Importance of Entropic Contribution to Electrochemical Water

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Importance of Entropic Contribution to Electrochemical Water Oxidation Catalysis Kang-Gyu Lee, Mani Balamurugan, Sunghak Park, Heonjin Ha, Kyoungsuk Jin, Hongmin Seo, and Ki Tae Nam ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.9b00541 • Publication Date (Web): 08 Jul 2019 Downloaded from pubs.acs.org on July 19, 2019

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ACS Energy Letters

Importance of Entropic Contribution to Electrochemical Water Oxidation Catalysis

Kang-Gyu Lee,† Mani Balamurugan,† Sunghak Park,† Heonjin Ha,† Kyoungsuk Jin,† Hongmin Seo† and Ki Tae Nam*,†

†Department

of Materials Science and Engineering, Seoul National University, Seoul 151-744,

Korea

ACS Paragon Plus Environment

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[Abstract] Electrochemical water oxidation is considered as one of the most important reactions for a sustainable future. However, in heterogeneous water oxidation, atomistic understanding of this four-electron reaction is still elusive. In particular, the mechanism of O-O bond formation, the coupled transfer of proton and electron, and the hopping of high-valent metal-oxo intermediates are challenging issues. To date, binding energy related descriptors have been used successfully to predict the catalytic activities by matching the experimental data and quantitatively comparing the performances. In this perspective, we attempt to emphasize the significance of entropic contribution, like enthalpy, by highlighting recent literature and calculating the entropy from the available data. We envision that this perspective can suggest a new research direction towards the catalyst design to control the entropy during water oxidation and be technologically useful for characterizing the temperature effects.

[Table of Contents]


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The electrochemical oxygen evolution reaction (OER) plays a central role in the development of energy conversion and storage systems,1 and acts as an electron/proton source in hydrogen production,2 and carbon dioxide reduction3 as like natural photosynthesis, in which the protons and electrons extracted from the water are used for reducing CO2 to glucose. However, OER is often considered to be one of the most challenging reactions due to its sluggish kinetics, which involve O-H bond cleavage and O-O bond formation steps as well as multiple proton coupled electron transfer (PCET).4,5 Therefore, huge efforts have been devoted to not only increasing the efficiency of the OER catalysts, but also elucidating the reaction mechanisms. Several good reviews describing the achievements in this field are available in previous literature.6,7 For the heterogeneously catalyzed OER, new active catalysts have been discovered via the construction of theoretical models and the identification of the relationship between the electronic and atomic structure of the active sites with their catalytic activity. This universal approach follows Sabatier principle,8 where catalysts with optimal (neither too strong nor too weak) interaction with adsorbate show the best performance, and introduces a single descriptor to quantify the interaction strength.9 Based on this approach, the volcano-shaped relations of the catalytic activities was determined as a function of single descriptor, so-called volcano plot. Because all the reaction intermediates (*OH, *OOH, and *O) can bind with the catalyst surface through the O atom, the binding strength of the substrate with oxygen on the surface is considered to be prime descriptor for the catalytic OER activity.10–12 Practically, an outstanding agreement has been reported in the experimental activity trends using descriptors, such as the reaction enthalpy of oxide transition (MOx+ 1/2O2 MOx+1),13,14 or the surface oxygen binding energy,11 which should be related to the enthalpic contribution of oxygen adsorption energy on the catalyst surface.



The advancement of descriptor-based approach can enhance the ability to predict the catalytic activity and reaction mechanism of various materials. For a wide range of oxide materials, Man et al. observed the universal scaling relationship between the binding energy of *OH and *OOH (ΔEOOH = ΔEOH + 3.2 eV), and proposed that the material with *O binding energy in between the binding energy of *OH and *OOH shows the best performance.15 CalleVallejo et al. reported that the surface adsorption energy can be described by the formation energy of the bulk oxide on transition metal perovskites and monoxides.16 In calculation results of both researches, SrCoO3, and LaNiO3 with the optimum oxygen binding energy/formation energy were predicted to have the lowest overpotential in perovskites materials, which is in agreement with the experimental overpotential. Recently, Seitz and coworkers identified the IrO3 or the anatase IrO2 layer as the active site on a strontium iridium oxide by calculating the overpotential of the possible active surface structures.17 Furthermore, the reaction mechanism for the electrochemical OER was predicted at the atomic scale for various metal oxides, such as Co oxide clusters (Plaisance et al.18 and Li et al.19), RuO2 (Fang et al.20), and IrO2 (Ping et al.21), by calculating the reaction energy barrier. Although significant advances have been made in the catalytic performance and theoretical understanding of heterogeneous catalysts for OER, comparing the synthetic reaction systems with biological photosystem, there are many fundamental questions that are yet to be answered to alter the strategy of catalyst design. For examples, 1) How does the water/oxygen transferred to and from the active site? 2) How does the conformation of the active site change during the reaction and influence the reaction kinetics? 3) What is the detailed mechanism of the O-O bond formation? 4) Does the local hydrophobic/hydrophilic environment play a significant role in proton transfer? 5) How does the incorporation of a redox active/inactive heteroatom affect the overall electronic properties of the active sites? In an effort to analyze these correlated parameters, we envision that the thermodynamic parameter “entropy” can


