Toward Fundamentals of Confined Electrocatalysis in Nanoscale

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Towards Fundamentals of Confined Electrocatalysis in Nanoscale Reactors Haobo Li, Chenxi Guo, Qiang Fu, and Jianping Xiao J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b03448 • Publication Date (Web): 14 Jan 2019 Downloaded from http://pubs.acs.org on January 17, 2019

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The Journal of Physical Chemistry Letters

Towards Fundamentals of Confined Electrocatalysis in Nanoscale Reactors Haobo LiΔ, Chenxi GuoΔ, Qiang Fu§ and Jianping XiaoΔ,§,* Δ

Institute of Natural Science, Westlake Institute for Advanced Study, Westlake University,

Hangzhou 310024, People’s Republic of China. §

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of

Sciences, Dalian 116023, People’s Republic of China.

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Abstract: A number of experiments have demonstrated that electrochemical reactions are feasible in confined nanoscale reactors, while it is not clear what is the fundamentals of confined electrochemistry. Using first principles calculations and electrochemical modelling, we find that the capacitance in the confined nanoscale reactors can be significantly enhanced, compared to an open electrode interface, essentially promoting the electrochemical reactions and charge transfer efficiency in nanoscale reactors. More importantly, this is a general character, as found in a variety of electrochemical and thermochemical reactions. At the end, we define the new concept of "confinement energy" for understanding the nature of confined electrochemistry from both thermochemical and electrochemical points of view. Confined Reactors

IS Econ

vs.

Open Reactors

TS

TS

IS FS

FS

(a)

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Numerous experiments have provided solid evidences that chemical reactions and catalysis are exceptional in confined nanoscale reactors.1-3 First principles density functional theory calculations (DFT) have been employed to study the electronic nature of confined chemistry in one-dimensional (1D)4,5, and two-dimensional (2D)6–8 nanoscale reactors. As the electronic potential has been modified significantly in confined 2D space, the binding energies of various species on catalyst surfaces become weaker6. The previous theoretical studies explain well a number of experimental results, and also successfully predict some new chemical phenomena, as confirmed experimentally9. Similarly, some interesting experimental results show electrochemical reactions can also take place in confined nanoscale reactors.10-13 Feng et al.14 found that water can intercalate into the interface between a graphene cover and metals substrate, showing the feasibility of splitting graphene and substrate metals15,16. Then, a kind of experimental techniques by adopting an electrochemical scenario were developed successfully to split graphene and substrate catalysts. Wang et al.17 showed that it is an efficient electrochemical route for the delamination of epitaxial graphene grown on transition metal copper. Gao et al.18 also reported the bubbling transfer of graphene with platinum catalysts. However, it is unclear what is the nature of confined electrochemistry between graphene and transition metals Cu and Pt substrate. In addition, the interesting phenomena were not directly related with the specificity of Cu and Pt, because Dollekamp et al.19 have also observed that nanoscale bubbles can be efficiently created between graphene and mica, by reducing intercalated water to hydrogen. In short, the electrochemical reaction has been observed in many nanoscale reactors. Therefore, it is quite interesting to understand the general nature of confined electrochemistry in nanoscale reactors. However, the simulation of electrochemistry is quite difficult because electrochemical reactions are potential dependent. The standard DFT calculations are based on a closed system with constant electrons, while experimental electrochemical reactions are essentially an open system and the (electro)chemical potentials of electron are pinned via applied electric field. In other words, the experimental measurement of electrochemistry is usually performed at constant potential. Indeed, Chan et al.20 showed there is a variation of ~1.5 eV for DFT calculated work function of water layers from initial to transition states in a proton-coupled electron transfer process (PCET). Therefore, the standard DFT calculated energies at initial and transition states are not valid at the same potential. Moreover, Chan et al. found the electrochemical model is quite capacitor-like in the 3



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PECT process. Using the transient charge, work function, and electronic energies along reaction pathway, the reaction energies and barriers obtained from DFT calculations can be extrapolated to a constant potential, see more details in Ref. 20. In a word, an electrochemical process can be determined by the transient potential variation (ΔV), charge transfer (Δq), and electronic energies (E). The ab initio electrochemical simulation method above provides a possibility to study potential-dependent electrochemistry, e.g. reaction energies and barriers. However, there are many open questions for the general electrochemical characters in confined nanoscale reactors. It is not known what is the charge transfer efficiency in confined nanoscale reactors? Whether the (electro)chemical potential of reactants can be different from that on open systems, e.g. a confined proton? Are there any improvements or suppressions for the chemical kinetics in electrochemistry in nanoscale reactors?

