Supplementary Information. Rational Screening Low-Cost Counter Electrodes for Dye-Sensitized Solar Cells

Size: px

Start display at page:

Download "Supplementary Information. Rational Screening Low-Cost Counter Electrodes for Dye-Sensitized Solar Cells"

Clara Chapman
5 years ago
Views:

1 Supplementary Information Rational Screening Low-Cost Counter Electrodes for Dye-Sensitized Solar Cells Yu Hou, Dong Wang, Xiao Hua Yang, Wen Qi Fang, Bo Zhang, Hai Feng Wang, Guan Zhong Lu, P. Hu, Hui Jun Zhao & Hua Gui Yang Supplementary Figures Supplementary Figure S1: The electrostatic potential change along Z-axis for the transition state structure of I desorption from CH 3 CN/Pt(111). Φ is the work function of the corresponding system. According to formula, U=Φ/e - U SHE, it is clear that a certain electrode voltage corresponds to a certain work function. To get a relative precise barrier, one can adjust the work function of transition state system by tuning its net charge [40,41]. For the neutral system, Φ is calculated to be 5.08 ev, a little larger than the experimental value (5.04 ev) at the work voltage. After adding 0.1 electrons into this system, Φ is decreased to 4.94 ev. The more electrons were added, a lower work function would be. 1

2 a b Supplementary Figure S2: The desorption barrier changes as a function of electrode voltage and adsorption energy of I atom. a: Desorption barrier (E des a ) under different voltage (U). A linear relationship between the desorption barrier with the electrode voltage (U) was achieved; b: Variation of desorption barrier with chemisorption energy of I atom (E I ad ). As the binding strength of I-Pt bond increases, the corresponding desorption barrier is larger, being in agreement with the common sense. 2

3 a Energy Adsorption Desorption E a dis I 2 +2e - 2E ad I G 0 E a des 2I - 2I*+2e - Reaction Coordinate b Adsorption rate determining Desorption rate determining Real volcano curve Activity E ad,r, 1 E ad,r,max 2 E ad,r, Adsorption Energy of I atom Supplementary Figure S3: Schematic energy profile and diagram of the activity variation. a) Schematic profile of the two-step model taking into consideration the dissociative adsorption of reactants and associative desorption of products on a heterogeneous catalyst surface. b) Schematic diagram of the activity variation as the function of adsorption energy of I atom. The adsorption (red) and desorption (blue) processes are rate determining together with the real volcano curve (black). 3

4 a) TiN (100) b) CeO 2 (110) c) FeS (101) d) Fe 2 N (001) e) MoO 3 (010) f) MoN (010) g) WO 3 (001) h) TiO 2 (101) i) MS (100); M=Co, Ni j) MO 2 (110); M=Mn, Sn k) MC (010); M=Mo, W l) M 2 O 3 (012); M=Fe, Al, Cr m) Ga 2 O 3 (100) n) La 2 O 3 (001) o) Ta 2 O 5 (100) p) ZrO 2 (100) Supplementary Figure S4: Visualization of related surface structures of the considered materials. a) TiN (100): a uniformly arranged flat surface, exposed five-coordinated Ti 5c (gray balls) and N 5c (blue balls) atoms. b) CeO 2 (110): Ce 6c (white balls); O 3c (red balls). c) FeS (101): Fe 5c (purple-gray balls); S 3c (yellow balls). d) Fe 2 N (001): Fe 5c (purple-gray balls); N 4c (blue balls). e) MoO 3 (010): Mo 6c (blue-green balls); O 1c and O 3c (red balls). f) MoN (010): Mo 5c (blue-green balls); N 4c (blue balls). g) WO 3 (001): W 6c (light-blue balls); O 1c and O 2c (red balls). h) anatase-tio 2 (101): Ti 5c (grey balls); O 2c and O 3c (red balls). i) MS (100) surface with 4

M = Co, Ni. M 5c (blue balls); S 4c (yellow balls). j) Rutile-like stoichiometric MO 2 (110) surface with M = Sn, Mn. M 5c and M 6c (grey balls); O 2c and O 3c (red balls).

m) Ga 2 O 3 (100): Ga 5c (light-brown balls); O 3c (red balls). n) La 2 O 3 (001): La 5c (sky-blue balls); O 3c (red balls).

