Influence of Redox-Inactive Cations on the Structure and

Sep 30, 2015 - The elimination of prolonged cation-exchange submersion dramatically reduced the synthesis time scale from days to minutes...
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Influence of Redox-Inactive Cations on the Structure and Electrochemical Reactivity of Synthetic Birnessite, a Heterogeneous Analog for the Oxygen-Evolving Complex Jack H. Baricuatro,*,† Fadl H. Saadi,†,‡ Azhar I. Carim,§ Jesus M. Velazquez,† Youn-Geun Kim,† and Manuel P. Soriaga*,†,∥ †

Joint Center for Artificial Photosynthesis, ‡Division of Engineering and Applied Sciences, and §Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States ∥ Department of Chemistry, Texas A&M University, College Station, Texas 77843, United States ABSTRACT: Electrochemical protocols were developed for the facile potentiostatic deposition of birnessite films, supported on Au substrates, to serve as a structural motif for oxygen evolution reaction electrocatalysts. The elimination of prolonged cation-exchange submersion dramatically reduced the synthesis time scale from days to minutes. The electrodeposited films were characterized using a combination of X-ray diffraction, scanning electron microscopy, transmission electron microscopy, scanning tunneling microscopy, and X-ray photoelectron spectroscopy. The prepared birnessite films were crystalline, monophasic oxide materials that contained Mn3+, Mn4+, traces of Mn2+, and the intercalant of choice. Redox-inactive Na+, Ca2+, Sr2+, Y3+, and Zn2+ cations showed minimal influence on the voltammetric behavior of birnessite in the presence of Mn2+(aq). Slightly more significant effects emerged during potential cycling and chronopotentiometry of birnessite films in 0.1 M NaOH. The potential needed to sustain a current density of 10 mA cm−2 in 0.1 M NaOH increased according to the sequence Na+ < Ca2+ < Sr2+ < Y3+ < Zn2+. The sequence, with slight inversions in the order, was reminiscent of the trend in the heterometal-dependent modulation of the half-wave potential of the redox couple Mn3+Mn24+/Mn34+ in nonaqueous solutions of heterometallic manganese−dioxido cluster systems. Unlike the case of the homogeneous cluster catalysts, the electrochemical reactivity of intercalated birnessite films did not vary linearly with the pKa of the redox-inactive cations.

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the Mn4CaO5 cluster; the “dangler” exocuboidal Mn in the OEC, however, remains unduplicated by the birnessite motif. Mx+ ions, like Ca2+, can be judiciously introduced into the motif via intercalation. The role of many redox-inactive cations in biological electron-transfer systems is not fully understood. Specifically, the debate on the exact role of Ca2+ in the OEC is premised on the following propositions: (i) Ca2+ participates in a network of hydrogen bonds14,15 that facilitates efficient proton transfer and contributes to the binding, delivering, and reloading of water to the OEC active site.16,17 (ii) The coordination of Ca2+ with water enhances the nucleophilicity of oxygen and aligns it properly for an effective O−O bond formation.18 (iii) Ca2+ favorably regulates the protonation state and the reduction potential of the Ca2+-bound water that is hydrogen-bonded to the μ-oxo-Mn units.19 All three assertions are anchored, in part, to the observation that the replacement of Ca2+ by its close chemical congener, Sr2+, leads to dramatic changes in the catalytic performance of the OEC.16,20,21 Property modulation by the systematic variation of redox-inactive cations is,

INTRODUCTION Birnessite is a natural mineral of manganese characterized by stacked sheets of edge-sharing octahedral MnO6 units.1 The open layered structure of birnessite provides a high surface area accessible to a variety of ion-exchange processes2,3 and redox reactions4,5 responsible for the transport, concentration, and transformation of Mn, along with its mineralogical associates, in aquatic and terrestrial environments.6 Natural samples are poorly crystalline, but synthetic strategies are available to produce hexagonal, monoclinic,7 and triclinic crystals8 of birnessite. Intercalated cations generally preserve the electroneutrality of the sheets,9,10 which attain excess negative charges due to Mn vacancies and the coexistence of Mn3+ and Mn4+ centers11 in the polyhedra. The compositional and structural similarity of birnessite to the oxygen-evolving center (OEC) in Photosystem II provides an attractive motif for the creation of multimetallic electrocatalysts active toward the oxygen evolution reaction (OER) of water. The structural resemblance stems from the abundance of mono-μ-oxo and di-μ-oxo bridges in the edge-sharing octahedra of birnessite. The same connectivity is present between three Mn3+,4+ ions and one Ca2+ in the Mn4CaO5 cluster of the OEC.12 Metal ions, Mx+, in the intersheet region can coordinate with defect-free sections of the birnessite sheets, giving rise to an Mx+Mn3O4 cuboidal configuration13 imitative of the core of © XXXX American Chemical Society

