Cu2ZnSnS4 Nanocrystals as Highly Active and Stable Electrocatalysts

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CuZnSnS Nanocrystals as Highly Active and Stable Electrocatalysts for the Oxygen Reduction Reaction Xuelian Yu, Da Wang, Jingjing Liu, Zhishan Luo, Ruifeng Du, Li-Min Liu, Guangjin Zhang, Yihe Zhang, and Andreu Cabot J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b08577 • Publication Date (Web): 10 Oct 2016 Downloaded from http://pubs.acs.org on October 15, 2016

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The Journal of Physical Chemistry

Cu2ZnSnS4 Nanocrystals as Highly Active and Stable Electrocatalysts for the Oxygen Reduction Reaction Xuelian Yu,*,a,† Da Wang,b,† Jingjing Liu,c,† Zhishan Luo,d Ruifeng Du,a Li-Min Liu,*,b Guangjin Zhang,*,c Yihe Zhanga and Andreu Cabotd,e a. Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences, 100083 Beijing, P. R. China. b. Beijing Computational Science Research Center, Beijing 100193, P. R. China. c. Key Laboratory of Green Process and Engineering, Chinese Academy of Sciences, 100190 Beijing, P. R. China. d. Catalonia Institute for Energy Research - IREC, 08930 Sant Adria del Besos, Barcelona, Spain. e. ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Spain

Abstract

ACS Paragon Plus Environment

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The implementation of cost-effective fuel cells and metal-air batteries requires developing environmental friendly, cost-effective and high performance electrocatalyst for the oxygen reduction reaction (ORR). While several oxides has been proposed as electrocatalysts for the ORR in basic conditions, some sulfides, particularly copper sulfide, have been shown to provide some of the highest electrical conductivities and overall improved electrocatalytic properties. We use here first-principle calculations to study the oxygen adsorption and ORR kinetics on alternative copper-based sulfides and find out that the inclusion of cations with a higher oxidation state, such as Zn2+ and Sn4+, provides more energetically favorable sites for oxygen adsorption and subsequent formation of hydroxyls. DFT calculations show all species in the OOH* dissociation pathway to preferentially adsorb on Sn4+ and Zn2+ sites, providing low energy barriers of the rate-determining step. Supported by these calculations, we synthesized Cu2ZnSnS4 (CZTS) nanocrystals (NCs) with controlled crystallographic phase by high-yield colloidal synthesis routes, and investigated their electrocatalytic properties toward ORR in alkaline solution. Kesterite CZTS NCs show exceptional electrocatalytic performance for ORR with high current densities (5.45 mA cm-2 at 0.1 V vs. RHE) and low onset reduction potential (0.89 V vs. RHE), comparable to commercial Pt/C electrocatalysts. Equally important, CZTS NCs exhibit superior stability and resistance to methanol, thus avoiding the poisoning limitation of commercial Pt/C catalyst in the particular case of direct methanol fuel cells.

Introduction

The oxygen reduction reaction (ORR) is a limiting step in several energy conversion and storage technologies, such as fuel cells, metal-air batteries and certain electrolysers.1,2 To accelerate this reaction, platinum-based catalysts are generally required. However, the low abundance and high


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cost of platinum strongly limits the cost-effective commercialization of these technologies. In addition, platinum-based electrocatalysts suffer from limited durability associated to surface poisoning by certain substances. This is the case, for instance, in direct methanol fuel cells where Pt stability becomes an issue due to methanol crossover.3 Alloying Pt with low-cost transition metals which maintain proper ORR activities with a lower usage of Pt represents an insufficient step in the correct direction.4-6 Another approach to overcome the extraordinary challenge that the cost-effective activation of the ORR represents is the use of alkaline electrolytes. While the implementation of alkaline solutions still requires further electrolyte or membrane optimization in terms of performance and especially tolerance to CO2, it has a main advantage in enabling the use of much more cost-effective catalysts. In this regard, a wide range of noble metal-free electrocatalysts have been explored for ORR in alkaline media, such as transition metal oxides, chalcogenides, nitrides, oxynitrides and modified graphene.7-10 Among them, copper chalcogenides have been demonstrated to provide outstanding performance and stability, especially when supported on different forms of carbon.11-13