In this work, a nanoscale reactor model was built with a p(6×3) metal surface [Cu(211), Cu(111), Pt(211) and Pt(111)] supercell and a p(3√3×3) R90o 2D overlayer (graphene and h-BN). A layer of water with one hydronium were added into the reactor to study the electrochemical characters in confined space. All the structures have been fully optimized with one bottom layer of metal fixed and all the other atoms relaxed (optimized structural model for open and confined electrode see Figure S1). The equilibrium distance between the Cu(211) surface and graphene cover is ~5.5 Å in average. All reaction energetics were calculated based on DFT, as implemented in the Vienna ab initio simulation package (VASP)21. In addition, the projector-augmented wave (PAW)22 scheme and generalized gradient approximation (GGA) method at the level of revised Perdew-Burke-Ernzerhof (RPBE) functional23 were used. The convergence criteria of energy and force were set to 10-4 eV and 0.1 eV Å-1 in structural optimizations, respectively (the comparison of convergence criteria of force shown in Figure S2). Plane-wave cutoff was set to 400 eV. Moreover, the dispersion-corrected DFT-D324 was adopted for a more reliable description of van der Waals interactions. All transition states were calculated by using climbing image nudged elastic band (NEB)25 method. A (2×3×1) and (4×6×1) k-point grids generated with the Monkhorst-Pack26 scheme was used in structural optimization and the analysis of electrostatic potential and Bader charge, respectively. The interface charge transfer has been calculated by the charge amount of the double electric solvent layer, as well as the surface adsorbate. The electrostatic potential of the

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double electric layer has been calculated as the average between two extreme points in the 1D potential curve vertical to the surface.

We have first examined the electrochemical behavior of Volmer and Heyrovsky reactions, which are two of the simplest electrochemical reactions with charge transfer process. Then, the Volmer reaction in the presence of pre-adsorbed *CO and the competing CO protonation process (CO→CHO), which is an important step in CO2 electroreduction, were studied too. Cu(211) surface is chosen to set out in this work because it is a representative step surface with activity for hydrogen evolution, CO protonation, and CO coupling. We employed the charge-extrapolation scheme to study the correlation between electrochemical charge transfer and local electronic potential variation at initial, transition, and final states. In the earlier charge-extrapolation scheme by Chan et al.20, DFT calculated work functions were adopted to represent transient electronic potential. As calculated work function is quite sensitive to water structures and orientations, polarizable adsorbates and solvents might cause significant variations of work-function. Hence, we employed the averaged electrostatic potentials of the local water layer, instead of work functions, to represent the transient electronic potentials. Calculated Bader charge and electrostatic potentials were referred to the neutral final states.

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Figure 1. Calculated charge transfer and potential variation at initial (IS), transition states (TS) with respect to the final states (FS) of a variety of elementary steps in hydrogen evolution reactions. (a) Volmer reaction in the absence of *CO, (b) Heyrovsky reaction in the absence of *CO, (c) Volmer reaction in the presence of *CO on Cu(211), (d) Volmer reaction on Cu(111). The red and blue lines are corresponding to the open and confined electrodes, respectively.