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ACS Energy Letters

provide a guideline for understanding the mechanism and reaction kinetics. Furthermore, entropy-based analysis may represent a quantitative understanding of the microscopic configurations of electrochemical OER. Additionally, inspired by the recent experimental observations (discussed in next section) on the effect of entropy in biological systems, we believe that this perspective will summarize the entropic contribution to the reaction kinetics of enzymes, including photosystem II, and point out the significant entropic contribution in heterogeneous OER catalysis by analyzing the reported data of well-known cobalt oxide-based catalyst. We expect that this perspective will provide a deeper insight into understanding the reaction kinetics and mechanism based on entropic changes. In addition, the perspective will provide future guidelines for the design of heterogeneous OER catalysts with high performance beyond the single-descriptor based volcano plot. In the first section, starting the discussion with the case of photosystem II, we summarize recent reports on the entropic contribution in the enzyme reactions, followed by the theoretical basis for identifying the entropic contribution decoupled with enthalpy. In third section, the entropic effect on heterogeneously catalyzed OER is discussed based on our simple analysis of the well-known Co-Pi catalyst. Finally, based on previously reported mechanistic investigations, we show how the extracted entropy parameter can be linked with the catalytic mechanism of heterogeneous catalysts.



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Figure 1. Entropic contribution to water oxidation catalysis of photosystem II (PS II). (a) The structure of PS II and detailed atomic structure of active site (Mn4CaO5 cluster). Reproduced with permission from ref 23. Copyright 2011 Nature Publishing Group. (b) Catalytic Kok cycle of PS II. The cycle consists of five redox states (S0-S4) depending on the oxidation state of Mn. Reproduced with permission from ref 24. Copyright 2018 American Chemical Society. (c) Three water channels and involved amino-acid residues in oxygen 2+

evolving complex (OEC). Ca , Asp61, and Val185 are marked in red circles, which accelerates reaction rate through favorable entropic contribution. Reproduced with permission from ref 41. ‡

‡

Copyright 2016 Elsevier BV. (d) Comparison of activation energy barrier (Δ H and -TΔ S) in •

S3YZ – S0YZ transition for wild type and mutant PS II. The blue and red arrows show the ‡

‡

difference of Δ H and -TΔ S with the value of wild type PS II, respectively (T = 25°C). Kinetic data for PS II(wild type), PS II(ValAsn), and PS II(AspAla) were derived from Ref 39. And data for PS II(Cs2+Sr2+) were derived from Ref 40.

Significance of the Entropic Contribution to OER Kinetics in Biological System. Photosystem II (PS II) utilizes water as an electron/proton source for the photosynthesis containing an oxygen evolving center (OEC), which efficiently catalyzes the OER under neutral condition with markedly higher turnover frequency.22 The OEC consists of an inorganic Mn4CaO5 cluster and contains a unique ligand environment to stabilize the cluster (Figure 1a).


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From the recent X-ray crystallographic analysis, detailed structural information of Mn4CaO5 cluster was obtained.23 The cluster has a low-symmetric structure containing a distorted cubane core of Mn3CaO4 with a fourth Mn bridged through one oxygen of the cubane core. This Mn4CaO5 cluster undergoes successive changes in its oxidation state and geometry, taking part in a so-called Kok cycle (Figure 1b).24 The distorted structure of the cluster has been believed to play an important role in the stabilization of high valent Mn oxidation states and catalytic activity. The structural insights of the Mn4CaO5 cluster continue to inspire the design of highly active Mn-based catalysts in both molecular and heterogeneous catalysis and many people have tried to incorporate the lessons learned from the natural system to synthetic catalysts. For example, the Driess group developed amorphous MnOX nanoparticles with high catalytic activity, in which the local structure and the oxidation state of Mn are similar to the Mn4CaO5 cluster.25 Later, the Dismukes group made λ–MnO2 by removing Li ions from LiMn2O4, in which the structure is similar to the Mn3CaO4 core of the cluster. Interestingly, the λ–MnO2 showed catalytic activity for OER, whereas the parent LiMn2O4 was inactive.26 The Dau group utilized an electrodeposition method by switching the voltage and achieved improved OER activity and from X-ray adsorption spectroscopy (XAS) analysis, they found that the active catalyst has a disordered Mn geometry with mixed oxidation states of Mn(III) and (IV).27 Inspired by the unique properties of OEC, many other manganese-based heterogeneous OER catalysts have been reported.28–38 In an effort to transform the principles of the biological OEC to the synthetic heterogeneous manganese oxide-based catalysts, our group synthesized several manganese-based heterogeneous OER catalysts such as hydrated manganese(II) phosphate,35 lithium manganese pyrophosphate,36 and Mn oxide nanoparticles.37,38 Through a series of experiments, we suggested that the stabilization of Mn(III) by the distorted geometric structure plays an important role in the OER activity. In addition, we found that Mn(III) species can be



