (Web): 20 Jul 2017 Downloaded from http://pubs.acs.org on July 21, 2017

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Nanoporous Metals as Electrocatalysts: State-of-the-Art, Opportunities, and Challenges Wesley Luc and Feng Jiao* Center of Catalytic Science and Technology, Department and Biomolecular Engineering, University of Delaware, Newark, DE 19716 USA *Corresponding author: [email protected] Abstract Nanoporous metals with their distinct three-dimensional interconnected porous networks are promising materials as electrocatalysts for fundamental studies and practical applications because they have highly conductive self-supporting porous structures with large electrochemical surface areas. This perspective provides an overview of the recent developments and state-ofthe-art nanoporous metals as electrocatalysts for various important electrochemical systems. Potential strategies and opportunities of utilizing the unique characteristics of nanoporous metals to overcome typical problems faced in electrocatalysis are presented. Lastly, challenges regarding the synthesis of nanoporous metals with controlled porous structure and targeted surface catalytic sites are also discussed to stimulate new ideas and interests for nanoporous metallic electrocatalysts. Keywords electrocatalysts; nanoporous; self-supported; nanomaterials; bimetallic

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Introduction Electrocatalysts are central in designing efficient electrochemical systems because they help lower the activation barriers and facilitate the transformation of one chemical form to another. Many efforts have been made towards improving the performances of electrocatalysts such as increasing surface area, improving the intrinsic activity of the active sites, and manipulating the transport of reactants and products to and from the electrolyte/electrode interface.1-2 Nanoporous metals with their three-dimensional interconnected porous networks consisting of interconnected backbones (ligaments) and pores (channels/voids) on the nanoscale as shown in Figure 1, are promising materials as electrocatalysts because they provide an ideal catalytic structure for fundamental studies and practical applications.3-5

Figure 1. (a) SEM image of nanoporous-Ag with (b) cross-sectional SEM imaging. (c) SEM images of bimetallic nanoporous-CuCo and d) partially-oxidized nanoporous-Cu with hierarchical porous structures. Nanoporous metals possess several distinct features that make them highly desirable as electrocatalysts. First, the metallic structure is interconnected resulting in a highly conductive network that allows easy transport of electrons. Second, the highly internal curve structure may expose various different crystal facets on the ligament surfaces which may result in favorable reaction sites depending on reaction conditions and surface composition. Third, the synthesis procedure can be applied to not only monometallic catalysts, but also bimetallic catalysts with highly porous structures and large surface areas, thus providing flexibility in creating and engineering target catalytic sites. Fourth, the manifolds and confined spaces in the nanoporous structure offer an interesting strategy to control diffusive fluxes of reactants and/or products, thus enhancing selectivity towards desired products while suppressing unwanted ones. Lastly, from a fundamental perspective, the monolithic structure of nanoporous metals offer a unique 2 ACS Paragon Plus Environment