Special Issue: Kohei Uosaki Festschrift Received: July 21, 2015 Revised: September 10, 2015

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allowed the insertion of a reference electrode, a large-surfacearea Au foil auxiliary electrode (99.9975%, Alfa Aesar), and a working electrode, either made of Au foil (99.9975%, Alfa Aesar) or an ITO-coated glass slide (Sigma-Aldrich, St. Louis, MO) with a surface resistivity of 8−12 Ω cm. The reference electrode for voltammetry in acidic media was Ag/AgCl (1 M KCl) (CH Instruments, Austin, TX); Hg/HgO (1 M NaOH) (CH Instruments, Austin, TX) was used for chronopotentiometry in alkaline solution. Electrolytic solutions were bubbled, at least 30 min prior to use, with either N2(g) for voltammetry or O2(g) for chronopotentiometry. The same gases were used to blanket the electrochemical cell headspace during each experiment. All precursor solutions for birnessite contained 2 mM MnSO4 (99.99%, Sigma-Aldrich). Sulfates of Na+, Ca2+, Sr2+, Y3+, and Zn2+ (at least 99%, Aldrich) were used as supporting electrolyte. The concentration of the supporting electrolyte was 0.2 M, except for SrSO4 and CaSO4 wherein, due to solubility limitations, their saturated solutions were used instead. The pH of the solution was adjusted to 5.3 ± 0.3 using a 0.1 M solution of the hydroxide of the cation under investigation; a saturated solution of Y2O3 (99.99%, Alfa Aesar) was used instead of Y(OH)3. X-ray Photoelectron Spectroscopy. Elemental composition and valence information were acquired from an AXIS Ultra DLD instrument (Kratos Analytical, Chestnut Ridge, NY) at a background pressure of 1 × 10−9 Torr. High-intensity excitation was provided by monochromatic Al Kα X-rays, 1486.6 eV in energy, with a 0.2 eV resolution at full width at half-maximum (fwhm) value. Photoelectrons were collected at the surface normal using a retarding (pass) energy of 20 eV. The peak energies were calibrated against the binding energy of the adventitious C 1s peak, which was taken as 284.65 eV. Peak fitting was performed using CasaXPS software (SurfaceSpectra Ltd., Manchester, UK) assuming a Shirley background and symmetric Voigt line shapes composed of Gaussian (70%) and Lorentzian (30%) components. Scanning Electron Microscopy. The gross morphology of the electrochemically deposited birnessite was analyzed using an FEI Nova NanoSEM 450 scanning electron microscope (FEI, Hillsboro, OR) at an accelerating voltage of 15 kV, with a working distance of 5 mm and an in-lens secondary electron detector. A 400 μm thick, single-side polished wafer of Asdoped n+-Si(111) with a resistivity range of 0.004−0.006 Ω cm (Addison Engineering, San Jose, CA) was coated with a sputtered film of Au atop a Cr adhesion layer. Approximately 10 × 5 mm2 sections of the Au-coated n+-Si(111) wafer were used as substrates for the electrodeposition of various surface coverages of birnessite. The sections were cleaved parallel to the short axis approximately midway along the long axis. Micrographs were then acquired along the freshly cleaved edges. X-ray Diffraction. X-ray diffraction patterns of birnessite electrodeposited on ITO substrates were acquired using a Bruker D8 Discover diffractometer (Bruker Inc., Madison, WI) equipped with a Cu Kα X-ray source (λ = 1.5418 Å) and VÅNTEC-500 2D detector. X-ray diffraction patterns of birnessite on Au substrates and of birnessite powder harvested from ITO substrates (by exfoliation with a razor blade) were acquired using a Bruker D2 Phaser diffractometer equipped with a Cu Kα X-ray source (λ = 1.5418 Å) and LYNXEYE 1D detector. All X-ray diffraction patterns were acquired using a θ− 2θ geometry.

