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Integrating Perovskite Photovoltaics and noble-metal-free Catalysts towards Efficient Solar Energy Conversion and H2S Splitting Weiguang Ma, Jingfeng Han, Wei Yu, Dong Yang, Hong Wang, Xu Zong, and Can Li ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01772 • Publication Date (Web): 09 Aug 2016 Downloaded from http://pubs.acs.org on August 13, 2016
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Integrating Perovskite Photovoltaics and noble-metal-free Catalysts towards Efficient Solar Energy Conversion and H2S Splitting Weiguang Ma, †‡ Jingfeng Han, † ‡ Wei Yu, † Dong Yang, † Hong Wang, †,§ Xu Zong,* †
and Can Li* †
†
.State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian National Laboratory for Clean Energy, The Collaborative Innovation Center of Chemistry for Energy Materials (iChEM)
Zhongshan Road 457, Dalian, 116023, China. §
.University of Chinese Academy of Sciences
Beijing 100049 China.
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ABSTRACT: Hydrogen sulfide (H2S) has been considered as a potential hydrogen source. Identifying efficient solar-driven processes and low cost materials that can extract hydrogen from H2S is highly attractive. Herein, for the first time, we reported the establishment of a perovskite photovoltaic-electrolysis (PV-EC) H2S splitting system by integrating single perovskite solar cell, noble-metal-free catalysts, and H2S splitting reaction with the aid of mediators. The as-established system delivered a solar-to-chemical energy conversion efficiency of up to 13.5 % during the PV-EC step by using molybdenum-tungsten phosphide (Mo-W-P) as the catalyst for hydrogen evolution reaction (HER) and graphite carbon sheet as the catalyst for the oxidation of mediators, respectively. To the best of our knowledge, this is among the highest value ever reported for the artificial conversion of solar to chemical energy using perovskite solar cells. Moreover, when integrated with the PV-EC system, H2S splitting reaction with a net energy conversion efficiency of 3.5 % can be accomplished and the overall energy consumption to obtain equivalent amount of H2 from H2S is reduced by ca. 43.3 % compared with that from water splitting. This paradigm of producing value-added chemicals by consuming negative value waste products is solely based on low cost materials and simpler system configuration, which significantly improves the economic sustainability of the process.
KEYWORDS: Perovskite photovoltaics, noble-metal-free catalysts, redox mediators, solar energy conversion, H2S splitting
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1. INTRODUCTION Producing solar hydrogen by consuming negative value waste products represents an attractive pathway that presents both economic and environmental benefits.1-4 Hydrogen sulfide (H2S) is a byproduct produced in large qualities from anthropogenic activities such as natural gas production and oil refinery.5,6 For example, it was reported that H2S accounts for 8.6 % of the nature gas from Puguang Natural gas field (the largest natural gas field in China) and the concentration of H2S can even reach up to 90 % in some field.7 Similarly, by treating H2S from oil refineries with Clause process, more than 70 Mt elemental sulphur is produced every year in USA.8 With simple estimation, this means that ca. 4.4 Mt of valuable H2 is wasted with this traditional technology that can only exact elemental sulphur form H2S. Moreover, compared with the H2O splitting reaction (∆G0 = 237 kJ/mol), the H2S splitting reaction (∆G0 = 33 kJ/mol) is thermodynamically less stringent and much less energy is required to obtain equivalent amount of H2.9 Therefore, although producing solar H2 from H2O is our ultimate objective, extracting H2 from H2S is also highly attractive in the scheme of solar energy conversion. Up to now, numerous efforts have been devoted to the production of H2 from H2S by utilizing solar energy.3,10-14 Although continuous progress has been witnessed in this field, the difficulty in the simultaneous extraction of sulfur species in amiable forms introduces new environmental and technical challenges, which makes these approaches less appealing. To tackle this dilemma, we recently proposed a photoelectrochemical-chemical loop strategy for the overall splitting of H2S to H2 and 3 ACS Paragon Plus Environment
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elemental sulphur using solar energy.15 With this strategy, solar energy is converted to chemical energy in the form of H2 and Fe3+ (or I3-) in photoelectrochemical reactions. The chemical energy stored in Fe3+ (or I3-) is then readily liberated to oxidize H2S selectively to elemental sulfur and protons in a simple chemical reaction. With the aid of mediators, the photoelectrochemical and chemical reactions can be integrated to achieve a loop and enable the simultaneous production of H2 and S from H2S. Although this proof-concept study opens a new avenue for the mineralization of H2S by utilizing solar energy, we note that several important challenges remain unresolved. In the proposed approach, the most important step is to deposit solar energy in the form of mediators and H2. However, the solar energy conversion efficiency delivered