Intrinsic Mechanical Effects on the Activation of Carbon Catalysts

Intrinsic Mechanical Effects on the Activation of Carbon Catalysts | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Intrinsic Mechanical Effects on the Activation of Carbon Catalysts Bin Wang, Bowen Liu, Shuaishuai Xu, Xinying Luo, Junjie Xiong, and 9 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-2295214/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract The mechanical effects on carbon-based metal-free catalysts (C-MFCs) have rarely been explored although the C-MFCs have attracted worldwide interest as alternatives to the noble metal catalysts. Stress is everywhere, but a specialized study is strongly limited because the stress usually intermingles with other structural variables, including the dopants, defects, and interfaces in catalysis. Herein, we report a proof-of-concept study by establishing a platform to apply strain to a highly oriented pyrolytic graphite (HOPG) lamina continuously and collecting the electrochemical signals simultaneously. For the first time, the correlation between the surface strain of a graphitic carbon and its oxygen reduction reaction (ORR) activation effect is established. Results show that the in-plane and edge carbon sites in HOPG could not be further activated by applying tensile strain, but when the in-plane defects were involved in the structure, a strong and repeatable dependence of the catalytic activity on the tensile strain was observed, wherein ~ 35.0% improvement in ORR current density was realized by applying ~ 0.6% tensile strain. The density function theory (DFT) simulation shows that appropriate strain on the specific defect can optimize the adsorption of reaction intermediates, and the Stone-Wales defect on graphene correlates with the mechanical effect. Moreover, the effect was further authenticated by preparing a powdered graphene-based catalyst with varied strain-involved, which showed an apparent improvement of the ORR activity with ~ 0.4% surface strain. This work clarifies some basic principles of strain effects on graphitic carbon’s catalytic activities towards ORR, and may lay the foundation for developing carbon-based mechanoelectrocatalysis. Physical sciences/Energy science and technology/Fuel cells Physical sciences/Energy science and technology/Energy storage/Batteries Physical sciences/Materials science/Materials for energy and catalysis/Fuel cells Mechanical effects Catalysis Carbon Strain engineering Mechanoelectrochemistry Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 1. Introduction The electrochemical application is inherently interdisciplinary crossing chemistry, electronics, materials, and even mechanics 1 – 3 . For example, strain engineering is emerging as an efficient tool to modulate the catalytic activity of metallic materials when being used in reactions, such as oxygen reduction reaction (ORR) 4 , 5 , hydrogen evolution reaction (HER) 6 , and others 7 , 8 . In contrast to metals, the mechanical effects on carbon-based metal-free catalysts (C-MFCs) have rarely been explored although C-MFCs have attracted worldwide interest as alternatives to the noble metal catalysts 9 . On the one hand, identification of the mechanical effects on C-MFCs is critical as it directly works on the coordination of neighboring carbon atoms and thus the electronic states (a key factor affecting the catalytic activity), but the clarification is strongly limited by the fact that the mechanical effect usually intermingles with other structural variables, including the dopants, defects, and interfaces 10 . On the other hand, applying strain to the C-MFCs accurately and continuously is a big challenge because the strain in metals could be inputted by alloying and read out by referring to the lattice variation, which is not feasible in pure carbon materials (e.g., graphene). A delicate design has been proposed to study the mechanical effect on the HER properties of MoS 2 by deforming these two-dimensional (2D) materials on a patterned gold nanocone support, but the strain cannot be applied in a continuous and uniform manner 11 . To our best knowledge, the continuous application and accurate detection of strain and its electrochemical effects on carbon-based catalysts haven’t been achieved to date. Therefore, the intrinsic mechanical effects on the catalytic behaviors of a pure carbon material remain unknown. Herein, we report a proof-of-concept study using highly oriented pyrolytic graphite (HOPG) as the model catalyst. HOPG is selected due to its well-defined π conjugation, defect/dopant-less structure, flat surface, and ordered stacking state 12 . Note that we have attempted to use single-layer graphene films (chemical vapor deposition grown, CVD) as the model but failed because the transfer process was usually not defect-free and, more critically, a conductive, elastic, and catalytic-inert substrate was required to support graphene in this experiment, which limited its feasibility and caused more uncertainties. Based on a horizontal drawing/compressing machine and an electrochemical station, we established a platform to apply strain to the exfoliated HOPG lamina continuously and collected the electrochemical signals simultaneously (ORR in this work). For the first time, the correlation between the surface tensile strain (ε) of a graphitic carbon and its ORR activation effect was established. It was found that there is a negligible correlation between the applied strain and ORR performance in the strain range from − 1.0–2.0% (‘-’ refers to compressive strain, otherwise the tensile strain), in which only ~ 13.0% improvement in the reduction current was achieved by applying ~-0.8% compressive strain on a fresh HOPG surface. In the case of a defective HOPG surface without other elemental dopants, a repeatable ~ 35.0% improvement of current density was observed when applying a ~ 0.6% tensile strain. We further explored an in-depth understanding of the relationship between tensile strain and ORR performance based on density function theory (DFT) calculations. Both adsorption sites of reaction intermediates and charge redistribution were found related with the strain, and the Stone-Wales defect was identified as the site that correlates with the mechanical effect strongly due to the force induced lattice variation, and thus charge redistribution. Finally, a graphene-wrapped spherical powder material with varied radius, and thus different surface strain, was prepared and tested. Apparent improvement of the ORR activity over 100% was observed with ~ 0.4% surface strain in graphene, suggesting the significance and potential of clarifying the mechanical effect on the catalytic performance of the C-MFCs. This work could open up an entirely new way of developing functional carbon materials, including carbon catalysts. 2. Results And Discussion 2.1 Accurate detection of strain applied on the surface of HOPG A freshly exfoliated HOPG lamina is sandwiched between a polycarbonate (PC) substrate and an elastic mask (a PI tape, polyimide) with a hole in the center that is used to define the HOPG surface area exposed to the electrolyte (Fig. 1a). We have tried to stretch this assembly in the electrolyte using the drawing machine to induce the tensile strain, but the detachment between the mask and HOPG occurred when the strain read by the machine exceeded ~ 1.0%. Alternatively, the compression mode of the machine was used to bend the assembly, which caused the surface stretching of HOPG that exposed to the electrolyte. In this case, the detachment was avoid obviously, and the degree of deformation was much higher. Detailed experimental information can be found in the Methods section. The next problem needs to be solved is the accurate detection of the surface strain applied to HOPG. There are several reported methods to extract the strain of a 2D material, such as the measurement of the lattice difference by transmission electron microscopy (TEM), the visualization of the geometric deformation of the supporting substrate, and the band variation in Raman spectra. The TEM method is typically localized in a range of few nanometers that cannot reflect the entire view of the strain distribution across hundreds of micrometers. We then used the substrate geometric deformation method to extract the strain applied on the HOPG as shown in Figure S1 (see the Supplementary Materials, Table S1 ). In the literature the deformation of the substrate is usually considered as that of the sample on the surface. However, in our case, the results are not convincing as it exceeded 20.0% without damaging the HOPG surface as observed using the optical microscope. Note that the theoretical maximum strain of graphene is around 20.0% 13 , and the experimental value is much lower, ~ 1.0% 14 . Since the HOPG we used is about 10 µm in thickness, there are thousands of graphene layers stacking on the PC substrate. The interlayer slippage results in a weak force delivery from the PC substrate to the top HOPG surface, so we conclude that the geometric deformation method is not applicable in this experiment. Raman spectroscopy has been used to identify the number of layers and the information about doping, edges, defects, and disorders of graphene. Given that strain can effectively modify the electronic structure of graphene and soften the optical-phonon branches, it is expected to induce variations in Raman spectra 15 , 16 . In contrast to the Raman shift changes of single-layer graphene under uniaxial or biaxial stretching, the study of strain effects on the Raman spectrum of HOPG is still