Molecular Photoelectrocatalysts for Light-Driven Hydrogen Production

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Molecular Photoelectrocatalysts for Light-Driven Hydrogen Production Kelsey R Brereton, Annabell G. Bonn, and Alexander J. M. Miller ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b00255 • Publication Date (Web): 06 Apr 2018 Downloaded from http://pubs.acs.org on April 6, 2018

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ACS Energy Letters

Molecular Photoelectrocatalysts for Light-Driven Hydrogen Production Kelsey R. Brereton, Annabell G. Bonn, and Alexander J. M. Miller* Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290, United States Abstract. The light-driven synthesis of fuels requires efficient coupling of photon absorption with bond-forming chemical reactions. Molecular photoelectrocatalysis is an emerging approach in solar fuel production based on single molecules that support electrochemical hydride formation and photochemical bond-forming fuel synthesis. This Perspective article outlines the design requirements for transition metal candidates and describes the development of the first molecular photoelectrocatalyst for dihydrogen (H2) evolution. Mechanistic aspects are discussed in the context of electronic tuning of the catalyst, and the outlook for future development is considered.

TOC Graphic. Background The utilization of solar photons to drive the catalytic synthesis of molecular fuels could play a crucial role in future energy infrastructure.1,2 The efficient synthesis of solar fuels relies on

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two key processes: light absorption and chemical bond formation. Most molecular approaches to solar fuels utilize multiple components.3 A photosensitizing chromophore absorbs light, a redox mediator separates charge, and a catalyst facilitates the bond-forming reactions of fuel synthesis.4 Single-component molecular systems have also been developed. In most of these cases, a dinuclear transition metal complex achieves HX splitting by photochemical X2 release (X = halide) followed by protonation and subsequent H2 release (which is sometimes also photochemically driven).5,6 This Perspective article considers an emerging approach to light-driven fuel synthesis: molecular photoelectrocatalysis utilizing a single molecular framework to support electrochemical hydride generation and photochemical H2 release. This approach does not rely on sacrificial electron donors or semiconductor light harvesters, relying instead on simple electrodes and molecular photochemistry. Molecular photoelectrocatalysts offer attractive tunability and opportunity for detailed mechanistic examination and optimization. Furthermore, decreasing the number of charge transfer interfaces relative to multi-component systems could improve photonto-fuel efficiency. The key considerations that led to the development of the first molecular photoelectrocatalysts for H2 evolution are introduced, followed by a mechanistic analysis that highlights the ability of synthetic modification to tune relevant thermodynamic parameters. The future of this synergistic approach is then considered.

Designing a molecular photoelectrocatalyst A conceptual starting point for molecular photoelectrocatalyst design is the need for a single system that is capable of (a) electrochemical hydride formation and (b) photochemical H2 release. As shown in Scheme 1, many transition metal complexes can carry out one of these steps, but far fewer systems are able to complete both under the same conditions.

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xH+/2e–

Electrochemical hydride formation most common for x = 1 q+

(q+2–x)+

Ln M

Ln M

Hx

Photochemical H2 evolution

most common for x = 2 x/2 H H

Scheme 1. Key steps needed for a new molecular approach to photoelectrocatalysis.

In screening potential candidates for a molecular photoelectrocatalytic system, the divergent reactivity trends for monohydrides and dihydrides illustrate the challenge in design. A wide array of transition metal complexes can undergo two-electron reduction followed by protonation to generate a monohydride complex. In contrast, there are few examples of electrochemical formation of dihydrides. Clean and efficient photochemical H2 evolution by dihydrides via a reductive elimination pathway is well documented; the photochemistry of monohydrides is much less developed.7 As is so often the case in catalysis, designing to optimize one step of a process runs the risk of impinging other steps. Several strategies can be employed in the search for transition metal complexes capable of balancing these two modes of reactivity. Metal monohydride complexes with promising photochemical reactivity are intriguing, given the likelihood of successfully generating the hydrides electrochemically. Conversely, metal dihydrides that can be generated electrochemically would be promising because they are likely to exhibit the desired photochemical H2 release reactivity. Another area ripe for exploration are reports of “sensitizer-free” or “unsensitized” photochemical H2 evolution, as these systems demonstrate photochemical reactivity without

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needing additional light-absorbing components; here, the major challenge is replacing a sacrificial reductant with an electrode.

