Efficient Photocatalytic Production of Hydrogen Peroxide from Water and Dioxygen with Bismuth Vanadate and a Cobalt(II) Chlorin Complex Kentaro Mase,† Masaki Yoneda,† Yusuke Yamada,‡ and Shunichi Fukuzumi*,§,∥ †
Department of Material and Life Science, Graduate School of Engineering, Osaka University, ALCA and SENTAN, Japan Science and Technology (JST), Suita, Osaka 565-0871, Japan ‡ Department of Applied Chemistry and Bioengineering, Graduate School of Engineering, Osaka City University, Osaka 558-8585, Japan § Department of Chemistry and Nano Science, Ewha Womans University, Seoul 120-750, Korea ∥ Faculty of Science and Engineering, Meijo University, ALCA and SENTAN, Japan Science and Technology Agency (JST), Nagoya, Aichi 468-8502, Japan S Supporting Information *
ABSTRACT: Efficient photocatalytic production of H2O2 as a promising solar fuel from H2O and O2 in water has been achieved by the combination of bismuth vanadate (BiVO4) as a durable photocatalyst with a narrow band gap for the water oxidation and a cobalt chlorin complex (CoII(Ch)) as a selective electrocatalyst for the two-electron reduction of O2 in a two-compartment photoelectrochemical cell separated by a Nafion membrane under simulated solar light illumination. The concentration of H2O2 produced in the reaction solution of the cathode cell reached as high as 61 mM, when surfacemodified BiVO4 with iron(III) oxide(hydroxide) (FeO(OH)) and CoII(Ch) were employed as a water oxidation catalyst in the photoanode and as an O2 reduction catalyst in the cathode, respectively. The highest solar energy conversion efficiency was determined to be 6.6% under simulated solar illumination adjusted to 0.05 sun after 1 h of photocatalytic reaction (0.89% under 1 sun illumination). The conversion of chemical energy into electric energy was conducted using H2O2 produced by photocatalytic reaction by an H2O2 fuel cell, where open-circuit potential and maximum power density were recorded as 0.79 V and 2.0 mW cm−2, respectively.
D
alternative solar fuel because the reduction of dioxygen (O2) is thermodynamically easier than that of protons.20,21 In addition, H2O2 is liquid at ambient temperature and pressure and thereby has much higher energy density per unit volume without the use of a high-pressure tank. H2O2, stored as chemical energy, provides electrical energy on demand through H2O2 fuel cells with theoretical output voltage of 1.09 V by utilizing H2O2 as both a reductant at a cathode and an oxidant at an anode, emitting only water and oxygen after power generation.20−28 Thus, H2O2 produced by the reduction of
irect conversion of solar energy into easily storable chemical energy has attracted much attention as the key technology to ensure a stable supply of energy because the amount of electric energy, generated by photovoltaic cells, depends directly on intermittent solar energy.1,2 In this context, hydrogen (H2) has been long regarded as a promising chemical energy carrier producible by the utilization of solar energy.3−9 Extensive efforts have so far been devoted to achieving photochemical water splitting with visible light responsive photocatalysts for the efficient utilization of solar energy.4−19 However, because of the narrower band gap of those photocatalysts, a two-step photoexcitation system with a redox mediator is required to implement both water oxidation and reduction, simultaneously, resulting in an uninspiring efficiency mainly due to back electron transfer. Recently, hydrogen peroxide (H2O2) has been considered as an © XXXX American Chemical Society
Received: September 6, 2016 Accepted: October 10, 2016
913
DOI: 10.1021/acsenergylett.6b00415 ACS Energy Lett. 2016, 1, 913−919
Letter
http://pubs.acs.org/journal/aelccp
Letter
ACS Energy Letters
HO2•) = −0.05 V vs RHE].46−48 To overcome the poor reducing ability of the electron in CB of BiVO4, a cobalt chlorin complex (CoII(Ch)), which has been previously proven to catalyze the selective O2 reduction via the two-electron reduction process (E°(O2/H2O2) = 0.68 V vs RHE) with a small overpotential in PhCN under homogeneous conditions (eq 2), was employed as a cathode catalyst.49 The performance of CoII(Ch) adsorbed on a carbon paper (CoII(Ch)/CP) in an aqueous solution was compared with cobalt octaethylporphyrin (CoII(OEP)), which is commonly used as a electrocatalyst for O2 reduction,50 adsorbed on CP (CoII(OEP)/CP). The overall photocatalytic reaction is given by eq 3, where H2O2 can be produced by two-electron reduction of earth-abundant O2 by water that acts as an electron donor.
earth-abundant O2 with water utilizing solar energy is highly desirable. High concentration of H2O2 has been obtained in the photocatalytic reaction system,29−37 where the highest H2O2 concentration is reported to be 69 mM by using an organic photocatalyst.36 In these cases, a sacrificial electron donor, such as methanol, oxalic acid, formic acid, or other organic electron donor, is necessary. The moderate reduction potential of O2 enables photocatalytic production of H2O2 from H2O and O2 by the single-step photoexcitation of visible-light-driven photocatalysts without the use of a redox mediator. Photocatalytic production of H2O2 from H2O and O2 by the combination of a ruthenium complex as a homogeneous photocatalyst and water oxidation catalyst under single-step visible light illumination in a one-compartment cell has been reported.37 However, the concentration of H2O2 produced in this system is limited to as low as ∼1 mM because of the poor stability of the ruthenium complex under illumination and the decomposition of H2O2 by disproportionation on the metal oxide water oxidation catalyst. For the practical conversion of chemical to electric energy with an H2O2 fuel cell, the concentration of H2O2 should be higher than ∼50 mM.22−25 We have previously demonstrated a photocatalytic production of H2O2 from H2O and O2 in seawater attained by the combination of mesoporus WO3 (mWO3) as a durable photocatalyst for the water oxidation and a cobalt chlorin complex (CoII(Ch)) as a catalyst for the efficient and selective two-electron reduction of O2 in a two-compartment photoelectrochemical cell separated by a Nafion membrane.38 The physically separated two-compartment configuration prevented the decomposition of H2O2 by WO3. A high H2O2 concentration of ca. 48 mM was obtained in seawater by the enhancement effect of Cl− on the photocatalytic water oxidation under simulated 1 sun (AM 1.5G, 100 mW cm−2) illumination without the degradation of the photocatalyst. However, further improvement in solar energy conversion efficiency is needed to cover increasing global energy demand, where it is estimated that conversion efficiency of 10% would be required to supply one-third of the expected global energy demand in 2050.1,39 We report herein photocatalytic production of H2O2 from H2O and O2 with much improved turnover number and solar energy conversion efficiency by harvesting a wider range of solar energy with monoclinic scheelite structured bismuth vanadate (BiVO4), which is an n-type semiconductor with smaller band gap (2.4 eV) driven by visible light (