missing. Figure S2 shows the Raman spectra of HOPG under the uniaxial strain from 0.0–3.1%. The absence of D band at ~ 1350 cm − 1 corresponding to defects indicates the high-quality HOPG surface. The red shifts of G (~ 1580 cm − 1 ) and 2D (~ 2700 cm − 1 ) bands are observed in Fig. 1b-c under the increased tensile strain due to the softening of the E 2g phonon associated with the G band and the TO phonon between the Γ and K points associated with the 2D band 17 . The fitted relationship of ω/G and ω/2D versus ε are plotted in Fig. 1d. The change of Raman shift of HOPG during the stretching process is thus established, and it is found that the Raman shift offset is much smaller than the single-layer graphene reported in the literature possibly due to the interlayer interactions 18 . As the strain increases from 0.0–3.1%, the G and 2D shifts are found to be reduced from 1579.8 cm − 1 to 1579.0 cm − 1 , and from 2725.5 cm − 1 to 2723.0 cm − 1 , respectively. Although the relationship between the Raman signals and the strain on HOPG can be established, the indistinctive variations and particularly the absence of a standard relationship from the literature indicate that this method could not identify the strain values confidently. Inspired by the standard tensile testing process, we finally used the digital image correlation (DIC) method 19 – 21 to map the strain field on the surface of HOPG. Specifically, a certain amount of ink was sprayed on the HOPG surface as the positioning mark, that is, the relative position of the ink dots changes with the deformation of the HOPG surface and then the actual strain on the surface can be clearly measured combined with Ncorr software 22 (Fig. 1e-f). The correspondence between the as-measured surface strain (ε) and the actual movement of the machine clamps (ΔL) was thus established (Fig. 1g). In detail, the optical microscope was used to observe the relative positions of the ink marks when the PC substrate was bent due to the shortening of the clamp spacing (Fig. 1e). The images positioning and program recognition were performed to depict strain distribution map with the color variation (Fig. 1f), and the details about the program recognition are shown in the Methods section. Note that the ‘inhomogeneous’ yellow color distribution is normally caused by the formation of shear bands at ~ 45° with respect to the stretch axis by the neighbouring ink marks, which will not affect the strain distribution in a clean HOPG sample that was used for the mechanical-electrochemical investigation in this work 23 , 24 . Thus, the corresponding fitted curve for the clamp shortening distance (ΔL) and the calculated average strain (ε) was obtained, as shown in Fig. 1g. With the gradual change of the distance between the clamps, the surface of the HOPG deformed accordingly. It is noted that when ΔL is 6 mm, the strain reached a maximum of 3.4%. The HOPG surface was vulnerable to destruction with further bending, which would affect the observation of strain, so 3.4% was controlled as the threshold of tensile strain in this study. Moreover, we have used the same method to characterize the compressive strain after changing the bending direction of the PC substrate. The specific topography pictures are shown in Figure S3 . However, due to the squeezing effect, the HOPG surface is peculiarly prone to crack during the compression process, so the threshold of the compressive strain was set as low as -0.8%, below which the HOPG surface was well preserved. 2.2 Mechanical effect on the catalytic performance of clean-HOPG The dependence of the ORR activity of the pure C-MFCs on the strain was firstly investigated using the clean-HOPG, which is highly regular with limited defects ( Figures S2 and S4 a). The ORR catalytic performance was measured by linear sweep voltammetry (LSV) in alkaline electrolyte (0.1 M KOH). All electrochemical data were presented without iR correction. Figure 2 a shows the ORR curves obtained under O 2 -saturated conditions with the subtraction of data under N 2 -saturated conditions as the background, in which the currents are divided by the geometric surface area of the hole in the mask (~ 3.14 mm 2 ) (the same as the exposed HOPG catalyst surface areas). Before testing the strain effects, a repeated LSV measurement over 20 cycles was conducted to ensure the curves were in complete agreement and thus getting rid of the possible influences from the testing conditions ( Figure S5 ). Results show that the ORR activities varied under different tensile strain conditions. The current densities at a specific potential ( versus RHE, reversible hydrogen electrode) were extracted and plotted versus the corresponding tensile strain in Fig. 2 b. As the applied tensile strain increases, the current density under different specific potential decreases gradually until reaching the limit at ~ 2.0% strain, indicating the slight inhibition of ORR activity. The onset potential (potential versus RHE at current density of 1 µA cm − 2 ), representing the intrinsic activity of catalyst, also demonstrated a negligible negative correlation relative to the tensile strain ( Figure S6 ), corresponding to the enlargement of the overpotential. In contrast, it is found that the compressive strain shows an opposite promotion effect on the ORR activity, for which the current density increases along with increasing compressive strain. Since the sample was more prone to surface fragmentation when being subjected to the compressive strain, however, we can only control the threshold of compressive strain at ~ 0.8% without further exploration ( Figure S7 ). In brief, in the case of a clean-HOPG surface (there should also be many graphene edges on the HOPG surface when the tested area is across mm 2 scale), there is no obvious correlation between its ORR catalytic activity and the tensile strain, while the compressive strain was observed to promote the activity slightly. It should be noted that although surface defects were rare in the HOPG model material, edges still existed on the surface of HOPG. Therefore, the strain effects could be ascribed to the variation of the graphene skeleton (in-plane structure), or the graphene edges, or a combination of both structures. A theoretical simulation was then used to analyze the relationship of the strain and the ORR activity. A pure graphene nanobelt model (G) without defects was established (Fig. 2 d) and the adsorption capacity of the intermediates *OOH, *O, and *OH was studied by DFT method. The free energy of adsorption was used as the descriptor to characterize the catalytic activity of different carbon sites ( C1-C6 ) from edge to in-plane in the G with zigzag edge. In Fig. 2 c, it shows a volcano relationship between the overpotential of ORR and the adsorption energy of *OH (ΔG(*OH)). Accordingly, the sites on the edges ( C1 ) are the optimal active sites, which have lower overpotential than those sites picked up in the middle of the graphene. Note that the carbon sites on the armchair edge were not used because of the large overpotentials caused by the adsorption of *OOH ( Figure S8 ). The free energy diagrams for the ORR at C1 site show that the rate-determining step on G is the desorption of *OH ( Figure S9 ), where the energy involved was taken as the overpotential. From the overpotential vs . strain plots in Fig. 2 d, a negligible negative correlation between tensile strain and ORR performance is observed although the effect is extremely weak and can be ignored, which is consistent with the experimental results. Moreover, the effect of compressive strain extended to -5.0% on the catalytic performance was also modulated in Figure S10. Interestingly, an optimized compressive strain of ~ -1.2% is found beneficial to the catalysis, but overpotential increases dramatically with further increase of the compressive strain. Our experiments verified such an effect partially by applying the compressive strain to -0.8%, although further compression was inhibited by the surface fragmentation of HOPG under large deformations, which may be solved in the future by establishing another platform to test the mechanoelectrochemcial effects of ultrathin and flexible graphene samples. 