Chart 1. Chart of photoelectrocatalyst candidates.

A selection of promising candidates is present in the chemical literature (Chart 1). Examples of “unsensitized” photochemical H2 evolution date back to 1983, when Hawecker, Lehn, and Ziessel reported Re(bpy)(CO)3(Cl) to catalyze H2 evolution from DMF solutions containing triethanolamine under 250 W halogen lamp illumination. 8 This system is not particularly efficient for H2 evolution, exhibiting highly selective CO formation when an atmosphere of CO 2 is admitted to the system. More recently, Ertem, Fujita, and coworkers reported unsensitized photochemical CO2 reduction using isomers of [Ir(terpyridine)(phenylpyridine)(H)]+ wherein small amounts of H2 were observed in addition to the dominant product CO. 9 Rauchfuss and coworkers recently discovered a dithiolate- and hydride-bridged diiron system capable of unsensitized photochemical H2 evolution from CH2Cl2 solutions containing octamethylferrocene as an electron donor and triflic acid as the proton donor.10 Subsequent studies have examined the mechanism of H 2

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evolution in these systems, but the dominant photochemical pathway appears to be CO photodissociation.11,12 It is noteworthy that each of these discoveries appears to have arisen by careful control reactions of sensitized photocatalysis, where the researchers discovered activity remained in the absence of a photosensitizer. Although the photochemistry of monohydride complexes is dominated by ligand isomerization or dissociation pathways,7 there are some promising examples of photochemical H2 evolution reactivity. Two reports in 1981 provide early precedent for photochemical H2 release involving monohydride intermediates. Eidem, Maverick, and Gray showed that solutions of hexachloroiridate, IrCl63–, in 12 M HCl release H2 upon ultraviolet irradiation (254 nm).13 The mechanism was proposed to involve photochemical H2 release from HIrCl62–. In the same year, Jones and Cole-Hamilton reported that Rh, Pd, and Pt complexes supported by simple monodentate phosphine ligands undergo ultraviolet light-induced H2 evolution from acidic aqueous solutions.14,15 While monohydrides are invoked as intermediates, the photochemistry is proposed to proceed only after protonation to form a dihydride. A series of studies highlighting H2 release involving monohydrides were performed by Ziessel in the 1980s and 1990s. Ziessel prepared organometallic iridium complexes of the type [Cp*Ir(L2)(X)]+ (L2 is bipyridine derivative, X is halide) and discovered these species to be active catalysts in the photochemical water-gas shift reaction that converts CO and H2O to H2 and CO2.16 Spectroscopic and mechanistic studies suggested that the photochemically active species in both water-gas shift reactivity and formic acid dehydrogenation was the hydride complex [Cp*Ir(L2)(H)]+.17–20 Considering the promising electrochemical behavior of this class of complex,21–23 the Cp*Ir-based system was identified as an ideal starting point for developing a combined electrochemical/photochemical system.

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Development of Cp*Ir-based photoelectrocatalysts The air-, light- and moisture-stable chloride complex [Cp*Ir(bpy)(Cl)][Cl] is readily synthesized by addition of 2,2'-bipyridine to [Cp*Ir(Cl)2]2. Electrochemical reduction of [Cp*Ir(bpy)(Cl)]+ in aqueous pH 7 phosphate buffer (Ep,c ~ –0.61 V vs NHE at 250 mV/s) is assigned as a 2e–/1H+ process coupled with ligand dissociation that forms the hydride complex [Cp*Ir(bpy)(H)]+. The nature of the electrochemical reduction was confirmed by controlled potential electrolysis (CPE) of a phosphate-buffered solution (pH 7) at –1 V vs NHE in the dark, which formed [Cp*Ir(bpy)(H)]+ in high yield according to cyclic voltammetry (CV), NMR spectroscopy, and UV-vis absorbance spectroscopy. A second, quasi-reversible 1e– reduction wave at –1.25 V vs NHE is assigned to the IrIII/II couple of the generated hydride.24 These studies illustrate that electrochemical generation of the hydride is possible in aqueous solutions at potentials only 200 mV more than the thermodynamic potential for H2 evolution (EH+/H2 = –0.41 V vs NHE at pH 7). The photochemistry of isolated hydride [Cp*Ir(bpy)(H)][Cl] was also explored. Photolysis of aqueous solutions of [Cp*Ir(bpy)(H)]+ leads to rapid H2 release, as Ziessel had observed.17 In the absence of light, [Cp*Ir(bpy)(H)]+ is stable in neutral water for several hours. Sequential electrolysis/photolysis experiments confirmed that both key processes of Scheme 1 (above) could be carried out in the same vessel under identical conditions less harsh than reported for previous photochemical H2 evolution by monohydrides.24 [Cp*Ir(bpy)(H)]+ can also be regenerated using formate, leading to a chemical system for photochemical H2 evolution from formic acid.25