As shown in Figure 1a, the solvated proton (in the form of hydronium) has a charge of about +0.65 e at the initial state of Volmer reactions, consistent with what found previously20. Correspondingly, the Cu(211) surface has been negatively charged, resulting in a local electric field. Along the proton approaches to the Cu(211) surface (Figure 1a, red line), the charge of ~0.3 e was transferred from the Cu(211) surface to the proton, and the transient potential at the transition state (TS) was enhanced by 0.27 V compared to the initial state (IS). In the confined nanoscale reactor, the charge of transition state is comparable to that without graphene cover. However, the ΔV (0.14 V) from initial to transition states is much smaller than 0.27 V (Figure 1a, blue line). On the other hand, the charge of confined proton is smaller by 0.1e at initial state with respect to the 6


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open Cu(211) surface. The comparisons above indicate the confined reactor can effectively enhance the capacitance of electrochemical interface and improve charge transfer efficiency in the Volmer reaction. Moreover, similar trends have been also observed in other proton transfer processes, such as Heyrovsky reaction (Figure 1b) and *CO→*CHO (Figure 1c), as well as on other metal facets, such as Cu(111) surface (Figure 1d), which demonstrate the universality of such confinement effects in the confined nanoreactors. The linear correlation between charge transfer amount and potential variation reflects that the electrochemical interface is similar to a capacitor, consistent with the previous findings in Ref. 20. According to the capacitor model, the standard DFT calculated energies can be extrapolated to constant potentials. Hence, the reaction energies in nanoscale reactor is much more preferable (-0.76 eV) in comparison to 0.24 eV on the open Cu(211) surface at 0 V vs. RHE, as shown in Figure 2. Although the charge transfer amount on the open Cu(211) surface is slightly more than that in nanoscale reactors, the confined Volmer reaction is more favorable in the region with negative potentials. This indicates the (electro)chemical potential of a confined proton is higher compared to open Cu(211) surface. Calculated electrochemical barriers on the open Cu(211) surface and confined nanoscale reactors are shown in Figure 2. The barrier of a proton approaching to the Cu(211) surface in Volmer reaction is about 0.86 eV and the corresponding one is 0.51 eV in confined environment at ΔV = 0 vs. RHE. A similar trend has been both observed in confined reactors formed with graphene and h-BN overlayers (Figure S3). In addition, the double-layer graphene on Cu(211) surface was also examined, showing similar trends (Figure S4). Note that the forward kinetic barrier of Volmer reaction on an open Cu(211) surface is lower, in comparison to the backward barrier at applied voltages more negative than -0.4 V vs. RHE (the crossing point). However, the confinement can efficiently enhance the forward Volmer reaction and suppress the backward process, making the overall Volmer reaction feasible without overpotential, rationalizing well the observed efficient electrochemical hydrogen evolution reaction at the interface between graphene and copper17. In addition to the first protonation on Cu(211) surface, we have examined the second proton-coupled electron transfer in Heyrovsky process too. For the open Cu(211) surface, the transient potential variation (ΔV) from initial to final states is 0.26 V, while it is only 0.14 V vs. RHE, as long as the Heyrovsky reaction under a graphene cover, as shown in Figure 2b, consistent with the Volmer process above (Figure 1a and 2a). Calculated reaction energy and forward barrier 7



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on the open Cu(211) surface is 0.49 eV and 0.77 eV, while the confinement can improve the Heyrovsky reaction with the reaction energy of -0.22 eV and a lower barrier of 0.30 eV, respectively, showing the (electro)chemical potential of a confined proton was enhanced by ~0.7 eV. Hence, the applied voltage must be close to ~0.7 V vs. RHE to drive the overall Volmer and Heyrovsky process on the open Cu(211) surface (the crossing point), while it does not need overpotential on confined Cu(211) surface.

Figure 2. Calculated potential-dependent forward reaction energies (ΔE), forward kinetic barriers (Ea), and backward kinetic barriers (Eb) on a Cu(211) surface with respect to the same electrochemical process in confined nanoscale reactors. (a) Volmer process in the absence of *CO, (b) Heyrovsky process in the absence of *CO, (c) Volmer process in the presence of *CO. All dashed lines in (a), (b), and (c) indicate confined electrochemical nanoscale reactors. (d) Microkinetic modelling of hydrogen evolution reaction with the Volmer and Heyrovsky path. The purple and blue lines are the TOFs of reactions on the open Cu(211) and Cu(211) with graphene cover, respectively.