stabilized on the surface of the manganese oxide nanoparticles (MnO37 and Mn5O838), which exhibit superior catalytic performance compared to bulk counterparts. Furthermore, through electrochemical and in situ spectroscopic analysis, we demonstrated the OER mechanism of the nanoparticles. In our study, the involvement of high-valent Mn(IV)=O species formed by a PCET followed by a chemical O-O bond formation was suggested,34 which is very different from the conventional Mn catalysts. Collectively, these reports show that the lessons from biological Mn4CaO5 cluster can be successfully applied to synthetic heterogeneous OER electrocatalysts. While the effect of structural factors on the OER catalyzed by the OEC is well documented via X-ray crystallographic and spectroscopic analysis, thermodynamic and kinetic parameters of the reactions are also calculated or measured to show how the natural system is choosing a low energy pathway to carry out this kinetically sluggish reaction. The thermal activation parameters, such as activation enthalpy (Δ‡H) and entropy (Δ‡S) provide effective information for discovering the functional role of co-factors in OEC, such as surrounding amino-acid residues, and redox-inactive ions (Ca2+ and Cl-). Recently, new evidence was provided for the contribution of entropy to the rapid oxidation of the Kok cycle,39,40 which is another lesson we can learn from the natural system. In previous reports, the decrease in reaction rate was observed upon structural modification of PS II by replacement of specific amino acids or the exchange of redox-inactive ions, resulting in a higher entropic energy barrier compared to the wild-type PS II. Furthermore, the entropy effect was significant when the local environment of the water channel changed. Specifically, Val185, Asp6139 and the Ca2+ ion40 involved in water channel41 (Figure 1c) seemed to have a central role in facilitating the Kok cycle via favorable entropic contributions, notably in the S3Yz* to S0YZ transition. In the following paragraphs, we summarize the detailed results and discuss the proposed hypothesis


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for the entropy effect in S3Yz* to S0YZ transition involving O-O bond formation as well as deprotonation step and water/oxygen transfer. In Figure 1d, we reorganize the reported kinetic energy barrier (Δ‡G, Δ‡H, and TΔ‡S) by comparing the structurally perturbed PS II with the wild type. Here, the kinetic parameters were measured by analyzing the temperature dependence of O2 evolving kinetics under the third flashing light. Controlling the reaction step by flashing light, the kinetic energy barrier of S3Yz* to S0YZ transition step can be measured separately. According to Bao et al. (Ref 39), the Gibbs energy of activation (Δ‡G) for the wild-type PS II was measured to 0.6 eV (Δ‡H = 0.45 eV and -TΔ‡S = 0.15 eV). For the mutant PS II, in which the Val185 was exchanged to Asn, the overall kinetic barrier is increased to 0.67 eV, where the entropic energy barrier (-TΔ‡S) is increased to 0.47 eV, which is 0.32 eV larger than the value of wild-type PS II. In this case, the activation enthalpy decreased to 0.20 eV (Δ(Δ‡H) = -0.25 eV). Additionally, in the mutant PS II, replacement of Asp61 by Ala caused the -TΔ‡S increase to 0.34 eV (Δ(-TΔ‡S) = 0.19 eV), which resulted in an increase of the entire kinetic barrier to 0.67 eV. This result clearly indicates that the Val185 and Asp61 plays a vital role in constructing the favorable entropic environment for the OER. In addition, the slowdown of the S3Yz* to S0YZ transition was also observed upon replacing Ca2+ by Sr2+ in the mutant PS II. Rappaport et al. reported that Sr-PS II showed a high entropic energy barrier (Δ(-TΔ‡S) = 0.08 eV) compared with Ca-PS II, resulting in the decrease of the oxygen evolving rate.40 The origin of the observed entropic effect was investigated by the additional experiments involving H/D isotope and pH dependent analysis. The observed KIE and pH dependence for S3Yz* to S0YZ transition indicated the mutation induces the change of rate determining step, in which the reaction kinetics is limited by disturbed rearrangement of water molecules.

39

These results were consistent with the analysis of activation parameters,



indicating that the structural rearrangement of the amino-acid (ValAsn, or AspAla) induces the unfavorable arrangement of water molecules related to O-O bond formation, and therefore, the additional entropic energy is required to rearrange the most favorable transition state configuration. For the reason why Δ‡H is slight decreased, it was hypothesized that the change of the bonding interactions would be caused by the replacement of amino acid.39 Additionally, in the case of Ca2+ replacement by Sr2+, the Boussac group proposed that the increased entropic energy barrier is a result of the reorientation of water molecules bound to Ca2+.40 Furthermore, it was revealed that the Sr-PS II showed a faster water exchange rate than Ca-PS II in the last transient state before the O-O bond formation,42 indicating that the Ca2+ ion is important in setting the correct configuration of the H-bond network required for an efficient proton transfer as well as contributing to the delivery of at least one water molecule to its catalytic site. These reports suggest that the favorable entropic contribution of PS II play an important role in the overall reaction kinetics of OER, and is possibly associated in the delivery and rearrangement of water molecules.