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To probe whether [Cp*Ir(bpy)(Cl)]+ could support sustained photoelectrocatalysis, controlled potential electrolysis with simultaneous 460 nm illumination was carried out. Continuous photocurrent (>25-fold above the dark background current) was observed over 1 h, with gas chromatographic analysis of the headspace confirming that the bubbles forming during the experiment were indeed H2. The reaction proceeds with high Faradaic efficiency (>90 %) and converts 460 nm photons incident at the electrode surface to equivalents of H2 fuel with high photochemical quantum efficiency (~10%).24 A shutter experiment, toggling between light and dark during CPE, further established the necessity of light for catalysis and the stability of the hydride (Figure 1B).

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Figure 1. Overall photoelectrocatalytic water reduction process (A), controlled potential electrolysis with alternating dark (gray) and light (white) conditions (B), and individual reaction steps in the catalytic cycle (C). Adapted from reference 24.

Figure 1C depicts the proposed mechanism of photoelectrocatalytic H2 evolution, combining electrochemical formation of [Cp*Ir(bpy)(H)]+ in the dark and light-triggered H2 production with regeneration of [Cp*Ir(bpy)(Cl)]+. The observed catalytic rate constant was obtained using electrochemical methods. Chronoamperometry (CA), in which short-term unstirred

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controlled-potential electrolysis is monitored over time, was carried out to determine a rate constant for the catalytic process from the ratio of current passed under illumination and in the dark.26 The prototypical complex [Cp*Ir(bpy)(Cl)]+ produced H2 with a rate constant of 0.039 s–1 at potentials where the kinetics are expected to be limited by the photochemical process (Figure 1C, steps iii and iv).

Mechanistic aspects of molecular photoelectrocatalysts To help identify design principles for the development of future molecular photoelectrocatalysts, thermodynamic and kinetic studies have been undertaken. Using the first generation of molecular photoelectrocatalysts as a model, this section discusses each step of the mechanistic cycle in more detail and highlights how synthetic modifications can tune the individual steps to approach more efficient catalysis. Figure 1C depicts the individual steps for photoeletrocatalytic H2 evolution by [Cp*Ir(bpy-R)(H)]+ (bpy-R indicates substituents at the 4 and 4’ positions on bipyridine). The first step shown in the cycle of Figure 1C involves electrochemical reduction of an Ir(III) complex. Two-electron reduction (which can also proceed via a parallel route of Ir(II) disproportionation)21 and ligand dissociation produces a reduced species that is formally an Ir(I) complex. At high pH, Cp*Ir(bpy-R) species can be isolated and fully characterized.27,28 A characteristic dark purple color is attributed to significant delocalization of electron density into the bipyridine  system,29 with the reduced state having character expected from both IrI(bpy0) and IrII(bpy∙–). Most Cp*Ir(bpy-R) species are neutral, leading to poor solubility in water and precipitation or electrode adsorption problems; charged substituents (such as carboxylate) can improve solubility to support reversible electrochemistry.27,30