Current density (ma cm -2 ) 12 10 8 6 4 2 a b c 200 nm 200 nm 200 nm a) CeO 2 b) Ta 2 O 5 c ) TiO 2 0 0.0 0.2 0.4 0.6 0.

5 M = Co, Ni. M 5c (blue balls); S 4c (yellow balls). j) Rutile-like stoichiometric MO 2 (110) surface with M = Sn, Mn. M 5c and M 6c (grey balls); O 2c and O 3c (red balls). k) MC (010) surface with M = Mo, W. M 4c (blue-green balls); C 6c (grey balls). l) M 2 O 3 (012) surface with M = Fe, Al, Cr. M 5c (light-purple balls); O 3c (red balls). m) Ga 2 O 3 (100): Ga 5c (light-brown balls); O 3c (red balls). n) La 2 O 3 (001): La 5c (sky-blue balls); O 3c (red balls). o) Ta 2 O 5 (100): Ta 5c (blue balls); O 2c (red balls). p) ZrO 2 (100): Zr 6c (blue-green balls); O 2c (red balls). Current density (ma cm -2 ) a b c 200 nm 200 nm 200 nm a) CeO 2 b) Ta 2 O 5 c ) TiO Voltage (V) Supplementary Figure S5: Photocurrent voltage (J-V) curves of DSCs with different CEs. CeO 2 (blue line), Ta 2 O 5 (red line), and TiO 2 (green line), which were measured in the dark and under illumination of AM 1.5G full sunlight (100 mw cm -2 ). The scanning electron microscopy (SEM) images of these nanoparticles are shown in the inset. 5

0015. There is no orbital overlap between the p-orbital of ion-like I and d-orbital of surface metal atom, indicating a typical ionic bond of Pt-I.

6 Supplementary Figure S6: The charge density differences for both Pt (111) system (left) and Fe 2 O 3 (012) system (right) at transition state. The electrons accumulate at I atom resulting from the electron depletion at the surface metal atom and the surface adsorbed CH 3 CN molecules shown at the iso-value of There is no orbital overlap between the p-orbital of ion-like I and d-orbital of surface metal atom, indicating a typical ionic bond of Pt-I. Bader charge analysis showed that I* at TS is charged with 0.55 and 0.69 e at CH 3 CN/Pt(111) and Fe 2 O 3 (012) interfaces respectively, indicating that it is in forms of iodine ions when detached from the surface. 6

7 Vacuum Level CB I 3 - /I - Equilibrium Potential VB Supplementary Figure S7: The calculated projected DOS of Fe 2 O 3 (012) surface and the relative energy level comparison using hybrid HSE06 functional. The redox level of the oxidant and the energy level of vacuum level are shown in the figure as dash lines. The valence band maximum (VBM) and the conduction band minimum (CBM) of an electrode are the two most important energy levels, reflecting the redox capacity. It is clear that the CBM level of Fe 2 O 3 (012) surface is more negative than the I - 3 /I - equilibrium potential, indicating that it has the ability to reduce I - 3 into 3I - thermodynamically. 7

8 Intensity (arbitrary units) θ (degrees) Supplementary Figure S8: Typical XRD pattern of α-fe 2 O 3 nanoparticles. 8

9 6 µm 5 µm Supplementary Figure S9: SEM image of α-fe 2 O 3 electrode films on FTO substrate (cross-section). 9

10 Current density (ma cm -2 ) Fe 2 O 3 Pt Potential (V) vs. Ag/AgCl Supplementary Figure S10: Cyclic voltammogram of triiodide/iodide redox couple for α-fe 2 O 3 (blue line) and Pt (red line) electrodes. The experiment was performed under simulated 1 sun illumination at a scan rate of 20 mv s -1 in 10 mm LiI, 1 mm I 2 acetonitrile solution containing 0.1 M LiClO 4 as the supporting electrolyte. Pt electrode is used as CE and Ag/Ag + works as reference electrode. 10

