Bioinspired Metal Selenolate Polymers with Tunable Mechanistic

Subscriber access provided by UNIV OF CALIFORNIA SAN DIEGO LIBRARIES

Article

Bioinspired Metal Selenolate Polymers with Tunable Mechanistic Pathways for Efficient H2 Evolution Courtney A. Downes, and Smaranda C. Marinescu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b03161 • Publication Date (Web): 13 Dec 2016 Downloaded from http://pubs.acs.org on December 13, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Catalysis is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 24

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

ACS Catalysis

Bioinspired Metal Selenolate Polymers with Tunable Mechanistic Pathways for Efficient H2 Evolution Courtney A. Downes, and Smaranda C. Marinescu* University of Southern California, Department of Chemistry, Los Angeles, CA, 90089, USA.

ABSTRACT. The efficient reduction of water into hydrogen has emerged as an attractive strategy for the conversion of solar energy into chemical bonds. Hydrogenase enzymes efficiently catalyze this reaction. The [NiFeSe] hydrogenases, a subclass of the [NiFe] hydrogenase with a selenocysteine residue replacing a cysteine residue, display higher activities and O2-tolerance than the conventional sulfur only [NiFe] hydrogenases. Inspired by the enhanced activity upon replacement of sulfur with selenium seen in nature, we report here the syntheses and characterization of cobalt and nickel selenolate coordination polymers (CP) based on benzene-1,2,4,5-tetraselenolate (BTSe), which are efficient catalysts for the hydrogen evolution reaction (HER) from water. To reach a current density of 10 mA/cm2, the benchmarking metric for HER, both cobalt and nickel systems display overpotentials of only ~350 mV, displaying a reduction in overpotential compared to the previously reported cobalt and nickel CPs based on benzene-1,2,4,5-tetrathiolate (BTT). In addition, the cobalt selenolate polymer displays a 217 mV improvement in the overpotential compared to its sulfur only analogue that arises from the ability to promote an alternative mechanism at high catalyst loadings that was not available for the cobalt BTT CP.

ACS Paragon Plus Environment

1

ACS Catalysis

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

KEYWORDS:

electrocatalysis,

[NiFeSe]

hydrogenases,

Page 2 of 24

coordination

polymers,

HER

mechanism, water, solar-to-fuel technologies, artificial photosynthesis, solar energy conversion.

INTRODUCTION The ability to efficiently and cost-effectively convert solar energy into molecular hydrogen through water splitting is necessary for meeting rising energy demands and to mitigate the adverse effects of carbon-based fuels on the environment.1-3 Numerous homogeneous4-8 and heterogeneous9-12 systems have been developed to catalyze HER in order to replace noble metals such as platinum. Hydrogenase enzymes catalyze reversible HER from water with remarkably high turnover frequencies at low overpotentials and have inspired the design of synthetic catalysts based on earth-abundant metals.13-18 [NiFeSe] hydrogenases are a subclass of [NiFe] hydrogenases with a selenocysteine (Sec) replacing a cysteine (Cys) residue terminally bound to the Ni center (Figure 1).19 These [NiFeSe] hydrogenases have emerged as efficient catalysts for HER, due to their increased activity (by a factor of 40) and oxygen-tolerant H2 production in comparison with the conventional [NiFe] hydrogenases.20,21 Additionally, integration of [NiFeSe] hydrogenases into photocatalytic and photoelectrochemical systems has proven an effective and efficient method for direct light-to-hydrogen conversion displaying the viability of photobiological fuel production.22,23 Because of the successful utilization of [NiFeSe] hydrogenases as catalysts in light-driven H2 production devices, these enzymes have emerged as an attractive blueprint for designing synthetic H2 evolution catalysts. Although the high catalytic activity of [NiFeSe] hydrogenases are well known, the role selenium plays in catalysis is currently unclear, however, it has been proposed that the Sec residue acts as a proton relay during H2 cycling similar to the Cys residue in [NiFe]-


2

Page 3 of 24

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

ACS Catalysis

hydrogenases.21 The enhanced H2 production activity of [NiFeSe] hydrogenases has been attributed to the higher nucleophilicity of selenocysteine than cysteine and the lower pKa of a selenocysteine selenol than a cysteine thiol.21 Detection of protonated selenocysteine in hydrogenase during H2 production is challenging, therefore biomimetic molecular complexes bearing the Ni-Se moiety have been developed to facilitate a better understanding of the function of selenium during HER.21,24-26 However, under electrocatalytic conditions, a solid deposit formed on the electrode surface and it was established that the Ni-Se molecular mimics in solution were not responsible for the observed HER activity.21,24-26 Therefore, molecular systems active for electrocatalytic HER with a Ni-Se motif have yet to be developed. Heterogeneous systems based on pyrite-phase transition metal chalcogenides have been identified as earth-abundant and low-cost alternatives to platinum that display high efficiencies and stability for HER in both acidic and basic media.27-31 Interestingly, it has been shown that CoSe2 displays increased HER activity in comparison to CoS2 similar to the improvement in H2 production for [NiFe] hydrogenases when one of the cysteine residues is replaced by selenocysteine.21,24-26,29 It has been proposed that the strength of the E-H bond (where E = S or Se) can influence the rate of hydrogen desorption from the catalytic site during HER.32 Therefore, the enhanced activity of selenides versus sulfides could arise because the Se-H bond is weaker than the S-H bond.32 Cys S