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The reduction potential has a direct bearing on electrocatalytic performance through its relationship with electrochemical overpotential.31–34 As the reduction potential shifts to more negative potentials, the overpotential necessary to achieve a given catalytic rate increases. Synthetic modifications can have a significant impact on the reduction potential in complexes of the type [Cp*Ir(bpy-R)(X)]+ because of the ligand-based reduction. Introducing electronwithdrawing substituents at the 4,4' positions on the bipyridine ligand (R in Figure 1C) shifts the reduction feature to less negative potentials.35,36 Density Functional Theory (DFT) calculations of the reduction potential for complexes of the type [Cp*Ir(bpy-R)(X)]2+ correlate well with the experimental observations and allow a quantitative measure of the electronic influence on the reduction potentials based on the slope of the Hammett plot ( = 18.7).37 Altering the identity of the monodentate ligand X that dissociates upon reduction also has a significant influence on the reduction potential. CV of [Cp*Ir(bpy)(Cl)]+ in aqueous pH 7 phosphate buffer reveals a broad reduction feature with multiple peaks, which sharpens into a single peak upon addition of excess NaCl.24,27 Along with NMR spectroscopic evidence,27 this behavior indicates that partial chloride displacement from [Cp*Ir(bpy)(Cl)]+ upon dissolution in phosphate buffer forms significant amounts of [Cp*Ir(bpy)(H2O)]2+ and [Cp*Ir(bpy)(H2PO4)]+. The chloride complex [Cp*Ir(bpy)(Cl)]+ is reduced at potentials ~200 mV negative of the aquo complex [Cp*Ir(bpy)(OH2)]2+,27 emphasizing the importance of controlling speciation processes to provide the desired reduction potential. The second step of the cycle is protonation to generate the hydride (step ii). At pH 7, Cp*Ir(bpy) is rapidly protonated to form the hydride complex [Cp*Ir(bpy)(H)]+ in high yield. (Under these conditions on the CV timescale, so little Cp*Ir(bpy) builds up that adsorption due to insolubility is not a problem.) Broadly speaking, the medium must be sufficiently acidic to

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protonate the reduced metal complex and form the M–H bond. The pKa of the metal hydride is the thermodynamic parameter that controls step ii in the catalytic cycle. As with the reduction potential, acidity for this family of complexes is strongly influenced by the bipyridine supporting ligand, perhaps driven by the high level of electronic delocalization between the metal center and the bpy π system.30 Whereas [Cp*Ir(bpy)(H)]+ can be accessed at pH 7, the ester-substituted bipyridine-containing hydride complex [Cp*Ir(bpy-CO2Me)(H)]+ can only be accessed below pH 4. The more electron-donating methoxy-substituted bipyridine ligand supports a hydride complex [Cp*Ir(bpy-OMe)(H)]+ that cannot be deprotonated even in pH 14 water. DFT has also provided some insight into the electronic influence of these substitutions on acidity, with = 11.5 for hydride pKa values.37 Once formed, a transition metal hydride can react with a proton source to generate H2, either in the dark (a background reaction; step v) or in a light-driven process (step iv). In the dark, negligible amounts of H2 are produced at the first reduction feature near –1 V vs NHE, indicating that neither the Ir catalyst nor the electrode are competent for H2 evolution under these conditions. This lack of reactivity can be rationalized according to the experimentally determined thermodynamic hydricity, a measure of the free energy required to heterolytically cleave the M– H bond and release a hydride ion (H–).38 The hydricity can be used to calculate the pH at which H2 generation from the (ground state of the) hydride is thermoneutral (blue line in Figure 2).38 For a series of hydrides [Cp*Ir(bpy-R)(H)]+ with varying substitution at the 4,4' positions on the bipyridine ligand, thermodynamic hydricity has been shown to correlate with the Hammett parameter p– both experimentally and computationally.27,37 As the bipyridine ligand becomes more electron rich, the hydride complex becomes a thermodynamically stronger hydride donor (smaller ∆GºH– value). The pKa and hydricity of the hydride govern the region of hydride stability:

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the pH region in which the hydride can be generated but is stable to protonation and H2 release in the ground state (white area in Figure 2). For complexes with known hydricities and acidities (like [Cp*Ir(bpy-CO2)(H)]–; pKa = 12.4, ∆GºH–(Cl) = 27.6 kcal/mol), the region of stability can be accurately predicted (green line in Figure 2). The most valuable photoelectrocatalysts will feature thermodynamically unfavorable H2 evolution in the dark and the reaction will be driven using photon energy.

Figure 2. Plot of pH vs. hydricity (∆GºH–) illustrating the regions of stability of transition metal hydrides. The dashed red line is an estimation of the relationship between the hydride pKa and the hydricity, based on DFT calculated trends.37 At higher pH values the hydride will be deprotonated (red shading), precluding further reactivity. The blue solid line is the pH at which H2 release in the dark is thermoneutral for a hydride with a given ∆GºH–; in the white region, the hydride is stable (green lines), while in the blue shaded region, the hydride will release H2 in the dark (black lines). Black circles represent the stability boundaries of [Cp*Ir(bpy-CO2)(H)]– and [Cp*Ir(bpyOMe)(H)]+.