In view of the exceptional electrocatalytic properties of copper sulfides and selenides, it is surprising that other copper-based chalcogenides have not been explored for such a technologically relevant reaction, especially when taking into account that some multinary copper chalcogenides have been demonstrated to overcome binary copper chalcogenide performances in other electrocatalytic applications. This is the case on the reduction reaction of the oxidized electrolyte at the counter electrode of quantum dot sensitized solar cells, for instance.14-19


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The ample chemical and structural freedom of multinary chalcogenide enable optimizing this class of materials in multiple applications, including photovoltaics, thermoelectrics and catalysis.20,21 The small size and electronegativity differences between transition metals and chalcogens makes them strongly covalent compounds. This covalency, the chalcogen ability to catenate, the variable valences of transition metals, and the higher ability of metal atoms to form metal-metal bonds within chalcogenides result in a very large diversity of multinary chalcogenide stoichiometries and structures, including abundant non-stoichiometric phases and a wide range of solid solutions. This structural and valence variability yields materials with large densities of defects, large charge carrier concentrations and potentially excellent and tunable photocatalytic and electrocatalytic properties.22,23

In this work, we investigate multinary copper-based chalcogenides as low-cost, non-toxic, potentially highly reactive and robust electrocatalysts for ORR. We first use first-principles calculations to determine the potential of Cu2ZnSnS4 (CZTS) for ORR. Then, we produce CZTS nanocrystals (NCs) with two different phases and test their catalytic properties toward ORR, including stability and tolerance to methanol crossover. We show kesterite CZTS NCs to display excellent catalytic properties toward ORR, and therefore, to provide a reasonable alternative electrocatalysts based on non-noble metal multinary chalcogenides.

Experimental section

Computational methods: DFT calculations were implemented in the Vienna ab initio package (VASP).24,25 For the exchange-correlation functional, the general gradient approximation of Perdew-Burke-Ernzerhof (GGA-PBE) was adopted.26 Moreover, the electron wave functions were expanded by a 400 eV plane wave cutoff. The (4×4×2) and (4×3×3) supercell kesterite and


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wurtzite CZTS with the stoichiometry of Cu64Zn32Sn32S128 and Cu72Zn36Sn36S144, respectively, were constructed by periodic boundary condition (PBC). To avoid periodic interaction, the vacuum layers were set to be larger than 20 Å. Reciprocal space was performed by the Monkhorst-Pack special k-point scheme with 2×2×2 grid meshes for structure relaxation for both kesterite and wurtzite CZTS. Atomic relaxation until the total energy variation was smaller than 10-6 eV and all forces on each atom were less than 0.01 eV/Å. We determined the free energy barrier for ORR on the CZTS using the climb image nudged elastic band (CL-NEB) method.27 Synthesis of kesterite CZTS NCs: In a typical synthesis, 1.8 mmol of CuCl2·2H2O, 1.6 mmol of ZnO and 0.6 mmol of SnCl4·5H2O, and 0.5 mmol of octadecylphosphonic acid (ODPA) were mixed using a magnetic stirrer with 30 mL of oleylamine (OLA). The solution was heated under argon flow to 200 °C and it was kept 1 hour at this temperature. Then it was heated to 300 °C and 1 mL of a 2 M solution of tetraethylthiuram disulfide in dichlorobenzene was injected. NCs were allowed to grow at 300 ºC for 30 min, before the solution was cooled down to ambient temperature. NCs were finally purified by several precipitation and redispersion steps using 2propanol and chloroform.