therefore, explored herein with the birnessite motif. The effects of Na+, Ca2+, Sr2+, Y3+, and Zn2+ on the electrochemistry of birnessite are a convolution of many factors that may also be operative in the OEC.22 The trends observed in this work resemble those of molecular cluster mimics of the OEC,23 with contradistinctions in the magnitude and origin of the effects. The results are notably nuanced by the fundamental differences between homogeneous and heterogeneous catalyses that proceed under intrinsically different reaction conditions. The wide array of synthesis protocols for birnessite in the mineralogical literature demonstrates the rich redox chemistry of Mn. Conventional methods are hinged either on the oxidation of Mn2+ precursors24−26 or on the reduction of permanganate ion (MnO4−)27−29 in solution. A comproportionation reaction between Mn2+ and MnO4− is also an established alternative route.30,31 Prolonged hydrothermal treatments are customary to ensure structural order of the product and to circumvent its conversion into other polymorphs of manganese oxide. As a structural motif, birnessite allows the conversion of its layered structure into any of over 20 polymorphs of manganese oxide.24,32,33 Oxides of manganese with lower oxidation states, such as MnO(OH)2 and MnO·Mn2O3, are also electrochemically accessible on both metallic34 and semiconductor35 substrates. Electrodeposition provides a nonthermal reaction pathway based on the interplay of the chosen electrode, supporting electrolyte, solution pH, and potential−current density schemes. Typical synthesis of birnessite takes 1−3 days for completion;24,26,27,29,33 in the present work, birnessite films are prepared within minutes to an hour, depending on the desired surface coverage. Because electrodes may act as electron “sink” and “source”, conventional redox reagents in the synthesis of birnessite are bypassed and their byproducts, which often beset purification procedures, are also avoided. Electrodeposited birnessite readily conforms to the unique geometry and size of the working electrode. Unlike samples generated by precipitation, electrodeposits do not require drop-casting procedures or surface binders to prepare the material for further electrochemical characterization. The facile synthesis of near-isostructural members of the birnessite family is invaluable in the isolation of compositional effects from the closely intertwined structural effects in the rational design of electrocatalysts. To generate functional structures with tailorable physicochemical properties, the birnessite motif offers two sites for compositional modification: the interconnected framework of octahedral units and the interlayer space. This paper reports the implementation of the latter option via intercalation under electrochemical conditions.

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EXPERIMENTAL SECTION Electrochemistry. Birnessite films were potentiostatically electrodeposited onto a 1 cm2 Au foil substrate (99.9975%, Alfa Aesar, Ward Hill, MA), unless otherwise specified. The Au foil was cleaned by a 10 min soak in hot concentrated HNO3 (EMD Chemicals, Gibbstown, NJ) followed by a thorough rinse with Nanopure water (ThermoFisher Scientific, Waltham, MA). The electrode was slowly heated to yellow-orange incandescence using a fine jet of hydrogen flame and was then quenched in water. Electrochemical measurements were performed using an SP200 Bio-Logic potentiostat (Bio-Logic, Knoxville, TN) in a three-electrode configuration. The electrochemical cell is a four-neck round-bottom flask (Chemglass, Vineland, NJ) that B

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Figure 1. Current−potential plots of Au in 2 mM MnSO4(aq) with different supporting electrolytes: (a) 0.2 M Na2SO4, (b) saturated CaSO4, and (c) saturated SrSO4 during the initial and fourth cycles. Voltammetric profile of Mn-free supporting electrolyte is included for comparison. Scan rate, 10 mV s−1.

Figure 2. Current−potential plots of Au in 2 mM MnSO4(aq) with (a) 0.2 M Y2(SO4)3 and (b) 0.2 M ZnSO4 as supporting electrolytes, during the initial and fourth cycles. Voltammetric profile of Mn-free supporting electrolyte is included for comparison. Scan rate, 10 mV s−1.

Figure 3. Current−potential plots of Au and ITO electrodes in 2 mM MnSO4 with (a) 0.2 M Na2SO4, (b) 0.2 M Y2(SO4)3, and (c) 0.2 M ZnSO4 at a scan rate of 10 mV s−1. Superimposed for reference is the voltammogram of the Au electrode in blank (Mn-free) electrolyte.

Transmission Electron Microscopy. Thin films of electrodeposited birnessite were cut with a razor blade into cross sections of ca. 0.2 mm in diameter and then embedded in

an EpoKwick epoxy system (Buehler). After the epoxy cured for about 90 min, the epoxy-embedded samples were mounted on a Reichert Ultramicrotome and sliced to a thickness of ca. 70 C

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RESULTS AND DISCUSSION Voltammetric Behavior of Mn2+ on Au in the Presence of Redox-Inactive Cations. Figures 1 and 2 display the Table 1. Peak Potential, EP, for the Anodic Oxidation of Mn2+ into Birnessite in the Presence of Redox-Inactive Cations EP for Mn2+ → birnessite

intercalant +

0.97 0.97 1.04 1.08 1.04

Na Ca2+ Sr2+ Y3+ Zn2+

± ± ± ± ±

0.04 0.02 0.04 0.04 0.01

V V V V V

Table 2. Charge Equivalent to 1 ML of Intercalated Birnessite Depends, in Part, on the Mole Ratio of Mn4+:Mn3+ in Each Birnessite Samplea sample