11 Supplementary Tables Supplementary Table S1: Surface calculation data of the considered materials. E ad I is short for the adsorption energy of iodine atom. Materials Symmetry Group Typical Surface E ad I / ev Efficiency (Pt) Lattice Parameters of Bulk unit cell TiN FM-3M (100) (5.68) a=b=c= CoS P63/MMC (100) (6.5) a=b=3.3487,c= FeS P21/M (101) (2.19) a=5.6822,b=3.3400,c= Reported Fe 2 N PBCN (001) (6.56) a=4.3303,b=5.4557,c= NiS P63/MMC (100) (7.01) a=b=3.4411,c= MoC P-6M2 (010) (7.89) a=b=2.9156,c= MoN P63/MC (010) (6.56) a=b=5.7364,c= WC P-6M2 (010) (7.89) a=b=2.9158,c= WO 3 P63/MCM (001) (7.57) a=b=7.4085,c= TiO 2 I41/AMD (101) (gas) \ a=b=3.8013,c= CeO 2 FM-3M (110) (gas) \ a=b=c= MnO 2 P42/MNM (110) (gas) \ a=b=4.4111,c= SnO 2 P42/MNM (110) (gas) \ a=b=4.7645,c= MoO 3 PNMA (010) (gas) \ a=3.9173,b= ,c= Unreported Al 2 O 3 R-3C (012) (gas) \ a=b=4.7595,c= Ga 2 O 3 C2/M (100) (gas) \ a= ,b=3.0494,c= La 2 O 3 P-3M1 (001) (gas) \ a=b=3.9185,c= Cr 2 O 3 R-3C (012) 1.70 \ a=b=4.9466,c= Ta 2 O 5 C2/C (100) 2.06 \ a= ,b=4.8510,c= ZrO 2 P21/C (100) 0.22 \ a=5.1400,b=5.2550,c= Fe 2 O 3 R-3C (012) 0.51 \ \ (104) 0.42 \ a=b= Å, c=

12 Supplementary Table S2: Photovoltaic parameters of DSCs with different counter electrodes. CEs J sc (ma cm -2 ) V oc (mv) FF (%) Eff. (%) CeO Ta 2 O TiO Supplementary Table S3: Influence of solvent adsorption coverage on I E ad. E I ad is short for the adsorption energy of iodine atom and d I-Pt is the bond distance between I and the close bonded Pt atom. Number of adsorbed CH 3 CN nearby I atom I E ad / ev d I-Pt / Å No solvent (gas phase)

13 Supplemental Notes Supplementary Note 1: Surface models. In the theoretical screening, we considered ~20 kinds of compounds and take their stable surface as representative to estimate the activity, which are constructed based on the pre-optimized bulk (see Supplementary Table S1), emphasizing at Pt and α-fe 2 O 3. The Pt (111) surface was modeled as a p(4 4) periodic slab with 4 atomic layers and the vacuum between slabs is ~20 Å. The atoms in the bottom one layer are fixed, and all other atoms are fully relaxed. A corresponding k-points mesh was used during optimizations after a mesh pre-checking tests. Likely, a large p(2 2) α-fe 2 O 3 (012) and p(1 2) (104) were modeled with the bottom one layer fixed, corresponding a size of Å 3 and Å 3, and a k-points mesh was applied to the two cases accordingly. Expect Pt and α-fe 2 O 3, a detailed description of all the candidate materials, including the crystal structure, optimized lattice parameters and the common surface, are summarized in Supplementary Table S1, and their surface configurations are demonstrated in Supplementary Fig. S4. Supplementary Note 2: Effect of the coverage of the adsorbed solvent molecule on the material properties. Based on our calculation results, we found that solvent molecule CH 3 CN has a relatively strong adsorption energy (0.42, 0.45, 0.46 ev on Pt(111), Fe 2 O 3 (012), Fe 2 O 3 (104) respectively). When I atom was introduced on the electrode surface, there should exist a competing adsorption effect between I atom and CH 3 CN molecule. We explored various co-adsorption configurations of I and CH 3 CN on the electrode surface by adjusting the number of CH 3 CN molecule adsorbed around I*. We found that it is the most favored thermodynamically when three CH 3 CN molecules adsorb nearby I*. As a result, this configuration was selected as the model to calculate the binding strength of I atom on the electrode surface, which gives an adsorption energy 13