2.3 Mechanical effect on the catalytic performance of defective-HOPG As can be seen from above, the response to strain is relatively feeble for HOPG with a smooth surface and edges. In contrast to the edge defects, the in-plane defects have been demonstrated to obviously promote ORR catalytic activities. In this regard, we treated the HOPG with an Ar plasma for 5 mins to produce the in-plane defects. The Raman spectrum ( Figure S11 ) shows that the defects on the HOPG surface have increased significantly after the Ar plasma treatment, which were observed in the aberration-corrected atomic-resolution high-angle annular dark field scanning transmission electron microscopy (AC-HAADF-STEM) images (Fig. 3 a and S4). LSV curves shown in Fig. 3 b exhibit the dependence of the ORR activity for the defective HOPG (named D-HOPG) on the tensile strain (ε). It is obvious that the D-HOPG showed enhanced catalytic activity with higher current density and more positive onset potential compared to the clean-HOPG. More interestingly, the strain effect on the D-HOPG exhibits a classic volcano relationship, that is, with the gradual increase of strain, the D-HOPG firstly showed gradually enhanced ORR activity until the strain up to ~ 0.5%, followed by a gradual decrease and then leveled off. To be more intuitive, the current densities at various specific potentials ( versus RHE) were extracted and plotted versus the corresponding tensile strain in Fig. 3 c. The strain effect was found to be recoverable and the similar behavior was observed under all the potentials (from 0.0V to 0.7V versus RHE), suggesting the restorability of the HOPG structure under the testing conditions and thus the reliability of the observed strain effects. The volcano relationship was also obtained by plotting the onset potentials at 1 µA cm − 2 vs . the strain (Fig. 3 d). As a result, a ~ 35.0% current improvement was realized with ~ 0.6% tensile strain in the case of D-HOPG, verifying the great promoting ability of strain to the ORR activity of the defective carbon. We have also tried to measure the compressive strain effects on D-HOPG, but the sample became more brittle after inducing defects than the clean-HOPG samples, severely limiting the compression range that could be tested so that no convincing result for the compression deformation could be obtained through this testing method. The underlying mechanism for the strain-catalytic activity relationship on defected HOPG was then investigated by DFT simulations. Firstly, various graphene structures with different defects were created, including the previously discussed clean-graphene with zigzag edges (G), mono-vacancy graphene (MVG), di-vacancy graphene (DVG), and Stone-Wales graphene (SWG), as shown in Fig. 4 a. Then, we simulated the ORR processes to identify the possible active sites for each structure and calculated the corresponding overpotential value. Figure S12 a shows the free energies and ORR reaction pathways at the optimal active site (marked with a red circle in Fig. 4 a) on each structure under strain-free condition, where MVG has the lowest overpotential (0.53V) while DVG has the highest (0.74 V). The strain effect simulation was then conducted by changing the size of the a-axis direction of each structural unit cell. For example, the a-axis size of the unit cell in the pristine structure is 9.85 Å, which is increased to 9.90 Å when the strain is 0.5%. By simulating the free energy diagrams of the optimal active sites as mentioned above under varied strain conditions (G-1, MVG-5, DVG-1, and SWG-5 under strain of 0.0%, 0.5%, 1.0%, 1.5%, and 2.0%, respectively), it was found there is a linear relationship between adsorption free energy of *OOH and *OH during the ORR process (Figure S12b). We then used the adsorption energy of *OH as the descriptor to characterize the catalytic activity of these graphene-based structures. The free energy diagrams of all the carbon sites shown in Fig. 4 a were simulated, and only the possible active sites under suitable strains are involved in Fig. 4 b to compare their catalytic activities. In the case of MVG and DVG, the optimal sites are MVG-5 and DVG-1, respectively, and the free energy diagrams of which show very limited variation by changing the strain values (Figure S12c-d). Also, other sites in MVG and DVG show almost negligible response to the strain (Figure S12e). However, the catalytic activities of SWG sites show significant responses to the applied strains ( Figure S13 ). According to the volcano-shaped relationship of the overpotentials of the ORR v s. ΔG(*OH) as established in Fig. 4 b, we can identify that the catalytic activity reaches the volcanic peak by adjusting the strain in SWG sites to be 0.5% for SWG-3 and SWG-6. Specifically, the comparison of the overpotential of each site under different strain (Fig. 4 c) show SWG-3, SWG-4, SWG-6, and SWG-9 share the similar trend that the overpotential decreases firstly and then increases along with increasing strain, except for the SWG-5 that has the lowest initial overpotential but keeps rising later. The minimum overpotential of 0.42 V is achieved at the strain of 0.5% for SWG-3, while SWG-4 and SWG-6 also have values of 0.52 V and 0.43 V at the strain of 0.5%, respectively, which are lower than the best initial overpotential of 0.53 for MVG-5. These results are in good agreement with the experimental data. To gain the mechanistic understanding of the influence of strain on the catalytic activity of SWG, we compared the free energies and reaction pathways shown in Figure S13 and found that the change of the rate-limiting step is perhaps one of the responsible factors. The reaction rate is limited by the last step for almost all the SWG sites (desorption of *OH to form H 2 O) in the initial structure, which turns to the first step (adsorption of *OOH) when the strain passes 0.5% (1.0% for SWG-9). However, for the sites (SWG-5 and SWG-8) whose first step is the rate-limiting step in the initial structure, the energy barrier continues to increase with the increase of strain, making the catalytic activity worse. To understand this observation, we modulated the charge density variation before and after applying strain ( Figure S14 , the one for SWG is also moved to Fig. 4 d). As shown in the initial structure of SWG, the carbon sites generally have low charge density, which means more positive charges for the adsorption of *OH inhibiting the desorption to form H 2 O. When the strain is increased to 0.5%, the charge density increases and thus the adsorption of *OH is weakened, corresponding to the promoted catalytic performance. An apparent increase trend of the charge density is observed along with the stretching process for SWG, which refers to the weakening interaction of carbon sites and the negatively charged reaction intermediates, including *OH, and *OOH. The weak adsorption of *OOH makes the first reaction step as the rate-limiting step and increases the overpotential. Like SWG-5 and SWG-8 whose first step is the rate-limiting, overpotential of MVG-5 increases when the charge density increases by applying strain (Figure S12c and Figure S13). However, since the charge density changes very little, the effect on the overpotential of MVG-5 is almost negligible. The charge density around DVG-1 decreases slightly with the application of strain, leading to the enhancement of *OH adsorption, but the effect is minimal (Figure S12d). For other sites (MVG-6, DVG-3 and DVG-6), their overpotentials are higher than those for the aforementioned sites (MVG-5 and DVG-1) and could not become ideal active sites, although they decrease gradually with the application of strain (Figure S12e). Therefore, the applied strain affects the charge density distribution of SWG obviously, modifies the interaction with the reaction intermediates, alters the rate-determining steps and the energy barriers, and thus endows SWG with the highest catalytic activity. As a common defect type, the activation of Stone-Wales defects could boost the catalytic efficiency of defective carbon materials by waking up more inert sites, which opens a new way to developing C-MFCs. 2.4 Carbon powder catalyst with intrinsic strain To demonstrate and investigate the aforementioned strain-induced catalytic activity for carbon materials of practical significance, we present here a spherical diameter engineering strategy, in which various PS@RG-O (PS: polystyrene, RG-O: reduced graphene oxide) carbon powder catalysts with different curvature-induced strain were prepared. Figure 5 a depicts the preparation of the PS@RG-O carbon catalysts. Briefly, the G-O dispersion was added dropwise into an aqueous dispersion of PS nanospheres to form the surface coating via the electrostatic and π-π / interactions (see details in Methods ). Hydrazine hydrate (N 2 H 4 ·H 2 O) was used to reduce the material to RG-O, which was verified by the decreased O1s intensity as shown in the X-ray photoelectron spectroscopy (XPS) surveys ( Figure S15 ) 25 . Nitrogen was also observed in the spectra, suggesting the sample is an O/N co-doped RG-O material ( Table S2 ). Note that the reduction of G-O at high temperature usually results in much purer graphene samples with less hetero-dopants, but it is not applicable in our experiment due to the thermal-decomposition of PS and thus structural collapse. By changing the PS spheres with varied spherical diameter (400 nm, 1.3 µm, and 2.5 µm, Figure S16 ), the curvature-induced strain (0.38%, 0.12%, and 0.06%, respectively) of the RG-O on PS surface can be realized. The first two images in Fig. 5 b show the 1.3 µm PS@RG-O spheres, which have a regular spherical shape with smooth surface and homogeneous graphene coating. The high-resolution transmission electron microscopy (HRTEM) image exhibits that the plane of RG-O coating is continuous along the curvy edge with shell thickness about ~ 3 nm containing ~ 8 graphene layers 26 , and the thickness of coating RG-O is approximately consistent with that of 400 nm and 2.5 µm PS@RG-O ( Figure S17 ). The typical X-ray diffraction (XRD) patterns of n-PS@RG-O (n = 400 nm, 1.3 µm, and 2.5 µm) show broad diffraction peaks at ~ 10.8° and ~ 20.0° (Fig. 5 c), and the Raman spectra show apparent D (1358 cm − 1 ) and G (~ 1594 cm − 1 ) bands for graphene (Fig. 5 d). These results further prove the successful preparation of RG-O coated PS-sphere samples 25 27 . The ORR performance of the n-PS@RG-O was evaluated by LSV measurements carried out on a rotating disk electrode (RDE) using a three-electrode system in an O 2 saturated 0.1 M KOH. The ORR activity of the n-PS@RG-O was found to be strongly correlated with the PS diameters. As shown in Fig. 5 e, the LSV measurements exhibit an increased trend for the diffusion-limiting current density with increasing curvature from n = 2.5 µm to 500 nm. However, the surface area is also a key parameter to consider. In order to study the effect of curvature-induced strain on the ORR catalytic activity more convincingly, the electrochemical double-layer capacitance (C dl ), which is positively proportional to electrochemical active surface area (ECSA), was obtained from cyclic voltammetry (CV) curves in a non-faradic potential range ( Figures S18 and 5f) 28 . ECSA represents the effective area that the catalysts work in the catalytic process, as a factor positively related to electrochemical performance. After normalization of the current density j at 0.60V ( vs . RHE) by ECSA, the activity of n-PS@RG-O still decrease distinctly as the diameter of PS spheres increases (Fig. 5 g), suggesting that the difference in current density was not caused by the variation of the surface areas. Due to the inevitable defects in chemically reduced graphene oxide, the strain-induced enhancement of oxygen reduction activity of defective graphene could thus be expected. These results reflect that the increased curvature-induced strain of n-PS@RG-O is most probably one of the key reasons for the observed enhancement in the ORR performance. 