Editorial pubs.acs.org/IC
The Way Forward in Molecular Electrocatalysis
E
a dicobalt complex, in acetonitrile and DMF.12 Detection of small amounts of gases evolved during small-scale electrolysis is a formidable challenge. However, the readily constructed collector−generator cells developed by Meyer and co-workers are straightforward to use as demonstrated by the successful detection of a small amount of O2 evolved during water oxidation by two ruthenium polypyridal complexes.13 Homogeneous electrocatalysis is not always free from artifacts. Costentin, Dridi, and Savéant demonstrate how even weak chemisorption of small amounts of an iron porphyrin electrocatalyst on a glassy carbon electrode can contribute substantially to the observed current.14 Decomposition of a molecular electrocatalyst on the electrode under an applied potential is a much more serious issue, as demonstrated by Artero and co-workers.15 In their paper, they show how cobalt diamine−dioxime molecular electrocatalysts decompose under acidic conditions under cathodic potentials to generate cobalt nanoparticles, as originally claimed by Saveant and Aukauloo and co-workers,16,17 which then act as electrocatalysts for the hydrogen evolution reaction. Thus far, this reactivity has been predominantly attributed to molecular electrocatalysts. Identifying the rate-limiting steps in a catalytic process is essential toward further development of the catalyst, and this can only be accomplished via mechanistic investigations. Previously, simulation of the electrochemical response was the primary mechanistic tool. However, recently mechanistic investigations of catalytic reactions in operando using different spectroscopic methods have provided deeper insights into the structure and chemical nature of the intermediates involved. Using in situ X-ray absorption spectroscopy to probe water oxidation by cobalt oxide based materials, Jeong, Joo, and coworkers discovered that mainly Co3O4 and CoOOH species are involved.18 Chen, Liu, and colleagues could substitute the CoIII and CoII ions in a Co3O4 spinel with AlIII and ZnII to demonstrate that the oxygen evolution reaction of such a spinel is mainly derived from the tetrahedral CoII centers where this proposed CoOOH species is formed.19 Tilley and co-workers support their erstwhile mechanistic hypothesis for water oxidation from results obtained using a cobalt cubane cluster and stress the importance of stabilizing high-valent cobalt species for efficient water oxidation. Using labeled substrates and kinetic modeling, the authors propose that a cobalt(V) species with a terminal oxo ligand is catalytically active in oxygen evolution.20 Reduction of CO2 can result in the formation of either CO or HCOOH. Figueiredo, Ledezma-Yanez, and Koper use vibrational spectroscopy to investigate the mechanism of CO2 reduction in acetonitrile on copper electrodes and find that the presence of H2O, even in trace amounts, leads to the preferential formation of carbonate and bicarbonate over CO.21 Lau, Robert, and co-workers find that, for the same pentadentate ligand system, a cobalt complex preferentially reduces CO2 to CO, while the iron analogue is selective for the
lectrochemistry is, arguably, one of the oldest forms of chemistry and is unique in that it boasts an electron/hole as a reagent. Noble metals such as platinum, palladium, silver, and gold and transition metals like iron, nickel, and copper have been used as electrodes for more than a century, and, inevitably, almost all electrochemical processes are electrocatalyzed by the electrode material. Since the early 1800s, experiments by Michael Faraday, Christian Friedrich Schönbein, and others laid the foundation for what would later become modern fuel cell technology. The adsorption of electroactive species on an electrode surface was first noticed by Lorenz and Anson in the 1950s, while molecular heterogeneous electrocatalysis may be considered to have originated in the 1960s with the work of Anson on the irreversible adsorption of inorganic complexes on graphite and their reversible electrochemical behavior.1,2 Similarly, the initial report of chemisorption of allyl compounds on a platinum electrode by Lane and Hubbard in 1973, and the rational attachment of organic moieties to SnOx electrodes by Murray, opened up a new dimension in electrochemistry that would later allow attachment of proteins, DNA, and large organic molecules and enable control of charge-transfer kinetics.3−6 The tools and resources of the electrochemist are affordable and thus enable groups all over the world to contribute to the further development