Synthesis of wurtzite CZTS NCs: In a typical synthesis, 1.8 mmol of CuCl2·2H2O, 1.6 mmol of ZnO and 0.6 mmol of SnCl4·5H2O were dissolved in a minimum amount of the tetrahydrofuran (THF). Afterward, 8 mmol of OLA and 6.5 g of 1-octadecene were added. The solution was heated under flow of argon to 175 °C and maintained for 1 hour. Then it was cooled to 100 °C and 16 mmol of tert-dodecylmercaptan and 1.6 mmol of dodecanethiol were injected. Then the solution was heated to 250 °C and maintained for 1 hour. The purified process was the same as the kesterite ones.


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Characterization: Transmission electron microscopy (TEM) analysis was carried out using a JEOL 2010F field emission gun microscope at 200 keV. Powder x-ray diffraction (XRD) patterns were obtained with Cu Kα1(λ= 1.5406 Å) radiation in a reflection geometry on a Bruker D8 operating at 40 kV and 40 mA. Raman scattering measurements were carried out in backscattering configuration with a LabRam Model HR800-UV Horiba−Jobin Yvon spectrometer, using the 532 nm line of a Nd:YAG solid-state laser.

Electrochemistry Experiments: To study the material electrocatalytic performance, NCs were supported on carbon black (Ketjen carbon, C) with a weight ratio of 3:7 (NCs/C) through sonication of the NCs dispersion and C in chloroform. The C−NCs catalysts were activated via thermal annealing under Ar atmosphere at 400 °C for 1 h and then dispersed in a mixture of 0.5 mL deionized water, 0.5 mL isopropanol and 8.75 µL Nafion (10 wt%). Finally, 10 µL of this solution (5 mg/mL) was deposited on the glassy carbon (GC, 4 mm in diameter) rotating disk electrode (RDE) for electrochemical characterization. Electrodes were dried overnight at ambient temperature before electrochemical characterization. Following a similar procedure, a GC electrode coated with Pt/C (20 wt% Pt, purchased from Alfa Aesar) was also fabricated for comparison.

Cyclic

voltammetry

(CV),

linear

sweep

voltammetry

(LSV)

and

chronoamperometric experiments were performed on a CHI760e electrochemical workstation (CHI Instrument Inc.) using a standard three-electrode cell. Temperature was maintained at 25 °C using a water bath. The films prepared on GC were used as working electrode, and Platinum foil as the counter electrode. As a reference electrode, Ag/AgCl calibrated with respect to the reversible hydrogen electrode (RHE) in all measurements (+0.949 V vs. RHE) was used. The electrolyte, consisting of a solution of 0.1 M KOH, was saturated with ultrahigh-purity Ar for 30


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min before the measurements. ORR was performed after saturating the electrolyte with ultrahigh-purity O2 for 30 min. Results and discussion First-principles calculations were carried out to determine the potential of multinary copperbased sulfides toward the ORR. After preliminary analysis of several potentially suitable ternary compounds, e.g. Cu2SnS3, Cu3SbS4, we focused our attention on quaternary Cu2−II−IV−S4 compounds and particularly on CZTS. CZTS is composed of four relatively earth-abundant, nontoxic and low-cost elements. In CZTS, the electronic hybridization is very strong due to the atomic levels of Cu-3d states are just slightly above the S-3p orbitals. This strong p-d hybridization results in a very significant contribution of the Cu-3d states to the near-edge valence band, while Sn mainly contributes to the near-edge conduction band. The presence of multiple elements and with different valences at their surface also provides CZTS with a wide range of adsorption sites. Catalytic and electrocatalytic properties not only depend on material composition, but also on its crystal phase and particularly on their surface facets. CZTS crystallizes in three possible phases: kesterite, wurtzite and stannite, being the two former the most commonly encountered. These phases have sensibly different physical and chemical characteristics, which have an impact on the material functionality. As an example, kesterite phase is generally considered the best candidate for photovoltaic application,28 but better results were obtained with wurtzite CZTS in the photocatalysis field.22 To obtain an initial estimation of the potential of CZTS for the ORR, we undertook the modelling of the two main phases, kesterite and wurtzite. We modelled the (112) surface of kesterite and the (100) surface of wurtzite CZTS following previous literature


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reports

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considering these two as the most probably surface facets (as shown in Figure S1).