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opportunity to decouple support effects and allow for the study of the true origin of catalytic activity. This perspective highlights the most recent developments and current state-of-the-art nanoporous metals as electrocatalysts. In addition, the perspective will discuss potential strategies of utilizing nanoporous metals to overcome typical challenges faced in electrocatalysis such as: 1) support effects that convolute and prevent the understanding of the true intrinsic activity of nanostructured electrocatalysts, 2) scaling relationships that limit the development of more efficient catalysts for multi-electron multi-step electrochemical reactions, and 3) problematic side reactions such as hydrogen evolution that often competes with desired reactions in aqueous environment. Lastly, the perspective will highlight the grand challenges faced in the field of nanoporous metals. In such, we hope to convey the on-going developments as well as to stimulate new ideas and interests. Current State-of-the-Art Self-supported Electrocatalysts In general, catalysts in electrochemical systems are often consisted of nanoparticles deposited on conductive supports to maximize catalyst dispersion. However, under prolonged working conditions, the electrocatalysts often suffer from support degradation and nanoparticle aggregation, consequentially leading to decrease in system performance.6 Distinct from nanoparticulate catalysts, recent developments in nanoporous monolithic metal catalysts showed enhanced electrocatalytic properties. For example, Zhang et al. first studied the prototypic nanoporous-Au catalyst for the electro-oxidation of methanol for direct methanol fuel cell applications as a potential alternative to the commercial Pt/C catalyst that typically suffers from CO poisoning and support degradation under oxidative conditions.7 The nanoporous-Au catalyst showed enhanced activity which was evident by a more negative onset potential, and this enhancement was attributed to the exposure of low-coordinated step, edge, and kink surface sites that have been previously shown to be favorable catalytic sites in gas phase heterogeneous catalysis.8-11 In another work, Lu et al. developed a high-performing monolithic nanoporous-Ag electrocatalyst for CO2 reduction to CO, achieving high activity of 20 mA cm-2 and over 90% selectivity at moderate overpotentials. In comparison to other nanostructured Ag catalysts such as nanoparticles and nanowires, the nanoporous-Ag catalyst exhibited better performances in both overall and per surface site activity. The enhanced performance was attributed to the exposure of highly active internal surfaces of the nanopores, large electrochemical surface area, and the monolithic self-supported structure that allows for fast electron transport.12-13 Nanoporous Bimetallics Bimetallic materials offer unprecedented opportunities because the properties of a bimetallic catalyst can be tuned to either be between those of two individual metals as a mixed alloy or to enhance the surface properties of one metal through a core-shell structure. The unique and emergent properties of the latter arise from the hybridization of the atomic orbitals that shift the d-band center with the respect to the Fermi level in a non-intuitive way.14-15 As a result, bimetallics can access catalytic properties that cannot be mimicked by monometallics. More 3 ACS Paragon Plus Environment

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importantly, it may be possible to design a nanoporous bimetallic catalyst to take advantage of both bimetallic active sites and nanoporous structure. For example, Lu et al. recently constructed a high-performing nanoporous-CuTi bimetallic catalyst for hydrogen evolution reaction in base. Interestingly, the catalytic performance of the nanoporous-CuTi bimetallic catalyst exceeded that of the state-of-the-art Pt/C with more than two-fold enhancement.16 The combination of the Cu and Ti created highly active Cu-Cu-Ti hollow sites possessing similar hydrogen binding energy close to Pt whereas the unique hierarchical porous structure allowed for high surface area as well as gas diffusion channels to enhance mass transport properties. In another work, Chen et al. demonstrated that a nanoporous-PdAu bimetallic catalyst showed enhanced catalytic performance for the electrooxidation of ethanol for fuel cell applications.17 Besides excellent conductivity and transport of molecules due to the interconnected porous structure, the synergistic interactions between Pd and Au atoms led to a surface-specific activity as well as the mass activity that far exceeded those of nanoporous-Pd, nanoporous-Au, and commercial Pd/C catalysts. The interplay between Pd and Au sites where Pd served as the primary sites for alcohol dehydrogenation while Au sites facilitated intermediate species and prevented surface poisoning, enhanced catalytic performance as well as the overall stability. Synder et al. also utilized the advantages of both bimetallic sites and nanoporous structure to engineer a nanoporous-PtNi core-shell catalyst that exceeded the performance of the state-of-the-art Pt/C catalyst for the oxygen reduction reaction.18 The surface strain and alloying effects of Ni in the under layer induced a downward shift of the d-band center of the outermost surface Pt atoms, thus weakening the interactions of oxygenated intermediate species and enhancing oxygen reduction activity. In addition, the nanoporous-PtNi catalyst was impregnated with ionic liquid to enhance O2 solubility, thus further improving catalytic performance. Furthermore, the self-supported monolithic nature of the nanoporous-PtNi core-shell catalyst circumvented issues associated with nanoparticle aggregation and support detachment, leading to greater stability then commercial Pt/C catalyst as shown in long term cycling tests conducted by Wang et al.19 Geometric Effects of Nanoporosity Another important feature of nanoporous metallic electrocatalysts is their unique transport properties due to their three-dimensional interconnected porous structure, which is distinct from other nanostructured catalysts. In a recent work, Sen et al. showed that a Cu nanofoam catalyst improved formate production and suppressed CO, CH4, and C2H4 in comparison with polycrystalline Cu for the reduction of CO2.19-20 Interestingly, the nanofoam catalyst showed detectable quantities of C2 and C3 products such as ethane and propylene. The authors suggested that the interconnected porous structure helped facilitate reactions between adsorbed CO2 and CO2 reduction intermediates to form higher-order hydrocarbons by increasing the residence time of the intermediates through the catalytic structure. In another work, Hall et al. demonstrated that the diffusional manifolds of an ordered nanoporous-Au catalyst can be used to induce a diffusional gradient by manipulating fluxes of reactants/products into and out of its porous networks.21 By tuning the thickness of the nanoporous structure, the undesirable hydrogen evolution was suppressed without altering CO2 reduction activity. In a similar context, Benn et al. studied a nanoporous-NiPt as an oxygen reduction reaction electrocatalyst under proton diffusion-limited conditions in buffering solution.22 Interestingly, at low potentials of -0.4 V vs RHE, the nanoporous-NiPt catalyst suppressed hydrogen evolution while still sustaining 4 ACS Paragon Plus Environment