Mn4+:Mn3+ Ratio

Q1ML (μC cm‑2)

Na -birnessite Ca2+-birnessite Sr2+-birnessite Y3+-birnessite Zn2+-birnessite

3.94 5.29 2.52 2.91 1.69

345 350 330 335 310

+

Figure 4. Cross-sectional scanning electron micrographs of Na+birnessite at different surface coverages.

a

The ratios were calculated from the deconvolution of the Mn 2p3/2 peak in the XPS spectrum.

voltammograms of Au electrodes in 2 mM MnSO4 solutions that contain sulfates of Na+, Ca2+, Sr2+, Y3+, and Zn2+. The chosen cations were electrochemically inactive within the potential window of interest; in particular, they were incapable of forming high-valent metal-oxo electrodeposits during the anodic formation of birnessite. The concentration of the intercalant was about 100-fold higher than that of Mn2+, thereby enforcing the cationic predominance of the intercalant in the precursor solution; however, this was not the case for Sr2+ and Ca2+ because of the limited solubility of their sulfates in water. The initial pH of all the precursor solutions was adjusted to 5.3 ± 0.3 using the corresponding hydroxide of the intercalant. Voltammetric scans were programmed to proceed in the positive direction, with the open-circuit potential as the starting point. For all cases, the first anodic peak in cycle 1 appeared within the potential range of 0.9 to 1.1 V. This major feature marked the conversion of Mn2+ to birnessite as verified by a suite of characterization techniques (vide infra). The peak potential, EP, became the basis for the electrodeposition potential used in the preparation of each intercalated birnessite.

Figure 5. X-ray diffraction patterns of birnessite potentiostatically deposited in the presence of different cations. Peaks marked with an asterisk are associated with the ITO substrate.

In the presence of Na+, Ca2+, and Sr2+ (Figure 1a−c), a small anodic feature centered at ca. 1.03 V jutted out from the more positive tail-end of the birnessite peak and became indiscernible in subsequent cycles. The position of this small peak straddled the potential region, ca. between 0.70 and 1.16 V, for the D

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Figure 6. Transmission electron micrographs of electrodeposited birnessite films intercalated with different cations.

Figure 7. Scanning electron micrographs of birnessite films electrodeposited on Au in the presence of different cations.

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only anodic peak of cycle 1 on ITO (Figure 3c) rendered support to the assignment that this peak belonged to the Mn2+to-birnessite reaction. The second anodic feature at ca. 1.10 V on Au was absent on ITO; this peak belonged to the anodic oxidation of Au and, therefore, did not pertain to any oxidative processes of Mn. The area of this peak diminished with additional potential cycles as the oxides of Mn progressively covered the Au surface. The persistence of the anodic feature of Au in the voltammogram of Zn-birnessite clearly indicated its poor adherence to the substrate. The generally accepted chemical nobility of Au in a wide pH range often favors its use as an electrochemical substrate. ITO, on the other hand, is unstable in concentrated alkaline solutions, in which chronopotentiometry was performed. Additional insights into the nature of the anodic features of Au in Mn2+ can be gleaned from the replacement of the Au substrate with ITO: the voltammetric profile of the initial scans for the two substrates resembled each other, except for the Auspecific anodic features, as revealed in Figure 3a−c. No substrate effects were evident during the electrodeposition of birnessite on a noble metal electrode like Au and on a semiconductor oxide electrode like ITO. Subsequent potential excursions into the negative direction (cycle 1 of Figures 1 and 2) revealed a complex array of reduction processes. From the switching potential of ca. 1.24 V, the current did not immediately cross over into the cathodic (negative) regime, indicating sustained oxidation within the birnessite potential region. Elucidation of the cathodic features was not vigorously pursued because the present investigation primarily focused on the anodic formation of birnessite. Proposed peak assignments for the reduction features have been discussed elsewhere.35 In brief, the conversion of Mn4+ species into lower oxidation states, viz. Mn3+ and Mn2+, was generally observed in concordance with the Pourbaix diagram and the known redox chemistry of Mn in solution.36 Also, the cathodic features for the reduction of the surface oxides and hydroxides of Au intricately overlapped with those of Mn. The return scan in the positive direction, typified by cycle 4 in Figures 1 and 2, gave rise to a new anodic landscape that included the formation of MnO·Mn2O3 and its conversion to MnO(OH)2 at slightly variable potential windows. Systematic truncation of Figures 1 and 2 at potentials more negative than 0.20 V excised the region at which MnO·Mn2O3 was formed but retained the section between 0.50 to 0.70 V that contained the characteristic anodic peak of MnO(OH)2. Despite this truncation, it is evident from the evolution of the voltammograms that, after multiple potential cycles, electrochemically generated MnO·Mn2O3 and MnO(OH)2 served as anodic intermediates for birnessite production; this role was reminiscent of the use of these oxides or hydroxides as precursors for the hydrothermal synthesis of birnessite.37,38 These two anodic precursors were voltammetrically nondetectable during the initial scan. The formation of birnessite in the first potential scan involved the immediate conversion of Mn2+ into the mixed Mn3+−Mn4+ oxidation states of birnessite without passing through the formation of MnO·Mn2O3 and MnO(OH)2. Birnessite films potentiodynamically generated under the influence of the two transition-metal ions in Figure 2 did not readily increase their surface coverages after multiple potential cycles. The deposition rate for birnessite in the presence of Y3+ and Zn2+ was noticeably slow: the resulting films were visibly thinner under the same deposition time and less adherent to