Several possible adsorption configurations and the associated energies of various ORR species on the CZTS surfaces were examined. The corresponding most stable adsorption configurations of several ORR species on the kesterite (112) surface are shown in Figure S2. Due to their higher oxidation state, all ORR species preferentially adsorb on either Sn4+ or Zn2+ instead of Cu+ ions. The kinetically most favorable reaction pathway for the ORR process catalyzed by non-preciousmetal catalysts in an alkaline solution is considered to be the OOH* dissociation pathway,32 which can be divided into four steps: I) the adsorption of an O2 molecule onto the Cu2ZnSnS4 surface site (O2*); II) the reduction of O2* with H2O* to an OOH group adsorbed on the Cu2ZnSnS4 site; III) the transition from the OOH* to an adsorbed O* atom; IV) the transition from the O* to an adsorbed OH*. The latter three steps in our DFT calculations were considered as follows: H2O* + O2*→OH* + OOH*

(II)

OOH* → OH + O*

(III)

O* + H2O*→ OH* + OH*

(IV)

The relative energy profile of the ORR process on each CZTS surface was calculated, shown in Figure 1. To exhibit the reaction process more clearly, the molecular level of the ORR pathway is also provided in Figures S3 and S4. As illustrated, once the O2* and H2O* were adsorbed, O2* is reduced to OOH* while the H2O* is simultaneously oxidized to OH*. In this case, the calculated energy barrier is 0.36 eV for the kesterite (112) surface and 0.56 eV for the wurtzite (100) surface. The rate-determining step corresponds to the transition from the OOH* to an adsorbed O* atom (Equation III), which has a relatively low energy barrier of 0.65 eV for


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kesterite CZTS. This barrier is close to the overall barrier calculated for Pd and PdCu alloys33 and similar to the considered as limiting reaction in FeN4-graphene(O2OOH)34. The calculated energy for (100) wurtzite CZTS was still moderate, 0.87 eV, but sensibly higher than that of (211) kesterite CZTS. The final step in the ORR is the O* reduction to OH* aided by a H2O molecule, for which we calculated energy barriers of 0.38 eV and 0.59 eV for kesterite (112) surface and wurtzite (100) surface, respectively. Overall, DFT results clearly show that CZTS, particularly kesterite CZTS, has an excellent potential for ORR.

Figure 1. The chemical reactivity of kesterite and wurtzite CZTS. (a) Energy profiles of ORR on kesterite (blue) and wurtzite CZTS (red). (b) The initial state (IS), transition state (TS) and final state (FS) of the rate-determining step on kesterite (112) surface. The blue, yellow, grey, lavender, red, and pink balls represent Cu, S, Sn, Zn, O, and H atoms, respectively. Motivated by the excellent expectations given by DFT calculations, we produced CZTS nanocrystals using a facile and high yield colloidal synthesis protocol. By modifying the synthesis conditions (see experimental part) we synthesized CZTS NCs with kesterite and wurtzite crystal structures. Figure 2 shows representative TEM micrographs of as-synthesized