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Figure 2. Favorable entropic contribution in enzyme reaction. (a) Free energy diagram of chemical reaction with (lower) and without (upper) the enzyme. (b) Comparison of activation ‡

‡

‡

parameters (Δ G, Δ H, and -TΔ S) with (right) or without (left) the involvement of the enzyme (T = 25°C) for peptide bond formation (Ref 45), protein modification (Ref 46), cytidine deamination (Ref 44), and GTP hydrolysis (Ref 47 and 50).

In addition to PS II, the role of the entropic effect has been considered as a crucial factor in other enzymatic reaction. To date, there have been many observations to show that the free energy barrier (Δ‡G) of an uncatalyzed chemical reaction is always larger than the value of an enzyme-catalyzed chemical reaction due to the considerable negative activation



entropy.43–47 As presented in Figure 2a, the enzyme can reduce the entropic energy barrier (TΔ‡S) to form the transition state compared with the uncatalyzed chemical reaction that occurred in the aqueous solution. As presented in Figure 2a, the catalytic rate constant is characterized by a reduced entropic energy barrier (-TΔ‡S) to form the transition state compared with the corresponding uncatalyzed rate. According to the Page and Jencks, it was postulated that the key catalytic effect of enzyme is associated with the enzyme-substrate binding. When enzyme binds to the substrate (described as [E-S] in Figure 2a), the favorable binding free energy is spent on restricting the substrate motions and correctly aligning the substrate for reaction, which can induce the loss of translational and rotational entropy of the substrates. Therefore, in this condition where the entropic penalty for the reaction has already been paid upon binding with enzyme, substrate would climb the activation barrier of the activated complex ([E-R]*) with a smaller entropic energy barrier compared with the corresponding uncatalyzed reaction ([R]*).43 Although this suggestion is focused on the entropic contribution of the substrate only, many people have accepted this hypothesis. Recently, the computational simulation approach made it possible to calculate not only the activation entropy of substrates, but also the entropic contribution of the enzyme and the surrounding solvents. According to the calculation results, it was revealed that the surrounding proteins cofactors or their hydrogen bond networks highly contribute to the entropically favorable reaction kinetics of enzymecatalyzed reaction, which is not observed in homogeneous or heterogeneous solution reaction.51 Especially, the elimination of entropic energy barrier for solvent reorganization can be achieved by the hydrogen bond networks of enzyme active sites, which favors the reaction kinetics.46,48 In addition, Åqvist et al. confirmed that reaction can be proceeded through the entropically favored mechanism by stabilizing the reaction intermediate species in preorganized enzyme environment, which is not allowed in the uncatalyzed solution reaction.49 For the uncatalyzed (left) and enzymatic (right) chemical reactions, the experimentally


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measured activation energy parameters, including Δ‡G, Δ‡H, and -TΔ‡S, are shown in Figure 2b.44–47,50 Different from photon-controlled example of PS II, the measured kinetic parameters represent overall reaction, not the particular single step. In all the cases, the entropy of enzymatic reactions are extraordinarily favorable compared with uncatalyzed reaction in aqueous solutions. Wolfenden and coworkers observed a decrease of the entropic energy barrier of the amide bond formation with the puromycin substrate catalyzed by ribosome (8 kcal/mol) compared with the uncatalyzed reaction in water (15 kcal/mol).45 Based on a simulation study, Trobro et al. revealed the origin of the decreased entropy energy barrier in this reaction and explained how the solvent reorganization by hydrogen bond network of ribosome is involved in the mechanism of the reaction.48 In the case of the ubiquitin-like protein modification, Chen and coworkers showed that the entropy energy barrier of protein modification decreased approximately 4 kcal/mol from 17 kcal/mol when E3 ligase participated in the reaction. Computational study suggested that a number of bound water molecules near the active site can be stabilized by E3 ligase, and contribute to the low entropy energy barrier at the transition state.46 Similarly, Snider et al. observed the decrease in both the enthalpy (7 kcal/mol) and entropy (9 kcal/mol) energy barrier in the cytidine deaminase catalyzed deamination reaction compared to the uncatalyzed reaction.44 Furthermore, in the case of the hydrolysis of guanosine triphosphate (GTP) catalyzed by EF-Tu, the decrease of both the entropic (~ 10 kcal/mol) and enthalpic (~ 4 kcal/mol) energy barrier was observed.47,50 Åqvist and coworkers suggested that, for both cytidine deamination and GTP hydrolysis, the reaction kinetics can be highly accelerated with positive entropic contribution through the stabilization of the charged intermediate states, such as OH-, by the preorganized enzyme.51 Indeed, the characteristics of photosystem II and other enzymatic reactions demonstrate that the entropic contribution has a crucial role towards the enhancement of the reaction kinetics. In this aspect, we will discuss how the entropy affects the reaction kinetics



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of synthetic water oxidation catalysis, especially in heterogeneous systems. For this purpose, in the next section, we are going to introduce the theoretical basis for the investigation of the entropic contribution of electrochemical OER catalyzed by heterogeneous catalysts.