NC Fe OC NC

Cys S

Cys

S

S

NC

Ni

Se Sec

Fe S

Cys

[NiFe]-hydrogenase

Cys

OC NC

Ni S

S

Cys

Cys

[NiFeSe]-hydrogenase

Figure 1. Structures of [NiFe] and [NiFeSe] hydrogenases active sites.


3

ACS Catalysis

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

Page 4 of 24

We have recently reported the immobilization of metal dithiolene units into extended frameworks as a method of heterogenizing molecular catalysts.33-35 These systems have been shown to retain their molecular nature while displaying enhanced stability, activity, and durability in acidic aqueous media in comparison to their molecular analogues. Cobalt and nickel coordination polymers (CPs) based on benzene-1,2,4,5-tetrathiolate (BTT) exhibit high electrocatalytic activities and efficiencies for HER in pH 1.3 aqueous solutions.34,35 Theoretical studies performed on the metal dithiolenes complexes have suggested that the mechanism of H2 evolution involves protonation of the sulfur moiety of the dithiolene ligand,36-38 which overtime may result in ligand loss and catalyst decomposition. We have shown that incorporation of metal dithiolenes into extended frameworks can reduce the prevalence of decomposition pathways and eliminates the use of organic solvents,33-35 which have been known to contribute to catalyst decomposition. Although the polymers were shown to operate at high efficiencies with no evidence of chemical decomposition, the overpotentials of 560 mV and 470 mV necessary to reach 10 mA/cm2 of activity in pH 1.3 aqueous solutions for the cobalt34 and nickel35 BTT polymers, respectively, were undesirable. Because of the enhanced activity observed for selenium-based catalysts in comparison to their sulfur counterparts, we set out to synthesize bioinspired selenolate analogues of our cobalt and nickel coordination polymers as a strategy to improve the electrocatalytic activity and reduce the overpotential for HER. Herein, we report that a cobalt CP based on benzene-1,2,4,5-tetraselenolate (BTSe) is an active HER electrocatalyst that displays enhanced activity and a reduced overpotential in comparison to its sulfur only analogue because of the availability of an alternate mechanism of catalysis at high catalyst loadings. Additional electrochemical studies on a nickel CP based on BTSe revealed activity toward HER in acidic


4

Page 5 of 24

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

ACS Catalysis

aqueous solutions offering possible insight into the role of selenium in the Ni-Se motif present in [NiFeSe] hydrogenase.

RESULTS AND DISCUSSION Characterization Cobalt and nickel CPs 1 and 2 were synthesized using a liquid-liquid interfacial reaction modified from the reported BTT polymers previously synthesized in our laboratory (eq 1).33,34 1,2,4,5-tetrakis(acetylseleno)benzene (BTSeAc4) was synthesized following a reported procedure and the isolation of the acetyl-protected selenol affords air-stability and reduces the difficulty associated with handling air-sensitive selenols.39 The acetyl groups in BTSeAc4 can be easily hydrolyzed in the presence of base, such as NaOtBu to form sodium BTSe (C6H2Se4Na4). An acetonitrile/ethyl acetate solution of [M(MeCN)6][BF4]2 (where M = Co, Ni) was gently layered on top of the aqueous solution of sodium BTSe. The organic solvents were allowed to evaporate over several hours leaving a black film (1 and 2) at the gas-liquid interface, which was subsequently deposited on the support of interest. Deposition was carried out by two methods: (a) immersing the support face down in the reaction mixture and (b) synthesizing 1 and 2 with the support at the bottom of the reaction vessel which was subsequently lifted up and through the film formed at the gas-liquid interface. Following deposition, the modified electrodes were washed with water and methanol to remove the residual starting materials. The poor film quality and low yield of 2 due to the interfacial reaction conditions limited its extensive characterization; therefore, electrochemical studies were focused on 1 whose synthetic preparation resulted in a high quality film and yield.