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oxygen reduction even on a highly active hydrogen evolution reaction catalyst at potentials where hydrogen evolution would typically dominate. In addition, a shift in reaction mechanism from a proton-limited (O2 + 4H+ + 4e- → H2O) to a proton-free (O2 + 2H2O + 4e- → OH-) oxygen reduction reaction was observed at high overpotentials in unbuffering solution. This suggested that the surfaces within the nanopores of the catalyst were depleted of protons and that the flux of oxygen from the bulk solution nominated the flux of protons. Strategies of Utilizing Nanoporous Metals Elucidating the Origin of Catalytic Activity Majority of nanostructured materials such as nanoparticles are usually synthesized using organic surfactants and eventually deposited on supports. Thus, when studying the intrinsic catalytic activity of these nanostructured materials, the question whether the enhanced catalytic activity of the material originates from its nanostructure, surfactant, support, or the combination of either arises as shown in Figure 2a. In these cases, the origin of catalytic activity may be convoluted and difficult to determine, preventing the fundamental understanding needed to design more efficient catalysts.

Figure 2: a) Activity of nanoparticles are often convoluted with surfactant and/or support effects. b) Free-standing nanoporous metals eliminate surfactant and support effects which provide an ideal platform for studying the intrinsic catalytic activity of nanostructured materials. On the contrary, nanoporous metals offer significant advantages of probing the true origin of catalytic activity of nanostructured materials. The monolithic structure of nanoporous metal eliminates support effects that can influence catalytic activity as illustrated in Figure 2b. In addition, the dealloying process, the most widely used method to create nanoporosity, occurs under a corrosive environment which leaves the metal surfaces clean of organic surfactants. In the previous example of the nanoporous-Ag catalyst, the genuine catalytic properties of unsupported Ag were clearly originated from its nanostructure. The same conclusion may have been more difficult to draw if a supported nanoparticulate Ag catalyst was studied where the presence of surfactants and/or support could affect the CO2 reduction performance in a significant way.23-24 5 ACS Paragon Plus Environment

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Breaking Scaling Relationships for Multi-electron, Multi-step Electrochemical Reactions Multi-electron, multi-step electrochemical reactions that involve various intermediates and complex reaction pathways offer opportunities to further take advantage of the unique properties of nanoporous bimetallic catalysts. Currently, bimetallics are being extensively studied in CO2 reduction because it has been proposed that a bimetallic surface containing two different metal sites can bind key intermediates (CHO* and CO*) in different manners through interactions with the carbon and/or oxygen atoms.25-26 However, in electrochemical reactions where key intermediates bond in a similar nature, such as nitrogen reduction, a simple bimetallic system is insufficient to break scaling relationships. In more details, an ideal catalyst for nitrogen reduction for ammonia synthesis should be able to adsorb and break the nitrogen to nitrogen (N-N) bond as well as to facilitate the transformation of intermediates and desorption of the ammonia product. One catalyst design approach that stems from the volcano relationship between nitrogen binding energy and metal surfaces, is to combine two metals with high and low nitrogen binding energies as a bimetallic system to obtain an intermediate binding strength.27 However, a key technical barrier is that nitrogen reduction intermediates all bind to the surface through nitrogen. Therefore, due to interpolation effects, it is difficult for a simple bimetallic system to simultaneously possess a strong nitrogen binding energy to sufficiently break the N-N bond while not adsorbing NHx intermediates too strongly.28 This principle suggests that the catalyst needs to have two very distinct active sites, and because of scaling relationships, a simple bimetallic system may not be able to electrochemically reduce nitrogen for ammonia synthesis.

