Homogeneous Catalysts Based on First-Row Transition-Metals for Electrochemical Water Oxidation

Abstract

Strategies that enable the renewable production of storable fuels (i. e. hydrogen or hydrocarbons) through electrocatalysis continue to generate interest in the scientific community. Of central importance to this pursuit is obtaining the requisite chemical (H⁺ ) and electronic (e^- ) inputs for fuel-forming reduction reactions, which can be met sustainably by water oxidation catalysis. Further possibility exists to couple these redox transformations to renewable energy sources (i. e. solar), thus creating a carbon neutral solution for long-term energy storage. Nature uses a Mn-Ca cluster for water oxidation catalysis via multiple proton-coupled electron-transfers (PCETs) with a photogenerated bias to perform this process with TOF 100∼300 s^-1 . Synthetic molecular catalysts that efficiently perform this conversion commonly utilize rare metals (e. g., Ru, Ir), whose low abundance are associated to higher costs and scalability limitations. Inspired by nature's use of 1^st row transition metal (TM) complexes for water oxidation catalysts (WOCs), attempts to use these abundant metals have been intensively explored but met with limited success. The smaller atomic size of 1^st row TM ions lowers its ability to accommodate the oxidative equivalents required in the 4e^- /4H⁺ water oxidation catalysis process, unlike noble metal catalysts that perform single-site electrocatalysis at lower overpotentials (η). Overcoming the limitations of 1^st row TMs requires developing molecular catalysts that exploit biomimetic phenomena - multiple-metal redox-cooperativity, PCET and second-sphere interactions - to lower the overpotential, preorganize substrates and maintain stability. Thus, the ultimate goal of developing efficient, robust and scalable WOCs remains a challenge. This Review provides a summary of previous research works highlighting 1^st row TM-based homogeneous WOCs, catalytic mechanisms, followed by strategies for catalytic activity improvements, before closing with a future outlook for this field.

Keywords: Homogeneous catalysis; Molecular electrochemistry; Oxidation; Transition metals; Water Splitting.

Figure 1

Thermodynamic arguments against 1^st row TMs as water oxidation catalysts.

Figure 2

Kinetic arguments against 1^st row TMs as water oxidation catalysts.

Figure 3

The proposed catalytic mechanism of Fe‐1 ^[37] (η=700 mV, TOF=0.15 s⁻¹) in propylene carbonate/water.

Figure 4

Ball‐and‐stick representations of the molecular structure, the Fe₅O core structure of [Fe^II ₄Fe^III(μ₃‐O)(μ‐L)₆]³⁺ (Fe‐2, η>500 mV, TOF>1000 s⁻¹) ^[11] and the chemical structure of LH (left), deprotonated to L upon catalyst formation. The proposed catalytic mechanism of Fe‐2 (right). (Copyright 2016 Nature Publishing Group, adapted with permission).

Figure 5

Structure of WOCs Fe‐3, Fe‐4 and Fe‐5 (pH 7.5 aqueous system). ^[40]

Figure 6

Structure of WOCs Fe‐6 and Fe‐7 (pH 1 aqueous buffer), ^[42] and Fe‐8 (0.1 M pH 7 Na₂SO₄ solution). ^[44]

Figure 7

Structures of WOCs Cu‐1 (0.1 M NaOH/NaOAc at pH 13.1), ^[49] Cu‐2 (pH 12.6), ^[50] and Cu‐3, Cu‐4 and Cu‐5 (pH 12.6). ^[52]

Figure 8

Proposed mechanism of Cu‐2 for electrochemical water oxidation. (Figure reproduced from ref. [50], Copyright 2014 American Chemical Society, adapted with permission).

Figure 9

(a) Structures of WOCs Cu‐6 (0.1 M NaOH/NaOAc at pH 12), ^[53] and Cu‐7 to Cu‐10 (0.1 M phosphate buffer at pH 11.5). ^[45] (b) The proposed catalytic mechanism of Cu‐7 to Cu‐10 for water oxidation.

Figure 10

Structural representation of Cu‐7 and Cu‐11 and the hybrid materials upon adsorption by π‐stacking to graphitic electrodes (pH 12). ^[54] (Figure reproduced from ref. [54], Copyright 2017 American Chemical Society, adapted with permission).

Figure 11

(a) Structures of WOC Cu‐12 (0.1 m phosphate buffer at pH 11.5) ^[55] and Cu‐13 (0.1 m KNO₃/0.1 m KOH at pH 10.4). ^[57] (b) trans/cis‐conversion of Cu‐13. ^[58]

Figure 12

a) Structures of WOC Cu‐14 (0.25 m phosphate buffer at pH 11), ^[59] Cu‐14 and Cu‐15 (0.15 m phosphate buffer at pH 11), ^[60] and Cu‐17 (0.10 m phosphate buffer at pH 8). ^[61] b) Interaction between H⁺ and glycine in Cu‐16.

Scheme 1

The proposed APT mechanism for electrochemical water oxidation.

Figure 13

Structures of Cu‐18, ^[62] Cu‐19„ ^[63] Cu‐20„ ^[64] Cu‐21, ^[66] (0.10 m phosphate buffer at pH 7) and Cu‐22 ^[67] (0.1 m NaNO₃ at pH 7).

Figure 14

Structures of Ni‐1[ ⁷⁰ , ⁷¹ ] to Ni‐4 ^[73] (0.10 m phosphate buffer at pH 7).

Figure 15

Structures of Ni‐5 to Ni‐9 working in neutral conditions,[ ⁷⁴ , ⁷⁶ , ⁷⁷ ] and Ni‐10 at pH 7–10.8. ^[78]

Figure 16

Structures of Ni‐11, Ni‐12, and Ni‐13 ^[79] (phosphate buffer solution at pH 11).

Figure 17

(a) The key rdox‐active species involved in the catalytic cycle of galactose oxidase; (b) the structure of nickel‐phenolate WOC Ni‐14 ^[81] (phosphate buffer solution at pH 11).

Figure 18

Structures of Co‐1 ^[85] to Co‐6 ^[87] (neutral buffer solution).

Figure 19

Intramolecular preorganization (Co‐2, left) and intermolecular base (B:) effect (Co‐7, right) facilitating O−O bond formation for structure of Co‐2 ^[85] and Co‐7. ^[88]

Figure 20

(a) Structures of Co‐8, Co‐9, Co‐10 (b) Structure of inactive Co‐11 and (c) the catalytic cycle of Co‐8 (neutral buffer solution). ^[89]