5

ACS Catalysis

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

AcSe

SeAc SeAc

AcSe BTSeAc 4 Se Se M Se Se

Page 6 of 24

[M(MeCN) 6][BF4]2 NaOt-Bu H 2O 1:4 MeCN/EtOAc Se Se M Se Se

Se Se M Se Se

3–

(1)

M = Co (1), Ni (2)

The FTIR spectrum of 1 shows the disappearance of the strong C=O stretch of the acetyl protecting group of BTSeAc4 around ~1720 cm–1 demonstrating the successful hydrolysis of the acetyl group (Figure S1). XPS studies of 1 reveal the presence of Co, Se (Figure 2), and Na (Figure S3). In the cobalt region, two peaks at binding energies of 778.5 and 793.6 eV are observed for the 2p3/2 and 2p1/2 levels with the corresponding satellite features similar to the previously reported cobalt CPs based on BTT, benzenehexathiolate (BHT), and triphenylene2,3,6,7,10,11-hexathiolate (THT).33,34 Two additional peaks are observed at 1071.3 and 55.1 eV which arise from the presence of Na 1s and Se 3d. Deconvolution of the Se 3d region reveals two peaks at 54.7 and 55.6 eV corresponding to 3d5/2 and 3d3/2 levels in agreement with the expected metal-selenolate binding motif.40 XPS studies of 2 reveal the presence of Ni with two peaks at binding energies of 853.5 and 870.7 eV along with the corresponding satellite features (Figure 2c), which is similar with the XPS spectra of reported Ni2+ complexes.41 Deconvolution of the selenium signal results in two peaks at 55.1 and 56 eV for the 3d5/2 and 3d3/2 levels (Figure 2d). A peak at 1071.1 eV indicates the presence of Na (Figure S4).


6

Page 7 of 24

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

ACS Catalysis

Figure 2. XPS analysis of 1 (a) Co 2p (b) Se 3d and 2 (c) Ni 2p (d) Se 3d core level XPS spectra.

Electrochemical studies The electrochemistry of 1 and 2 were investigated by cyclic voltammetry (CV) on glassy carbon electrodes (GCE). No differences in electrochemistry were observed for electrodes prepared from different synthetic batches or using deposition methods (a) or (b). A broad electrochemical wave is observed for 1 in pH 10.0 aqueous solutions between 0 and –0.3 V vs SHE (Figure S5). This wave is negatively shifted compared to the analogous cobalt BTT CP34, which is consistent with previous reports.24-26 Because selenium increases the electron density on the cobalt center, the reduction process is thermodynamically less favorable. Electrochemical investigation of the related molecular complexes, [Co(bdt)2]– and [Co(bds)2]– (where bdt = 1,2-benzenedithiolate and bds = 1,2-benzenediselenolate), revealed the reduction of [Co(bds)2]– occurred 40 mV more negative than [Co(bdt)2]–.42


7

ACS Catalysis

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

Page 8 of 24

The cathodic wave of 1 in pH 10.0 solutions varied linearly with the scan rate as expected for a surface confined process (Figures S5–S6). Large cathodic currents were generated as the pH of the solution was decreased (Figures 3a, and S7–S11), which is attributed to HER from water, as described below. This catalytic onset begins in pH 4.4 solutions (Figure S9) and reaches significant activity in the most acidic media tested, pH 1.3. Interestingly, the onset of catalysis emerges at the potential of the cathodic wave seen in pH 10.0 solutions (~ –0.25 V vs. SHE) in comparison to the sulfur analog, which occurs ~0.5 V more negative than its corresponding cathodic wave. The first onset is followed by a second at approximately –0.6 V vs. SHE possibly indicating a change in the mechanism of catalysis, as described below. Similar enhancements in catalytic current upon lowering the pH were seen for 2 which displayed HER activity beginning in pH 4.4 aqueous solutions (Figure 3b). However, only one onset is observed for polymer 2 indicating that the change in mechanism as a function of applied potential is unique to 1.


8

Page 9 of 24

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

ACS Catalysis

Figure 3. Polarization curves of 1 (a) (7.3×10–7 molCo/cm2) and 2 (b) (4.5×10–7 molNi/cm2) in 0.1 M NaClO4 aqueous solutions at pH 10.0 (orange), pH 7.0 (purple), 4.4 (green), 2.6 (blue), 1.3 (red), and of blank GCE at pH 1.3 (dashed black); scan rate for (a) in pH 10.0–2.6: 20 mV/s and in pH 1.3: 100 mV/s; scan rate for (b) in pH 10.0–7.0: 20 mV/s and in pH 4.4–1.3: 100 mV/s. The catalytic activity at –0.8 V vs SHE was directly proportional to the catalyst loading for 1 (Figure 4), which was estimated through integration of the electrochemical wave in pH 10.0 aqueous solutions. As the catalyst concentration increased, the intensity of the first onset at approximately –0.25 V vs SHE was greatly enhanced (Figure 5) suggesting the prominence of the mechanism of HER at more positive potentials is related to the catalyst loading. The highest catalyst loading achieved, 9.2×10–7 molCo/cm2, generated the benchmarking metric of 10 mA/cm2 of activity11 at an overpotential ( / ) of only 343 mV in pH 1.3 aqueous


9

ACS Catalysis

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

Page 10 of 24

solutions. The / for 1 ranges from ~600–350 mV as a function of its corresponding catalyst loading (Table 1).

Figure 4. Current densities of 1 measured at –0.80 V vs SHE at pH 1.3 as a function of the surface catalyst concentration. The surface concentration was quantified by integrating the peaks area at pH 10.0.