Figure 3. a) A highly curved internal surface can bring two activity sites in close proximity such that the two sites can work cooperatively. b) Two catalytic sites sitting on an open surface are less likely to work cooperatively. On the contrary, a nanoporous bimetallic catalysts with more sophisticated catalytic structure can bring two very distinct active sites in close proximity to one another such that the two sites can work cooperatively. Similar to nature’s biological catalytic systems where catalytic clusters are often oriented in a three-dimensional environment to simultaneously activate and facilitate chemical transformations, the highly curved internal surfaces of the nanopores can bring two very distinct sites in close proximity such that the two sites can work harmoniously as 6 ACS Paragon Plus Environment

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illustrated in Figure 3a. As envisioned, one site can activate dinitrogen and bind to the *N2H intermediate while the other site can bind to the *NH2 intermediate and facilitate the desorption of ammonia. This geometric configuration differs from traditional two-dimensional reaction sites (i.e. open surface) on bulk and nanoparticulate catalysts as shown in Figure 3b. This collaborative interaction between two distinct sites creates a bifunctional surface that can significantly break scaling relationships, which cannot be achieved by simply alloying two metals. Tuning Selectivity and Suppressing Side-Reactions with Nanoporosity Electrocatalysis is an interfacial phenomenon in which the rates of reactions are often dictated by both the intrinsic catalytic properties of the active sites as well as the local concentration of reactants at the electrolyte/electrode interface. A potential strategy to manipulate catalytic activity and selectivity of an electrocatalyst is to control the local concentration of reactants at the interface to either promote desirable reactions or suppress undesirable ones. As alluded previously, nanoporous metals have unique three-dimensional interconnected nanoporous structure and offer interesting opportunities to utilize the diffusional manifolds and confined spaces to manipulate local concentrations and improve catalytic performance.

Figure 4. a) The interconnected networks and confined spaces of a nanoporous metal can enhance total conversion and selectivity by confining intermediates within the catalytic structure. b) The diffusional manifolds of a nanoporous metal can manipulate fluxes of desired and undesired reactants to improve selectivity towards a desired product. For example, the interconnected networks and confined spaces of the nanoporous structure can enhance multi-electron, multi-step reactions by confining intermediates at the 7 ACS Paragon Plus Environment