in this step is less than 2 % in the previous effort, which is far below the level of 10 % required in practical applications. Moreover, mainly precious materials such as platinum were used to catalyze the hydrogen evolution reaction (HER) and the oxidation of mediators, which substantially increases the cost of the system. To realize a solar energy economy, it is crucial to improve the economical sustainability of the overall process. Therefore, identifying more efficient solar-driven processes and low cost materials for H2S splitting is highly desirable. In recent years, perovskite solar cells have emerged as promising candidates for the conversion of solar energy. The solar energy conversion efficiency obtained on perovskite solar cells has increased from the initial 3.8 % to over 20 % in less than 5 years.16-20 More interestingly, when coupled with electrocatalytic system, perovskite photovoltaic-electrolysis (PV-EC) system can deliver inspiring efficiencies of 12.3 % 4 ACS Paragon Plus Environment
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and 6.5 % for water splitting and CO2 reduction reactions, respectively, which provide an attractive route for the storage of solar energy.21,22 We hypothesize that if we can successfully introduce perovskite PV-EC system to the approaches we proposed, we could realize more efficient H2S splitting reaction. Herein we reported the construction of a perovskite PV-EC system by combining single perovskite solar cell and noble-metal-free electrocatalysts for the charge and discharge of solar energy towards H2S splitting. An impressive solar-to-chemical energy conversion efficiency of up to 13.5 % was obtained during the PV-EC step by using molybdenum-tungsten phosphide (Mo-W-P) as the catalyst for hydrogen evolution reaction (HER) and graphite carbon sheet (GCS) as the catalyst for the oxidation of mediators (Fe2+ to Fe3+ or H5(PMo2VMo10VIO40) to H3(PMo12VIO40)), respectively. Moreover, when integrated with the PV-EC system, H2S splitting reaction with a net energy conversion efficiency of 3.5 % can be accomplished. 2. METHODS Reagent. hydrogen
Ammonium
heptamolybdate
((NH4)6MoO4·4H2O),
phosphate
((NH4)2HPO4),
Ammonium
Diammonium
tungstate
hydrate
((NH4)5H5[H2(WO4)6]·H2O), Iron(III) chloride hexahydrate (FeCl3·6H2O) were obtained from Sinopharm Chemical Reagent Co., Ltd. Phosphomolybdic acid (H3PMo12O40) and Iron(II) chloride tetrahydrate (FeCl2·4H2O) was received from Alfa Aesar. Nafion 117 (183 µm) was purchased from DuPont. The titanium foam was obtained from Kunshan Jiayisheng Electronic Co., Ltd. Other chemicals were used as received without further purification unless otherwise noted. All solutions 5 ACS Paragon Plus Environment
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were prepared with 18.2 MΩ·cm deionized water from a Millipore deionized water system. Preparation of HER Catalysts. Mo-W-P catalysts were synthesized on titanium (Ti) foam and used as the HER catalyst. In a typical synthesis, 0.66 g of (NH4)6Mo7O24·4H2O, 0.77 g of (NH4)5H5[H2(WO4)6]·H2O and 0.73 g of (NH4)2HPO4 were dissolved in 30 ml of deionized water. 50 µL of the mixed solution was then drop-cast onto 1 cm2 Ti foam substrate followed by drying at room temperature. The samples were then reduced in a quartz tube furnace at 750 oC under approximately 120 sccm mixture of 5 %|95 % H2|Ar flowing for a period of 4 h. After reduction treatment, the as-obtained Mo-W-P/Ti electrodes were allowed to cool down to room temperature and followed by passivation in a flow of 1 %|99 % O2|Ar for 4 h at room temperature. The MoP and WP catalysts were synthesized with a similar way. The MoP/Ti, WP/Ti, and Mo-W-P/Ti electrode were connected with a silver wire by conductive silver paste. Finally, the silver wire was sealed in a glass tube with epoxy resin. Fabrication of Perovskite Solar Cell. FTO glasses (NSG Inc., Model name TEC-7, sheet resistance is lower than 10 Ω·sq.-1) were cleaned with acetone, isopropyl alcohol, ethanol and deionized water successively in ultrasonic baths for 30 min, respectively. The TiO2 blocking layer was sputtered on FTO glass substrates by the PVD-75 vacuum system (Kurt J. Lesker, U.S.A). DC magnetron sputtering power was 400 W using a 3-inch-diameter metallic plate of Ti (99.995 %) in an atmosphere of 85 %|15 % Ar|O2. The thickness of TiO2 film was ca. 60-70 nm determined by a Bruker 6 ACS Paragon Plus Environment
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150 surface profiler. The thin TiO2 film samples were annealed at 500 °C for 30 min before deposition of perovskite films. The perovskite films were deposited by our reported method.23 Firstly, the substrate was deposited a layer of 150 nm PbCl2 at 310 °C through evaporation way with a deposition rate of ca. 1 Å·s-1. After cooling down to the room temperature, the PbCl2 samples were transferred to the nitrogen-filled glove-box for the following process. CH3NH3I powder were placed on the PbCl2 sample at 150 °C and allowed to stand for 30 min to ensure that all PbCl2 was transformed into perovskite. Then, the perovskite samples were transferred into a petri dish and cooled down to the room temperature. After washing with 50 mL isopropanol and drying by flowing nitrogen, the samples were annealed at 70 °C for 5 min.