Figure 5. Polarization curves of 1 in 0.1 M NaOCl4 aqueous solutions at pH 1.3 at different catalyst loadings; 3.7×10–7 molCo/cm2 (green), 7.3×10–7 molCo/cm2 (blue),9.2×10–7 molCo/cm2 (red), and blank GCE (black dashed). Scan rate: 100 mV/s Table 1. Overpotential necessary to reach 10 mA/cm2 of activity in pH 1.3 aqueous solutions at various catalyst loadings of 1.


10

Page 11 of 24

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

ACS Catalysis

Catalyst Loading

@ /

× / 3.7

602

5.0

572

7.3

517

9.2

343

To ensure production of H2 is responsible for the increase in current density in pH 1.3 solutions, controlled potential electrolysis experiments (CPE) were performed. CPE of 1 performed at –0.8 V vs. SHE in pH 1.3 aqueous solutions showed the material is stable for 1 hour under reductive and acidic conditions (Figure 6). Over 1 hour, 81 coulombs of charge were consumed and analysis of the gaseous headspace of the electrolysis cell by gas chromatography confirmed the production of H2 with a Faradaic efficiency (FE) of 97±2%. Additionally, because two onsets of catalysis are observed in CV experiments, it was important to determine if the earlier onset is also due to HER by 1. CPE experiments were conducted at –0.44 V vs. SHE in pH 1.3 aqueous solutions. 53 coulombs of charge were passed after 1 hour with a 98±2% FE for H2 production, suggesting that 1 is an efficient catalyst for HER at low ɳ.


11

ACS Catalysis

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

Page 12 of 24

Figure 6. CPE of 1 in pH 1.3 H2SO4 aqueous solution at –0.8 V vs. SHE (1.2×10–7 molCo/cm2; red), –0.44 V vs. SHE (4.0×10–7 molCo/cm2; blue), and blank GCE (black dashed). Since electrocatalytic proton reduction is responsible for the two onsets seen for 1 and not a secondary process, we propose the two onsets are a result of a change in mechanism as the potential is swept more negative (Figure 7). Following a one-electron reduction and a single protonation at one of the selenolate moieties of the ligand, catalysis can proceed through two pathways. At more positive potentials, a second protonation followed by the final reduction to produce H2 is favored (ECCE where E = electron transfer and C = chemical reaction). As the potential is scanned more negative, reduction followed by protonation and release of H2 is preferred (ECEC). Similar changes in mechanism as a function of applied potential have been previously observed for nickel bis(aryldithiolene) complexes for HER.43 Because the intensity of


12

Page 13 of 24

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

ACS Catalysis

the first onset increases as a function of catalyst loading (Table 1, Figure 5), we propose that the first mechanism is preferred as the catalyst loading increases. Theoretical studies of the cobalt dithiolene complex, [Co(bdt)2]–, suggest that upon oneelectron reduction of [Co(bdt)2]–, two protonations occur on different sulfur moieties of the benzenedithiolate ligand followed by a second one-electron reduction.37 A subsequent intramolecular proton shift to form a cobalt hydride adjacent to a protonated sulfur leads to release of H2.37 Two mechanistic pathways for HER have been proposed for nickel dithiolenes, ECEC and ECCE (E = electron transfer and C = chemical reaction) with protonation occurring solely on the sulfur moiety of the dithiolene ligand.43 Calculations have suggested that geometric distortions occur to accommodate the protonated ligand and to allow for H2 evolution.43 The difference in mechanistic pathways for the cobalt and nickel dithiolenes arises from the difference in the frontier orbitals of these complexes. For nickel bis(aryldithiolene) complexes, it is proposed that the protonation and redox events occur primarily on the ligand because the frontier orbitals are mainly ligand based.43 No stable species protonated at both nickel and sulfur have been observed for nickel dithiolenes. For the cobalt bis(dithiolene) complexes, however, the cobalt orbitals contribute more significantly to the frontier orbitals so protonation and redox events occur on both the ligand and the metal center.37 We propose that the ECCE mechanism, favored at higher catalyst loadings, involves protonation of the selenium moieties, since selenium is known to be more readily protonated than its corresponding sulfur analogue.21,24,32 Similar with the mechanism of H2 evolution by nickel bis(aryldithiolene) complexes, the ECCE mechanism by cobalt selenolate polymers to evolve H2 observed here may involve the recombination of two protonated selenium moieties, which are in closer proximity at high catalyst loadings. This type of mechanism does not


