Supporting information Temperature dependence of electrocatalytic and photocatalytic oxygen evolution reaction rates using NiFe oxide Ela Nurlaela, Tatsuya Shinagawa, Muhammad Qureshi, Dattatray S. Dhawale, Kazuhiro Takanabe* [ ] These authors contributed equally to this work Division of Physical Sciences and Engineering KAUST Catalysis Center (KCC) King Abdullah University of Science and Technology (KAUST) 4700 KAUST, Thuwal, 3955-6900 (Saudi Arabia) E-mail: kazuhiro.takanabe@kaust.edu.sa S1
Characterization This section summarizes the characterization of various materials, including Scanning electron microscopy (SEM) Transmission electron microscopy (TEM) Scanning transmission electron microscopy (STEM) electron energy-loss spectroscopy (EELS) X-ray diffraction (XRD) X-ray photoelectron spectroscopy (XPS) Inductively coupled plasma (ICP) atomic emission spectroscopy (AES) Diffuse reflectance ultraviolet-visible (DR UV-Vis) spectroscopy S
Figure S1. (A) SEM image and (B) XRD patterns of bare Ni foam (NF). S3
Figure S. (A) TEM image, (B) corresponding electron diffraction pattern and (C) STEM image of NiFeOx/NF. S4
Figure S3. (A, B) TEM images, (C) electron diffraction and (D) EELS of. wt% NiFeOx/Ta3N5 before reaction. S5
Figure S4. (A) TEM image and (B) STEM image of. wt% NiFeOx/Ta3N5 after photocatalytic reaction. Figure S5. XRD patterns of bare Ta3N5 and NiFeOx/Ta3N5. S6
np Figure S6. XPS spectra of NiFeOx powder and NiFeOx/Ta3N5 before/after photocatalytic reaction: (A) Ni p, (B) Fe p, (C) O 1s (D) C 1s, (E) Ta 4f and (F) N 1s, Ta 4p3/. S7
Figure S7. DR UV-Vis spectra of bare Ta3N5 and NiFeOx/Ta3N5. S8
Electrochemistry Figure S8. Linear sweep voltammograms (LSVs) using various catalysts in 1.0 M of (A) Na and (B) K. (C) LSVs using NiOx/NF catalysts in 0.01, 0.1 and 1.0 M of Na. All measurements were carried out with bubbling O at a scan rate of 1 mv s 1 and at 98 K. S9
Figure S9. Tafel relation using NiFeOx/NF electrocatalysts in (A) Na and K at 0.1 or 1 M and in (B) 1.0 M of M (M: Li, Na, K and Cs). Figure S10. Experimentally observed Tafel slope from Figure in the main text against temperature. S10
Detailed discussion on the OER description over NiFeOx In conjunction with micro-kinetic analyses, the theoretical aspect of the observed OER dependence on the temperature described in the main text will be addressed. The elementary steps for the oxygen evolution reaction (OER) with proton coupled electron transfer (PCET) are as follows: 1, M M e, (1) M MO H O e, () MO MO e, (3) where M denotes a site on the surface. Regarding the coverage expression, θ0, θ1 and θ denote the surface coverage by empty sites, M and MO, respectively. Based on the observed Tafel slope of 40 mv dec 1 as shown in Figure 3A, Equation 3 is assumed to be the rate determining step (rds) to describe the electric currents. Under the considered condition, the forward reaction rate in Equation 1: r k a k a, (4) 1 1 0 1 and the backward reaction rate in Equation 1: r k a, (5) 1 1 1 are in equilibrium, which gives the following coverage description: 1 0, (6) 0 K1 exp f1 a where r is the reaction rate, k is the reaction rate constant, and ax represents the activity of ion x. The same argument is applied to Equation : r k a, (7) 1 r k a, (8) HO S11
a, (9) HO 1 0 a K exp f where K 0 x is the ratio of k 0 x/k 0 x and k 0 is the standard rate constant of k. Additionally, the following relationship is true among the coverage terms: i 1. (10) i0 Combining Equations 6, 9 and 10 gives the following coverage expression: 0 0 K1 K exp f 1 a exp 0 0 0 aho K1 exp f 1 a a HO K1 K f 1 a, (11) where f is F/RT and F is Faraday s constant. Equation 11 yields the following kinetic rate description: r k a, (1) 3 3 exp I nfa K K exp f k exp 1 f a a K f a a K K f a 0 0 0 3 1 1 3 3 4 0 0 0 HO 1 exp 1 HO 1 1. (13) When Equation 3 is the rds with M (θ1) being the surface predominant species, the theoretical Tafel slope is 40 mv dec 1. 1, Under such conditions, the following assumption can be made: 1 0 and 1, (14) which corresponds to and 0 HO a a K exp f, (15) K exp f a 1, (16) 0 1 1 respectively. With these inequalities, Equation 11 is simplified to: HO 0 K exp f a. (17) a S1