150
µL
of
(2,2`,7,7`-tetrakis(N,N-di-p-methoxyphenylamine)-
9,9-spirobifluorene) (spiro-MeOTAD) solution was spin-coated on the perovskite layer at 4000 rpm for 30 s. The spiro-MeOTAD solution was prepared by dissolving 90 mg of spiro-MeOTAD in 1 mL of chlorobenzene, to which 36 µL of 4-tert-butyl pyridine and 22 µL of lithium bis(trifluoromethanesulfonyl) imide (Li-TFSI) solution (520 mg LI-TSFI in 1 mL acetonitrile) were added. Finally, 80 nm of gold layer was thermally evaporated on the spiro-MeOTAD coated film. Establishment of Perovskite PV-EC cell for H2S Splitting. The perovskite PV-EC cell for H2S splitting is established by connecting a perovskite solar cell with electrocatalysts for catalysing HER and oxidation of mediators. The schematic configuration and the physical picture of the cell are shown in Scheme S1 and Figure S1 (Supporting Information), respectively. The active area of the perovskite solar cell 7 ACS Paragon Plus Environment
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is ca. 0.1134 cm-2. Mo-W-P/Ti and commercial graphite carbon sheet with area of both ca. 4 cm2 (2×2 cm) were used as the electrodes for catalysing HER and oxidation of mediators, respectively. The amount of Mo-W-P loaded on Ti is ca. 0.043 mg·cm-2 (based on the metal contents). Material Characterizations. The catalysts were characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). XRD analysis of the samples was performed using a Rigaku D/Max-2500/PC powder diffractometer in the range of 10-80° (2θ). The samples were scanned using Cu-Kα radiation with an operating voltage of 40 kV and current of 200 mA. SEM images were collected on a field emission scanning electron microscopy (FE-SEM, Quanta 200 FEG). XPS analysis was carried out on a Thermo Esclab 250Xi photoelectron spectroscopy with a monochromatic Al Kα X-ray radiation as the X-ray source for excitation and normalized to the C 1s peak (284.6 eV) for the sample. The crossover of Fe and Mo species through the Nafion membrane were analyzed by inductively coupled plasma atomic emission spectrometry (ICP-AES, ICPS-8100). Electrochemical Measurements. The electrochemical performance of catalysts was evaluated in a home-made reactor with two compartments on a CHI 660E electrochemical workstation. A Nafion 117 proton exchange membrane was sandwiched between the anodic and cathodic compartments, which will allow proton transfer and prevent the mixing of the electrolytes in the two compartments. For the measurements performed in a three-electrode system, saturated calomel electrode 8 ACS Paragon Plus Environment
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(SCE, saturated KCl) with a glass frit was used as the reference electrode. Pt foil was used as the counter electrode (To preclude the influence of Pt contamination from the counter electrode, catalysts were also tested using Ti foam as the counter electrode and we obtained similar results). Pt foil, WP/Ti, MoP/Ti or Mo-W-P/Ti electrodes was used as the working electrode for HER in 0.5 M H2SO4. For the Fe2+ oxidation reaction, Pt foil or GCS was applied as the working electrode in 0.5 M H2SO4 containing 1 M FeCl2. The HER and Fe2+ oxidation reaction were investigated by linear sweep voltammetry at a scan rate of 5 mV·s-1. For the measurements performed in a two-electrode configuration, Mo-W-P/Ti and GCS electrodes were used for HER and Fe2+ oxidation reaction, respectively. The electrochemical performance of the Mo-W-P/Ti-GCS system was investigated by linear sweep voltammetry at a scan rate of 5 mV·s-1 from 0.3 V to 1.0 V. The long-term durability tests were performed by using
both
chronopotentiometry
and
linear
sweep
voltammetry
(LSV).