3. Conclusion In summary, we have performed a proof-of-concept study to explore the mechanical effects on pure carbon catalysts. Using HOPG as the model catalyst, we established a platform to apply strain to the HOPG continuously and collected the electrochemical signals simultaneously, thus realized the ORR activity characterization of HOPG lamina under a controllable/continuous strain deformation. For the first time, the correlation between the surface tensile strain (ε) of a graphitic carbon and its ORR activation effect was established experimentally and theoretically. It was found that there was a negligible negative correlation between tensile strain and ORR performance for a clean HOPG lamina in the strain range from − 1.0–2.0% with only ~ 13.0% improvement by applying ~-0.8% compressive strain. In the case of a defective HOPG lamina, however, a great current density boost of ~ 35.0% was observed when ~ 0.6% tensile strain was applied on the surface. Combined experimental and theoretical studies indicate that the initial inert sites, the Stone-Wales defects, can be transformed into optimal active sites by applying tensile strain of ~ 1.0%, which was realized by reducing the reaction energy barrier and altering the rate-determination steps. Finally, the relationship between the strain modulation and catalytic activity in carbon materials for ORR was authenticated by a spherical diameter engineering strategy with carbon powder catalysts, and an apparent improvement of the ORR activity was achieved with ~ 0.4% surface strain in graphene. This work clarifies the intrinsic strain effects on the electrochemical catalytic performance for C-MFCs, which lays the foundation for mechanistic understanding of some complex catalytic reactions and triggers the invention of new concepts for carbon catalysis. Methods 1. Assembly Of The Bendable HOPG Configuration Figure 1a depicts the preparation of the bendable HOPG assembly. We firstly peeled off a piece of HOPG squared in 0.5 × 0.5 cm with the mechanical peeling tape purchased from Minnesota Mining and Manufacturing company, and then stuck it on the sticky PC substrate, keeping the smooth HOPG side up. Then a copper wire was bonded to the edge of the HOPG surface using the conductive silver glue. Finally, a PI tape with a 2 mm diameter hole was pasted on the smooth surface of the HOPG to expose a fixed area. 2. Preparation Of Polystyrene Spheres (PS) The polystyrene spheres with three different sizes (PS-a: ~400 nm, PS-b: ~1.3 µm, PS-c: ~2.5 µm) were synthesized according to the reported procedure with some modifications 29 . In a typical synthesis of 400 nm PS spheres as an example, 50 mL of styrene was first washed thoroughly with 50 mL of 10 wt. % NaOH solution and deionized water successively to remove the stabilizer. Then, the washed styrene was added to a triple-neck, 500 mL round-bottomed flask with 200 mL water containing 0.25 mg of PVP. After bubbling with nitrogen for 15 min, the mixture was then fluxed at 95℃ under magnetic stirring for 30 min. Subsequently, 20 mL of aqueous solution containing 0.75 mg K 2 S 2 O 8 was added quickly into the flask to initiate the polymerization of styrene. After stirring at 400 rpm for 24 h at 95℃ temperature, the mixture was cooled down. Then the obtained milk-like product was centrifuged three times with ethanol and lyophilized to obtain the PS nanospheres powder. 3. Preparation Of PS@RG-O Graphene oxide was synthesized from natural graphite flakes by a modified Hummers method 30 . The 300 mg PS powder was added into 10 mL deionized water and ultrasonicated for 30 min to form uniform colloidal dispersion, followed by dropping 5 mL 0.5M G-O aqueous dispersion. The mixture was then ultrasonicated for 30 min and the excess G-O was removed by centrifugation. Finally, the matted yellow powder harvested after lyophilization was reduced by 5mL hydrazine hydrate at 95℃ for 24 hours to convert into grey powder eventually. 4. Materials Characterization The morphologies of the HOPG and PS@RG-O were examined by optical microscope (RamanMicro 200), scanning electron microscope (SEM, Hitachi S8220), transmission electron microscope (TEM, FEI Tecnai G2 F20 U-TWIN), and aberration-corrected atomic-resolution high-angle annular dark field scanning transmission electron microscopy (AC-HAADF-STEM, probe-corrected JEOL ARM200F with an acceleration voltage of 80 kV). The structure of the HOPG and PS@RG-O was characterized by Raman spectrometer (Renishaw inVia Raman spectrometer) with a 514.5 nm excitation laser and X-ray diffractometer (Bruker D2 Phaser, Cu Kα radiation γ = 1.54184 Å). The elemental composition of the PS@RG-O was measured by an X-ray photoelectron spectrometer (ESCALAB 250XI, VG Instruments, CA, USA) using a non-monochromatized Mg-Kα X-ray source. The deformation loading of HOPG was operated on horizontal stretching machine (HY-0230, Shanghai Hengyi). 5. Electrochemical Measurements All the electrochemical measurements for HOPG were performed at room temperature on a CS350H potentiostat workstation using a designed polytetrafluoroethylene (PTFE) three-electrode cell in 0.1 M KOH electrolytes. A Pt wire, a saturated Ag/AgCl electrode, and the HOPG connected with a copper wire were served as the counter, reference, and working electrode, respectively. For PS@RG-O, the electrochemical measurements were performed at room temperature on Bio-logic VMP potentiostat workstation using a standard three-electrode cell with a graphite rod, a saturated Ag/AgCl electrode, a rotating disk electrode (RDE) with a glassy carbon (GC) disk (5.0 mm diameter) as the counter, reference and working electrode, respectively. To deposit the catalyst onto a GC disk electrode, 1.0 mg of PS@RG-O catalyst was ultrasonically dispersed in 245 µL water, 245 µL isopropanol, and 10 µL 5 wt% Nafion solution for 1 h to form a uniform catalyst ink. In order to ensure that the PS@RG-O catalysts of different particle sizes have the same active surface area, the specific volume is controlled (5.00 µL for 400 nm PS@RG-O, 16.25 µL for 1.3 µm PS@G-O, and 31.25 µL for 2.5 µm PS@RG-O) onto GC electrode. Before data collection, cyclic voltammetry (CV) curves were repeatedly recorded at 50 mV s − 1 , until the signals stabilized. Then, linear sweep voltammograms (LSV) curves were measured at 10 mV s − 1 . The ORR polarization curves were corrected by subtracting the background current for the N 2 -saturated electrolyte. All electrochemical data were presented without IR compensation. All the potentials were referred to the reversible hydrogen electrode (RHE). E (RHE) = E (Ag/AgCl) + 0.0591pH + 0.197 6. Digital Image Correlation (DIC) Method Digital Image Correlation (DIC) is a non-contact optical measurement method that uses random speckle sprayed on the surface of an object to accurately match corresponding points in the speckle images before and after deformation of the object, and measure deformation displacement and other data. It can also be used to analyze the mechanical properties of objects in the deformation process, and is mainly used in the measurements of full-field displacement, deformation, amplitude, mode and other information. 