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Assuming the hydrogen evolution process with the Volmer and Heyrovsky path, in the following expressions, the rf and rb are the rates of forward and backward reactions, respectively. We use the same value (1013) for the exponential prefactor in all reactions. The letter, kf and kb, indicate reaction constant for Volmer and Heyrovsky, considering both forward and backward reactions. θ* and θad are the coverage of free active sites and adsorbates on the electrode surface, respectively. Microkinetic modelling of the reaction rate (r) for hydrogen evolution reaction with the Volmer and Heyrovsky path is carried out with the following equations: 𝑟 = 𝑟𝑓 − 𝑟𝑏

(1)

𝑟𝑓,𝑉𝑜𝑙𝑚𝑒𝑟 = 𝑘𝑓,𝑉𝑜𝑙𝑚𝑒𝑟 × 𝜃∗

(2)

𝑟𝑓,𝐻𝑒𝑦𝑟𝑜𝑣𝑠𝑘𝑦 = 𝑘𝑓,𝐻𝑒𝑦𝑟𝑜𝑣𝑠𝑘𝑦 × 𝜃𝑎𝑑

(3)

𝑟𝑏,𝑉𝑜𝑙𝑚𝑒𝑟 = 𝑘𝑏,𝑉𝑜𝑙𝑚𝑒𝑟 × 𝜃𝑎𝑑

(4)

𝑟𝑏,𝐻𝑒𝑦𝑟𝑜𝑣𝑠𝑘𝑦 = 𝑘𝑏,𝐻𝑒𝑦𝑟𝑜𝑣𝑠𝑘𝑦 × 𝜃∗

(5)

As shown in blue and purple lines in Figure 2d, we find the reaction rates at the confined nanoscale reactors can be enhanced significantly in comparison to that on open Cu(211) surface, rationalizing the rapid bubbling of electrochemically catalyzed hydrogen evolution and delamination of epitaxial graphene from copper17. Note that at the potential lower than -0.6 V vs. RHE, the forward reactions on open Cu(211) surface become faster with respect to the backward reaction, while the confinement effects can significantly enhance overall reaction rate in all potentials, consistent with our discussions above. Eventually, we should be able to observe hydrogen evolution reaction at 0 V vs. RHE with confined reactors. On the realistic electrode surfaces, all electrochemical reactions occur on more complicated surfaces. Usually the electrode surfaces are not clean with many pre-adsorbed species. Furthermore, there should be some competing reactions in the same reaction condition. Therefore, we have examined the Cu(211) surface with pre-adsorbed *CO to study the proton electroreduction to *H in the presence of *CO, as shown in Figure 2c. In the presence of *CO, the reaction energies of Volmer is -0.78 eV on the open Cu(211) surface, comparable to -0.76 eV on the same surface in the absence of *CO. This indicates the pre-adsorbed *CO do not affect significantly the nature of confined electrochemistry. All electrochemical behaviors for the Volmer reaction with the presence of *CO are similar to that without pre-adsorbed *CO. 9



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In order to understand the generality of the confined electrochemical phenomena shown above, we have also examined potential variation in the Volmer reaction on a Cu(111) surface and a Pt(211) surface, as shown in Figure 3c and 3d, respectively. For both cases the general trend of variational potential is consistent in the confined reactors of Cu(211) (Figure 3a and 3b). For the Cu(111) surface, the reaction energy and barrier for proton adsorption are more favorable by ~0.37 eV (see Figure 4b), showing the enhanced chemical potential of a confined proton is not facet-dependent. Furthermore, we also find the chemical potential of confined proton on a Pt(211) electrode is enhanced, resulting in lower reaction energy by 0.15 eV and barrier by 0.11 eV (see Figure 4a), respectively, explaining well observed experiments regarding platinum 18. In addition, a similar phenomenon has also been observed on the alloyed CuPt(211) surface (Figure S5), thus indicating the generality of such confinement effects.