of experimental techniques and analysis methods. The equilibrium constant, K, and the rate, k, characterizing a homogeneous reaction are embedded in the formal potential and the current behavior in an electrochemical process. While Nicholson laid out the theoretical treatment of the electrochemical response for homogeneous electrochemistry, Savéant and Koutecky−Levich developed elegant mathematical models for describing the kinetic and mass-transferlimited currents in homogeneous (static) and heterogeneous (dynamic) electrocatalysis.7−10 These phenomenal discoveries laid the foundations of the modern era of electrocatalysis covered in this virtual issue. The articles highlighted here were published in Inorganic Chemistry, Journal of the American Chemical Society, and ACS Catalysis within the past 2 years and present significant advancements in the areas of water splitting and CO2 fixation related to (a) general principles, (b) mechanistic insights, (c) factors controlling selectivity, and (d) new molecules. The ability to handle molecular electrocatalysts in organic solvents offers certain advantages with regard to the solubility of nonpolar substrates such as O2, H2, and CO2. However, critical analysis of the electrochemical data is quite susceptible to errors arising from the use of incorrect standard potentials for these electrochemical processes. Appel, Mayer, and colleagues have reported these standard potentials in acetonitrile and N,N-dimethylformamide (DMF) with respect to the H+/H2 potentials in these solvents.11 Similarly, the potential of processes requiring both electrons and protons in organic solvents is expressed as a function of pKa of the proton donor in the respective solvent. This is elegantly demonstrated by Nocera and co-workers for O2 reduction to H2O or H2O2 by © 2016 American Chemical Society
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reduction of CO2 to formate.22 The authors postulate that enhanced back-bonding from the metal center to the C−O π* orbital in a proposed CoII-COOH intermediate leads to C−O bond scission. However, in the case of an analogous FeIIICOOH intermediate, π donation from FeIII is weak, and therefore bond cleavage is less facile. Taheri and Berben discuss the pivotal role played by the hydricity of metal hydrides involved in the reduction of CO2.23 Investigating a series of metal carbonyl clusters with differing hydricities, they demonstrate the competition between CO2 reduction and the hydrogen evolution reaction. In a similar investigation, Yang and co-workers find that the hydricities of [HNi(DHMPE)2][BF4] (DHMPE = 1,2-bis(dihydroxymethylphosphino)ethane], formic acid, and H2 are strongly dependent on the solvent polarity, suggesting the importance of such considerations in catalyst design.24 The role of second-coordination-sphere residues in catalysis has been gaining gradual attention. In their publication, Raugei, Bullock, and colleagues systematically vary the number of pendant amines in nickel-based hydrogen evolution reaction catalysts to conclude that a single amine group is ideal for proton reduction, while multiple protonation sites may just open more nonreactive channels.25 Density functional theory calculations and experimental data on nickel and iron complexes with pendant amines suggest that all of the steps involved in catalysis need to be considered during catalyst optimization. McNamara and co-workers introduce pendant sulfonate groups to an iron-based molecular complex to generate a compound that is quite efficient in catalyzing the hydrogen evolution reaction in acetonitrile.26 Marinescu and co-workers find that a pendant NH is 2 orders of magnitude more efficient in the reduction of CO2 to CO than the alkylated version in a series of cobalt amminopyridine complexes.27 In electrocatalysis, there is always a need for newer “dimensions”, i.e., new catalysts, new catalyst−electrode combinations, or new reaction setups. Kundu and co-workers integrate platinum nanoparticles into DNA self-assemblies, and the resulting material efficiently catalyzes the hydrogen evolution reaction with very low quantities of the precious metal.28 Catalysis carried out by molecular species immobilized within a metal−organic framework (MOF) can afford enhancements relative to homogeneous reactions. Indeed, in their study of an iron porphyrin based MOF deposited on FTO thin films, Kubiak, Farha, Hupp, and colleagues achieve high catalyst concentration and catalytic currents for CO2 reduction that are three times greater than those for the molecular iron species in solution.29 Mayer, Grätzel, and co-workers