Research ArticleELECTROCHEMISTRY

Combining scaling relationships overcomes rate versus overpotential trade-offs in O2 molecular electrocatalysis

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Science Advances  13 Mar 2020:
Vol. 6, no. 11, eaaz3318
DOI: 10.1126/sciadv.aaz3318
  • Fig. 1 Catalytic system efficiencies, reaction mechanism, and structure of catalyst 1.

    (A) Plot of log(TOFmax)/ηeff values and fits (dashed lines) for 1 (diamonds; data and conditions in Table 1) and for previously reported iron porphyrin [Fe(por)] catalysts (circles; 0.1 M [DMF-H+] in DMF or MeCN) (5). The uncertainties are smaller than the data points. The yellow shading indicates an aspirational region. (B) Fe(por) catalyzed O2 reduction mechanism, as described in the main text (11). (C) Drawings of 1 and of the cation in the solid-state x-ray crystal structure, [1•2H2O]OTf5 (Fe, orange; N, blue; C, white; O, red; H atoms omitted for clarity; thermal ellipsoids at 50% probability).

  • Fig. 2 Electrochemical studies of 1 under noncatalytic and catalytic conditions.

    (A) CVs of 1 showing the shift of E1/2 with increasing concentrations of 1:1 AcOH/AcO buffer. (B) Plot of E1/2 versus the acid pKa at 0.1 M buffer. (C) Linear sweep voltammograms under catalytic conditions with different BzOH buffer concentrations. (D) FOWA for voltammograms in (C), with fits between ic/ip = 1 to 4.

  • Fig. 3 Vector analysis to predict the inverse scaling relationship for 1.

    Predicted coordinates using the vectors are shown as red squares. (A) Plot of log(TOFmax) versus ηeff for catalytic systems of 1 and varying carboxylic acid buffers (data points match those in Fig. 1). Superimposed vectors (gold, black, and blue) show predicted changes caused by pKa, E1/2, and summative effects, respectively. (B) Buffers with cationic acids follow only the pKa vector (gold). Prior Fe(por) data and E1/2 scaling relationships (7) are included for reference (gray). Uncertainties are smaller than the data points.

  • Table 1 Properties of catalytic systems with 1 and different buffers.

    Experimental conditions: 0.1 mM 1, 0.1 M buffer (1:1 HA/A or HB+/B), 0.1 M [n-Bu4N][BF4] in MeCN (~15 mM H2O), 1 atm O2. [DMF-H+], N,N′-dimethylformamidium triflate; TFAH, trifluoroacetic acid; [Lut-H+], lutidinium tetrafluoroborate; SalOH, salicylic acid; BzOH, benzoic acid; AcOH, acetic acid.

    BufferpKa*E1/2 (V)ηeff (V)TOFmax (s−1)§log(TOFmax)
    None−0.295
    [DMF-H+]/DMF6.1−0.25*1.168.50.91
    TFAH/TFA12.6−0.3490.883.20.51
    [Lut-H+]/Lut14.1−0.23*0.680.07−1.17
    SalOH/SalO16.7−0.5360.82121.08
    BzOH/BzO21.5−0.6530.67631.80
    AcOH/AcO23.5−0.6510.541702.23

    *See the Supplementary Materials.

    E1/2(FeIII/II) reduction potential (versus ferrocenium/ferrocene, under Ar, with 100 mM buffer).

    ‡From Eq. 2; ±0.02 V. §From FOWA.

    Supplementary Materials

    • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/6/11/eaaz3318/DC1

      Section S1. Spectroscopic characterization of 1

      Section S2. Voltammetry of 1

      Section S3. UV-vis spectroscopy of 1 + varying buffers

      Section S4. Homoconjugation

      Section S5. Kinetic analysis

      Section S6. Effective overpotential determination

      Section S7. Selectivity for H2O versus H2O2

      Section S8. Conceptual background for E1/2 and pKa scaling relationships

      Section S9. Single crystal x-ray structure

      Fig. S1. Full high-resolution ESI mass spectrum of 1 with identified peaks, as labeled.

      Fig. S2. High-resolution ESI mass spectrum and isotopic fits for the [M5+ + 3OTf] ion.

      Fig. S3. UV-vis spectrum of 1 in N,N′-dimethylformamide.