Figure 3. Calculated potential profiles for (a) Volmer and (b) Heyrovsky reactions on the open and confined Cu(211). Volmer reaction on Cu(111) and (d) Pt(211) surfaces (red lines), respectively. 10


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Taking the Heyrovsky process as an example, we can understand that an electrochemical reaction should be affected by two factors. The first one is tightly correlated to the electrochemical proton-coupled electron transfer, attributing to the electronic potential variation and charge transfer efficiency. The second contributing component is closely related to thermochemistry, namely, the binding strength and structure of adsorbate on surface. Hence, we define the “confinement energy”, Econ, as the interaction energetic difference [Econ = Ead (w/Gr) - Ead (w/o Gr)] in electrochemistry, as defined in solid-gas interface6. The two factors in the Heyrovsky reaction are schematically shown in Figure 5a. Econ-TC and Econ-EC is the thermochemical and electrochemical factor, respectively. Econ = Econ-TC + Econ-EC

(6)

Figure 4. Calculated potential-dependent forward reaction energies (ΔE), forward kinetic barriers (Ea), and backward kinetic barriers (Eb) on (a) Pt(211) and (b) Cu(111) surfaces with respect to the same electrochemical process in confined nanoscale reactors.

For chemical reactions at solid-gas interfaces, it has shown the binding strength of a variety of species on solid surfaces was weakened in confined 2D space, rationalized by the overlapping of potential smearing between 2D slabs, as we have discussed previously6. The Econ-TC has been also confirmed to be positive in our solid-liquid systems, as shown in Figure 5b. In other words, the binding strength of hydrogen is weaker in the nanoscale reactor compared to that on the open Cu(211) surface. Herein, the Econ-TC was calculated with a neutral water structure in the absence of proton. As the hydrogen atom is small in size, the Econ-TC is not quite sensitive to the size of nanoscale reactors. In addition, the Econ-EC is the unique parameter for electrochemical systems, resulted from electronic potential variations, charge transfer, and electric field effects, exhibiting enhanced (electro)chemical potential for a confined proton. For the charge transfer and potential 11



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variation, shown in Figure 1, the slope (Δq/ΔV) on confined electrodes (blue lines) is larger than the open electrode systems (red lines) in Volmer and Heyrovsky processes. In other words, the confined nanoscale space equivalently produces an additional ‘confinement field’, which can enhance the capacitance of electrochemical capacitor, essentially improving the efficiency of electrochemical charge transfer. For a confined electrode, a smaller potential variation, namely, applied voltage, is needed to drive the same amount of charge transfer. Generally speaking, the electrochemical reaction energies become more favorable and forward kinetic barriers are lower in nanoscale reactor compared to the same reactions on an open Cu(211) surface. Moreover, Figure 5b has shown the Econ is correlated with confinement strength, namely, the size of reactors, indicating the feasibility of tuning the (electro)chemical potential of reactants, reaction energies, and kinetic barriers in electrochemical processes.

Figure 5. (a) Scheme of confinement energies (Econ) formation shown in the case of Heyrovsky reaction, resulting in more favorable reaction energies and lower electrochemical kinetic barrier. Econ is defined as the difference between an open system (red dots) and the confined electrochemical system (blue dots), which is composed of two components including the enhanced (electro)chemical potential of proton and weakening the adsorption energies of adsorbates. (b) Econ-TC and Econ-EC were shown at different confinement strengths, namely, the distances between graphene and electrode. The Econ-TC is calculated with a neutral water structure.

In the following, we will show you a competing reaction to the Volmer process in the presence of *CO, namely, CO electroreduction to CHO (CO→CHO). This reaction is also a great example to study the coexistence of Econ-TC and Econ-EC. Compared to the Volmer reaction in the presence of *CO, CO→CHO is less favorable on the open Cu(211) surface. Moreover, we find the CO→CHO 12


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process has been improved in the confined reactors too, as shown in Figure 6, consistent well with the Volmer and Heyrovsky reactions above. On the one hand, it confirms the (electro)chemical potential of a confined proton at the nanoscale reactors is higher in comparison to the open Cu(211) surface. On the other hand, we find the binding energy of CO is weakened by ~0.45 eV in the confined aqueous environment, consistent again with the previous theoretical findings in solid-gas systems6. In the recent studies of CO2 electroreduction27,28, it was found CO→CHO is the potential-limiting step over a number of electrodes. Our results indicate the confined nanoscale reactor might be one of the promising ways to improve the activity or selectivity of CO 2 electroreduction (Figure 6a). The electrochemical CO protonation is slightly different from what shown in Volmer and Heyrovsky reactions, mainly resulted from the thermochemical effects of CO→CHO process. The *CO is straight adsorbed on the surface of electrode, while *CHO is bidentate lying down on the surface. Apart from the proton approaching to the Cu(211) surface, the *CO needs a significant rotation in order to expose the carbon end to proton for C-H bond formation. The *CO rotation is purely thermochemical process without charge transfer. This is also consistent with what has been observed in the previous work27 that the electrochemical barrier of CO→CHO over a variety of electrodes is tightly correlated with the *CO binding strength. In other words, the transition state structure is quite initial-state like. Thus, the CO→CHO process in nanoscale reactor is affected by both Econ-TC and Econ-EC.