13

ACS Catalysis

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

Page 14 of 24

necessitate an intramolecular proton transfer to form a metal hydride. For nickel bis(aryldithiolene) complexes, following a geometric distortion, the two protonated sulfurs recombine from the same catalytic unit to evolve H2.43 Because of the rigidity of the coordination polymer network, we suggest that two protonated selenium moieties from two different catalytic units can combine following a second one-electron reduction and evolve H2. As the catalyst loading increases, the availability of protonated selenium moieties in close enough proximity to recombine upon one-electron reduction increases. The appearance of two onsets and the possibility of two available mechanisms was not observed for the related cobalt BTT CP indicating that this dual mechanism is unique to 1. The / of 343 mV for 1 is a significant improvement from that of the cobalt BTT derivative of 560 mV (Figure 7).34 We attribute this enhanced activity to the larger exchange current density of 1, further discussed below, and the availability of a mechanism that is preferred at higher catalyst loadings. Once activated, catalysis occurs at the potential of the cathodic wave seen in pH 10.0 solutions (~ –0.25 V vs. SHE) in comparison to HER for the sulfur analog, which only occurs at potentials ~0.5 V more negative than its corresponding cathodic wave. The activity of 1 is similar to that of the cobalt BHT framework reported by us ( / = 340 mV).33 At a catalyst loading of 9.0×10–7 molNi/cm2, 2 displays an / of 353 mV in pH 1.3 solution (Figure S12). This is an improvement from the 470 mV ɳ necessary for the related nickel BTT CPs at a catalyst loading of 6.8×10–7 molNi/cm2.35 An improvement in catalytic activity has previously been reported for [NiFeSe] hydrogenase and CoSe2 in comparison to their sulfur only analogues.21,24-26,29


14

Page 15 of 24

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

ACS Catalysis

Figure 7. Polarization curves of 1 (9.2×10–7 molCo/cm2, red), cobalt BTT CP (CoBTT, 5.5×10–7 molCo/cm2, blue), and blank GCE (black dashed) in 0.1 M NaOCl4 aqueous solutions at pH 1.3. Tafel analysis of 1 gave a Tafel slope of 97 mV/dec and an exchange current density of 10–4.4 A/cm2 (Figure S13). Tafel slopes and exchange current densities of the previously reported cobalt and nickel coordination polymers have been included in Table 2.33-35 The cobalt BTT CP exhibits the lowest Tafel slope which indicates the highest intrinsic activity for HER, however, it is the least active of the four cobalt based polymers studied in our laboratory. The higher Tafel slopes of 1, cobalt BHT, and cobalt THT signifies the rate-determining step in the HER mechanism is the Volmer reaction, the discharge step that converts protons into absorbed hydrogen on the catalyst surface. The reduction in the Tafel slope for the cobalt BTT polymers suggests a reduction in the energy barrier associated with the Volmer reaction. Further investigation is necessary to determine the origin of the large variance (70–160 mV/dec) in the experimentally determined Tafel slopes.


15

ACS Catalysis

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

Page 16 of 24

Table 2. Catalytic HER activity of metal diselenolate and dithiolate coordination polymers. M

Co Ni Co Ni Co Co

Liganda

BTSe (1) BTSe (2) BTT BTT BHT THT

Catalyst Loading

× / 9.2 9.0 5.5 6.8 7.0 11.0

@ / 343 353 560 470 340 530

Tafel slope (mV/dec) 97 (pH = 1.3) ̶ 70 (pH = 1.3) 76 (pH = 2.6) 108 (pH = 2.6) 161 (pH 2.6)

i0 (A/cm2)

Reference

10–4.4 ̶ –9.4 10 10–8.1 10–5.3 10–5.4

this work this work 34 35 33 33

a

Where BTSe = benzene-1,2,4,5-tetraselenolate, BTT = benzene-1,2,4,5-tetrathiolate, BHT = benzenhexathiolate, and THT = triphenylene-2,3,6,7,10,11-hexathiolate.

Although the Tafel slope predicts high intrinsic activity, the exchange current density for the cobalt BTT polymer is the lowest of the four materials. High exchange current densities are characteristic of high electrocatalytic activity. The cobalt BTSe, BHT, and THT polymers possess similar exchange current densities. This can explain the comparative activity of the BTSe and BHT frameworks that exhibit similar Tafel slopes and exchange current densities, and can reach 10 mA/cm2 of activity at overpotentials ~350 mV. The THT system, on the other hand, requires an overpotential of 530 mV. Although the THT system has a high exchange current density in comparison to the cobalt BTT system, the overpotential to reach 10 mA/cm2 is very similar (530–560 mV) which may be attributed to its large Tafel slope of 161 mV/dec. The Tafel slope of 161 mV/dec is the largest of the coordination polymers synthesized in our laboratory suggesting the energy barrier of desorption of H2 from the catalyst surface (Volmer reaction) is greatest for the cobalt THT system. Further CPE studies of 2 also revealed continuous charge build up over 1 hour in pH 1.3 at –0.55 V vs. SHE, however, the system operated at a reduced efficiency of ~70% (Figure S14). This reduction in efficiency for the nickel analogue relative to cobalt was also observed for the BTT derivatives, with the nickel BTT CP operating at an 85% FE compared to 97% for


16

Page 17 of 24

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

ACS Catalysis

cobalt.34,35 Polymer 1 was also subjected to 4 hours of CPE in pH 1.3 at –0.6 V vs SHE and displayed moderate stability (Figure S15). CV experiments before and after exposure to strong reductive acidic conditions revealed no significant change (Figures S16–17). XPS analyses following 5 hours of CPE for 1 and one hour CPE for 2 in pH 1.3 revealed no change in the features observed prior to electrochemical testing (Figures S18–19). Negligible amounts of cobalt and selenium were observed in ICP-OES measurements of the pH 1.3 solution used for longer duration CPE experiments of 1 indicating no cobalt or selenium species are solubilized over the course of electrochemical analysis. Negligible activity in pH 1.3 solutions was observed for all starting materials and potential byproducts, therefore the observed activity for HER was attributed to polymers 1 and 2 (Figure S20).