14 of 0.52 ev. Compared to the case of I adsorption in the absence of CH 3 CN solvent, it is clear that the solvent effect plays a negative role on I adsorption. To be systematic, we did a series of tests about the influence of solvent adsorption coverage on E I ad. From Supplementary Table S3, it is obvious that as the number of CH 3 CN molecules adsorbed around I*, the weakening on I adsorption is more evident, and the bond length ( d I-Pt ) of I-Pt increases accordingly. Considered that Fe 2 O 3 has almost the chemisorption energy of CH 3 CN, we simulated the solvent environment with the same coverage model as Pt(111) surface. Supplementary Note 3: Free energy calculation. Here we would show how to calculate the free energy change ( G0 ) of reaction (S1). I + 2e 2I 2(sol) (sol) (S1) It is hard to directly calculate the energy of the charged periodic system accurately. However, it is known that the Gibbs free energy change of the standard hydrogen electrode (SHE) reaction is zero ( G = 0), that is: + H /H 2 1 H 2(gas) H + (aq) e + ( SHE ) (S2) 2 By combining reaction (S1) and (S2), we can get reaction (S3), in which the eu term represents the electron free energy shift in the counter electrodes at the voltage U relative to the SHE. It is clear that the Gibbs free energy change G1 of reaction (S3) is equivalent to G0, i.e. G0 = G1. I + H 2I + 2H + 2eU (S3) + 2(sol) 2(gas) (sol) (aq) As shown below, we can design a thermodynamic cycle based on Hess s Law to calculate G1 indirectly: (S4) 14

15 For the above cycle, it is obvious that I + H 1 2 sol sol I2 G = G 2(Ε + Ε ) + µ. We used Gaussian 03 software to calculate G2 in reaction cycle (S4), and the salvation energies of I - in acetonitrile solvent and H + in water were taken from experimental + I H value( Ε, Ε ), giving -2.86, ev, respectively [42,43]. For the value of sol sol µ I 2, that is chemical potential difference of I 2 molecule in between gas phase and CH 3 CN solvent, we can calculate it according to the ideal solution model by considering the phase equilibrium at the gas/liquid interface during I 2 dissolvation, and we can get * p ο ο 2 µ 2 2 ( ) 2 ( ) ln I I = µ I sol µ I gas = RT (S5) ο p Where p is the saturated vapor pressure of I 2 molecule, which can be calculated * I2 according to the Antonie Equation (taken from the NIST WebBook). Supplementary Note 4: Kinetic analysis. I 2 (sol) + 2* 2I* (S6) I* + e - I - (sol) + * (S7) To explore the key catalytic parameter in affecting the whole catalytic activity, we took the two-step model reported in our recent work [24,25] as a starting point to illustrate heterogeneously catalyzed reactions, namely adsorption of reactants to form the intermediates and the desorption of the intermediate to the products. The energy profile containing the two steps is shown in Figure S3a. According to BEP relation of dissociative adsorption, one can obtain: E dis a = α E + β, α 1 <0 (S8) I 1 ad 1 With respect to the dissociation of bi-atomic molecule, α 1 is usually ~-0.9. For the desorption of surface intermediate, it makes sense that the desorption barrier would generally become higher as its binding strength with the catalyst surface increases. Particularly, we consider the desorption barrier of I* from the Pt(111) surface at different adsorption energy through simulating a various of electrode surface properties (see Supplementary Fig. S2b), which give that 15