7. Ncorr Ncorr 22 is an open source 2D digital image correlation MATLAB program to obtain displacement and strain fields within a region of interest (ROI) for a material sample undergoing deformation. DIC does this by taking small subsections of the reference image, called subsets, and determining their respective locations in the current configuration. The transformation of initial reference subset points to the current configuration is typically constrained to a linear, first order transformation as shown below: $${\tilde{x}}_{{cur}_{i}}={x}_{{ref}_{i}}+{u}_{rc}+\frac{\partial u}{\partial {x}_{rc}}\left({x}_{{ref}_{i}}-{x}_{{ref}_{c}}\right)+\frac{\partial u}{\partial {y}_{rc}}\left({y}_{{ref}_{j}}-{y}_{{ref}_{c}}\right)$$ $${\tilde{y}}_{{cur}_{j}}={y}_{{ref}_{j}}+{v}_{rc}+\frac{\partial v}{\partial {x}_{rc}}\left({x}_{{ref}_{i}}-{x}_{{ref}_{c}}\right)+\frac{\partial v}{\partial {y}_{rc}}\left({y}_{{ref}_{ϵSj}}-{y}_{{ref}_{c}}\right) (i,j)\in S$$ 1 where \(, {x}_{{ref}_{c}}\) and \({y}_{{ref}_{c}}\) are the x and y coordinates of an initial reference subset point, \({x}_{{ref}_{c}}\) and \({y}_{{ref}_{c}}\) are the x and y coordinates of the center of the initial reference subset, \({\tilde{x}}_{{cur}_{i}}\) and \({\tilde{y}}_{{cur}_{j}}\) are the x and y coordinates of a final current subset point, (i,j) are indices used for the relative location of the subset points with respect to the center of the subset, as well as for correspondences between subset points in the current and reference configuration, and S is a set which contains all of the subset points. The subscript "rc" is used to signify that the transformation is from the reference to the current coordinate system. The full field strains are calculated by Green-Lagrangian strain in Ncorr, which is obtained by using the four displacement gradients as shown below: \({E}_{xx}=\frac{1}{2}(2\frac{\partial u}{\partial x}+{\left(\frac{\partial u}{\partial x}\right)}^{2}+{\left(\frac{\partial v}{\partial x}\right)}^{2}\) (2) \(\) $${E}_{xy}=\frac{1}{2}(\frac{\partial u}{\partial y}+\frac{\partial v}{\partial x}+\frac{\partial u}{\partial x}\frac{\partial u}{\partial y}+\frac{\partial v}{\partial x}\frac{\partial v}{\partial y})$$ 3 $${E}_{yy}=\frac{1}{2}(2\frac{\partial v}{\partial y}+{\left(\frac{\partial u}{\partial y}\right)}^{2}+{\left(\frac{\partial v}{\partial y}\right)}^{2})$$ 4 8. Computational Details The density functional theory (DFT) calculations were performed with Vienna Ab initio Simulation Package (VASP) using a plane-wave basis set 31 – 33 . Plane-augmented wave (PAW) pseudopotential is used to describe the valence electrons-nuclei interactions 34 , 35 , while the electronic exchange and correlation effects are demonstrated within the generalized gradient approximation (GGA) as introduced by Perdew et al. 36 A 4×1×1 grid centered at the gamma (Γ) point is used as k-point sampling of the Brillioun zone using Monkhorst Pack Scheme. All calculations are nono-spin polarized and the cut off energy was set as 450 eV. Structure optimization was carried out until the force converges to 0.02 eV/Å and the energy converges to 10 − 5 eV. A series of graphene-based model structures with different kinds of defect were built, e.g. mono-vacancy, di-vacancy, and Stone-Wales. The unit cell sizes for strain-free periodical graphene nanobelt with zigzag and armchair edge were 9.85 Å × 24 Å × 18 Å and 8.60 Å×24 Å×18 Å, respectively. The periodicity of all structures is along the a-axis, and the strain was realized by changing the size of the a-axis in the unit cell. A strain range of 0.0–2.0% was studied. Declarations Data availability The data that support the findings of this study are included in the published article (and its Supplementary Information) or available from the corresponding authors on reasonable request. Acknowledgements We thank the financial support from the National key R&D Program of China (2021YFA1202802), Natural Science Foundation of China (Grants No. 12102098, 52073020), China Postdoctoral Science Foundation (Grant No. 2020M680479, 2021M690801), the Chinese Academy of Sciences, and the Australian Research Council (ARC, DP 190103881, FL 190100126 and CE230100032). Author contributions B. L. and S.X. contributed equally to this work. B.W. conceived the idea. B.W. and L.D. directed the project. B.L. mainly performed experiments while S.X. and L.Z. did DFT calculations. X.L., J.X., H.L., Z.Y., Y.G., Q.Z., S.Z., B.Z. and Z.X. contributed to the experimental and theoretical data analysis such as electrochemical measurements, strain distribution, TEM, and others. B.W., L.D. and B.L. wrote the manuscript with input from all authors. Competing interests The authors declare no competing interests. Additional information Supplementary information The Supplementary Material is available from the online version. Correspondence and requests for materials should be addressed to B.W. References Hu, C., Paul, R., Dai, Q. & Dai, L. Carbon-based metal-free electrocatalysts: from oxygen reduction to multifunctional electrocatalysis. Chem. Soc. Rev. 50 , 11785–11843 (2021). Xu, X., Liang, T., Kong, D., Wang, B. & Zhi, L. Strain engineering of two-dimensional materials for advanced electrocatalysts. Mater.Today Nano 14 , 100111 (2021). Gao, Y., Zheng, F., Wang, D. & Wang, B. Mechanoelectrochemical issues involved in current lithium-ion batteries. 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Ordered macro-microporous metalorganic framework single crystals. Science 359 , 206–210 (2018). Marcano, D. C., Kosynkin, D. V., Berlin, J. M. & Sinitskii, A. Improved Synthesis ofGraphene Oxide. ACS Nano 4 , 4806 (2010). Kresse, G. & Hafner, J. Ab initio molecular-dynamics simulation of the liquid-metal-amorphous-semiconductor transition in germanium. Phys. Rev. B. 49 , 14251–14269 (1994). Kresse, G. & Furthmiiller, J. Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set. Comput. Mater. Sci. 6 , 15–50 (1996). Kresse, G. & Furthmuller, J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B. 54 , 11169–11186 (1996). E.Blochl, P. Projector augmented-wave method. Phys. Rev. B. 50 , 17953–17979 (1994). Kresse, G. & Joubert, D. From ultrasoft pseudopotentials to the projector augmented-wave method. Phys. Rev. B 59 , 1758–1775 (1999). Perdew, J. P., Burke, K. & Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 77 , 3865–3868 (1996). Additional Declarations There is NO Competing Interest. Supplementary Files SupplementaryMaterials.docx Intrinsic Mechanical Effects on the Activation of Carbon Catalysts Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-2295214","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":157507577,"identity":"4cf93531-87ac-445b-90a6-55a5f48e1e43","order_by":0,"name":"Bin 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Wales","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Liming","middleName":"","lastName":"Dai","suffix":""}],"badges":[],"createdAt":"2022-11-21 04:20:36","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-2295214/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-2295214/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":29971391,"identity":"72bfe190-f76c-45e8-85ff-752205cfb860","added_by":"auto","created_at":"2022-12-06 15:47:13","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":214730,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSchematic illustration of the testing configuration that enables the detection of electrochemical signals when applying a tensile strain on a defined HOPG surface continuously and the strain characterization of HOPG surface by Raman spectra and optical microscopy.\u003c/strong\u003e (a) Schematic illustration of the assembly of the working electrode and the electrochemical measurement process in a modified three-electrode system in 0.1M KOH combining with a Pt wire and Ag/AgCl electrode as the counter (CE) and reference electrode (RE), respectively. (b) Evolution of the G band of HOPG under the strain. (c) Convolution and evolution of the 2D band of HOPG under the strain. (d) Correlation between the strain ε and the Raman shift of the G and 2D bands. (e) Optical microscope pictures of strained HOPG with shortened distance between two clamps from 0 to 5mm, in which the black ink dots are used as the marks to realize position-determination. (f) Strain distribution map corresponding to (e). (g) Corresponding fitted curve of clamp shortening distance and strain. Error bars indicate standard errors obtained from three or more independent replicates.\u003c/p\u003e","description":"","filename":"1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-2295214/v1/1e891890916c80edf3b9cc29.jpg"},{"id":29971392,"identity":"482ca80d-dffa-4237-921b-bb895231a159","added_by":"auto","created_at":"2022-12-06 15:47:13","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":161185,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eTensile strain effect on catalytic performance of clean-HOPG model catalysts. \u003c/strong\u003e(a) ORR results obtained for the clean-HOPG with the increased tensile strain ε from 0.0% to ~2.1%. (b) Correlation between current densities of ORR at 0.05 V \u003cem\u003eversus\u003c/em\u003e RHE and the tensile strains. (c) Correlation between overpotential of ORR and adsorption free energy of OH (ΔG(*OH)) of \u003cstrong\u003eC1-C6\u003c/strong\u003e sites shown in (d). (d) The variation of overpotential of \u003cstrong\u003eC1\u003c/strong\u003e site in (c) with strain. Inset shows the graphene model with zigzag edge.\u003c/p\u003e","description":"","filename":"2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-2295214/v1/b9fafcb97f72e6f952c131e1.jpg"},{"id":29971393,"identity":"00d53f16-0ecd-492c-97b1-862c3093b1bf","added_by":"auto","created_at":"2022-12-06 15:47:13","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":228500,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eTensile strain effect on catalytic performance of D-HOPG model catalysts.\u003c/strong\u003e (a) The HAADF-STEM image of D-HOPG. Insets show the partially zoomed-in images of various defects: mono-vacancy graphene (MV-G), di-vacancy graphene (DV-G), and Stone-Wales graphene (SW-G). (b) ORR results obtained for the D-HOPG with the increased tensile strain ε from 0.0% to ~2.4%. (c) Correlation between current densities of ORR at 0.0 - 0.7V \u003cem\u003eversus\u003c/em\u003e RHE and the tensile strain. (d) Correlation between onset potentials at 1 μA cm\u003csup\u003e−2\u003c/sup\u003e and the tensile strain of D-HOPG. The symmetry of the x axis represents the recovery process of applying strain.\u003c/p\u003e","description":"","filename":"3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-2295214/v1/68723286b2955dffeb95742c.jpg"},{"id":29971394,"identity":"cbdc7ed3-905e-4a39-a1eb-97ec86bc0b6d","added_by":"auto","created_at":"2022-12-06 15:47:13","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":722388,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eTheoretical simulation of overpotentials as a function of strain for ORR.