Figure 3. Concept of temperature dependent kinetic analysis to measure activation ‡

‡

enthalpy (Δ H) and entropy (Δ S). (a) Free energy diagram of a typical electrochemical oxygen evolution reaction under neutral condition. The initial state is presented to M-OH according to the previous mechanistic studies conducted under neutral condition.34,69,72 It is assumed that reaction proceeded by an acid-base mechanism, and O-O bond formation step is the rate determining step (RDS). The black bars present the free energy profile at equilibrium potential (Eeq). The blue bars present the free energy profile when an overpotential (η) is ‡

applied. (b) The Butler-Volmer equation and Eyring plot for electrochemical reaction. (Δ G: ‡

‡

Gibbs free energy of activation, Δ H: enthalpy of activation, Δ S: entropy of activation, α: transfer coefficient and Γact: the number of active site).

Theoretical basis for Identifying the Kinetic Barrier of Heterogeneous OER. The free energy diagram of electrochemical OER is shown in Figure 3a, in which the reaction mechanism is composed off our sequential electron and proton transfer steps. The energy states of each intermediate is depicted considering *OH, *O, and *OOH as the reaction intermediates. The rate determined step (RDS) is assumed to be O-O bond formation and the additional


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transition state (TS) of M-OOH formation is added at highest free energy. In this diagram, the energy barrier of O-O formation step (RDS) was set to be the largest compared to those of the other steps. Note that the energy barrier of pre-equilibrium step, such as deprotonation of hydroxide, can be larger than that of RDS in several cases when the intermediate (M=O) is unstable. The position of the TS is determined by the transfer coefficient (α) defined as the fraction of the electrochemical potential energy affecting the electrochemical reaction rate.52 In the free energy diagram, the energy states were depicted in both cases of the equilibrium potential and under the applied potential. The deviation of the electrode potential (E) from the equilibrium potential (Eeq) defines the overpotential (η = E − Eeq) At the equilibrium potential (black bars), the kinetic energy barrier is determined to be as high as the standard Gibbs energy of activation (Δ‡G°), which defines the exchange current density (j0) in proportion to exp(-Δ‡G°/RT). As shown in Figure 3a, Δ‡G° is defined as the difference of energy state between the initial state and the transition state of the RDS at equilibrium potential. Then, on the applied overpotential (η), the free energy of each step decreases depending on the number of transferred electrons. The energy state of each step decreases by nFη, where n is the number of electrons transferred from the initial state. The kinetic energy barrier is also reduced from Δ‡G° in proportion to αFη (Δ‡G°- αFη). In multistep electron transfer reactions, the transfer coefficient of the oxidation reaction is defined as below (1), where np is the number of electrons transferred prior to the RDS, nq is the number of electrons transferred in the RDS, and β is the symmetric factor for RDS.53,86 The electrochemical reaction rate is expressed by the Butler-Volmer equation (2), where Γact is the surface concentration of active sites as follows.53 𝛼 = 𝑛𝑝 + 𝑛𝑞𝛽 (1)



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𝑘𝐵𝑇 ― 𝛥 ‡ 𝐺𝑜 𝛼𝐹𝜂 𝑗 = 𝑛𝐹 𝛤𝑎𝑐𝑡𝑒𝑥𝑝 𝑒𝑥𝑝 ℎ 𝑅𝑇 𝑅𝑇

(

𝑘𝐵𝑇

= 𝑛𝐹

ℎ

𝛤𝑎𝑐𝑡𝑒𝑥𝑝

(

) ( )

― 𝛥 ‡ 𝐻𝑜

𝛥 ‡ 𝑆𝑜

𝛼𝐹𝜂

𝑅𝑇

𝑅

𝑅𝑇

)𝑒𝑥𝑝 ( )𝑒𝑥𝑝 ( )

(2)