Thus, the overall rate description can also be rewritten as the following: k K a 0 0 3 3 exp f 1 3 3 aho r. (18) Notably, for Equation 3 to be the rds, the conditions k 0 3 << k 0 and k 0 3 << k 0 have to be met. With the following well-known equations: k 0 Ea Aexp RT, (19) K 0 G exp RT, (0) Equation 18 is also written as: G Ea F 1 a,3 3 3 r3 A exp a HO RT. (1) Equation 1 is regarded as the final form of the theoretical reaction rate description under the considered condition. The following relation is always applicable: I nfar, () from which the following electric current description can be derived: G Ea F 1 a,3 3 3 i3 nfaa exp a HO RT. (3) Equation 3 can be compared with the following simplified relation: 3 i i exp 1 f. 0 (4) Equation 3 can be further converted into the following form, which corresponds to the Tafel relation: ln G Ea F 1 a,3 3 3 i3 ln nfaa a HO RT, (5) S13
E 0 0 RT a 1 3 E3 E Ea,3 G ln nfaa RT F a HO ln i, (6) 3 3 F 3 E RT ln i 3 F. (7) This equation was applied to our measured Tafel slope (Figures 3A and S10), and the calculated electron transfer coefficient against temperature is summarized in Figure S11. The electron transfer coefficient was found to decrease with temperature according to the following relation: α = 7.1 10 3 T +.8, R² = 0.96. (8) Although there are few reliable transfer coefficient values measured for relevant oxidation reactions, the obtained temperature dependence was in agreement with the literature for other reactions, i.e., a cathodic charge transfer coefficient for the oxygen reduction reaction over Pt/Nafion =.3 10 3, 4 and an anodic charge transfer coefficient for V 4+/5+ over graphite: 3 10 3. 5 Importantly, when the temperature term is first isolated before the temperature from the experimental results, the apparent activation energy corresponds to: E, a app E a,3 G F 1 3 3. (9) The further isolation of temperature eventually provides the apparent activation energy: E E G. (30) a, app a,3 For a photocatalytic system where the overpotential cannot be explicitly identified, the apparent activation energy corresponding to the surface OER rate constant should be considered as that defined in Equation 9. Because F, α and η are positive, Ea,app η=η in Equation 9 is naturally smaller than Ea,app in Equation 30. If we simply assume Ea,app η=η S14
corresponds to the measured photocatalytic OER activation energy (16 kj mol 1 ), then η is 108 mv (1.34 V vs. RHE). We are not claiming that this is the actual surface potential. However, the potential needs to be isolated to accurately describe the activation energy in the photocatalytic system although in practice it is difficult to do so. Figure S11. Calculated charge transfer coefficients based on the observed Tafel slope in Figure S10. S15
Photocatalytic study Table S1. Quantification of Ni and Fe in the NiFeOx/Ta3N5 by ICP-AES. Targeted total Fe Ni Actual total (Fe+Ni) wt% wt% wt% (Fe+Ni) wt% 0.7 0.40 0.30 0.7 1.4 0.66 0.64 1.3.1 1.10 1.08. 4. 1.4 1.34.8 Figure S1. Photocatalytic activity time courses of O evolution for NiFeOx/Ta3N5 with various loading amount of NiFeOx (0.1 M NaSO8, 1.0 M Na, 100 ml, under visible light irradiation (40 < λ<800 nm)). S16
Figure S13. Quantum efficiency of NiFeOx/Ta3N5 (50 mg of. wt%nifeox/ta3n5, 0.1 M NaSO8, 1.0 M Na, 100 ml, 300 W Xe lamp). S17
Figure S14. Photocatalytic stability test of NiFeOx/Ta3N5 (0.1 M NaSO8, 1.0 M Na, 100 ml, under visible light irradiation (40 < λ<800 nm)). S18
Figure S15. Photocatalytic OER on as-prepared and hydrothermally treated Ta3N5 (0.1 M NaSO8, 1.0 M Na, 100 ml, under visible light irradiation (40 < λ<800 nm)). Figure S16. Mott Schottky plot of Ta3N5 in Na at (A) ph 1 and (B) ph 13.5 with Ar bubbling. Reaction rate description of the photocatalytic OER The following illustration represents a considered schematic illustration of kinetics of photoexcited carriers. 6 Figure S17. Scheme representing kinetics of photoexcited carriers in photocatalytic water splitting. S19