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kesterite (Figure 2a) and wurtzite CZTS (Figure 2b) NCs. Both CZTS kesterite and wurtzite NCs were highly monodisperse, with average sizes of 15 ± 2 nm. The NC composition, obtained using energy-dispersive X-ray spectroscopy (EDS), was found to be consistent, within the technique error (∼10 %), with that of stoichiometric CZTS, [Cu]: [Zn]: [Sn]: [S]=2.0: 1.0: 1.0: 4.0. Figure 2c shows the XRD patterns of the two different NC phases. In the kesterite sample, the main diffraction peaks correspond to the (112), (200), (220), and (312) lattice planes of the tetragonal CZTS phase (JCPDS 00-026-0575). For the wurtzite sample, the diffraction peaks match well with the (100), (002), (101), (102) and (110) planes of the hexagonal CZTS phase.35 The Raman spectra in Figure 2d obtained with a 532 nm excitation shows a band at 327 cm−1 and 331 cm−1 for wurtzite and kesterite CZTS, respectively, which agrees well with the reported spectra of CZTS.36 The presence of CuS, SnS, SnS3, SnS2 or Cu2SnS3, which would have associated bands at 475 cm−1, 190-220 cm−1, 310 cm−1, 315 cm−1, or 350 cm−1, was not identified. a)

CZTS-KE

b)

CZTS-WZ

100 nm

c)

CZTS-KE

30

40

50

2θ (Degree)

60

70

Intensity (a.u.)

CZTS-WZ

20

-1

d)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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327 cm

CZTS-WZ -1

331 cm

CZTS-KE

200

300

400

500 -1

Raman Shift cm


10

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Figure 2. TEM micrographs (a,b), powder XRD patterns (c) and Raman spectra (d) of kesterite and wurtzite CZTS NCs. To study the electrocatalytic performance of CZTS toward ORR, CZTS NCs were supported on Ketjen carbon (C) up to a weight ratio of 3:7 (NCs/C) through sonication and subsequent annealing at 400 °C. Thermogravimetric analysis (TGA) of the CZTS NCs was shown in Figure S5 and it showed organics could decompose at the temperature below 300 °C. Following this procedure, homogeneous nanocomposites with a high NC dispersion were systematically obtained, as shown in Figure 3a. During this process, a slight increase of the particle size is observed with the diameter of ~30 nm. XRD analysis of the supported materials revealed that the NCs preserved their crystallographic phase after the annealing process (Figure S6). These nanocomposites were deposited over GC electrodes for electrocatalytic measurements. Figure S7 displays CVs curves of the GC electrodes modified with kesterite and wurtzite CZTS NCs/C hybrids in a 0.1 M KOH solution saturated with Ar or O2. Both materials exhibited considerable electrocatalytic activities toward ORR in the O2-saturated solution. The wurtzite CZTS/C electrocatalyst showed a faradaic current onset potential of Eonset= +0.86 V vs. RHE, a potential of the cathodic peak current of Epc= +0.66 V vs. RHE, and a cathodic peak current density of ipc= 1.12 mA cm-2. Slightly improved values were obtained for kesterite CZTS/C electrocatalyst, with a Eonset= +0.89 V vs. RHE, a Epc= +0.70 V vs. RHE and a ipc= 1.44 mA cm2

.

RDE experiments and Koutecky–Levich analysis were used to study the ORR kinetics. Figure 3b and 3c show the LSV curves of the wurtzite and kesterite CZTS/C catalysts at different rotation speeds, with a scan rate of 10 mV s-1 and in an O2-saturated 0.1 M KOH solution. As expected,


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increasing the rotation speed promotes diffusion of oxygen at the surface of electrodes, resulting in larger current densities. The limiting current density of kesterite CZTS/C modified electrode at 0.1 V vs. RHE reached 5.45 mA cm-2, well above that of pure carbon (2.0 mA cm-2) and a very similar value to the one obtained with a commercial Pt/C electrocatalyst. On the other hand, the wurtzite CZTS/C showed significantly lower current densities, with a saturation value at 1600 rpm of 4.0 mA cm-2. b)

a)

0

-2

j (mA cm )