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catalytic interface as well as increasing their residence time through the catalytic structure. Although significant efforts have been made in recent years to develop electrocatalysts that can reduce CO2 to higher-value C2 and C3 hydrocarbon and alcohol products, current catalysts still suffer from low performances. To overcome this, novel strategies have been proposed to enhance production of higher-order products by breaking down the multi-step CO2 reduction reaction into two consecutive intermediate reactions that are catalyzed on two tandem catalytic sites on the same surface.29-30 As envisioned, the first site can selectivity and efficiently reduce CO2 to CO while the second site can further catalyze the newly formed CO to produce higher-value products. For this strategy to function properly, the CO intermediate must be able to find and interact with the second site. By using a nanoporous structure, the confined spaces in the nanopores will prevent CO intermediates from escaping; and thus the local concentration of intermediates at the catalytic interface can be sustained or enhanced. In addition, with the interconnected porous networks, the reactants and intermediates must diffuse and maneuver through the cavities of the nanoporous structure, therefore increasing the residence time. By doing so, the likelihood that a CO intermediate finds and interacts with the second site will be more probable, potentially shifting the reduction of CO2 towards higher-value products as illustrated in Figure 4a. Furthermore, the increased residence time through the interconnected networks can also enhance total conversion, behaving as a plug flow reactor. Lastly, the diffusional manifolds of the nanoporous structure can be used to manipulate diffusive fluxes of reactants and products in and out of its interconnected networks, potentially tuning the selectivity of electrocatalysts. In the case of CO2 reduction in aqueous environment, the hydrogen evolution reaction which operates in the same potential range as CO2 reduction is a problematic side reaction that often hinders the design of more efficient CO2 reduction catalysts.31-32 As mentioned in previous works, one potential strategy of suppressing unwanted hydrogen evolution is to limit the diffusion of protons from the bulk solution to the catalytic sites, consequentially decreasing the local concentration of protons at the electrolyte/electrode interface.21-22, 33 Although the electroreduction of CO2 also requires protons, the diffusional manifolds of the nanoporous structure can be used to limit the flux of excess protons into the porous networks in comparison to the flux of CO2 molecules as illustrated in Figure 4b. In such, excess protons not participating in the electroreduction of CO2 at the catalytic surface can be reduced so that more active sites are available for CO2 adsorption, thus potentially reducing undesirable hydrogen evolution and in vise-versa, promoting CO2 reduction. However, it must be noted that the intrinsic selectivity of the active sites, type of electrolyte, and applied potential are also important factors to consider, and that solely using nanoporosity to limit the flux of protons may not be sufficient to suppress hydrogen evolution alone. Having said, a similar strategy can be extended for other small molecule electrochemical reactions where unwanted side reactions are also problematic. Challenges of Nanoporous Metal Electrocatalysts While multiple strategies have been proposed to utilize nanoporous metals as novel electrocatalysts, open questions remain regarding the synthesis and scale-up of nanoporous metals with controlled porous structure and targeted surface catalytic sites. In more details, the dealloying process, which is the most common method to generate nanoporosity, involves the selectively dissolution of the less noble element(s) from a binary or ternary precursor alloy under 8 ACS Paragon Plus Environment

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a corrosive environment, either in a highly acidic or basic solution, while the more noble element(s) spontaneously rearranges and is eventually retained in a three-dimensional interconnected porous structure.34 With that said, it is critical to completely remove the less noble element(s) during dealloying since residuals can alter catalytic properties, and this degree of dealloying greatly depends on the nature of the precursor alloy and leaching conditions.35-38 For instance, the leaching of Al via dealloying from NiAl3 or Ni2Al3 alloys to activate the well-known Raney nickel is never complete and results in high surface area NiAl catalyst.39-41 In such cases, multistep dealloying may help effectively remove unwanted residuals or by selecting appropriate constituents in the precursor alloy such that the less noble element is limited to passivation during dealloying. In addition, a positive potential above the critical potential that is necessary for bulk corrosion can also be applied to help drive the dealloying process. This is also known as electrochemical dealloying. Other synthesis methods can also be used to generate nanoporosity which can potentially circumvent challenges associated with the more traditional dealloying technique. For example, electrodeposition in an aqueous environment can form threedimensional interconnected porous materials by utilizing the simultaneous reduction of metal ions and the generation of hydrogen bubbles as a dynamic template to form the nanoporous structure.42 With this technique, unwanted residuals can be avoided. Furthermore, liquid metal dealloying can also be used to access nanoporosity with transition metals that cannot typically form the nanoporous structure due to oxidation encountered with the more traditional dealloying techniques in aqueous environment.43 However, similar to both dealloying and electrodeposition that rely on the spontaneous development of the nanoporous structure, there is a lack of control with these synthesis techniques to generate nanoporous structure with ordered porous networks and specific surface morphologies. In such cases, hard templating can be used as an alternative technique to generate nanoporous metals with more control. In hard templating, an ordered-porous template such as porous silica or ordered colloidal crystals is used as a patterning substrate to guide the synthesis of the porous structure and is eventually etched away, leaving behind free-standing nanoporous metals with specific porosity and surface morphology.44 Nonetheless, with all synthesis techniques, surface-sensitive spectroscopy such as X-ray photoelectron spectroscopy is critical to correlate surface structure properties with catalytic behavior. Another challenge associated with nanoporous metals is that these materials are not mechanically strong due to large void volumes (>70%); and thus, the implementation of nanoporous metals in commercial applications remains a challenge. In addition, there are few studies of incorporating nanoporous metals in working devices such as electrolyzers and fuel cells. Though few in numbers, there are some works that have been done to explore nanoporous metals in working devices. Zeis et al. explored using nanoporous-Au leaf as a high surface area current collector where platinum nanoparticles were uniformly coated through an electroless plating method.45 The Pt-nanoporous-Au leaf was then stamped onto a Nafion membrane to form a membrane electrode assembly for a proton exchange membrane fuel cell. In another work, Luc et al. were able to scale up a 0.5 cm2 nanoporous-Ag catalyst to a 25 cm2 and incorporate it into a flow-cell type electrolyzer for CO2 reduction to CO, demonstrating the feasibility of scaling up nanoporous metals.46 Although nanoporous metals are promising materials as electrocatalysts, a 9 ACS Paragon Plus Environment