From Figure S17, four pathways toward recombination are observed between the electrons and holes within the bulk and at the surface. However, only one major pathway is considered for the kinetic calculations for n-type semiconductors, which consists of electrons from the bulk recombining with holes from the surface. This assumption is valid for the system proposed because Ta3N5 is an n-type semiconductor. At high light intensities, the rate of recombination follows second order kinetics. The dominant rate of recombination with an n-type semiconductor can be defined as: r k e e h, (31) r r bulk dark surf where rr is the rate of recombination for electrons and holes, kr is the rate constant for the recombination reaction, e bulk, e dark, and h surf are the concentrations of photoexcited electrons in the semiconductor bulk, the intrinsic electrons in an n-type semiconductor, and photoexcited holes at the surface of the semiconductor, respectively. The following equations represent the photoexcitation and consumption rate of electrons and holes under the steadystate conditions derived from Figure S17: d O 4 ko hcocat vreact, (3) dt d h dt cocat ' k h h surf k h h cocat k O h cocat, (33) d h dt surf ' ' k h1 h bulk k h1 h surf k h h surf k h h cocat r, (34) d h dt bulk ' I k h1 h bulk k h1 h surf, (35) d O dt k e v, (36) red. surf react d e dt surf ' k e e bulk k e e surf k red. e surf, (37) S0
d e dt bulk I k e k e r ' e bulk e surf r, (38) where [O] and [O] are the concentrations of consumed SO8 and evolved oxygen from the photocatalytic system, respectively; vreact is defined as the overall rate of photocatalytic reduction and oxidation reaction; kh1, k h1, kh, k h, ko, ke, k e, and kred. are the rate constants for the specified photoexcited carriers; α is the absorption coefficient of the semiconductor; I is the incident light intensity; e surf is the concentration of excited electrons on the surface; and h bulk and h cocat are the concentrations of holes in the bulk and cocatalyst, respectively. To solve for the overall rate description, Equation 36 was used, and the terms e surf, e bulk, and h surf were substituted with Equations 34, 35, and 31, respectively. After substituting the above reactions into Equation 38, the following equation was determined: k ' e kh k ' e k ' e kred. k ' h ko I ke vreact vreact kr vreact [e dark ] vreact 0. (39) kekred. kred. kekred. kh ko Expanding and simplifying this term results in a quadratic equation that can be solved using the quadratic formula where the a, b, and c terms shown below relative to vreact: a k ' k k ' k e H h O kr, (40) kekred. khko k ' k k ' k ' k b ke k [e ], (41) k k k k k c e red. e h O r dark e red. red. h O I. (4) Solving the quadratic term yields: v react [ ke( k ' e kred.)( khko ) ( kek ' e)( khko ) kr kekred.[e dark ]( k ' h ko )]( 1 1 b), k ( k ' k )( k ' k ) r e H h O (43) with S1
4 k k k k ' k ( k ' k )( k ' k ) b [ k ( k ' k )( k k ) ( k k ' )( k k ) k k k [e ]( k ' k )] r e red. h O e H h O e e red. h O e e h O r e red. dark h O I. (44) Equation 44 can be further simplified by first assuming b >> 1, i.e., the light intensity is very high and e dark and kr are small. Then, by assuming that kred. is smaller than k e,, and ko is smaller than k h, the following equation is derived: v react K K k k I e h O red., (45) k r where Ke and Kh are the equilibrium constants between the rates of ke/k e and kh/k h, respectively. The rate of photoexcited electrons and holes is equal to a half order of the equilibrium constant of the electrons between the surface and the bulk, the equilibrium constant of the holes between the surface and the cocatalyst, the oxygen and hydrogen evolution rates, the absorption coefficient of the material, the light intensity, and the rate of the recombination reaction. S
Figure S18. Calculated carrier mobility dependence on temperature in the experimental condition. S3
Figure S19. Calculated reversible voltage for water electrolysis as a function of temperature. S4
Figure S0. Experimentally measured relative solution resistance (closed symbols) and relative solution viscosity taken from literature (lines) with respect to temperature. The designated numbers correspond to the applied potential on the RHE scale during resistance measurement. The solution resistance was measured by impedance at 100 khz with 10 mv amplitude. S5
Figure S1. Calculated oxygen solubility compiled against temperature. The oxygen solubility is calculated with the extended Sechenov equation. S6
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