      Fig. S4. IR spectrum of 1.

      Fig. S5. A CV of an Ar-sparged solution of 1.

      Fig. S6. Scan rate investigation of the FeIII/FeII redox couple of 1 in unbuffered solution.

      Fig. S7. Voltammograms of an Ar-sparged solution of 1 before and after addition of buffered AcOH.

      Fig. S8. Investigation of the FeIII/FeII redox couple of 1 with titrations of AcOH buffer.

      Fig. S9. Scan rate investigation of the FeIII/FeII redox couple of 1 in AcOH-buffered solution.

      Fig. S10. Investigation of the FeIII/FeII redox couple of 1 with titrations of BzOH buffer.

      Fig. S11. Scan rate investigation of the FeIII/FeII redox couple of 1 in BzOH-buffered solution.

      Fig. S12. Investigation of the FeIII/FeII redox couple of 1 with titrations of SalOH buffer.

      Fig. S13. Scan rate investigation of the FeIII/FeII redox couple of 1 in SalOH-buffered solution.

      Fig. S14. Investigation of the FeIII/FeII redox couple of 1 with titrations of TFA buffer.

      Fig. S15. Scan rate investigation of the FeIII/FeII redox couple of 1 in TFA-buffered solution.

      Fig. S16. Investigation of the FeIII/FeII redox couple of 1 with titrations of [DMF-H]OTf-buffer.

      Fig. S17. Investigation of the FeIII/FeII redox couple of 1 with titrations of [Lut-H]BF4 buffer.

      Fig. S18. Changes in E1/2(FeIII/FeII) with varying buffers (and concentrations).

      Fig. S19. CVs of an O2-sparged MeCN solution containing 100 mM [AcOH] buffer before (black) and after (blue) adding 30 μM 1.

      Fig. S20. Voltammograms of 1 with AcOH buffer under various solution conditions.

      Fig. S21. Voltammograms of 1 with BzOH buffer under various solution conditions.

      Fig. S22. Voltammograms of 1 with SalOH buffer under various solution conditions.

      Fig. S23. Voltammograms of 1 with TFA buffer under various solution conditions.

      Fig. S24. Voltammograms of 1 with [Lut-H+] buffer under various solution conditions.

      Fig. S25. Voltammograms of 1 with [DMF-H]OTf buffer under various solution conditions.

      Fig. S26. Rinse tests for all of the buffers used in this study.

      Fig. S27. UV-vis spectra of MeCN solutions containing 1 (~0.05 mM), [n-Bu4N][BF4] (~0.05 M), and varying 1:1 buffers (~0.05 M, as identified).

      Fig. S28. Foot-of-the-wave analysis for the buffer concentrations used in this study (all at 1 atm O2).

      Fig. S29. Foot-of-the-wave analysis for all the partial pressure O2 measurements performed in this study (all at 20 mM buffer).

      Fig. S30. TOFmax versus [substrate] plots for the buffers used in this study.

      Fig. S31. Plot of kobs versus partial pressure of O2 for each of the buffers used in this study.

      Fig. S32. RRDE analysis using ferrocene to estimate collection efficiencies.

      Fig. S33. RRDE analysis for the ORR catalyzed by 1 using [DMF-H]OTf buffer.

      Fig. S34. RRDE analysis for the ORR catalyzed by 1 using TFAH buffer.

      Fig. S35. RRDE analysis for the ORR catalyzed by 1 using SalOH buffer.

      Fig. S36. RRDE analysis for the ORR catalyzed by 1 and BzOH buffer.

      Fig. S37. RRDE analysis for the ORR catalyzed by 1 and AcOH buffer.

      Fig. S38. The complete x-ray model of [Fe-o-TMA]OTf5•2H2O represented with balls and sticks.

      Fig. S39. The complete numbering scheme of the cation-only portion of [Fe-o-TMA]OTf5•2H2O with 50% thermal ellipsoid probability levels.

      Fig. S40. The x-ray model of the cation-only portion of [Fe-o-TMA]OTf5•2H2O from a secondary perspective showing the αβαβ isomer of porphyrin structure.

      Fig. S41. The complete numbering of the disordered triflate at a general position in the model of [Fe-o-TMA]OTf5•2H2O with 50% thermal ellipsoid probability levels.