Photochemical hydrogen release from a metal hydride begins with light absorption to reach an excited state (Step iii). UV-Vis absorbance spectroscopy provides the light-harvesting profile, with peaks characterized by the wavelength of maximum absorbance, max. As observed with the other thermodynamic parameters discussed, the absorbance maxima for the family of [Cp*Ir(bpyR)(H)]+ complexes has been shown to correlate with the p– Hammett parameter. Moving from 12


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methoxy substituents on bipyridine to methyl ester substituents leads to a red shift in max of 90 nm.27 The absorbance profile can significantly affect the rate of H2 production based on the degree of spectral overlap with the light source, particularly in laboratory settings where lamps or lasers with narrow excitation wavelengths lamps are often utilized. Light absorption leads to an excited state that must have a sufficiently long lifetime to undergo bimolecular reactivity.39 (This requirement may be relaxed for H2 evolution from dihydride complexes, which undergo intramolecular reductive elimination). If the excited state is emissive, the emission maximum provides insight into the nature of the excited state and how much photon energy was stored in the photoexcitation process. As the emission energy shifts to longer wavelengths, the luminescence lifetime tends to decrease according to the energy gap law. Irradiation of the visible region absorbance features of [Cp*Ir(bpy)(H)]+ generates an excited state with a broad emission band (max,em = 708 nm).20,39 Transient absorption studies in methanol identified characteristic bands in the excited state species between 490-730 nm that indicate significant charge distribution onto the bipyridine * system. Coupled with DFT calculations that predict a transition from an iridium d-orbital to bipyridine-based molecular orbital,29 the lowestenergy excited state of [Cp*Ir(bpy)(H)]+ can be assigned as a triplet metal-ligand charge transfer (MLCT) state. The final step in the catalytic cycle is H2 evolution from the excited state hydride (step iv). Photochemical H2 release from the triplet state can be extremely efficient: the quantum yield for H2 release from [Cp*Ir(bpy)(H)]+ is as high as 0.93 in acetonitrile solvent. 39 Thermodynamic parameters for various pathways starting from the high-energy excited state can be calculated. Heterolytic cleavage of the M–H bond to release a hydride ion would be termed excited state hydricity, and can be calculated using the excited state acidity (pKa*) and reduction potential. The

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excited state of [Cp*Ir(bpy)(H)]+ was estimated to be almost 50 kcal/mol more hydridic than the ground state in acetonitrile.40 Alternatively, heterolytic cleavage to release H+ is the excited state acidity: pKa* is –12,40 indicating a thermodynamically potent acid in the excited state. Finally, the excited state can undergo single electron transfer with Ir acting as an oxidant (IrIII*/II = +0.37 V vs Fc+/0) or as a reductant (EIV/III* = –1.67 V vs Fc+/0).39 Detailed photophysical studies have only been carried out on the parent hydride [Cp*Ir(bpy)(H)]+ so far. With several thermodynamically favorable pathways available, further mechanistic studies were required to establish which mechanism was operating in photoelectrocatalysis. Kinetic studies and a labeling experiment in acetonitrile solvent indicated that initial excited state electron transfer occurs as part of a bimetallic “self-quenching” electron transfer mechanism.39 Upon excitation, [Cp*Ir(bpy)H]+* undergoes electron transfer with a ground state hydride to generate a transient pair of Ir(II) and Ir(IV) hydrides that react in a bimetallic fashion to release H2. For [Cp*Ir(bpy)(H)]+, excited state electron transfer is kinetically favored over excited state Ir–H bond scission. Excited state electron transfer can be highly efficient, and the reactive hydride ligand can support a rapid and efficient follow-up reaction to release H2, which may explain the high quantum efficiency observed for this photochemical bond formation. This mechanism is noteworthy for utilizing a single photon to trigger the conversion of 2H+ and 2e– into H2. A thermodynamic analysis of the overall photoelectrochemical water reduction is possible. The overall energy input necessary to drive H2 release from water can be calculated based on the pH-dependent standard potential for proton reduction to H2, ∆Grxn = 19 kcal/mol at pH 7. This endergonic reaction is achieved using a combination electrochemical and photochemical energy input. The electrochemical contribution is defined by the reduction potential that converts [Cp*Ir(bpy)(Cl)]+ into [Cp*Ir(bpy)(H)]+, which requires approximately 27 kcal/mol. This is