Several kinetic parameters, including activation enthalpy (Δ‡H), activation entropy (Δ‡S) and the number of active sites (Γact), can be decoupled based on the temperature dependent kinetics of the Butler-Volmer equation as shown in equation (3). The temperature dependency of the reaction kinetics is shown in Figure 3b, the so-called Eyring plot, where the linear slope and y-intercept have kinetic meaning related to Δ‡H, Δ‡S, and Γact.54 With the assumption that temperature dependence of Δ‡H, Δ‡S is negligible in experimental condition, the potential dependent activation enthalpy (Δ‡H) could be easily measured, where the slope gradually decreases with increasing overpotential (η13 s-1), where the labeled oxygen (36O2 and 34O2) was detected after the first laser pulse (300 ms) in mass spectrometry. On the other hand, the 840 cm-1 band, without isotopic shift, was proposed to be a Co(IV)=O, and a slow decay time constant of 1 s-1 indicated that the cycle proceeded through irrelevant slower kinetics. Recently, the Nocera group reported that the O-O bond can be formed by a radicalcoupling mechanisms on the di-cobalt edge sites (Figure 5a).72 To probe the O-O bond formation mechanism, they synthesized the 18O-labeled molecular di-cobalt complexes, and analyzed the product oxygen molecules in a phosphate buffer solution (H216O) using differential electrochemical mass spectrometry (DEMS) and observed 32O2, 36O2 and 34O2 as products. Based on the results, they suggested that the O-O bond was formed by direct intramolecular coupling between two oxygen atoms at adjacent cobalt edge sites.

Table 2. Calculated Entropy Values, TS (eV), of H2O, H2 and Intermediate Species for Various Oxide Catalysts. (T = 25°C).

There have been previous literatures to estimate the free energy profiles of OER using theoretical models in the discussion of O-O bonding mechanism. To figure out the entropic



energy barrier during the water oxidation, we assessed the DFT studies, in which the OER mechanism were investigated based on the calculated free energy diagram for heterogeneous catalysts surface.15,78-83 In Table 2, we summarize the reported entropy values of water and hydrogen molecules, and reaction intermediate species. In all the previous studies, the entropy of gas-phase H2O at 0.035 bar was used, because at this state of H2O is in equilibrium with liquid H2O at 300 K.15,78-83 In case of reaction intermediates (*OOH, *OH, *O, *OO), the vibrational entropy was only considered,15,78-83 resulting in the similar entropy calculation results for various materials. Based on the calculation results, the entropy (TS) diagram is presented for different the O-O bond formation mechanisms on Co3O4 surface (Figure 5b).80 At first step, the entropy (S) increases during deprotonation step in which one proton and electron are released to form Co=O from Co-OH. Afterward, a huge difference in entropy value is observed in the O-O bond formation step depending on the involvement of water molecule in the mechanism. In the AB mechanism, the entropy highly decreases during the O-O bond formation between the water molecule in the bulk solution and the Co=O on the electrode surface through a nucleophilic attack of the water molecule. Considering the calculated entropy value of activated species (Co-OOH2) at transition state,80 the overall entropic energy barrier (-TΔ‡S) for the AB mechanism was calculated to be 0.48 eV (T = 25°C). On the other hand, in the RC mechanism, it was reported that the difference of vibrational entropy between the CoOO-Co and Co=O is negligible.20 Accordingly, reaction proceed with a positive entropic contribution (-TΔ‡S = -0.19 eV) in case of the RC mechanism (Figure 5b). As a result, the theoretical calculations predicts that there is distinct difference of entropic contribution between two kinds of O-O bond formation mechanisms (AB and RC). The meaningful information can be provided for understanding the detailed OER mechanisms by combining the theoretical prediction with the experimentally determined entropic contribution (TΔ‡S = -0.4 eV). However, solely based on this measured value of


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activation entropy, the additional investigation is required to understand an accurate atomicscale description of catalytic mechanism of Co-Pi catalysts. Especially, the initial state in the computational hydrogen electrode method applying a continuum model of the electrolyte presents several challenges for evaluating the atomic level description of interface between solid electrode and liquid electrolyte with the contribution from the solvent, ion, and adsorbates under the effects of electrostatic potential. Recently, Chen et al. verified that proton in the electrical double layer (outer Helmholtz plane) possesses fractional charge density, due to the overlap of electron density between the electrode and the solvent.85 Furthermore, entropy or energy state of H2O substrates crucially influence the energetics of OER, especially in AB mechanism. Therefore, the detail description of atomic or electronic structure of H2O in the electrical double layer is necessary for the accuracy of reaction energetics, rather than approximated to that of gas-phase H2O in bulk liquid phase.15,78-83 Moreover, whereas the vibrational entropy of intermediate species is mainly considered in most cases, other factors can determine the entropy change of reaction. For example, the entropy of spin-crossing for Co is not negligible,84 where spin-crossing is necessary in O-O bond formation for cobalt oxide complexes for both AB and RC mechanism.19 Besides, the multiple metal-oxo hopping on the surface of the catalysts, effect of solvation by water molecules, entropic contribution of electrolyte, movement of protons can also be possibilities.