      Fig. S42. The unit cell of [Fe-o-TMA]OTf5•2H2O, with a surface that represents a level of 1.5 e/Å3.

      Fig. S43. All orientations of the disordered triflate are shown in relation to the 4¯ rotation axis, represented with red lines.

      Table S1. Summary of E1/2(FeIII/FeII) (V) versus Fc+/Fc values measured under the conditions reported in this study; errors are ±0.005 V.

      Table S2. Q-band region λmax values for ~0.05 mM 1 in MeCN containing ~0.05 M [n-Bu4N][BF4] and ~0.05 M buffer.

      Table S3. Homoconjugation formation constants for the buffers used in this study.

      Table S4. Calculated values for nonhomoconjugated acid and conjugate base ([HA]free = [A]free and [HB+]free = [B]free) for varying buffer identities and concentrations.

      Table S5. Average percent H2O2 formed for 1-catalyzed ORR in MeCN containing various buffers.

      Table S6. Crystal data and structure refinement for [Fe-o-TMA]OTf5•2H2O.

      Table S7. Atomic coordinates (×104) and equivalent isotropic displacement parameters (Å2 × 103) for [Fe-o-TMA]OTf5•2H2O.

      Table S8. Bond lengths (Å) and angles (°) for [Fe-o-TMA]OTf5•2H2O.

      Table S9. Anisotropic displacement parameters (Å2 × 103) for [Fe-o-TMA]OTf5•2H2O.

      Table S10. Hydrogen coordinates (×104) and isotropic displacement parameters (Å2 × 103) for [Fe-o-TMA]OTf5•2H2O.

      Table S11. Torsion angles (°) for [Fe-o-TMA]OTf5•2H2O.

      References (2541)

    • Supplementary Materials

      This PDF file includes:

      • Section S1. Spectroscopic characterization of 1
      • Section S2. Voltammetry of 1
      • Section S3. UV-vis spectroscopy of 1 + varying buffers
      • Section S4. Homoconjugation
      • Section S5. Kinetic analysis
      • Section S6. Effective overpotential determination
      • Section S7. Selectivity for H2O versus H2O2
      • Section S8. Conceptual background for E1/2 and pKa scaling relationships
      • Section S9. Single crystal x-ray structure
      • Fig. S1. Full high-resolution ESI mass spectrum of 1 with identified peaks, as labeled.
      • Fig. S2. High-resolution ESI mass spectrum and isotopic fits for the M5+ + 3OTf ion.
      • Fig. S3. UV-vis spectrum of 1 in N,N′-dimethylformamide.
      • Fig. S4. IR spectrum of 1.
      • Fig. S5. A CV of an Ar-sparged solution of 1.
      • Fig. S6. Scan rate investigation of the FeIII/FeII redox couple of 1 in unbuffered solution.
      • Fig. S7. Voltammograms of an Ar-sparged solution of 1 before and after addition of buffered AcOH.
      • Fig. S8. Investigation of the FeIII/FeII redox couple of 1 with titrations of AcOH buffer.
      • Fig. S9. Scan rate investigation of the FeIII/FeII redox couple of 1 in AcOH-buffered solution.
      • Fig. S10. Investigation of the FeIII/FeII redox couple of 1 with titrations of BzOH buffer.
      • Fig. S11. Scan rate investigation of the FeIII/FeII redox couple of 1 in BzOH-buffered solution.
      • Fig. S12. Investigation of the FeIII/FeII redox couple of 1 with titrations of SalOH buffer.
      • Fig. S13. Scan rate investigation of the FeIII/FeII redox couple of 1 in SalOH-buffered solution.
      • Fig. S14. Investigation of the FeIII/FeII redox couple of 1 with titrations of TFA buffer.
      • Fig. S15. Scan rate investigation of the FeIII/FeII redox couple of 1 in TFA-buffered solution.
      • Fig. S16. Investigation of the FeIII/FeII redox couple of 1 with titrations of DMF-HOTf-buffer.
      • Fig. S17. Investigation of the FeIII/FeII redox couple of 1 with titrations of Lut-HBF4 buffer.
      • Fig. S18. Changes in E1/2(FeIII/FeII) with varying buffers (and concentrations).
      • Fig. S19. CVs of an O2-sparged MeCN solution containing 100 mM AcOH buffer before (black) and after (blue) adding 30 μM 1.
      • Fig. S20. Voltammograms of 1 with AcOH buffer under various solution conditions.
      • Fig. S21. Voltammograms of 1 with BzOH buffer under various solution conditions.
      • Fig. S22. Voltammograms of 1 with SalOH buffer under various solution conditions.
      • Fig. S23. Voltammograms of 1 with TFA buffer under various solution conditions.
      • Fig. S24. Voltammograms of 1 with Lut-H+ buffer under various solution conditions.
      • Fig. S25. Voltammograms of 1 with DMF-HOTf buffer under various solution conditions.
      • Fig. S26. Rinse tests for all of the buffers used in this study.
      • Fig. S27. UV-vis spectra of MeCN solutions containing 1 (~0.05 mM), n-Bu4NBF4 (~0.05 M), and varying 1:1 buffers (~0.05 M, as identified).
      • Fig. S28. Foot-of-the-wave analysis for the buffer concentrations used in this study (all at 1 atm O2).
      • Fig. S29. Foot-of-the-wave analysis for all the partial pressure O2 measurements performed in this study (all at 20 mM buffer).
      • Fig. S30. TOFmax versus substrate plots for the buffers used in this study.
      • Fig. S31. Plot of kobs versus partial pressure of O2 for each of the buffers used in this study.
      • Fig. S32. RRDE analysis using ferrocene to estimate collection efficiencies.
      • Fig. S33. RRDE analysis for the ORR catalyzed by 1 using DMF-HOTf buffer.
      • Fig. S34. RRDE analysis for the ORR catalyzed by 1 using TFAH buffer.
      • Fig. S35. RRDE analysis for the ORR catalyzed by 1 using SalOH buffer.
      • Fig. S36. RRDE analysis for the ORR catalyzed by 1 and BzOH buffer.
      • Fig. S37. RRDE analysis for the ORR catalyzed by 1 and AcOH buffer.
      • Fig. S38. The complete x-ray model of Fe-o-TMAOTf5•2H2O represented with balls and sticks.
      • Fig. S39. The complete numbering scheme of the cation-only portion of Fe-o-TMAOTf5•2H2O with 50% thermal ellipsoid probability levels.
      • Fig. S40. The x-ray model of the cation-only portion of Fe-o-TMAOTf5•2H2O from a secondary perspective showing the αβαβ isomer of porphyrin structure.
      • Fig. S41. The complete numbering of the disordered triflate at a general position in the model of Fe-o-TMAOTf5•2H2O with 50% thermal ellipsoid probability levels.
      • Fig. S42. The unit cell of Fe-o-TMAOTf5•2H2O, with a surface that represents a level of 1.5 e/Å3.
      • Fig. S43. All orientations of the disordered triflate are shown in relation to the 4¯ rotation axis, represented with red lines.
      • Table S1. Summary of E1/2(FeIII/FeII) (V) versus Fc+/Fc values measured under the conditions reported in this study; errors are ±0.005 V.
      • Table S2. Q-band region λmax values for ~0.05 mM 1 in MeCN containing ~0.05 M n-Bu4NBF4 and ~0.05 M buffer.
      • Table S3. Homoconjugation formation constants for the buffers used in this study.
      • Table S4. Calculated values for nonhomoconjugated acid and conjugate base (HAfree = Afree and HB+free = Bfree) for varying buffer identities and concentrations.
      • Table S5. Average percent H2O2 formed for 1-catalyzed ORR in MeCN containing various buffers.
      • Table S6. Crystal data and structure refinement for Fe-o-TMAOTf5•2H2O.
      • Table S7. Atomic coordinates (×104) and equivalent isotropic displacement parameters (Å2 × 103) for Fe-o-TMAOTf5•2H2O.
      • Table S8. Bond lengths (Å) and angles (°) for Fe-o-TMAOTf5•2H2O.
      • Table S9. Anisotropic displacement parameters (Å2 × 103) for Fe-o-TMAOTf5•2H2O.
      • Table S10. Hydrogen coordinates (×104) and isotropic displacement parameters (Å2 × 103) for Fe-o-TMAOTf5•2H2O.
      • Table S11. Torsion angles (°) for Fe-o-TMAOTf5•2H2O.
      • References (2541)

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