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thermodynamically sufficient for H2 release, but the reaction is not kinetically accessible in the dark (probably because the Ir hydride is not hydridic enough). 27 Substituting aquo for chloro or utilizing a carboxy-substituted bipyridine ligand leads to a ~200 mV shift in the reduction potential, lowering the electrochemical contribution to ergoneutrality, ~19 kcal/mol. The photochemical energy input is defined by the absorption profile of the Ir hydride. Blue (460 nm) light used for the photoexcitation of [Cp*Ir(bpy)(H)]+ provides an additional 62 kcal/mol (2.7 eV) energy input (which relaxes to a triplet excited state estimated to store 50 kcal/mol).39 In these cases, photochemical energy overcomes a kinetic barrier; if the electrochemical contribution is less than 19 kcal/mol at pH 7, the photochemical energy would be thermodynamically required. A more energy efficient process could be developed by (a) moving to less negative potentials to minimize electrochemical energy input and (b) achieving H2 release with longer wavelength (lower energy) light.

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less acidic

pKa(MH+)

sp–

EWG

more hydridic

lmax

∆GºH–

less hydridic

EDG

more acidic

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

easier harder reduction Eº(M+/MH+) reduction

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Figure 3. Correlation diagrams describing the general relationship between the electronic properties of the bipyridine ligand and the thermodynamic factors governing photoelectrocatalysis (EDG is electron donating group, EWG is electron withdrawing group).

The above analysis reveals several correlated factors (or scaling relationships) that can be considered in synthetic tuning involving the bipyridine ligand. Synthetic modifications will influence the observed reduction potential, hydride acidity, hydricity, and absorption profile for the system. Figure 3 illustrates the relationship between ligand electronics, as reflected in the p– Hammett parameter, and various thermochemical parameters. As is commonly encountered in catalyst development, optimizing one parameter will have effects elsewhere in the system. Consider the example of a substituted bipyridine ligand featuring electron-withdrawing substituents (larger p– value). According to the top portion of Figure 3, the electrochemical reduction in step i of the catalytic cycle will be facilitated (moving up and to the right along the

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red dotted line), occurring at less negative potentials. A concomitant diagonal shift (down and to the right) along the green dashed line in Figure 3 indicates that the less electron-rich ligand will also lead to formation of a hydride that is more acidic (smaller pKa value) than the parent complex. The increased acidity will make the product of reduction more difficult to protonate (step ii) during the electrochemical hydride generation process. For example, while the parent bipyridine hydride complex is readily formed and stable at pH 7, the methyl ester-substituted bipyridine hydride complex is unstable with respect to deprotonation above pH 4. An experimental pKa could not be determined due to low solubility, but DFT predicts that the methyl ester-substituted hydride is 6 orders of magnitude more acidic than the parent bipyridine hydride. 37 The lower portion of Figure 3 maps the electronic influence of the bipyridine ligand on the hydride donating ability (left y-axis) and the absorptivity (right y-axis) of these hydride complexes. A hydride featuring electron-withdrawing groups will exhibit decreased hydride donating ability (larger ∆GºH– value moving up and to the right along the black dashed line). The hydricity dictates the pH at which H2 release in the dark is thermodynamically favored. It has been recognized more widely that hydricity values also correlate strongly with reduction potential,38,41 which derives from a roughly constant M–H bond dissociation free energy (BDFE) in thermochemical cycles relating BDFE, Eº, and ∆GºH–. Along with a decrease in hydricity comes a red-shift in the max of the hydride, lowering the energy of light needed to access the excited state of the hydride. It should be noted that bipyridine complexes also often exhibit correlations between reduction potential and max.35 The overall ground state effect of moving to a more electron-poor bipyridine ligand is to render electrochemical reduction more facile, but at the expense of requiring more acidic