Figure 8. Unfiltered STM images of Ca2+-birnessite in air obtained at a bias voltage of −300 mV and a tunneling current of 2 nA.

surface oxidation of the Au surface. These results implied that the birnessite films formed at the initial cycle did not fully cover the substrate. After the second cycle, sufficient surface coverage was attained to fill in any previously exposed portions of the substrate. The growth of birnessite films in Y3+ and Zn2+ solutions was accompanied by a different set of voltammetric signatures (Figure 2a,b). The Ep for the formation of birnessite in the presence of Y3+ ions emerged at a more positive potential (1.12 V); i.e., the deposition of birnessite transpired after the gold surface had already initiated its anodic oxidization. In contrast, the onset potential for birnessite formation in the presence of Na+, Ca2+, and Sr2+ was less positive than that of the anodic oxidation of Au. The coexistence of the peaks for the Au anodic oxidation and birnessite formation was not readily discernible during cycle 1 of Figure 2a but became evident in advanced cycles, as indicated by the slight hump prior to the birnessite peak in cycle 4. The influence of Zn2+ on the electrodeposition of birnessite is best understood by comparing the voltammograms obtained on Au and ITO substrates. The superimposition of the first anodic peak of cycle 1 on Au (Figure 2b) at ca. 0.90 V with the F

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Figure 9. XPS narrow scans of the spectral regions for Na+, Ca2+, Sr2+, Y3+, and Zn2+ indicate the presence of the intercalated ions in the electrodeposited birnessite films. The binding energies were calibrated against the adventitious C 1s peak taken as 284.65 eV.

the substrate compared to the Na+-, Sr2+-, and Ca2+-birnessite films. The anodic peaks for MnO·Mn2O3 and MnO(OH)2 in the presence of Y3+ were ill-defined in advanced cycles. In the extreme case of Zn2+, the peak for the anodic formation of MnO(OH)2 at ca. 0.59 V became most prominent as the birnessite peak dwindled upon multiple cycling (cycle 4). The discrepant voltammetric features between Figures 1 and 2 clearly suggest that the growth mechanism of birnessite and its

electrochemical precursors in the presence of redox-inactive transition metal ions is different from that in the presence of alkali and alkaline earth metal ions. Further mechanistic studies are warranted to understand these phenomenological observations. Electrodeposition of Birnessite from MnSO4(aq) at Constant Potential. Previous work showed that Ca2+intercalated birnessite34 can be electrochemically prepared by G