Chronopotentiometric measurements were conducted by maintaining the current density of HER at 10 mA·cm-2 and Fe2+ oxidation at 4 mA·cm-2 for 12 h. LSV were scanned at certain potential range before and after 12 h chronopotentiometric tests at a scan rate of 5 mV·s-1. It is worth noting that the solution used for LSV before and after 12 h chronopotentiometric tests is the same to preclude the influence of the change of the solution. Furthermore, sequential bulk electrolysis was also applied for studying the stability of Fe3+/Fe2+ reduction-oxidation reaction in 0.5 M H2SO4 in a three-electrode system. The biases applied were 0.37 V and 1.22 V vs. RHE for the
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reductive and oxidative steps, respectively. The electrochemical characterization of phosphomolybdic acid is similar with that of Fe3+/Fe2+. All potentials measured were calibrated to RHE using the following Equation: E (RHE) =E (SCE) + 0.241 V + 0.059 × pH. All the electrochemical data were directly collected without IR compensation unless otherwise stated. The Faradaic efficiency for HER was obtained in an H-type electrochemical reactor with a Nafion separator. The sample line of the reactor was connected to online gas chromatography (Agilent 7890 GC, TCD, Argon carrier), allowing the real-time detection of the hydrogen evolved. A two-electrode setup was consisted of Mo-W-P/Ti as the working electrode, a Pt foil as the counter electrode, and deaerated 0.5 M H2SO4 was used as the electrolyte. Before measurement, the reaction system was thoroughly degassed with ultrapure argon to expel the air in the reactor. Constant current was applied to the electrodes using an electrochemical workstation. The concentration of hydrogen was analyzed with gas chromatography through automatic sampling every 3 min. Calibration was carried out using a similar setup but with two cleaned Pt foils as electrodes. To determine the Faradaic efficiency for Fe2+ oxidation, GCS, Pt, and SCE (in saturated KCl) were used as the working, counter and reference electrodes. Deaerated 0.5 M H2SO4 contained 1 M FeCl2 were used as the electrolytes in the cathodic compartment and anodic compartment, respectively. The amounts of Fe3+ produced during the reaction were analyzed with iodometric titration. The Faradaic efficiency for H5(PMo2VMo10VIO40) oxidation was evaluated with a way similar for Fe2+. The 10 ACS Paragon Plus Environment
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amounts of H5(PMo2VMo10VIO40) were analyzed with UV-visible spectrophotometer (JASCO V-650). For single perovskite solar cell, the current-voltage characteristics were measured using a computer-controlled Keithley 2400 source measure unit under 100 mW·cm-2 illumination (AM 1.5 G Oriel solar simulator, calibrated by a NREL-traceable KG5 filtered silicon reference cell). All devices scanned from negative bias to positive bias with standard test procedure at scan rate 0.2 V·s-1. The IPCE was characterized on the QTest Station 2000ADI system (Crowntech. Inc., USA). The monochromatic light intensity for IPCE was calibrated using a reference silicon photodiode. An aperture area of 0.1134 cm2 was used during the measurement to define the active area of the device and to avoid light scattering through the sides. The current of the present PV-EC system was recorded by i-t without applying an external bias for different time periods under chopped AM 1.5 G illumination. Calculation of the Solar-to-chemical Conversion Efficiency. In the present perovskite PV-EC system, the solar-to-chemical conversion efficiency (ηSTC) was calculated according to the following equation.24-26 =
∆∙ ∙
(1)
where ∆E is the full-cell formal potential difference (in volts) between hydrogen evolution and Fe2+ oxidation in the cell as measured at two nonpolarizable electrodes, Jop is the operating current density (in mA·cm2 and on the basis of the area of the perovskite solar cell), ηF is the faradaic efficiency for HER and Fe2+ (or H5(PMo2VMo10VIO40)) oxidation and Pin is the incident solar power density 11 ACS Paragon Plus Environment
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(mA·cm-2). We have used two Pt foil electrodes, which are highly reversible for hydrogen evolution and Fe2+ (or H5(PMo2VMo10VIO40)) oxidation, respectively, to explicitly measure ∆E in this system under conditions identical to those used in the perovskite PV-EC system. When calculating the net solar energy conversion efficiency during the H2S splitting loop reaction, equation (1) is also used except replacing ∆E that was calculated by the following equation: ∆ = ⁄ − ⁄
(2).