CONCLUSIONS In conclusion, we have demonstrated the preparation of novel bioinspired cobalt and nickel selenolate polymers, which act as efficient electrocatalysts for HER from water. We report here the first molecular system utilizing the Co-Se motif to be investigated as a highly active and stable catalyst for HER in acidic aqueous media representing an important step in understanding the role of selenium in facilitating and improving HER activity in aqueous solutions. Polymer 1 exhibits an / of only 343 mV in pH 1.3 aqueous solutions at a catalyst loading of 9.2×10–7 molCo/cm2. The replacement of sulfur with selenium results in a 217 mV improvement in the ɳ to reach the same H2-evolving activity. This improvement arises from the availability of an alternate mechanism of catalysis for 1 that is preferred at higher catalyst loadings. We propose two potential dependent mechanisms, ECCE and ECEC, with ECCE greatly reducing the


17

ACS Catalysis

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

Page 18 of 24

overpotential for HER at higher catalyst loadings. Improvements in H2-evolving activity were also observed for the nickel system (2), which exhibits a 117 mV enhancement in / compared to the analogous nickel BTT CP. The significant improvement in HER activity originating from the simple modification of the ligand framework displays the advantages of maintaining the molecular nature of catalysts, while the low ɳ and durability under prolonged reductive acidic conditions documents the advantages of heterogenization of molecular catalysts. The incorporation and immobilization of molecular catalysts via CPs has proven a viable method to systematically tune important catalytic metrics such as mechanistic pathways, overpotential, stability, and activity, aiding in the design of nonprecious and efficient HER catalysts.

EXPERIMENTAL SECTION General All manipulations of air and moisture sensitive materials were conducted under a nitrogen atmosphere in a Vacuum Atmospheres glovebox or on a dual manifold Schlenk line. The glassware was oven-dried prior to use. Water was deionized with the Millipore Synergy system (18.2 MΩ·cm resistivity). Ethyl acetate and water were placed under vacuum and refilled with nitrogen (10 ×). Acetonitrile was degassed with nitrogen and passed through activated alumina columns and stored over 4Å Linde-type molecular sieves 1,2,4,5-tetrakis(acetylseleno)benzene39 was prepared according to the reported procedure. All other chemical reagents were purchased from commercial vendors and used without further purification. The pHs of the aqueous solutions were measured with a benchtop Mettler Toledo pH meter.


18

Page 19 of 24

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

ACS Catalysis

Caution: Perchlorate salts are potential explosive chemicals and should only be used in very small amounts, especially in the presence of the reductant H2. Please handle these mixtures using the proper equipment and protection. Synthesis of 1 In a glove-box, NatOBu (0.18 mL of 0.5 M solution in methanol, 4 equiv.) was added to 1,2,4,5tetrakis(acetylseleno)benzene (12.88 mg, 0.023 mmol). Once dissolved, 20 mL of degassed H2O was added to the solution. The aqueous solution was transferred to a 70 mm × 50 mm crystallizing dish. [Co(MeCN)6][BF4]2 (36.3 mg, 0.146 mmol) was dissolved in a 1:4 mixture of acetonitrile and ethyl acetate and 1 mL of this solution was carefully layered via glass pipette on top of the aqueous solution to cover ~80% of the surface area. The organic solvents were allowed to evaporate over one to two hours at room temperature, leaving behind 1 as a black solid at the gas-liquid interface. The black solid was collected through centrifugation, washed with water, methanol, and ethyl acetate. The resultant black powder was dried under vacuum. Synthesis of 2 In a glove-box, NatOBu (0.18 mL of 0.5 M solution in methanol, 4 equiv.) was added to 1,2,4,5tetrakis(acetylseleno)benzene (12.88 mg, 0.023 mmol). Once dissolved, 20 mL of degassed H2O was added to the solution. The aqueous solution was transferred to a 70 mm × 50 mm crystallizing dish. [Ni(MeCN)6][BF4]2 (36.3 mg, 0.146 mmol) was dissolved in a 1:4 mixture of acetonitrile and ethyl acetate and 1 mL of this solution was carefully layered via glass pipette on top of the aqueous solution to cover ~80% of the surface area. The organic solvents were allowed to evaporate over one to two hours at room temperature, leaving behind 2 as a black solid at the gas-liquid interface.