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conditions to generate the hydride. As illustrated by the  values for Hammett plots of reduction potential ( = + 18.7) and acidity ( = + 11.5), electronic changes to the bipyridine ligand have dramatic effects on these two thermodynamic parameters. However, the reduction potential is subtracted from the acidity in the calculation of hydricity using thermochemical cycles, so ∆GºH– is less dramatically influenced ( = –1.7).27 A substitution that changed the hydride pKa by six orders of magnitude will only shift the pH at which ground state H2 release is thermoneutral by about 1 pH unit. The potential–acidity scaling relationship is problematic from a catalyst design perspective, but the presence of a photochemical reaction step helps to break scaling relationships: a hydride that is predicted based on these thermochemical arguments to be unreactive undergoes photochemical H2 release independent of the solution pH. As expected for a system for which ligand tuning impacts various thermodynamic parameters in different ways (Figure 3), rate of photoelectrocatalysis does not follow clear trends as a function of bipyridine substituent. The limiting observed rate constant (kobs) should reflect rate-determining photochemical processes at high overpotential for photoelectrocatalytic H2 evolution from water at pH 7 is 0.039 s–1. A catalyst with electron-donating methoxy substituents on the bipyridine ligand is more than twice as fast, kobs = 0.091 s–1. This is the expected trend for an electrocatalyst: the harder-to-reduce species is a faster catalyst. A catalyst with electronwithdrawing carboxylate substituents on the bipyridine ligand, however, is much faster still — kobs = 0.20 s–1. This runs counter to the expectations of an electrocatalyst. In this case, the improved performance is attributed to improved light harvesting, based on a red shift in the absorption profile of the hydride intermediate that aligns favorably with the 460 nm LED lamp employed and accelerates the photochemical H2 production step.

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Outlook The iridium complexes in focus here are prototype molecular photoelectrocatalysts for H 2 fuel generation. One new photoelectrocatalytic system based on metallocene complexes was recently discovered by Girault and co-workers: ruthenocene (Cp*2Ru) is protonated by the strong acid [H(OEt2)2][BArF4] in CH2Cl2 to form the redox-inactive hydride [Cp*2Ru(H)]+ and subsequent UV irradiation (385 nm) leads to formation of H2 and [Cp*2Ru]+.42 When photolysis is carried out during a CV sweep, a catalytic current response is observed at the potential expected for reduction of [Cp*2Ru]+. While the overall mechanism for photoelectrocatalysis is the same, the instability of the Ru(III) species leads to the hydride complex being the resting state. There are also significant differences in reaction conditions, with the ruthenocene system requiring the use of chlorinated organic solvents, a strong “designer” proton source (Brookhart’s acid) and UV light. Looking forward, there are several areas in need of practical improvement and fundamental studies. Practical considerations include (a) tuning the electrochemical potential of hydride formation to be as mild as possible, (b) tuning the absorption properties to cover a wide swatch of the solar spectrum, especially in the red, and (c) achieving electrocatalytic rates that align with the rate of solar photon absorption. Fundamental studies into the photochemistry of metal hydrides and new strategies to break thermodynamic scaling relationships will help achieve these goals, so these two aspects will be touched upon in more detail here. Understanding the photophysics and photochemistry of transition metal monohydrides will be important in the development of new photoelectrocatalysts. Whereas most photocatalysts and photosensitizers are explicitly designed with non-labile, non-reactive ligands, metal hydride complexes contain a reactive ligand site. The hydride is also strongly electron-donating, which may have a significant impact on the electronic structure of such complexes in the ground state

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and excited state. In terms of photochemistry, defining pathways for productive photochemical H2 evolution (such as the bimetallic self-quenching mechanism described above) will help identify optimal new catalyst designs and reaction conditions. A range of time-resolved spectroscopic techniques have been developed for such studies. Transient absorption and photoluminescence spectroscopy has proven valuable in elucidating the nature of bipyridine-supported excited states, and could also help identify and characterize photochemical intermediates. 43 Time-resolved infrared spectroscopy could be valuable in tracking the M–H stretching frequencies throughout a reaction. Even time-resolved NMR spectroscopy methods are becoming more accessible for organometallic photochemical reaction monitoring. 44,45 Along these lines, two recent studies examined the excited state properties of Rauchfuss’ unsensitized diiron hydride photocatalyst using infrared spectroscopy to monitor transiently photogenerated species. 11,12 Discovering catalyst designs that overcome scaling relationships will help enable photoelectrocatalytic H2 evolution at less negative potentials using a broad cross-section of the solar spectrum. The first generation of catalysts showed modest activity at the thermodynamic potential for H2O reduction to H2, but true energy storage could be possible if the electrochemical onset was substantially positive of the thermodynamic potential. The equivalence point of the hydride formation potential and the thermodynamic potential for H2 evolution is the crossover between a photocatalytic system that uses light to accelerate H2 release and a photosynthetic system that uses light to drive an uphill synthetic reaction.46 The scaling relationships described above underscore the need for new catalyst structures, as the use of bipyridine ligands with electron-withdrawing groups in Cp*Ir-based complexes leads to the desired potential shift but an undesired change in pKa of the hydride that makes protonation at neutral or basic pH challenging.