3. RESULTS AND DISCUSSION Working Principle of the Perovskite PV-EC H2S Splitting System. The working principle of the perovskite PV-EC H2S splitting system is schematically shown in Scheme 1. The whole process involves two integrated steps. In the first PV-EC step, solar energy is converted in the form of H2 via HER in the cathodic compartment and the oxidation of Fe2+ to Fe3+ in the anodic compartment. In the second step, the chemical energy stored in Fe3+ is liberated to oxidize H2S selectively to elemental sulfur and protons via a simple and fast chemical reaction and Fe3+ is restored to Fe2+ simultaneously. Protons generated from H2S oxidation in the anodic compartment transfer to the cathodic compartment through Nafion membrane and act as the proton source for H2 production in the cathodic compartment. Therefore, the perovskite PV-EC solar energy storage process and H2S overall splitting reaction can be well integrated to obtain H2 and elemental sulphur. Ideally, the whole process should be catalyzed by noble-metal-free materials to improve the economic sustainability of the 12 ACS Paragon Plus Environment
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approach. Moreover, it is worth noting that the approach shown in Scheme 1 is only a simplified demonstration of the overall process. To enable the continuous operation of the whole process, the reaction can be implemented according to the flowchart shown in Scheme S2 (Supporting Information).
Scheme 1. Working principle of the perovskite PV-EC H2S splitting system. Before starting experiments, we assessed the potential merits of the perovskite PV-EC H2S splitting system. In this system, solar energy is stored in the form of mediators instead of O2. As the thermodynamic potential for this reaction is only ca. 0.77 V, one pervoskite solar cell could provide sufficient voltage to drive the reaction. Compared with the water splitting and CO2 reduction systems that require two or more perovskite solar cells connected in series, the configuration of the system will be simpler. Moreover, the oxidation of the mediators represents one or two-electron processes and more rapid kinetics compared with the sluggish four-electron oxygen evolution reaction. Therefore, the energy loss during conversion could be reduced. Furthermore, compared with oxygen produced from water splitting or CO2 reduction, the as-produced mediators represent value-added chemicals that can be potentially used in a variety of application. Considering that both the photogenerated electrons 13 ACS Paragon Plus Environment
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and holes can be used to produce useful products, converting solar energy to H2 and mediators could represent a more attractive dual-function solar-to-chemical conversion process. HER Catalysts for H2S Splitting. In consideration of the two main target reactions that will occur in the system, we first aimed to develop low-cost materials with intrinsic electrocatalytic activity comparable with those containing noble metals, long-term stability, superior electrical conductivity, and chemical compatibility. Molybdenum phosphide (MoP), tungsten phosphide (WP), and molybdenum tungsten phosphide (Mo-W-P) catalysts were prepared onto titanium (Ti) foam and used for HER in the cathodic compartment. Figure 1 a-d shows the top-view scanning electron microscopy (SEM) images of bare Ti foam, MoP, WP, and Mo-W-P catalysts. Bare Ti foam exhibits a smooth surface with large pores (Figure 1a and Figure S2, Supporting Information). After preparing MoP, WP, and Mo-W-P catalysts on the Ti foam, the surface of the Ti foam turned rough and was covered with abundant nanoparticles. As for MoP, the nanoparticles are interconnected to form a compact film. As for WP and Mo-W-P electrodes, additional micropores were observed. The energy-dispersive X-ray (EDX) elemental mapping analysis (Figure 1e) indicates that Mo, W, and P elements are homogeneously distributed throughout the whole Mo-W-P film. The mean Mo:W:P ratio obtained from three different Mo-W-P samples is ca. 0.57:0.43:2.1. The high P content obtained in the material could be due to the formation of P4 on the surface of the catalyst during the high-temperature reduction treatment.27 14 ACS Paragon Plus Environment
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The chemical nature of Mo-W-P catalysts was then analyzed with X-ray photoelectron spectroscopy (XPS). Figure 1f-h shows the high-resolution XPS spectra for the Mo 3d, W 4f, and P 2p regions, respectively. The two peaks at 228.4 and 231.6 eV can be assigned to the Mo 3d5/2 and Mo 3d3/2 in MoP.28,29 The two peaks at 31.7 and 33.9 eV is attributed to W 4f7/2 and W 4f5/2 of the Wδ+ species (0