19

ACS Catalysis

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

Page 20 of 24

Deposition of 1 and 2 Deposition was carried out by two methods: (a) immersing the support face down in the reaction mixture and (b) synthesizing 1 or 2 with the support at the bottom of the reaction vessel which was subsequently lifted up and through the film formed at the gas-liquid interface. Following deposition, the substrate was washed with water and methanol. Electrochemical methods Electrochemistry experiments were carried out using a Pine potentiostat. Platinum wire used for the electrochemical studies was purchased from Alfa Aesar. The electrochemical experiments were carried out in a three electrode configuration electrochemical cell under an inert atmosphere using glassy carbon electrodes (GCE) as the working electrode. A platinum wire, placed in a separate compartment, connected by a Vycor tip, and filled with the electrolytic solution (0.1 M NaClO4) was used as the auxiliary (counter) electrode. The reference electrode, placed in a separate compartment and connected by a Vycor tip, was based on an aqueous Ag/AgCl/saturated KCl electrode. The reference electrode in aqueous media was calibrated externally relative to ferrocenecarboxylic acid (Fc-COOH) at pH 7.0, with the Fe3+/2+ couple at 0.28 V vs Ag/AgCl. All potentials reported in this paper were converted to standard hydrogen electrode by adding a value of 0.205 V, or to reversible hydrogen electrode (RHE) by adding a value of (0.205 + 0.059 × pH) V. Controlled potential electrolysis Controlled potential electrolysis measurements to determine Faradaic efficiency and study longterm stability were conducted in a sealed two-chambered H cell where the first chamber held the working and reference electrodes in 50 mL of 0.1 M NaClO4 (aq) at the corresponding pH, and


20

Page 21 of 24

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

ACS Catalysis

the second chamber held the auxiliary electrode in 25 mL of 0.1 M NaClO4 (aq). The two chambers, which were both under N2, were separated by a fine porosity glass frit. Glassy carbon plate electrodes (6 cm × 1 cm × 0.3 cm; Tokai Carbon USA) were used as the working and auxiliary electrodes. The reference electrode was a Ag/AgCl/saturated KCl(aq) electrode separated from the solution by a Vycor tip. Using a gas-tight syringe, 2 mL of gas were withdrawn from the headspace of the H cell and injected into a gas chromatography instrument (Shimadzu GC-2010-Plus) equipped with a BID detector and a Restek ShinCarbon ST Micropacked column. To determine the Faradaic efficiency, the theoretical H2 amount based on total charge flowed is compared with the GC-detected H2 produced from controlled-potential electrolysis.

ASSOCIATED CONTENT Supporting Information. Additional experimental procedures, spectroscopic characterization, and electrochemical data. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected] Notes The authors declare no competing financial interest.


21

ACS Catalysis

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

Page 22 of 24

ACKNOWLEDGMENTS We are grateful to the University of Southern California (USC) for funding and the USC Wrigley Institute for the Norma and Jerol Sonosky summer fellowship to CAD. XPS data were collected at the Center for Electron Microscopy and Microanalysis, USC. We are grateful to the Anton Burg Foundation for their sponsorship of the elemental analysis instrument.

REFERENCES (1)

Lewis, N. S.; Nocera, D. G. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15729-

15735. (2) Gray, H. B. Nat. Chem. 2009, 1, 7-7. (3) Eisenberg, R.; Nocera, D. G. Inorg. Chem. 2005, 44, 6799-6801. (4) McKone, J. R.; Marinescu, S. C.; Brunschwig, B. S.; Winkler, J. R.; Gray, H. B. Chem. Sci. 2014, 5, 865-878. (5) Dempsey, J. L.; Brunschwig, B. S.; Winkler, J. R.; Gray, H. B. Acc. Chem. Res. 2009, 42, 1995-2004. (6) Thoi, V. S.; Sun, Y.; Long, J. R.; Chang, C. J. Chem. Soc. Rev. 2013, 42, 23882400. (7) Du, P.; Eisenberg, R. Energy Environ. Sci. 2012, 5, 6012-6021. (8) Artero, V.; Chavarot-Kerlidou, M.; Fontecave, M. Angew. Chem. Int. Ed. 2011, 50, 7238-7266. (9) Morales-Guio, C. G.; Hu, X. Acc. Chem. Res. 2014, 47, 2671-2681. (10) Faber, M. S.; Jin, S. Energy Environ. Sci. 2014, 7, 3519-3542. (11) McCrory, C. C. L.; Jung, S.; Ferrer, I. M.; Chatman, S. M.; Peters, J. C.; Jaramillo, T. F. J. Am. Chem. Soc. 2015, 137, 4347-4357. (12) Merki, D.; Hu, X. Energy Environ. Sci. 2011, 4, 3878-3888. (13) Frey, M. ChemBioChem 2002, 3, 153-160. (14) Darensbourg, M. Y. L., E. J.; Smee, J. J. Coord. Chem. Rev. 2000, 206-207, 533– 561. (15) Darensbourg, M. Y.; Lyon, E. J.; Zhao, X.; Georgakaki, I. P. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 3683-3688. (16) Vincent, K. A.; Parkin, A.; Armstrong, F. A. Chem. Rev. 2007, 107, 4366-4413. (17) Cracknell, J. A.; Vincent, K. A.; Armstrong, F. A. Chem. Rev. 2008, 108, 2439– 2461. (18) Lubitz, W.; Ogata, H.; Rüdiger, O.; Reijerse, E. Chem. Rev. 2014, 114, 40814148.


