Hydrogen Peroxide as a Sustainable Energy Carrier: Electrocatalytic Production of Hydrogen Peroxide and the Fuel Cell

Abstract

This review describes homogeneous and heterogeneous catalytic reduction of dioxygen with metal complexes focusing on the catalytic two-electron reduction of dioxygen to produce hydrogen peroxide. Whether two-electron reduction of dioxygen to produce hydrogen peroxide or four-electron O₂-reduction to produce water occurs depends on the types of metals and ligands that are utilized. Those factors controlling the two processes are discussed in terms of metal-oxygen intermediates involved in the catalysis. Metal complexes acting as catalysts for selective two-electron reduction of oxygen can be utilized as metal complex-modified electrodes in the electrocatalytic reduction to produce hydrogen peroxide. Hydrogen peroxide thus produced can be used as a fuel in a hydrogen peroxide fuel cell. A hydrogen peroxide fuel cell can be operated with a one-compartment structure without a membrane, which is certainly more promising for the development of low-cost fuel cells as compared with two compartment hydrogen fuel cells that require membranes. Hydrogen peroxide is regarded as an environmentally benign energy carrier because it can be produced by the electrocatalytic two-electron reduction of O₂, which is abundant in air, using solar cells; the hydrogen peroxide thus produced could then be readily stored and then used as needed to generate electricity through the use of hydrogen peroxide fuel cells.

Fig. 1

Time profiles of formation of Fe(C₅H₄Me)₂⁺ monitored at 650 nm in electron transfer oxidation of Fe(C₅H₄Me)₂ (1.0 × 10⁻¹ mol L⁻¹) by O₂ (1.7 × 10⁻³ mol L⁻¹), catalyzed by Co₂(DPX) (2.0 × 10⁻⁵ mol L⁻¹), Co₂(DPA) (2.0 × 10⁻⁵ mol L⁻¹), Co₂(DPB) (2.0 × 10⁻⁵ mol L⁻¹), Co₂(DPD) (2.0 × 10⁻⁵ mol L⁻¹), and Co(OEP) (3.0 × 10⁻⁵ mol L⁻¹) in the presence of HClO₄ (2.0 × 10⁻² mol L⁻¹) in PhCN at 298 K.

Fig. 2

Cyclic voltammograms of (a) Co₂(DPX) and (b) Co(OEP) (1.0 × 10⁻³ mol L⁻¹) in PhCN containing 0.1 mol L⁻¹ TBAP; scan rate 100 mV s⁻¹.

Fig. 3

Selected distance (nm) in Co₂(DPB) [38], Co₂(DPA) [39], Co₂(DPX) [40,41], and Co₂(DPD)(2MeOH) [40,41].

Fig. 4

EPR spectra of the μ-superoxo complex (~10⁻³ mol L⁻¹) produced by adding iodine (~10⁻³ mol L⁻¹) to an air-saturated PhCN solution and ESR simulation of (a) Co₂(DPB), (b) Co₂(DPA) and (c) Co₂(DPX) in the presence of 1-tert-butyl-5-phenylimidazole (5 × 10⁻³ mol L⁻¹) at 298 K [32].

Fig. 5

Visible absorption spectra changes in the catalytic reduction of O₂ (1.7 × 10⁻³ mol L⁻¹) by Fe(C₅H₄Me)₂ (2.0 × 10⁻² mol L⁻¹) in the presence of HClO₄ (2.0 × 10⁻² mol L⁻¹) and 1 – 6 (2.0 × 10⁻⁵ mol L⁻¹) in PhCN at 298 K; 1 (red solid line), 2 (green solid line), 3 (blue solid line), 4 (red broken line), 5 (green broken line) and 6 (blue broken line). Black: in the absence of cobalt complex.

Fig. 6

Electrocatalytic reduction of O₂ in 1 mol L⁻¹ HClO₄ at a rotating graphite disk electrode coated with (PMes₂CO)Co₂ 1. (a) Values of the rotation rates of the electrode (w) are indicated on each curve. The disk potential was scanned at 5 mV s⁻¹. (b) Levich plots of the plateau currents of (a) vs. (rotation rate)^1/2. The dashed line refers to the theoretical curve expected for the diffusion-convection limited reduction of O₂ by 2e⁻. (c) Koutecky-Levich plots of the reciprocal plateau currents vs. (rotation rates)^−1/2. Supporting electrolyte: 1 mol L⁻¹ HClO₄ saturated with air.

Fig. 7

(a) X-ray structures of the fully reduced bimetallic heme a₃/Cu_B center in CcO from bovine heart (Fe^II···Cu^I = 0.519 nm), (b) heme/Cu synthetic model for CcO (⁶LFeCu), (c) Cu-free version of synthetic model for CcO (⁶LFe).

Fig. 8

(a) Levich plot from the plateau currents in plot of current vs. (rotation rate)^1/2. (b) Koutecky-Levich plot from the plateau currents in Fig. 2 [(current)⁻¹ vs. (rotation rate)^1/2]. The dashed lines in (a) and (b) were obtained from the calculated diffusion-convection controlled currents for the reduction of O₂ assuming the number of electrons as two and four.

Fig. 9

Complexes used as catalysts for the solution four-electron four-proton reduction of O₂ by ferrocene derivatives and the copper(I)-dioxygen derived complex intermediates known to form during the course of reaction. See text for discussions.

Fig. 10

UV-vis spectral changes in four-electron reduction of O₂ by Fc* (1.5 × 10⁻³ mol L⁻¹) with [(tmpa)Cu^II(H₂O)]²⁺ (9.0 × 10⁻⁵ mol L⁻¹) in the presence of HClO₄ in acetone at 298 K. The inset shows the changes in absorbance at 380 and 780 nm due to Fc*⁺ produced by stepwise addition of HClO₄ (0.18 – 1.44 × 10⁻³ mol L⁻¹) to an O₂-saturated acetone solution ([O₂] = 11 × 10⁻³ mol L⁻¹) of Fc* and [(tmpa)Cu^II(H₂O)]²⁺.