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the electrodeposition charge assayed from chronocoulometry. Figure 4 shows the charge−thickness relations of Na+-birnessite films, depicting a nonlinear deposition rate at EP. Structural and Compositional Characterization of Intercalated Birnessite Films. X-ray diffraction (XRD) patterns of birnessite intercalated with Na+, Sr2+, Ca2+, Y3+, and Zn2+ are compiled in Figure 5. ITO substrates were used to support ca. 100 ML of birnessite; peaks from the substrates were marked with asterisks. Y3+-birnessite and Zn2+-birnessite were poorly adherent to the ITO surface; the latter had to be carefully scraped off the surface to perform XRD analysis on the collected powder. All intercalated samples exhibited broad peaks that corresponded closely to the characteristic (001) and (002) facets of birnessite reported in the literature.1 Zn2+birnessite registered additional peaks for the (110) and (020) facets that were not prominent in other intercalated samples. The observed peak broadening was considered an aftermath of particle-size and shape effects rather than the preponderance of structural disorder. Natural birnessite is known to exist as finely dispersed crystallites7,39 that render precise structural characterization by XRD a challenging feat. Transmission electron micrographs of the electrodeposits shown in Figure 6 indicated that all electrodeposited samples were crystalline. Gross morphological studies by scanning electron microscopy (Figure 7) revealed rumpled sheet-like features typical of lamellar materials. The sheets appeared randomly oriented on the substrate, and no other distinct phases were evident. Similar morphological features were observed on birnessite electrodeposited on SnO2 substrates.35 Monoatomic edges of the stacked sheets of birnessite were discernible from scanning tunneling microscopy images acquired in air (Figure 8). Qualitative detection of the different intercalants in birnessite was afforded by X-ray photoelectron spectroscopy (XPS). A gallery of narrow-scan X-ray photoelectron spectra for Na+, Ca2+, Sr2+, Y3+, and Zn2+ is displayed in Figure 9. An overlay of Mn 2p3/2 spectral regions for all five samples is shown in Figure 10. Mn 2p3/2 spectral fitting parameters such as binding energies, fwhm values, multiplet splitting separations, and percentages of total area for Mn2+, Mn3+, and Mn4+ species have been extensively reviewed by Biesinger et al.;40 their results underpin the deconvolution procedure used to generate Figure 11. The calculated relative abundances of Mn3+ and Mn4+ listed in Table 2 imply that the average oxidation state of Mn is highest in Ca2+-birnessite (Mn4+:Mn3+ ratio = 5.29) and lowest in Zn2+-birnessite (Mn4+:Mn3+ ratio = 1.69). The presence of Mn2+ in low abundance ( 1.0 V) at pH 13, according to the Pourbaix diagram of Mn, the most stable species is MnO4−.36 The dissolution of birnessite was gradual during potential cycling because part of the birnessite was reformed from the comproportionation reaction of the electrogenerated MnO4− and the residual Mn2+ originally present in the sample prior to the voltammetric experiment in 0.1 M NaOH. The proximity of the anodic shoulder with the OER peak precluded the identification of the onset potential of the latter. An apparent onset OER potential can be nominally assigned for Na+-birnessite at ca. 1.20 V at an elevated current density of 12 mA cm−3, but no clear assignment can be made for the OER potential at the relevant current density of 10 mA cm−3. For Ca2+-birnessite and Sr2+-birnessite, the apparent inception of the OER peaks was at ca. 1.40 V with a current density of 8 mA cm−3; both required ca. 1.57 V to sustain 10 mA cm−2 of current density. Both Y3+-birnessite and Zn2+-birnessite suffered similar challenges in resolving the OER peak from its nearby anodic peak. The current−potential profile of Zn2+-birnessite resembled that of both Ca2+-birnessite and Sr2+-birnessite, but the OER potential at 10 mA cm−3 was obscured by the overlapping huge anodic shoulder. The anodic shoulder of Y3+birnessite was most dominant among all the intercalated samples, implying the greatest instability toward anodic oxidation in alkaline solution prior to OER. The voltammograms in Figure 12 did not achieve steadystate profiles even after cycle 10; thus, long-term chronopotentiometry was launched to ascertain with improved certainty the potentials of interest. Three current densities were maintained in the following sequence: 1, 5, and 10 mA cm−2; each current density was kept constant for 2 h, except for 10 mA cm−2 which was set for 18 h. Results in Figure 13 were obtained under an atmosphere of oxygen to fix its activity close to unity; fluctuations in the activity of oxygen were often ascribed to the intermittent release of oxygen bubbles during OER. At a low current density of 1 mA cm−2, Na+-birnessite and 2+ Sr -birnessite manifested the lowest potential at an almost steady-state value of 0.90 V. As indicated by the voltammograms in Figure 12, minimal, if not negligible, OER transpired in this potential region. The potential of Ca2+-birnessite barely stabilized at a value of ca. 1.0 V by the 2 h mark. Zn2+-birnessite took more than 80 min to reach a steady-state potential similar to that of Y3+-birnessite, which was 1.26 V. At a current density of 5 mA cm−2, all birnessite samples appeared chemically unstable in alkaline solution as evidenced by the rapid rise in the potentials during the first 40 min. The potentials never plateaued within the 2 h window. Notably, Na+-birnessite still gave the lowest potential. Both Zn2+-

Figure 12. Current−potential profile of intercalated birnessite in 0.1 M NaOH at a scan rate of 10 mV s−1.