Outlook. In this Perspective, we focus on the entropic contribution for the water oxidation particularly in context of difference between biological system and artificial heterogeneous system. We have provided the recent overview regarding the origins of entropically favorable kinetics in PS II, and the experimental and theoretical discussion about entropic contribution in heterogeneous OER. Such entropy-based approaches can provide new insight to develop fundamental understanding with regard to several challenges in OER,



especially how the water substrates transferred to active site and the detailed mechanism of OO bond formation. The effect of entropic contribution to electrochemical OER discussed above suggests that unfavorable activation entropy (Δ‡S) would be a key factor in the activity discrepancy between artificial heterogeneous catalysts and biological PS II. In this regard, investigating the contribution of entropic energy barrier of the catalytic OER is unavoidable to further develop the highly efficient catalysts close to PS II, as well as to optimize the oxygen binding energy (EO) of the material. We proposed a methodology for measuring the activation enthalpy (Δ‡H) and entropy (Δ‡S) based on temperature dependent analysis. Additionally, it is also emphasized to identify the kinetic parameters such as rate constants (k) and the number of active sites (Γact) to measure activation entropy in heterogeneous system. Providing the representative results of cobalt oxide catalyst, we experimentally confirmed the high negative entropic contribution in heterogeneous water oxidation and discussed the entropy effect on OO bond formation by both the AB and RC mechanisms based on various theoretical studies. We anticipate that the unfavorable entropic effect in heterogeneous system can be improved by a hydrogen-bond network stabilizing the active site, as shown in the enzyme catalysis, and by controlling the transportation of water molecules to the electrode surface in an ordered way.


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[Author Information] Corresponding Author *E-mail: [email protected] ORCID Kang-Gyu Lee: 0000-0002-0208-3714 Ki Tae Nam: 0000-0001-6353-8877

Notes The authors declare no competing financial interest.

Biographies Kang-Gyu Lee received his B.S. in Chemical and Biological Engineering from Korea University, and received M.S. in Materials Science and Engineering from Seoul National University under supervision of Prof. Ki Tae Nam. He currently joined the Institute of NanoBio Technology at Ewha Womans University as a researcher. His research focuses on materials for electrocatalysis.

Mani Balamurugan received his M.S. and Ph.D. degree from Bharathidasan University, India, and worked as a research associate at Central University of Tamil Nadu, India (2014). He is currently working as a postdoctoral fellow in Department of Materials Science and Engineering, Seoul National University. His research focuses on electrocatalysts for CO2 and nitrate reduction.

Sunghak Park is a Ph.D. candidate in the department of materials science and engineering at Seoul National University. He obtained B.S. in Materials Science and Engineering from Seoul National University. His research mainly focuses on materials for photocatalysts and electrocatalysts.

Kyoungsuk Jin obtained his B.S. and Ph.D. in Materials Science and Engineering at Seoul National University under supervision of Prof. Ki Tae Nam, and worked as a postdoctoral fellow in Nam’s group. He is currently working as a postdoctoral fellow in Massachusetts Institute of Technology. His research focuses on electrochemical water oxidation catalysis.

Heonjin Ha is a Ph.D. candidate in Materials Science and Engineering under supervision of Prof. Ki Tae Nam at Seoul National University. He obtained B.S. in Materials Science and



Engineering from Pohang University of Science and Technology (POSTECH) in 2016. His research focuses on electrocatalysts for water oxidation.

Hongmin Seo is Ph.D. candidate in Material Science and Engineering under supervision of Prof. Ki Tae Nam at Seoul National University. He obtained B.S. in same department in 2015. His research focuses on water oxidizing electrocatalysis.

Ki Tae Nam is an associate professor at Seoul National University (SNU) since 2010. He received his B.S. and M.S. at SNU and Ph.D. from Massachusetts Institute of Technology in 2007. His research group is investigating the interface between peptide and inorganic materials. Recently, his group developed photosystem inspired Mn nanocatalysts and peptide derived chiral materials (http://www.nkitae.org/).

[Acknowledgements] This research was supported by Creative Materials Discovery Program through the National Research Foundation of Korea (NRF) funded by Ministry of Science and ICT (NRF2017M3D1A1039377), the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (NRF-2017R1A2B3012003), and the KIST Institutional Program (2V06170).

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Parsons, R. General Equations for the Kinetics of Electrode Processes. Trans. Faraday Soc. 1951, 47, 1332–1344.


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Quotation “Not only surface oxygen binding energy, but ‘entropy’ can also provide a guideline to understand the mechanism and reaction kinetics, and entropy-based analysis may represent quantitative understanding of microscopic configurations of electrochemical OER”

“Indeed, the characteristics of photosystem II and other enzymatic reactions demonstrate that the entropic contribution has a crucial role towards the enhancement of the reaction kinetics”

“Information from surface kinetics of heterogeneous catalysis, such as active site concentration or rate constant of catalytic active species, provide the opportunity to identify the entropic contribution of heterogeneous catalysis through temperature dependent kinetic analysis.”

“The heterogeneous OER on Co-Pi showed a highly unfavorable entropic contribution to the reaction kinetics, where the entropic energy barrier took a huge part of the overall free energy barrier compared with the activation enthalpy.”