\u003c/strong\u003e (a) Structure of graphene with various defects: MVG, DVG, and SWG. Note: G refers to the graphene nanobelt with zigzag edge carbon sites, MVG is mono-vacancy graphene, DVG is di-vacancy graphene, and SWG is Stone-Wales graphene. Red circles indicate the optimal active sites in different defect structures for ORR catalysis under strain-free conditions. (b) The volcano relationship between the adsorption energy of OH (ΔG(*OH) and the overpotential of ORR for possible active sites as labled in the figure. These sites are selected by simulating the free energy diagrams of all the carbon sites under varied strains as illustrated in (a). (c) The relationship between overpotential and strain for the SWG sites. (d) Variation of charge density with strain in the SWG structure. The red and blue in the color scale stand for the increase and decrease of the charge density respectively on the carbons for strained structure compared with the initial strain free structure.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-2295214/v1/e198929991e5204a538b3e3d.png"},{"id":29971395,"identity":"dd0ad55e-eed3-4547-a12c-df25567aa29d","added_by":"auto","created_at":"2022-12-06 15:47:13","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":271981,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSchematic illustration of the material preparation with controllable surface-strain, and the structure and performance characterization of strain-involved PS@RG-O carbon catalysts.\u003c/strong\u003e (a) Schematic illustration of the PS@RG-O carbon catalysts with different curvature-induced strain. (b) Scanning electron microscopy (SEM), TEM, and HRTEM images of the 1.3 μm PS@RG-O. (c) X-ray diffraction (XRD) patterns. (d) Raman spectra. (e) LSV curves at 1600 rpm. (f) Plots of current densities (taken at 1.025 V \u003cem\u003evs\u003c/em\u003e. RHE) as a function of scan rates in 0.1 M KOH. (g) Current densities normalized by electrochemical active surface area (ECSA) at 0.60 V \u003cem\u003evs\u003c/em\u003e. RHE of various PS@RG-O carbon catalysts.\u003c/p\u003e","description":"","filename":"5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-2295214/v1/0119011f376167ab1fb6714a.jpg"},{"id":47629620,"identity":"9c388191-4f56-473f-b4e0-dc3c7b6bbe3c","added_by":"auto","created_at":"2023-12-05 10:34:12","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1962496,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-2295214/v1/448e60eb-20bf-4ca0-bb31-40338dee18f2.pdf"},{"id":29971396,"identity":"80e9d511-ecf9-4cbd-919c-0c7992547256","added_by":"auto","created_at":"2022-12-06 15:47:16","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":51131254,"visible":true,"origin":"","legend":"\u003cp\u003eIntrinsic Mechanical Effects on the Activation of Carbon Catalysts\u003c/p\u003e","description":"","filename":"SupplementaryMaterials.docx","url":"https://assets-eu.researchsquare.com/files/rs-2295214/v1/8b8782db1661568c864fcff5.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Intrinsic Mechanical Effects on the Activation of Carbon Catalysts","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eThe electrochemical application is inherently interdisciplinary crossing chemistry, electronics, materials, and even mechanics\u003csup\u003e\u003cspan additionalcitationids=\"CR2\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e. For example, strain engineering is emerging as an efficient tool to modulate the catalytic activity of metallic materials when being used in reactions, such as oxygen reduction reaction (ORR)\u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e,\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e, hydrogen evolution reaction (HER)\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e, and others\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e,\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e. In contrast to metals, the mechanical effects on carbon-based metal-free catalysts (C-MFCs) have rarely been explored although C-MFCs have attracted worldwide interest as alternatives to the noble metal catalysts\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e. On the one hand, identification of the mechanical effects on C-MFCs is critical as it directly works on the coordination of neighboring carbon atoms and thus the electronic states (a key factor affecting the catalytic activity), but the clarification is strongly limited by the fact that the mechanical effect usually intermingles with other structural variables, including the dopants, defects, and interfaces\u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e. On the other hand, applying strain to the C-MFCs accurately and continuously is a big challenge because the strain in metals could be inputted by alloying and read out by referring to the lattice variation, which is not feasible in pure carbon materials (e.g., graphene). A delicate design has been proposed to study the mechanical effect on the HER properties of MoS\u003csub\u003e2\u003c/sub\u003e by deforming these two-dimensional (2D) materials on a patterned gold nanocone support, but the strain cannot be applied in a continuous and uniform manner\u003csup\u003e\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u003c/sup\u003e. To our best knowledge, the continuous application and accurate detection of strain and its electrochemical effects on carbon-based catalysts haven\u0026rsquo;t been achieved to date. Therefore, the intrinsic mechanical effects on the catalytic behaviors of a pure carbon material remain unknown.\u003c/p\u003e \u003cp\u003eHerein, we report a proof-of-concept study using highly oriented pyrolytic graphite (HOPG) as the model catalyst. HOPG is selected due to its well-defined π conjugation, defect/dopant-less structure, flat surface, and ordered stacking state\u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e. Note that we have attempted to use single-layer graphene films (chemical vapor deposition grown, CVD) as the model but failed because the transfer process was usually not defect-free and, more critically, a conductive, elastic, and catalytic-inert substrate was required to support graphene in this experiment, which limited its feasibility and caused more uncertainties. Based on a horizontal drawing/compressing machine and an electrochemical station, we established a platform to apply strain to the exfoliated HOPG lamina continuously and collected the electrochemical signals simultaneously (ORR in this work). \u003cem\u003eFor the first time, the correlation between the surface tensile strain (ε) of a graphitic carbon and its ORR activation effect was established.\u003c/em\u003e It was found that there is a negligible correlation between the applied strain and ORR performance in the strain range from \u0026minus;\u0026thinsp;1.0\u0026ndash;2.0% (\u0026lsquo;-\u0026rsquo; refers to compressive strain, otherwise the tensile strain), in which only\u0026thinsp;~\u0026thinsp;13.0% improvement in the reduction current was achieved by applying ~-0.8% compressive strain on a fresh HOPG surface. In the case of a defective HOPG surface without other elemental dopants, a repeatable\u0026thinsp;~\u0026thinsp;35.0% improvement of current density was observed when applying a\u0026thinsp;~\u0026thinsp;0.6% tensile strain. We further explored an in-depth understanding of the relationship between tensile strain and ORR performance based on density function theory (DFT) calculations. Both adsorption sites of reaction intermediates and charge redistribution were found related with the strain, and the Stone-Wales defect was identified as the site that correlates with the mechanical effect strongly due to the force induced lattice variation, and thus charge redistribution. Finally, a graphene-wrapped spherical powder material with varied radius, and thus different surface strain, was prepared and tested. Apparent improvement of the ORR activity over 100% was observed with ~\u0026thinsp;0.4% surface strain in graphene, suggesting the significance and potential of clarifying the mechanical effect on the catalytic performance of the C-MFCs. This work could open up an entirely new way of developing functional carbon materials, including carbon catalysts.\u003c/p\u003e"},{"header":"2. Results And Discussion","content":"\u003cdiv class=\"Section2\" id=\"Sec3\"\u003e\n \u003ch2\u003e2.1 Accurate detection of strain applied on the surface of HOPG\u003c/h2\u003e\n \u003cp\u003eA freshly exfoliated HOPG lamina is sandwiched between a polycarbonate (PC) substrate and an elastic mask (a PI tape, polyimide) with a hole in the center that is used to define the HOPG surface area exposed to the electrolyte (Fig. 1a). We have tried to stretch this assembly in the electrolyte using the drawing machine to induce the tensile strain, but the detachment between the mask and HOPG occurred when the strain read by the machine exceeded\u0026thinsp;~\u0026thinsp;1.0%. Alternatively, the compression mode of the machine was used to bend the assembly, which caused the surface stretching of HOPG that exposed to the electrolyte. In this case, the detachment was avoid obviously, and the degree of deformation was much higher. Detailed experimental information can be found in the \u003cstrong\u003eMethods\u003c/strong\u003e section.