-1 C (1600 rpm) -2

400 rpm

-3

CZTS-WZ -4

1600 rpm

-5 -6 0.1

Pt (1600 rpm) 0.3

0.5

0.7

0.9

E (V vs. RHE)

d) 0.6

0.3

-2

CZTS-WZ

0.2 0.02

0.1V (n=4.15) 0.2V (n=3.97) 0.3V (n=3.90) 0.4V (n=3.90) 0.5V (n=3.90)

CZTS-KE

0.03

-0.5

ω

0 -1

j (mA cm )

-1

2 -1

0.4

c)

0.1V (n=4.12) 0.2V (n=3.90) 0.3V (n=3.80) 0.4V (n=3.81) 0.5V (n=3.60)

0.5

j (mA cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.04

-0.5

(rpm

0.05

)

-2

C (1600 rpm) 400 rpm CZTS-KE

-3 -4 -5 -6 0.1

Pt (1600 rpm)

1600 rpm

0.3

0.5

0.7

0.9

E (V vs. RHE)

Figure 3. (a) TEM micrograph of the CZTS NCs supported on Ketjen carbon. Arrows point at some of the CZTS NCs. (b-c) LSV curvesof the ORR at various rotation rates (400, 635, 900, 1225, 1600 rpm) from wurtzite CZTS/C (b) and kesterite CZTS/C (c) electrocatalysts in 0.1 M O2-saturated KOH solutions with a scan rate of 10 mV s-1. The RDE voltammograms of the Ketjen carbon and of commercial Pt/C, both obtained at 1600 rpm, are also plotted forreference. (d) Koutecky–Levich plots for wurtzite and kesterite CZTS at various potentials.


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While displaying slightly higher onset potentials than Pt/C, kesterite CZTS/C displayed very steep slopes in the kinetic region, pointing toward a highly favorable kinetic behavior. Figure 3d displays the inverse current density (j-1) as a function of the inverse of the square root of the rotation speed (ω-1/2) at different potentials (Koutecky–Levich plots). The good linearity and parallelism of the obtained curves is indicative of first-order reaction kinetics with the oxygen concentration and denotes that electron transfer numbers for ORR do not depend on the potential within the studied range.37 From the slopes of the Koutecky–Levich plots at potentials ranging from 0.1 to 0.5 V vs. RHE, the number of electrons transferred for the ORR was estimated to be in the range 3.7-4.0 for both electrodes. This high electron transfer numbers indicated that both electrocatalyst have high selectivity toward total oxygen reduction, and that the ORR is mainly dominated by the one-step, four electron pathway. In contrast, the electron-transfer number for C was close to 2, in agreement with the reported data for Vulcan XC-72 carbon support (Figure S8).13 The rotating ring-disk electrode (RRDE) technique was used to confirm the ORR pathway during the ORR process by monitoring the formation of intermediate peroxide species (see SI for details). As shown in Figure 4a and 4b, the measured HO2- yields were about 13–16% for wurtzite CZTS/C electrodes over the potential range of 0.1–0.6 V vs. RHE. These yields correspond to an electron transfer number of 3.65–3.75 and are in agreement with the RDE measurements. A significant drop of HO2− yield (4.3–10 %) was observed on kesterite CZTS/C electrodes, which confirmed the high electron transfer numbers and related excellent catalytic selectivity of this material.


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a)

1

Ring

CZTS-WZ CZTS-KE

-4

4.0 3.8 3.6

40

CZTS-KE

-

-3

80

60

HO2 yield (%)

-2

b)

Disk

-1

-2

j (mA cm )

0

CZTS-WZ

20

3.4 3.2

-5 -6

0.2

0.4

0.6

0.8

E (V vs. RHE)

1.0

0 0.1

0.2

0.3

0.4

0.5

Electron transfer number (n)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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3.0 0.6

E (V vs. RHE)