Figure 13. Chronopotentiograms of intercalated birnessite in 0.1 M NaOH under an atmosphere of O2 at constant current densities of 1 mA cm−2 (t = 0−2 h), 5 mA cm−2 (t = 2−4 h), and 10 mA cm−2 (t = 4−22 h).

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Figure 14. Wide-scan X-ray photoelectron spectra of electrodeposited birnessite intercalated with Na+, Ca2+, Sr2+, Zn2+, and Y3+ before and after 10 potential cycles in 0.1 M NaOH at a scan rate of 10 mV s−1. The potential window was set between 0.5 and 1.7 V. The spectral region for Na 1s, centered at 1071 eV, is marked for all the plots. Only Ca2+- and Y3+-intercalated birnessite showed the presence of Na+ ions after potential cycling. All intercalated ions, including Ca2+ and Y3+, were retained after long exposure to 0.1 M NaOH.

process was confuted by the reduction of the average oxidation state of Mn after chronopotentiometry at 10 mA cm−2 as shown in Table 3. Thus, the ordinate of Figure 13, particularly the section at 10 mA cm−2, can be converted into ηOER by taking the difference between the steady-state potential extracted from the chronopotentiogram (E vs SHE = E vs Hg/HgO(1 M OH−) + 0.098 V) and the thermodynamic potential of OER at the operational pH of 13 (E vs SHE = 1.23 V − 0.059•pH = 0.463 V). Notwithstanding that the Faradaic efficiencies of the suggested composite anodic reactions were not determined in the experiment, the influence of redoxinactive cations on the potentials needed to keep the current density at 10 mA cm−2 was conspicuous: The OER potential of intercalated birnessite in 0.1 M NaOH increased according to the sequence Na+ < Ca2+ < Sr2+ < Y3+ < Zn2+. The influence of these cations on the electrochemical behavior of birnessite is reminiscent of the redox-modulating effect of redox-inactive metals on the reduction potentials of heterometallic manganese−dioxido clusters, MMn3O2, where M = Na+, Ca2+, Sr2+, Y3+, and Zn2+.23 Follow-up work has further demonstrated similar redox-modulating effects on manganese−tetraoxido cubane clusters, which are more structurally related to the cubanoid oxygen-evolving complex in Photosystem II.42 The above sequence slightly varies with its cluster counterpart in terms of the reversed order of Ca2+ and Sr2+, which is not surprising because of the similarity in their chemical properties; and the reversed order of Y3+ and Zn2+. The origin of such reversal is not yet fully understood; the following aspects, however, distinguish the present results from those of model cluster studies: (i) The redox-inactive cations are present as intercalants in the birnessite motif. Cations in aqueous solutions are typically

Figure 15. XRD patterns of birnessite intercalated with Na+, Y3+, Ca2+, Sr2+, and Zn2+ after long-term chronopotentiometry in 0.1 M NaOH.

birnessite and Y3+-birnessite traced the same potential values and registered the highest potential among the intercalated samples. The anodic processes at 5 mA cm−2 were most likely a composite of the surface oxidation of gold, the formation of Mn species with oxidation states higher than +3, and the onset of OER. The aforementioned processes may also take place at a constant current density of 10 mA cm−2. On the basis of the fast bubble formation observed at this current density, it can be inferred that the major contributory reaction was OER, not the anodic oxidation of Mn3+ species. The dominance of the latter K

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in comparison with other Mn-based systems,29,46 the present work offers the prospect of property modulation via the use of cations that are classically labeled as spectator supporting ions in electrochemistry. For layered materials like birnessite, the influence of redox-inactive intercalants does not linearly correlate with pK a , unlike the case of heterometallic manganese−dioxido clusters. The discovery of new, relevant predictors of activity for electrocatalysts of multielectrontransfer processes such as OER necessitates the deployment of operando methodologies that do not only examine the phenomenology of the cation-sheet interactions for birnessite but also probe the evolving structure and composition of the electrode−electrolyte interface during electrocatalysis. The electrocatalytic surface is, after all, a dynamic, regenerable arena of bond-breaking and bond-forming events, in which many of the critical atomic details are either not yet fully developed at the beginning or are simply lost at the end when the electrocatalyst is restored to its initial state.