\u003c/p\u003e\n \u003cp\u003eThe next problem needs to be solved is the accurate detection of the surface strain applied to HOPG. There are several reported methods to extract the strain of a 2D material, such as the measurement of the lattice difference by transmission electron microscopy (TEM), the visualization of the geometric deformation of the supporting substrate, and the band variation in Raman spectra. The TEM method is typically localized in a range of few nanometers that cannot reflect the entire view of the strain distribution across hundreds of micrometers. We then used the substrate geometric deformation method to extract the strain applied on the HOPG as shown in \u003cstrong\u003eFigure S1\u003c/strong\u003e (see the Supplementary Materials, \u003cstrong\u003eTable S1\u003c/strong\u003e). In the literature the deformation of the substrate is usually considered as that of the sample on the surface. However, in our case, the results are not convincing as it exceeded 20.0% without damaging the HOPG surface as observed using the optical microscope. Note that the theoretical maximum strain of graphene is around 20.0%\u003csup\u003e13\u003c/sup\u003e, and the experimental value is much lower, ~\u0026thinsp;1.0%\u003csup\u003e14\u003c/sup\u003e. Since the HOPG we used is about 10 \u0026micro;m in thickness, there are thousands of graphene layers stacking on the PC substrate. The interlayer slippage results in a weak force delivery from the PC substrate to the top HOPG surface, so we conclude that the geometric deformation method is not applicable in this experiment.\u003c/p\u003e\n \u003cp\u003eRaman spectroscopy has been used to identify the number of layers and the information about doping, edges, defects, and disorders of graphene. Given that strain can effectively modify the electronic structure of graphene and soften the optical-phonon branches, it is expected to induce variations in Raman spectra\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e15\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e. In contrast to the Raman shift changes of single-layer graphene under uniaxial or biaxial stretching, the study of strain effects on the Raman spectrum of HOPG is still missing. \u003cstrong\u003eFigure S2\u003c/strong\u003e shows the Raman spectra of HOPG under the uniaxial strain from 0.0\u0026ndash;3.1%. The absence of D band at ~\u0026thinsp;1350 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e corresponding to defects indicates the high-quality HOPG surface. The red shifts of G (~\u0026thinsp;1580 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and 2D (~\u0026thinsp;2700 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) bands are observed in Fig.\u0026nbsp;1b-c under the increased tensile strain due to the softening of the E\u003csub\u003e2g\u003c/sub\u003e phonon associated with the G band and the TO phonon between the \u0026Gamma; and K points associated with the 2D band\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e. The fitted relationship of \u0026omega;/G and \u0026omega;/2D \u003cem\u003eversus\u003c/em\u003e \u0026epsilon; are plotted in Fig. 1d. The change of Raman shift of HOPG during the stretching process is thus established, and it is found that the Raman shift offset is much smaller than the single-layer graphene reported in the literature possibly due to the interlayer interactions\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e. As the strain increases from 0.0\u0026ndash;3.1%, the G and 2D shifts are found to be reduced from 1579.8 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e to 1579.0 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, and from 2725.5 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e to 2723.0 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, respectively. Although the relationship between the Raman signals and the strain on HOPG can be established, the indistinctive variations and particularly the absence of a standard relationship from the literature indicate that this method could not identify the strain values confidently.\u003c/p\u003e\n \u003cp\u003eInspired by the standard tensile testing process, we finally used the digital image correlation (DIC) method\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e19\u003c/span\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e to map the strain field on the surface of HOPG. Specifically, a certain amount of ink was sprayed on the HOPG surface as the positioning mark, that is, the relative position of the ink dots changes with the deformation of the HOPG surface and then the actual strain on the surface can be clearly measured combined with Ncorr software\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e (Fig. 1e-f). The correspondence between the as-measured surface strain (\u0026epsilon;) and the actual movement of the machine clamps (\u0026Delta;L) was thus established (Fig. 1g). In detail, the optical microscope was used to observe the relative positions of the ink marks when the PC substrate was bent due to the shortening of the clamp spacing (Fig. 1e). The images positioning and program recognition were performed to depict strain distribution map with the color variation (Fig. 1f), and the details about the program recognition are shown in the \u003cstrong\u003eMethods\u003c/strong\u003e section. Note that the \u0026lsquo;inhomogeneous\u0026rsquo; yellow color distribution is normally caused by the formation of shear bands at ~\u0026thinsp;45\u0026deg; with respect to the stretch axis by the neighbouring ink marks, which will not affect the strain distribution in a clean HOPG sample that was used for the mechanical-electrochemical investigation in this work\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e23\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e. Thus, the corresponding fitted curve for the clamp shortening distance (\u0026Delta;L) and the calculated average strain (\u0026epsilon;) was obtained, as shown in Fig. 1g. With the gradual change of the distance between the clamps, the surface of the HOPG deformed accordingly. It is noted that when \u0026Delta;L is 6 mm, the strain reached a maximum of 3.4%. The HOPG surface was vulnerable to destruction with further bending, which would affect the observation of strain, so 3.4% was controlled as the threshold of tensile strain in this study. Moreover, we have used the same method to characterize the compressive strain after changing the bending direction of the PC substrate. The specific topography pictures are shown in \u003cstrong\u003eFigure S3\u003c/strong\u003e. However, due to the squeezing effect, the HOPG surface is peculiarly prone to crack during the compression process, so the threshold of the compressive strain was set as low as -0.8%, below which the HOPG surface was well preserved.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv class=\"Section2\" id=\"Sec4\"\u003e\n \u003ch2\u003e2.2 Mechanical effect on the catalytic performance of clean-HOPG\u003c/h2\u003e\n \u003cp\u003eThe dependence of the ORR activity of the pure C-MFCs on the strain was firstly investigated using the clean-HOPG, which is highly regular with limited defects (\u003cstrong\u003eFigures S2 and S4\u003c/strong\u003ea). The ORR catalytic performance was measured by linear sweep voltammetry (LSV) in alkaline electrolyte (0.1 M KOH). All electrochemical data were presented without iR correction. Figure \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ea shows the ORR curves obtained under O\u003csub\u003e2\u003c/sub\u003e-saturated conditions with the subtraction of data under N\u003csub\u003e2\u003c/sub\u003e-saturated conditions as the background, in which the currents are divided by the geometric surface area of the hole in the mask (~\u0026thinsp;3.14 mm\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e) (the same as the exposed HOPG catalyst surface areas). Before testing the strain effects, a repeated LSV measurement over 20 cycles was conducted to ensure the curves were in complete agreement and thus getting rid of the possible influences from the testing conditions (\u003cstrong\u003eFigure S5\u003c/strong\u003e). Results show that the ORR activities varied under different tensile strain conditions. The current densities at a specific potential (\u003cem\u003eversus\u003c/em\u003e RHE, reversible hydrogen electrode) were extracted and plotted versus the corresponding tensile strain in Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003eb. As the applied tensile strain increases, the current density under different specific potential decreases gradually until reaching the limit at ~\u0026thinsp;2.0% strain, indicating the slight inhibition of ORR activity. The onset potential (potential \u003cem\u003eversus\u003c/em\u003e RHE at current density of 1 \u0026micro;A cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e), representing the intrinsic activity of catalyst, also demonstrated a negligible negative correlation relative to the tensile strain (\u003cstrong\u003eFigure S6\u003c/strong\u003e), corresponding to the enlargement of the overpotential. In contrast, it is found that the compressive strain shows an opposite promotion effect on the ORR activity, for which the current density increases along with increasing compressive strain. Since the sample was more prone to surface fragmentation when being subjected to the compressive strain, however, we can only control the threshold of compressive strain at ~\u0026thinsp;0.8% without further exploration (\u003cstrong\u003eFigure S7\u003c/strong\u003e). In brief, in the case of a clean-HOPG surface (there should also be many graphene edges on the HOPG surface when the tested area is across mm\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e scale), there is no obvious correlation between its ORR catalytic activity and the tensile strain, while the compressive strain was observed to promote the activity slightly.