16 E des a = α E + β, α 2 >0 (S9) I 2 ad 2 α 1, α 2, β 1 and β 2 are constants. Therefore, one can expect that the overall activity would be determined by the adsorption energy of I atom (E I ad ). Furthermore, the quantitative discussions are given based on the microkinetic analysis. Within the microkinetic framework, the reaction rate of step (S6) and (S7) can be written as: r = k c θ (1 Z ) (S10) I2 1 r = k θ (1 Z ) (S11) 2 2 I* 2 in which θ and θ Ι are the coverage of surface free site and I atom, respectively; ci 2 is the concentration of I 2 molecule in the solution; Z 1 and Z 2 are the reaction reversibility of step S6 and S7; k 1 and k 2 are the rate constant of reaction step S6 and S7, respectively, which can be determined following the transition state theory: k i = k B T h exp( S R )exp( E a RT ) (S12) where k B and h are constants, T is the reaction temperature. At steady state (TOF=2r 1 =r 2 ), by applying the condition of θ +θ Ι =1, we can solve the reaction turnover frequency TOF numerically. Analytically speaking, considering the relationship of (S8) and (S9), it is clear that TOF of the whole reaction would be mainly determined by the adsorption energy of I (E I ad ). If adsorption is the rate-determining step (Z 2 =1, Z 1 =Z 2 tot ): K TOF k c (1 Z ) 2 eq2 2 1 = 1 I 2 2 tot ( c + Keq2) I (S13) If desorption is the rate-determining step (Z 1 =1, Z 2 =Z tot ): c K TOF = k (1 Z ) 1/2 1/2 I2 eq /2 1/2 1+ ci K 2 eq1 tot (S14) K eq1 and K eq2 are the reaction equilibrium constant of step S6 and S7, determined by the adsorption energy of E ad I and G 0. 16

17 In real case, TOF would be determined following TOF=min{TOF 1, TOF 2 }, as shown in Supplementary Fig. S3b, which shows that the adsorption energy of I atom (E ad I ) is a key parameter to determine the whole catalytic activity. Supplementary Note 5: Theoretical deduction for good catalysts. As mentioned above in the text, equation µ I2 (sol) + 2µ e 2µ I* + 2µ e 2µ I - (sol) should be valid for good catalysts. The theoretical deductions are depicted in details as the following: µ + 2µ 2µ + 2µ 2µ I - 2 (sol) e I* e I (sol) c G = (µ + 2 µ ) 2µ 0 I 2 (sol) e I (sol) E T S + µ + 2E + 2µ 2E + 2µ (µ + 2 µ ) I * 2 (gas) I2 I2 sur(sol) e I e I 2 (sol) e G 0 It should be noted that, for simplicity, we neglected the minor contribution of I 2 and I - concentration in the solution (~0.02 ev, estimated from experimental condition) into the free energy, and the coverage-dependent term RT lnθ I * / θ* would lie in a small range on the best catalyst, about 0.06~0.12 ev at 298K, and is defined as ε. In other words, for good catalysts µ I* should satisfy Eq (S15). ο ο ο µ I* µ I* or I* I* I* µ ε µ µ + ε (S15) After deduction, we can get the suitable range of E ad I for good catalysts as follows: Neglecting ε term: 1 1 I T S µ Ead T S + G0 µ (S16) I2 I2 I2 I Taking ε into consideration: 1 1 I T ad 0 2 S 2 µ ε E T 2 S + 2 G 2 µ + ε (S17) I2 I2 I2 I2 17

18 Supplementary References: [40] Filhol, J. S. & Neurock, M. Elucidation of the electrochemical activation of water over Pd by first principles. Angew. Chem. Int. Ed. 45, (2006). [41] Yu, L., Pan, X., Cao, X., Hu, P. & Bao, X. Oxygen reduction reaction mechanism on nitrogen-doped graphene: a density functional theory study. J. Catal. 282, (2011). [42] Kelly, C. P., Cramer, C. J. & Truhlar, D. G. Single-Ion solvation free energies and the normal hydrogen electrode potential in methanol, acetonitrile, and dimethyl sulfoxide. J. Phys. Chem. B 111, (2006). [43] Crawford, E., McIndoe, J. S. & Tuck, D. G. The energetics of the X 2 + X X 3 equilibrium (X = Cl, Br, I) in aqueous and nonaqueous solution. Can. J. Chem. 84, (2006). 18

Molecular-Level Insight into Selective Catalytic Reduction of NO x with NH 3 to N 2

Supporting Information Molecular-Level Insight into Selective Catalytic Reduction of NO x with to N 2 over Highly Efficient Bifunctional V a Catalyst at Low Temperature Ying Xin, Hao Li, Nana Zhang, Qian