Figure 4. (a) RRDE curves of kesterite CZTS/C and wurtzite CZTS/C in 0.1 M O2-saturated KOH solution at a rotation rate of 1600 rpm with the ring potential kept at 1.45 V vs. RHE. (b) Calculated electron transfer number and yield of the peroxide species on the kesterite CZTS/C electrode and wurtzite CZTS/C electrode. Table S1 shows a comparison of the ORR activities of several potentially low cost electrocatalysts reported in the literature. Kesterite CZTS/C catalysts have one of the highest limiting current densities reported at 1600 rpm and comparable onset potentials. It is noteworthy that in the present work CZTS NCs were supported on low cost commercial carbon. Thus, the performances displayed here for CZTS NCs did not rely on the electrocatalytic activity enhancement from expensive carbon support with specific architectures. Conversely, in several previous works, other forms of carbon, with potentially higher performance but also higher costs, were used. This included doped graphene, carbon nanotubes, or highly ordered porous carbon matrices. All these results indicate that kesterite CZTS is indeed a strong and promising candidate as low cost electrocatalyst for ORR in alkaline electrolyte. The stability of the catalysts is also important parameters for practical applications. The stability of the kesterite CZTS/C catalyst was assessed by chronoamperometric analysis (Figure 5). These measurements revealed no apparent attenuation of the catalytic activity during a 40,000 s test. Additionally, to test its tolerance, the catalyst was exposed to methanol. When 1.2 mL of 3


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M methanol solution was added, no obvious response was observed for kesterite CZTS NCs/C, while the catalytic activity of Pt/C severely dropped in the same condition due to the methanol oxidation reaction. 200

Relative Current (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


150

3 M MeOH CZTS-KE/C

100

Pt/C

50

0 0.0

0.5

1.0

1.5

10

20

30

40

3

Time (x 10 s)

Figure 5. Stability and methanol-tolerance evaluation of kesterite CZTS/C tested by current– time chronoamperometric responses in O2-saturated 0.1 M KOH solution. Notice that after 490 s from the start of the test, 1.2 mL of a 3 M methanol solution was added. The commercial 20% Pt/C electrode was used for comparison

Conclusion

In summary, first-principles calculations pointed toward CZTS NCs as a potentially high performance electrocatalyst for ORR, showing a relatively low energy barrier for O2 adsorption and subsequent dissociation to OH* at Sn4+ and Zn2+ sites. Accordingly, CZTS NCs with controlled crystal phases, kesterite or wurtzite, were prepared by a high yield and easily upscalable colloidal synthesis routes. Such CZTS NCs, supported on conventional carbon, were demonstrated to be highly active for ORR and their activity to strongly depend on the crystal phase. We found kesterite CZTS NCs exhibited high current densities (5.45 mA cm-2 at 0.1 V vs. RHE) and relatively low onset reduction potential (0.89 V vs. RHE), comparable to commercial


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Pt/C electrocatalysts. Importantly, kesterite CZTS NCs showed not only excellent electrocatalytic activities, but also much better tolerances to methanol crossover than Pt/C.

ASSOCIATED CONTENT Supporting Information Top and side views of the initial state, transition state and final state structures for the ORR reactions, Detailed ORR experiments, TGA analysis and XRD patterns after supported on carbon, CV curve in Ar- and O2-saturated KOH solution, LSV and K-L plots of carbon black, Comparison of the performances between kesterite CZTS (this work) and other low cost catalysts reported in the literatures. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author +86-10-82322048, [email protected] +86-10-56981827, [email protected] +86-10-62528935, [email protected] Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval †

to the final version of the manuscript. These authors contributed equally to this work.

ACKNOWLEDGMENT


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This work was supported by the National Natural Science Foundation of China (Grant 51222212 and 51572016), Fundamental Research Funds for the Central Universities (2652015086). The computational resources from Tianhe2-JK support are greatly acknowledged. A.C. thanks financial support from the Spanish MINECO project BOOSTER (ENE2013-46624-C4-3-R).

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Cu2ZnSnS4 Nanocrystals as Highly Active and Stable Electrocatalysts

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