Figure 6. (a) Calculated potential-dependent forward reaction energies (ΔE), forward kinetic barriers (Ea), and backward kinetic barriers (Eb) of *CO electroreduction to *CHO (CO→CHO) on a Cu(211) surface with respect to the same electrochemical processes in confined nanoscale reactors. (b) CO-CO coupling barriers on the open Cu(211) surface and the same reaction in the nanoscale electrochemical environment. The insets show the transition state structures of CO-CO coupling on the open Cu(211) surface and in the nanoscale reactor, respectively. 13



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As the size of a nanoscale reactor is relative to confined species, the Econ is not only determined by the distance between graphene and surface, but also the size of confined species, explaining well that the Econ for *CO (0.45 eV) is significantly larger than *H (0.1 eV). In other words, the nanoscale reactors can possibly suppress some chemical reactions when the reactants or products are too large. We calculated explicitly the CO-CO coupling reaction with the same nanoscale reactor and solvation environment used for CO→CHO. Indeed, we found the C-C coupling reaction was suppressed in the nanoscale reactor. As the CO-CO coupling reaction does not have the proton-coupled electron charge transfer process, it can be considered as a purely thermochemical reaction in the presence of solution and nanoscale reactor. On the open Cu(211) surface, the CO-CO coupling is uphill with the reaction energies of 0.38 eV. In the confined nanoscale reactor, the initial state *CO adsorption can be weakened, while the *OCCO specie can be affected even more because the *OCCO is larger in size with stronger electronic interactions with the graphene cover. Therefore, the reaction energies can be reduced to 0.21 eV in nanoscale reactor, as shown in Figure 6. The forward barrier of CO-CO coupling in confined nanoscale reactor is enhanced. Eventually, the forward barrier of CO-CO coupling on the open Cu(211) surface is ~0.56 eV, while the forward barrier in the nanoscale reactor is higher by 0.19 eV. Hence, the net reaction activity of CO-CO coupling to *OCCO was significantly suppressed by Econ-TC. The main reason is the transition state of CO-CO coupling is quite like *OCCO in nanoscale reactor, as shown in Figure 6b. However, the CO→CHO process is significantly enhanced with the combination of Econ-TC and Econ-EC. Although enhanced CO→CHO reaction is not a sufficient condition to conclude the more facile C1 species formation, it still indicate the C1 formation will be possibly easier compared to the formation of C2 species in CO/CO2 electroreduction in nanoscale electrochemical reactors. The electrochemical protonation is equal to the size reduction of species, namely, the H3O+ dissociation to H2O and H+ (H*), associated with charge transfer with the electrode. As we have demonstrated the size of species is associated with confinement energies, the potential variation of initial state is larger than transition state. Therefore, the slope of Δq/ΔV on confined electrodes (blue lines) is larger than the open electrode systems (Figure 1), explaining well the enhanced capacitance in confined reactors. The charge transfer efficiency in confined space can be promoted because of enhanced capacitance in nanoscale reactors. The (electro)chemical potential of a confined proton is higher 14


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with respect to that on an open system. Correspondingly, the forward electrocatalytic barriers in nanoscale reactors become lower. At the end, we would propose the two important concepts, Econ-TC and Econ-EC, to understand the activity and selectivity for electrochemical reactions in nanoscale reactors. More importantly, we can rationally design catalysts and electrochemical reactors in terms of Econ-TC and Econ-EC.