Fig. 11

Depiction of the structure of the μ-η²:η²-peroxo dicopper(II) complex [Cu^II₂(N3)(O₂)]²⁺ (b) Formation of the η²:η²-peroxo complex (λ_max = 490 nm) in the reaction of [Cu^I₂(N3)]²⁺ (1.0 × 10⁻⁴ mol L⁻¹) with O₂ in the presence of Fc* (8.0 × 10⁻² mol L⁻¹) in acetone at 193 K. The Inset shows the time profiles of the absorbance at 490 nm (black line) and 780 nm (red line) due to [Cu^II₂(N3)(O₂)]²⁺ and Fc*⁺, respectively. (c) UV/Vis spectral changes observed in the four-electron reduction of O₂ (0.22 × 10⁻³ mol L⁻¹) by Fc* (3.0 × 10⁻³ mol L⁻¹) at 298 K and with TFA (1.0 × 10⁻² mol L⁻¹) catalyzed by [Cu^II₂(N3)(H₂O)₂]²⁺ (1.0 × 10⁻⁴ mol L⁻¹).

Fig. 11

Depiction of the structure of the μ-η²:η²-peroxo dicopper(II) complex [Cu^II₂(N3)(O₂)]²⁺ (b) Formation of the η²:η²-peroxo complex (λ_max = 490 nm) in the reaction of [Cu^I₂(N3)]²⁺ (1.0 × 10⁻⁴ mol L⁻¹) with O₂ in the presence of Fc* (8.0 × 10⁻² mol L⁻¹) in acetone at 193 K. The Inset shows the time profiles of the absorbance at 490 nm (black line) and 780 nm (red line) due to [Cu^II₂(N3)(O₂)]²⁺ and Fc*⁺, respectively. (c) UV/Vis spectral changes observed in the four-electron reduction of O₂ (0.22 × 10⁻³ mol L⁻¹) by Fc* (3.0 × 10⁻³ mol L⁻¹) at 298 K and with TFA (1.0 × 10⁻² mol L⁻¹) catalyzed by [Cu^II₂(N3)(H₂O)₂]²⁺ (1.0 × 10⁻⁴ mol L⁻¹).

Fig. 12

Cobalt porphyrins employed for electrocatalytic reduction of O₂ to H₂O₂ [24].

Fig. 13

(a) Electrocatalytic current at oxygen reduction. ([Co(DPP)] – –, [Co(OEP)] – · –, [Co(TPP)] - - -, [Co(TCPP)] —). (b) CV of [Co(TCPP)] at scan rate = 20 mV s⁻¹ (with 3 × 10⁻³ mol L⁻¹ H₂O₂ – –, with oxygen bubbling: ——). The measurements were performed in 1.0 × 10⁻¹ mol L⁻¹ hydrosulphuric acid at 298 K. To prevent contamination of the coordinating ion or ligands, ultrapure water was used in the experiments.

Fig. 14

TEM images of Ag or Ag-Pb alloy nanoparticles. (a) Ag nanoparticles, (b) Ag-Pb alloy (Ag:Pb = 9:1), (c) Ag-Pb alloy (7:3) and (d) Ag-Pb alloy (Ag:Pb = 6:4).

Fig. 15

I-V and I-P curves of a one-compartment H₂O₂ fuel cell with Ag or Ag-Pb alloy cathode. (Au anode. 1.0 mol L⁻¹ NaOH, 3.0 × 10⁻¹ mol L⁻¹ H₂O₂. black: Ag, green: Ag:Pb = 6:4, red: Ag:Pb = 7:3 and blue: Ag:Pb = 9:1).

Fig. 16

Chemical structures of porphyrin and phthalocyanine iron(III) complexes similar to active site structures of hydroperoxidases as candidates of cathodes for an H₂O₂ fuel cell. (a) [Fe^III(Pc)Cl], (b) [Fe^III(OEP)Cl] and (c) [Fe^III(TPP)Cl].

Fig. 17

Cyclic voltammograms of H₂O₂ on glassy carbon electrodes modified with Fe^III complexes. (a) [Fe^III(Pc)Cl], (b) [Fe^III(OEP)Cl] and (c) [Fe^III(TPP)Cl]. The measurements were performed in an acetate buffer solution (pH 4) containing 3.0 × 10⁻³ mol L⁻¹.

Fig. 18

I–V and I–P curves of a one-compartment H₂O₂ fuel cell with Ni anode and [Fe^III(Pc)Cl] cathode. Performance tests were conducted in an acetate buffer containing 3.0 × 10⁻¹ mol L⁻¹ H₂O₂. The pH of the solutions was fixed to 5 (a, blue), 4 (a, black) or 3 (b, red). Currents and powers were normalized by a geometric surface area of electrode.

Fig. 19

(a) UV-vis absorption change of a benzonitrile solution of [Fe^III(Pc)Cl] (4.0 × 10⁻⁵ mol L⁻¹) by adding trifluoroacetic acid (2 – 16 × 10⁻³ mol L⁻¹). (b) The Hill plot obtained from monitoring of the absorption changes at 520 nm.

Fig. 20

Potential changes by repetitive measurements of H₂O₂ fuel cells with the Nafion^® coated [Fe^III(Pc)Cl] cathode (red) and without Nafion^® coating (black). Potentials required to achieve the power density of 20 μA cm⁻² were recorded.

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Chart 1

Structures of biscobalt porphyrin-corrole complexes.