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platform to synthesize nanoporous materials at large-scale for industrial applications is still needed and this void may be filled with future engineering ingenuities. Conclusion Although significant progress has been made in developing advanced electrocatalysts, the commercialization of efficient electrochemical devices is still hampered by electrocatalysts that still operates far from ideal efficiency and selectivity; and thus significant efforts are still needed to further improve catalytic performances. Nanoporous metals with their unique threedimensional interconnected porous networks are promising materials as electrocatalysts because they are highly conductive, have large electrochemical surface area with highly active internal curved surfaces, and possess a self-supporting monolithic structure that eliminates overpotentials associated with catalyst/support interface. Herein, the state-of-the-art nanoporous metals as electrocatalysts were briefly summarized and potential strategies to utilize the distinct characteristics of nanoporous metals were proposed to overcome challenges commonly faced in electrocatalysis. Although multiple strategies were discussed, significant research efforts are still required to understand to what extent these strategies can enable for engineering more efficient electrocatalysts. Lastly, open questions remain regarding innovative techniques to synthesize and design novel nanoporous metals with controlled porous structure and specific surface morphologies with targeted active sites. Acknowledgement Acknowledgment is made to the National Science Foundation Faculty Early Career Development program (Award No. CBET-1350911). References 1. Seh, Z. W.; Kibsgaard, J.; Dickens, C. F.; Chorkendorff, I.; Nørskov, J. K.; Jaramillo, T. F. Science 2017, 355, eaad4998. 2. Mistry, H.; Varela, A. S.; Kühl, S.; Strasser, P.; Cuenya, B. R. Nat. Rev. Mater. 2016, 1, 16009. 3. Wittstock, A.; Wichmann, A.; Bäumer, M. ACS Catalysis 2012, 2, 2199-2215. 4. Biener, J.; Biener, M. M.; Madix, R. J.; Friend, C. M. ACS Catalysis 2015, 5, 6263-6270. 5. Zhang, Z.; Ding, Y., Nanoporous metals for advanced energy technologies. Springer Berlin Heidelberg: New York, NY, 2016; p 23. 6. Spöri, C.; Kwan, J. T. H.; Bonakdarpour, A.; Wilkinson, D. P.; Strasser, P. Angew. Chem. Int. Ed. 2017, 56, 5994-6021. 7. Zhang, J.; Liu, P.; Ma, H.; Ding, Y. J. Phys. Chem. C 2007, 111, 10382-10388. 8. Fujita, T.; Guan, P.; McKenna, K.; Lang, X.; Hirata, A.; Zhang, L.; Tokunaga, T.; Arai, S.; Yamamoto, Y.; Tanaka, N.; Ishikawa, Y.; Asao, N.; Yamamoto, Y.; Erlebacher, J.; Chen, M. Nat. Mater. 2012, 11, 775-80. 9. Zielasek, V.; Jürgens, B.; Schulz, C.; Biener, J.; Biener, M. M.; Hamza, A. V.; Bäumer, M. Angew. Chem. Int. Ed. 2006, 45, 8241-8244. 10. Xu, C.; Su, J.; Xu, X.; Liu, P.; Zhao, H.; Tian, F.; Ding, Y. J. Am. Chem. Soc. 2007, 129, 42-43. 10 ACS Paragon Plus Environment

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