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A number of strategies to break scaling relationships have emerged, including secondary sphere effects, cooperative catalytic structures, and medium tuning.47–50 M/MH*

semiconductor photoanode

H+/H2

conductor cathode

hn2

hn1

EF

M/MH

eV O2/H2O

Figure 4. Proposed device architecture for implementation of a photoelectrocatalyst into a solar cell. As more molecular photoelectrocatalysts are discovered, one can begin to imagine their role in solar fuels applications. One plausible device architecture that would utilize a molecular photoelectrocatalyst for the fuel-forming half-reaction is shown in Figure 4. A photoanode would absorb photons in one wavelength range, oxidizing water to oxygen while injecting electrons into a semiconductor conduction band. The injected electrons could then reduce the molecular species at the cathode at the conduction band potential, electrochemically generating a metal hydride. The molecular hydride could then absorb photons at another wavelength, triggering photochemical H2 evolution. If the metal hydride is formed at a potential that is thermodynamically unfavorable for H2 evolution, both photons contribute to energy storage. The schematic of Figure 4 shares some characteristics with photogalvanic cells that utilized transition metal complexes for photo-induced electron transfer to generate current (as opposed to driving fuel synthesis).51 Such a construct might exhibit efficient photochemical fuel synthesis by avoiding charge separation processes and interfacial transport in semiconductor-based photocathodes. The tunability and detailed 21



thermodynamic and kinetic insight provided by molecular systems are also valuable. The rate of photon absorption in molecular dye-sensitized devices has been estimated to be ~1 s–1,52 providing a target rate for combined photoelectrocatalysis — a relatively modest rate by electrocatalyst standards. Beyond new possible architectures for the synthesis of solar fuels, molecular photoelectrocatalysis offer opportunities for fundamental developments and mechanistic insight into energy storage processes.

AUTHOR INFORMATION Corresponding author e-mails: [email protected] Notes: The authors declare no competing financial interests.

Kelsey R. Brereton received her Bachelor’s degree from Pepperdine University in California. She is currently a Graduate Student in the Department of Chemistry and the University of North Carolina at Chapel Hill. Her research focuses on interrogating the factors influencing the thermodynamics of transition metal hydride bonds for hydrogen storage applications.

Annabell G. Bonn is a Postdoctoral Associate in the Department of Chemistry at the University of North Carolina at Chapel Hill. After finishing her diploma studies in Germany, she received her PhD degree from the University of Basel in Switzerland. Her research interests include lightdriven charge separation and accumulation, molecular catalyst design and their utilization for H2 evolution.

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Alexander J. M. Miller is an Assistant Professor of Chemistry at the University of North Carolina at Chapel Hill. Using the synthetic tools and mechanistic probes of organometallic chemistry, the Miller group (http://millergroup.web.unc.edu) is developing new catalytic strategies for the sustainable synthesis of fuels and chemicals. Alex received his B.S. from the University of Chicago in 2005 before obtaining his Ph.D. in Chemistry in the laboratories of John Bercaw and Jay Labinger at Caltech in 2011. Before joining the faculty at Carolina, he was a Dreyfus Environmental Chemistry Postdoctoral Fellow with Karen Goldberg and James Mayer at the University of Washington, Seattle.

ACKNOWLEDGEMENTS This work was supported by the Division of Chemical Sciences, Geosciences & Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through Grant DESC0014255. K. R. B. acknowledges support from a University of North Carolina Dissertation Completion Fellowship. The authors gratefully acknowledge Bethany M. Stratakes, Matthew B. Chambers, Catherine L. Pitman, Seth M. Barrett for their contributions to the highlighted research.

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Possible Quotations to Highlight “A single system that is capable of (a) electrochemical hydride formation and (b) photochemical H2 release” “The pKa and hydricity of the hydride govern the region of hydride stability” “Discovering catalyst designs that overcome scaling relationships will help enable photoelectrocatalytic H2 evolution at less negative potentials using a broad cross-section of the solar spectrum.”

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Molecular Photoelectrocatalysts for Light-Driven Hydrogen Production

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