In summary, we have examined two electrochemical elementary steps in hydrogen evolution reaction including Volmer and Heyrovsky processes. We find that the (electro)chemical potential of a confined proton was always higher in comparison to the open electrochemical interface. The enhanced (electro)chemical potential of a confined proton can be attributed to two contributing components. The first influence factor is tightly correlated with the charge transfer, local electric field, and potential variations. The nanoscale space equivalently produces an ‘confinement field’, which can enhance the capacitance of electrochemical capacitor, improving well the efficiency of charge transfer. The second one is a thermochemical factor, affected by the geometry and orientation of adsorbates, as well as the scale of chemical reactor. It was also found the CO→CHO electroreduction and CO-CO coupling can be tuned under confined nanoscale reactors.

Supporting Information Available: The confined electrochemical characters with a single layer of h-BN, a bilayer of graphene, and alloyed CuPt electrode are available.

Author Information Corresponding author: *[email protected] (J.X.)

Acknowledgements J.X. gratefully acknowledge Westlake Education Foundation and Supercomputing Cluster, Westlake Education Foundation, and the National Science Foundation of China (No. 21802124 and 91845103). Q.F. acknowledges the Ministry of Science and Technology of China (No. 2016YFA0200200), and Strategic Priority Research Program of the Chinese Academy of Sciences 15



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(No. XDB17020000). We also the useful discussions from Prof. Xinhe Bao (University of Science and Technology of China) and Prof. Chi-Yung Yam (Beijing Computational Science Research Center).

References (1) Pan, X.; Bao, X. The Effects of Confinement inside Carbon Nanotubes on Catalysis. Acc. Chem. Res. 2011, 44, 553-562. (2) Yao, Y.; Fu, Q.; Zhang, Y. Y.; Weng, X.; Li, H.; Chen, M.; Jin, L.; Dong, A.; Mu, R.; Jiang, P.; et al. Graphene Cover-Promoted Metal-Catalyzed Reactions. Proc. Natl. Acad. Sci. U.S.A. 2014, 111, 17023-17028. (3) Deng, D.; Novoselov, K. S.; Fu, Q.; Zheng, N.; Tian, Z.; Bao, X. Catalysis with Two-Dimensional Materials and Their Heterostructures. Nat. Nanotech. 2016, 11, 218-230. (4) Xiao, J.; Pan, X.; Guo, S.; Ren, P.; Bao, X. Toward Fundamentals of Confined Catalysis in Carbon Nanotubes. J. Am. Chem. Soc. 2015, 137, 477-482. (5) Xiao, J.; Pan, X.; Zhang, F.; Li, H.; Bao, X. Size-Dependence of Carbon Nanotube Confinement in Catalysis. Chem. Sci. 2017, 8, 278-283. (6) Li, H.; Xiao, J.; Fu, Q.; Bao, X. Confined Catalysis under Two-Dimensional Materials. Proc. Natl. Acad. Sci. U.S.A. 2017, 114, 5930-5934. (7) Ferrighi, L.; Datteo, M.; Fazio, G.; Di Valentin, C. Catalysis under Cover: Enhanced Reactivity at the Interface between (Doped) Graphene and Anatase TiO2. J. Am. Chem. Soc. 2016, 138, 7365-7376. (8) Doyle, A. D.; Montoya, J. H.; Vojvodic, A. Improving Oxygen Electrochemistry through Nanoscopic Confinement. ChemCatChem 2015, 7, 709. (9) Zhang, F.; Jiao, F.; Pan, X.; Gao, K.; Xiao, J.; Zhang, S.; Bao, X. Tailoring the Oxidation Activity of Pt Nanoclusters via Encapsulation. ACS Catalysis 2015, 5, 1381-1385. (10) Li, X.; Cai, W.; An, J.; Kim, S.; Nah, J.; Yang, D.; Piner, R.; Velamakanni, A.; Jung, I.; Tutuc, E.; et al. Large-Area Synthesis of High-Quality and Uniform Graphene Films on Copper Foils. Science 2009, 324, 1312-1314. (11) Bae, S.; Kim, H.; Lee, Y.; Xu, X.; Park, J. S.; Zheng, Y.; Balakrishnan, J.; Lei, T.; Kim, H. R.; Song, Y. I.; et al. Roll-to-Roll Production of 30-Inch Graphene Films for Transparent Electrodes. Nat. Nanotech. 2010, 5, 574-578. (12) Gao, L.; Ren, W.; Zhao, J.; Ma, L. P.; Chen, Z.; Cheng, H. M. Efficient Growth of High-Quality Graphene Films on Cu Foils by Ambient Pressure Chemical Vapor Deposition. Appl. Phys. Lett. 2010, 97, 183109. (13) Li, X.; Magnuson, C. W.; Venugopal, A.; An, J.; Suk, J. W.; Han, B.; Borysiak, M.; Cai, W.; Velamakanni, A.; Zhu, Y.; et al. Graphene Films with Large Domain Size by a Two-Step Chemical Vapor Deposition Process. Nano Lett. 2010, 10, 4328-4334. (14) Feng, X.; Maier, S.; Salmeron, M. Water Splits Epitaxial Graphene and Intercalates. J. Am. Chem. Soc. 2012, 134, 5662-5668. (15) Sotthewes, K.; Bampoulis, P.; Zandvliet, H. J. W.; Lohse, D.; Poelsema, B. Press-Induced Melting of Confined Ice. ACS Nano 2017, 11, 12723-12731. (16) Li, Q.; Song, J.; Besenbacher, F.; Dong, M. Two-Dimensional Material Confined Water. Acc. Chem. Res. 2015, 48, 119-127. (17) Wang, Y.; Zheng, Y.; Xu, X.; Dubuisson, E.; Bao, Q.; Lu, J.; Loh, K. P. Electrochemical Delamination of CVD-Grown Graphene Film: Toward the Recyclable Use of Copper Catalyst. ACS Nano 2011, 5, 9927-9933. 16