employ a rhenium complex adhered to TiO2-coated Cu2O photoelectrodes to reduce CO2 to CO. The high surface area provided by the nanostructured TiO2 allows 40 times greater photocurrent compared to planar surfaces.30 Copper is generally not the metal of choice for water splitting. However, Musaev, Hill, and colleagues show that a polyoxometalate complex of copper can catalyze the hydrogen evolution reaction photochemically when coupled to an appropriate sacrificial agent.31 Similarly, the potential of CuO-based materials as oxygen evolution reaction catalysts is demonstrated by Yeo and co-workers.32 While the reactivity of copper may not parallel the other transition metals, its ready availability may be a pertinent factor when large-scale applications are considered. Metalloporphyrins and metallocorroles have been extensively used in the past for the oxygen and hydrogen evolution reactions,33−35 and notably Lai, Cao, and co-workers demonstrate that a nickel porphyrin can
catalyze the oxygen evolution reaction at neutral pH.36 Along the same lines, the utility of a cobalt-based perfluorinated phthalocyanine complex for the reduction of CO2 to CO is shown by Morlanés, Takanabe, and Rodionov. This same compound can also promote water oxidation at the anode in a two-compartment cell.37 Ligand redox noninnocence has been used by Manbeck and co-workers in the design of mixed-valent ruthenium complexes that exhibit hydrogen evolution from a series of organic acids in a DMF solvent.38 Drawing on inspiration from hydrogenases in nature, Jones and co-workers designed a square-planar nickel complex with a bis(phosphino)ferrocene ligand that demonstrates high efficiency and turnover in the hydrogen evolution reaction at low overpotential.39 Notably, the Fe center does not appear to contribute to catalysis but rather enforces the geometry around the Ni center that is favorable for catalysis. Bren and co-workers also draw from nature and use a tetrapeptide as a ligand for cobalt to achieve hydrogen evolution in buffered aqueous solutions.40 In a rather interesting investigation, Wu and co-workers synthesize a series of molybdenum catalysts bearing persulfide ligands, which carry out hydrogen evolution in organic solvents.41 The basicity of the persulfide unit determines the efficiency of the catalytic process. Lotsch, Durrant, Reisner, and co-workers employ a cyanamide-functionalized carbon nitride material as a light harvester and couple oxidation of benzyl alcohols to hydrogen evolution in a highly efficient photocell.42 The area of molecular electrocatalysis has been rapidly expanding because of the growing demand of suitable catalysts for clean energy and environment applications. Over the last 5 years, several catalysts, some included in this virtual issue, have been reported to be selective, efficient, and durable, and they deserve the attention of industries hoping to gain from the growing renewable energy market. However, it is my view that the way forward is to try to challenge the limits of catalyst function. Intelligent modification of conventional spectroscopic and analytical techniques to accommodate electrochemical (both homogeneous and heterogeneous) experimental setups has begun to provide detailed insights into the chemical nature of intermediates involved. These mechanistic details, obtained in operando, are allowing meaningful “tailor made” synthetic modifications to catalysts for obtaining higher rates and selectivity. The investigations included in this virtual issue amply demonstrate that a comprehensive approach, although time-consuming, has a much higher chance of success than random trial and error. Thus, the way forward in molecular electrocatalysis entails interception- and interrogation-based innovations.
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Abhishek Dey
AUTHOR INFORMATION
Notes
Views expressed in this editorial are those of the author and not necessarily the views of the ACS.
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REFERENCES
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(41) Garrett, B. R.; Polen, S. M.; Click, K. A.; He, M.; Huang, Z.; Hadad, C. M.; Wu, Y. Tunable Molecular MoS2 Edge-Site Mimics for Catalytic Hydrogen Production. Inorg. Chem. 2016, 55, 3960−3966. (42) Kasap, H.; Caputo, C. A.; Martindale, B. C. M.; Godin, R.; Lau, V. W.; Lotsch, B. V.; Durrant, J. R.; Reisner, E. Solar-Driven Reduction of Aqueous Protons Coupled to Selective Alcohol Oxidation with a Carbon Nitride−Molecular Ni Catalyst System. J. Am. Chem. Soc. 2016, 138, 9183−9192.
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