22

Page 23 of 24

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

ACS Catalysis

(19) E., G.; Vernede, X.; Hatchikian, E. C.; Volbeda, A.; Frey, M.; Fontecilla-Camps, J. C. Structure 1999, 7, 557-566. (20) Parkin, A.; Goldet, G.; Cavazza, C.; Fontecilla-Camps, J. C.; Armstrong, F. A. J. Am. Chem. Soc. 2008, 130, 13410-13416. (21) Wombwell, C.; Caputo, C. A.; Reisner, E. Acc. Chem. Res. 2015, 48, 2858-2865. (22) Reisner, E.; Powell, D. J.; Cavazza, C.; Fontecilla-Camps, J. C.; Armstrong, F. A. J. Am. Chem. Soc. 2009, 131, 18457-18466. (23) Mersch, D.; Lee, C.-Y.; Zhang, J. Z.; Brinkert, K.; Fontecilla-Camps, J. C.; Rutherford, A. W.; Reisner, E. J. Am. Chem. Soc. 2015, 137, 8541-8549. (24) Wombwell, C.; Reisner, E. Chem. Eur. J. 2015, 21, 8096-8104. (25) Wombwell, C.; Reisner, E. Dalton Trans. 2014, 43, 4483-4493. (26) Figliola, C.; Male, L.; Horton, P. N.; Pitak, M. B.; Coles, S. J.; Horswell, S. L.; Grainger, R. S. Organometallics 2014, 33, 4449-4460. (27) Faber, M. S.; Lukowski, M. A.; Ding, Q.; Kaiser, N. S.; Jin, S. J. Phys. Chem. C 2014, 118, 21347-21356. (28) Faber, M. S.; Dziedzic, R.; Lukowski, M. A.; Kaiser, N. S.; Ding, Q.; Jin, S. J. Am. Chem. Soc. 2014, 136, 10053-10061. (29) Kong, D.; Cha, J. J.; Wang, H.; Lee, H. R.; Cui, Y. Energy Environ. Sci. 2013, 6, 3553-3558. (30) Kong, D.; Wang, H.; Lu, Z.; Cui, Y. J. Am. Chem. Soc. 2014, 136, 4897-4900. (31) Basu, M.; Zhang, Z. W.; Chen, C. J.; Chen, P. T.; Yang, K. C.; Ma, C. G.; Lin, C. C.; Hu, S. F.; Liu, R. S. Angew. Chem. Int. Ed. 2015, 54, 6211-6216. (32) Anantharaj, S.; Ede, S. R.; Sakthikumar, K.; Karthick, K.; Mishra, S.; Kundu, S. ACS Catal. 2016, 6, 8069-8097. (33) Clough, A. J.; Yoo, J. W.; Mecklenburg, M. H.; Marinescu, S. C. J. Am. Chem. Soc. 2015, 137, 118-121. (34) Downes, C. A.; Marinescu, S. C. J. Am. Chem. Soc. 2015, 137, 13740-13743. (35) Downes, C. A.; Marinescu, S. C. Dalton Trans. 2016, 45, 19311-19321. (36) Eckenhoff, W. T.; McNamara, W. R.; Du, P.; Eisenberg, R. Biochim. Biophys. Acta 2013, 1827, 958-973. (37) Solis, B. H.; Hammes-Schiffer, S. J. Am. Chem. Soc. 2012, 134, 15253-15256. (38) Letko, C. S.; Panetier, J. A.; Head-Gordon, M.; Tilley, T. D. J. Am. Chem. Soc. 2014, 136, 9364-9376. (39) Turner, D. L.; Vaid, T. P. J. Org. Chem. 2012, 77, 9397-9400. (40) Yagüe, J. L.; Agulló, N.; Fonder, G.; Delhalle, J.; Mekhalif, Z.; Borrós, S. Plasma Process. Polym. 2010, 7, 601-609. (41) Martin, D. J.; McCarthy, B. D.; Donley, C. L.; Dempsey, J. L. Chem. Commun. 2015, 51, 5290-5293. (42) Sandman, D. J.; Allen, G. W.; Acampora, L. A.; Stark, J. C.; Jansen, S.; Jones, M. T.; Ashwell, G. J.; Foxman, B. M. Inorg. Chem. 1987, 26, 1664-1669. (43) Zarkadoulas, A.; Field, M. J.; Papatriantafyllopoulou, C.; Fize, J.; Artero, V.; Mitsopoulou, C. A. Inorg. Chem. 2016, 55, 432-444.

TOC Graphic.


23

ACS Catalysis

Page 24 of 24

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 ACS Paragon Plus Environment

24

Bioinspired Metal Selenolate Polymers with Tunable Mechanistic

Recommend Documents