Page 17 of 17 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


(18) Gao, L.; Ren, W.; Xu, H.; Jin, L.; Wang, Z.; Ma, T.; Ma, L. P.; Zhang, Z.; Fu, Q.; Peng, L. M.; et al. Repeated Growth and Bubbling Transfer of Graphene with Millimetre-Size Single-Crystal Grains Using Platinum. Nat. Commun. 2012, 3, 699. (19) Dollekamp, E.; Bampoulis, P.; Poelsema, B.; Zandvliet, H. J. W.; Kooij, E. S. Electrochemically Induced Nanobubbles between Graphene and Mica. Langmuir 2016, 32, 6582-6590. (20) Chan, K.; Norskov, J. K. Electrochemical Barriers Made Simple. J. Phys. Chem. Lett. 2015, 6, 2663-2668. (21) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for ab initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169. (22) Blochl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953. (23) Hammer, B.; Hansen, L. B.; Norskov, J. K. Improved Adsorption Energetics within Density-Functional Theory using Revised Perdew-Burke-Ernzerhof Functionals. Phys. Rev. B 1999, 59, 7413. (24) Grimme, S.; Antony, J.; Ehrlich, S. Krieg, H. A Consistent and Accurate ab initio Parametrization of Density Functional Dispersion Correction (DFT-D) for the 94 Elements H-Pu. J. Chem. Phys. 2010, 132, 154104. (25) Henkelman, G.; Uberuaga, B. P.; Jonsson, H. A Climbing Image Nudged Elastic Band Method for Finding Saddle Points and Minimum Energy Paths. J. Chem. Phys. 2000, 113, 9901. (26) Monkhorst, H. J.; Pack, J. D. Special Points for Brillouin-Zone Integrations. Phys. Rev. B 1976, 13, 5188. (27) Liu, X., Xiao, J.; Peng, H.; Hong, X.; Chan, K.; Norskov, J. K. Understanding Trends in Electrochemical Carbon Dioxide Reduction Rates. Nat. Commun. 2017, 8, 15438. (28) Peterson, A. A.; Norskov, J. K. Activity Descriptors for CO2 Electroreduction to Methane on Transition-Metal Catalysts. J. Phys. Chem. Lett. 2012, 3, 251-258.

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Toward Fundamentals of Confined Electrocatalysis in Nanoscale

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