sheathed with hydration layers that determine, in part, the effective distance between interacting charged centers. On the other hand, the locus of these metals in the synthetic clusters is stabilized by an elaborate framework of ligands. (ii) The identity and amount of intercalants can be altered by classic cation exchange. Wide-scan XPS spectra (Figure 14) of intercalated birnessite after prolonged exposure to NaOH, either during potential cycling or chronopotentiometry, revealed the presence of Na+ in Ca2+-birnessite and Y3+birnessite; the other samples remained Na+-free. In contrast, the heterometal-to-manganese ratio in molecular clusters is stoichiometrically fixed. (iii) With manganese oxido complexes, the half-wave potential of the Mn3+Mn24+/Mn34+ redox couple in nonaqueous solvent varies with the pKa of the redox-inactive cation.23,42 The use of pKa as a scaling descriptor may not be straightforward in the aqueous environment because of the convolution of other relevant factors such as hydration energy, effective charge-tosize ratio of the intercalants, and the different surface sites available to the cations in the birnessite motif. (iv) The influence of the redox-inactive cations on the electrochemistry of birnessite is remarkably weak. The resultant OER potential range for birnessite spanned only 160 mV, whereas the heterometallic Mn oxido clusters covered a wide potential range of ca. 700 mV23 in the presence of the heterometal. The differences may emanate from the fluxional presence of the cations within the intersheet region of birnessite. The XRD patterns (Figure 15) of the intercalated birnessite after long-term chronopotentiometry were featureless, indicating either the absence of long-range order in the bulk or the formation of fine-grain crystallites. XPS analysis of the same samples indicated that, based on the deconvolution of the Mn 2p3/2 region (Tables 3), the average oxidation states of all samples decreased except for Zn-birnessite. The reduction of the birnessite samples implied that the predominant interfacial reaction at a current density of 10 mA cm−2 is the oxidation of water and that the reduction of birnessite constitutes its other half-reaction. Among the intercalated birnessite samples, Na+-birnessite is by far the best performing electrocatalyst in the group. Zn2+birnessite is least active toward OER in alkaline solution. The sorption mechanism of intercalants holds key information in understanding the cation-dependent electrocatalysis by birnessite. For Na+-birnessite, computational results from classical and electronic structure methods have unveiled a compact, ordered interlayer structure, wherein the electrostatic interactions between the sheet and Na+ predominate over the weak hydration energy of Na + (−96.7 kcal mol −1).43 Both competitive forces are almost equally contributory in the case of Ca2+, because of its large hydration energy of −380.6 kcal mol−1.43 Zn2+ has the largest hydration energy44 in this set, but spectroscopic studies and DFT computations have shown that Zn2+ resembles most transition-metal cations that like to form surface complexes with water and the O-corners of an octahedron with a Mn4+ vacancy.45 The major difference lies in the nonstereoactive, fully occupied 3d states of Zn2+ which disfavors its entry into the sheet to occupy Mn vacancy sites.45 Similar detailed calculations are, however, currently unavailable for Sr2+ and Y3+ to explain their position in the empirical sequence. Despite the nondescript performance of the prepared birnessite as an OER electrocatalyst in strongly alkaline media

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CONCLUSION Potentiostatic electrodeposition provides a facile route in the preparation of birnessite thin films, on Au substrates, intercalated with selected alkali, alkaline earth, and transition metal ions. The elimination of prolonged cation-exchange submersion has dramatically reduced the synthesis time scale from days to minutes. The electrodeposited birnessite films are crystalline, monophasic oxide materials that contain Mn3+, Mn4+, traces of Mn2+, and the intercalant ion of choice. Na+, Ca2+, Sr2+, Y3+, and Zn2+ ions exert minimal influence on the voltammetric behavior of birnessite in the presence of Mn2+: the anodic peak potential (EP) for the Mn2+-to-birnessite reaction spans a narrow 70 mV range, with Na+-birnessite and Ca2+-birnessite at the minimum and Y3+-birnessite at the maximum. Slightly more significant effects emerge during potential cycling and chronopotentiometry of birnessite films in alkaline solution. The potential needed to sustain a current density of 10 mA cm−2 in 0.1 M NaOH increases according to the sequence Na+ < Ca2+ < Sr2+ < Y3+ < Zn2+, which encompasses a 160 mV potential range. The sequence almost follows, with slight inversions in the order, the trend in the heterometal-dependent modulation of the half-wave potential of the Mn3+Mn24+/Mn34+ redox couple in nonaqueous solutions of heterometallic manganese−dioxido cluster systems. Unlike the case of the homogeneous cluster catalysts, the electrochemical reactivity of birnessite does not vary linearly with the pKa of the redox-inactive cations.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work performed by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award DE-SC0004993. L

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