“Deep understanding of OER mechanisms can be achieved by identifying the origin of entropic contribution, where the unfavorable activation entropy (Δ‡S) would be a key factor of activity discrepancy between artificial heterogeneous catalysts and biological PS II”



Figure 1. Entropic contribution to water oxidation catalysis of photosystem II (PS II). (a) The structure of PS II and detailed atomic structure of active site (Mn4CaO5 cluster). Reproduced with permission from ref 23. Copyright 2011 Nature Publishing Group. (b) Catalytic Kok cycle of PS II. The cycle consists of five redox states (S0-S4) depending on the oxidation state of Mn. Reproduced with permission from ref 24. Copyright 2018 American Chemical Society. (c) Three water channels and involved amino-acid residues in oxygen evolving complex (OEC). Ca2+, Asp61, and Val185 are marked in red circles, which accelerates reaction rate through favorable entropic contribution. Reproduced with permission from ref 41. Copyright 2016 Elsevier BV. (d) Comparison of activation energy barrier (Δ‡H and -TΔ‡S) in S3YZ• – S0YZ

transition for wild type and mutant PS II. The blue and red arrows show the difference of Δ‡H and -TΔ‡S with the value of wild type PS II, respectively (T = 25°C). Kinetic data for PS II(wild type), PS II(Val→Asn), and PS II(Asp→Ala) were derived from Ref 39. And data for PS II(Cs2+→Sr2+) were derived from Ref 40.


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Figure 2. Favorable entropic contribution in enzyme reaction. (a) Free energy diagram of chemical reaction with (lower) and without (upper) the enzyme. (b) Comparison of activation parameters (Δ‡G, Δ‡H, and -TΔ‡S) with (right) or without (left) the involvement of the enzyme (T = 25°C) for peptide bond formation (Ref 45), protein modification (Ref 46), cytidine deamination (Ref 44), and GTP hydrolysis (Ref 47 and 50).



Figure 3. Concept of temperature dependent kinetic analysis to measure activation enthalpy (Δ‡H) and entropy (Δ‡S). (a) Free energy diagram of a typical electrochemical oxygen evolution reaction under neutral condition. The initial state is presented to M-OH according to the previous mechanistic studies conducted under neutral condition.34,69,72 It is assumed that reaction proceeded by an acid-base mechanism, and O-O bond formation step is the rate determining step (RDS). The black bars present the free energy profile at equilibrium potential (Eeq). The blue bars present the free energy profile when an overpotential (η) is applied. (b) The Butler-Volmer equation and Eyring plot for electrochemical reaction. (Δ‡G: Gibbs free energy of activation, Δ‡H: enthalpy of activation, Δ‡S: entropy of activation, α: transfer coefficient and Γact: the number of active site).


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ACS Energy Letters

Table 1. Measured activation parameters (Δ‡H and Δ‡S) through temperature dependent kinetic analysis in heterogeneous and homogeneous catalyst systems (*T = 25°C).



Figure 4. Temperature dependent kinetic analysis of OER on Co-Pi. (a) Polarization curves in 0.5 M phosphate buffer solution (pH 7) at 20 mV/s scan rate and equilibrium potential (Eeq) of OER (inset). (b) Tafel plot from 20°C to 70°C. (c) Eyring plot with various applied overpotential from 0.41 V to 0.5 V. (d) Determined enthalpy of activation (Δ‡H) from the linear slope of Eyring plot. Transfer coefficient (α) was calculated from the linear relation of Δ‡Ho-αFη=Δ‡H. (e) Determined y-intercepts by extrapolation of the Eyring plot. (f) Calculated TΔ‡S from the equation (5) at T = 25°C. The green line presents the linear relation between log(Γact) and TΔ‡S at measured y-intercept (0.18). The TΔ‡S (red) is calculated based on the reported Γact (blue) of Co-Pi. We present the relation of equation (5) under arbitrary y-intercepts (±10, ±5, ±2) to evaluate the validity of our calculation.


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Figure 5. Comparison of entropy change in O-O bond formation mechanism for cobalt based catalyst. (a) Proposed mechanisms of O-O bond formation during OER on cobalt based catalysts. Two types of acid-base mechanisms were reported by the Frei group (mechanistic scheme was described based on Ref 77). The one is the slow cycle by a single Co site and the other is a fast cycle by the dual Co site. The radical-coupling mechanism was reported by the Nocera group (mechanistic scheme was described based on Ref 72). (b) The calculated entropy diagram (TS) for the acid-base, and radical-coupling mechanism at T = 25°C. S0 is the calculated entropy value at the initial state, and TΔ‡S is overall entropic contribution. The entropy values are derived from Ref 80.



Table 2. Calculated Entropy Values, TS (eV), of H2O, H2 and Intermediate Species for Various Oxide Catalysts. (T = 25°C).


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Importance of Entropic Contribution to Electrochemical Water

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