\u003c/p\u003e\n \u003cp\u003eIt should be noted that although surface defects were rare in the HOPG model material, edges still existed on the surface of HOPG. Therefore, the strain effects could be ascribed to the variation of the graphene skeleton (in-plane structure), or the graphene edges, or a combination of both structures. A theoretical simulation was then used to analyze the relationship of the strain and the ORR activity. A pure graphene nanobelt model (G) without defects was established (Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ed) and the adsorption capacity of the intermediates *OOH, *O, and *OH was studied by DFT method. The free energy of adsorption was used as the descriptor to characterize the catalytic activity of different carbon sites (\u003cstrong\u003eC1-C6\u003c/strong\u003e) from edge to in-plane in the G with zigzag edge. In Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ec, it shows a volcano relationship between the overpotential of ORR and the adsorption energy of *OH (\u0026Delta;G(*OH)). Accordingly, the sites on the edges (\u003cstrong\u003eC1\u003c/strong\u003e) are the optimal active sites, which have lower overpotential than those sites picked up in the middle of the graphene. Note that the carbon sites on the armchair edge were not used because of the large overpotentials caused by the adsorption of *OOH (\u003cstrong\u003eFigure S8\u003c/strong\u003e).\u003c/p\u003e\n \u003cp\u003eThe free energy diagrams for the ORR at \u003cstrong\u003eC1\u003c/strong\u003e site show that the rate-determining step on G is the desorption of *OH (\u003cstrong\u003eFigure S9\u003c/strong\u003e), where the energy involved was taken as the overpotential. From the overpotential \u003cem\u003evs\u003c/em\u003e. strain plots in Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ed, a negligible negative correlation between tensile strain and ORR performance is observed although the effect is extremely weak and can be ignored, which is consistent with the experimental results. Moreover, the effect of compressive strain extended to -5.0% on the catalytic performance was also modulated in \u003cstrong\u003eFigure S10.\u003c/strong\u003e Interestingly, an optimized compressive strain of ~ -1.2% is found beneficial to the catalysis, but overpotential increases dramatically with further increase of the compressive strain. Our experiments verified such an effect partially by applying the compressive strain to -0.8%, although further compression was inhibited by the surface fragmentation of HOPG under large deformations, which may be solved in the future by establishing another platform to test the mechanoelectrochemcial effects of ultrathin and flexible graphene samples.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv class=\"Section2\" id=\"Sec5\"\u003e\n \u003ch2\u003e2.3 Mechanical effect on the catalytic performance of defective-HOPG\u003c/h2\u003e\n \u003cp\u003eAs can be seen from above, the response to strain is relatively feeble for HOPG with a smooth surface and edges. In contrast to the edge defects, the in-plane defects have been demonstrated to obviously promote ORR catalytic activities. In this regard, we treated the HOPG with an Ar plasma for 5 mins to produce the in-plane defects. The Raman spectrum (\u003cstrong\u003eFigure S11\u003c/strong\u003e) shows that the defects on the HOPG surface have increased significantly after the Ar plasma treatment, which were observed in the aberration-corrected atomic-resolution high-angle annular dark field scanning transmission electron microscopy (AC-HAADF-STEM) images (Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ea and S4). LSV curves shown in Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eb exhibit the dependence of the ORR activity for the defective HOPG (named D-HOPG) on the tensile strain (\u0026epsilon;). It is obvious that the D-HOPG showed enhanced catalytic activity with higher current density and more positive onset potential compared to the clean-HOPG. More interestingly, the strain effect on the D-HOPG exhibits a classic volcano relationship, that is, with the gradual increase of strain, the D-HOPG firstly showed gradually enhanced ORR activity until the strain up to ~\u0026thinsp;0.5%, followed by a gradual decrease and then leveled off. To be more intuitive, the current densities at various specific potentials (\u003cem\u003eversus\u003c/em\u003e RHE) were extracted and plotted \u003cem\u003eversus\u003c/em\u003e the corresponding tensile strain in Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ec. The strain effect was found to be recoverable and the similar behavior was observed under all the potentials (from 0.0V to 0.7V \u003cem\u003eversus\u003c/em\u003e RHE), suggesting the restorability of the HOPG structure under the testing conditions and thus the reliability of the observed strain effects. The volcano relationship was also obtained by plotting the onset potentials at 1 \u0026micro;A cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e \u003cem\u003evs\u003c/em\u003e. the strain (Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ed). As a result, a\u0026thinsp;~\u0026thinsp;35.0% current improvement was realized with ~\u0026thinsp;0.6% tensile strain in the case of D-HOPG, verifying the great promoting ability of strain to the ORR activity of the defective carbon. We have also tried to measure the compressive strain effects on D-HOPG, but the sample became more brittle after inducing defects than the clean-HOPG samples, severely limiting the compression range that could be tested so that no convincing result for the compression deformation could be obtained through this testing method.\u003c/p\u003e\n \u003cp\u003eThe underlying mechanism for the strain-catalytic activity relationship on defected HOPG was then investigated by DFT simulations. Firstly, various graphene structures with different defects were created, including the previously discussed clean-graphene with zigzag edges (G), mono-vacancy graphene (MVG), di-vacancy graphene (DVG), and Stone-Wales graphene (SWG), as shown in Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ea. Then, we simulated the ORR processes to identify the possible active sites for each structure and calculated the corresponding overpotential value. \u003cstrong\u003eFigure S12\u003c/strong\u003ea shows the free energies and ORR reaction pathways at the optimal active site (marked with a red circle in Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ea) on each structure under strain-free condition, where MVG has the lowest overpotential (0.53V) while DVG has the highest (0.74 V).\u003c/p\u003e\n \u003cp\u003eThe strain effect simulation was then conducted by changing the size of the a-axis direction of each structural unit cell. For example, the a-axis size of the unit cell in the pristine structure is 9.85 \u0026Aring;, which is increased to 9.90 \u0026Aring; when the strain is 0.5%. By simulating the free energy diagrams of the optimal active sites as mentioned above under varied strain conditions (G-1, MVG-5, DVG-1, and SWG-5 under strain of 0.0%, 0.5%, 1.0%, 1.5%, and 2.0%, respectively), it was found there is a linear relationship between adsorption free energy of *OOH and *OH during the ORR process (Figure S12b). We then used the adsorption energy of *OH as the descriptor to characterize the catalytic activity of these graphene-based structures.\u003c/p\u003e\n \u003cp\u003eThe free energy diagrams of all the carbon sites shown in Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ea were simulated, and only the possible active sites under suitable strains are involved in Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003eb to compare their catalytic activities. In the case of MVG and DVG, the optimal sites are MVG-5 and DVG-1, respectively, and the free energy diagrams of which show very limited variation by changing the strain values (Figure S12c-d). Also, other sites in MVG and DVG show almost negligible response to the strain (Figure S12e). However, the catalytic activities of SWG sites show significant responses to the applied strains (\u003cstrong\u003eFigure S13\u003c/strong\u003e).\u003c/p\u003e\n \u003cp\u003eAccording to the volcano-shaped relationship of the overpotentials of the ORR v\u003cem\u003es.\u003c/em\u003e \u0026Delta;G(*OH) as established in Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003eb, we can identify that the catalytic activity reaches the volcanic peak by adjusting the strain in SWG sites to be 0.5% for SWG-3 and SWG-6. Specifically, the comparison of the overpotential of each site under different strain (Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ec) show SWG-3, SWG-4, SWG-6, and SWG-9 share the similar trend that the overpotential decreases firstly and then increases along with increasing strain, except for the SWG-5 that has the lowest initial overpotential but keeps rising later. The minimum overpotential of 0.42 V is achieved at the strain of 0.5% for SWG-3, while SWG-4 and SWG-6 also have values of 0.52 V and 0.43 V at the strain of 0.5%, respectively, which are lower than the best initial overpotential of 0.53 for MVG-5. These results are in good agreement with the experimental data.\u003c/p\u003e\n \u003cp\u003eTo gain the mechanistic understanding of the influence of strain on the catalytic activity of SWG, we compared the free energies and reaction pathways shown in Figure S13 and found that the change of the rate-limiting step is perhaps one of the responsible factors. The reaction rate is limited by the last step for almost all the SWG sites (desorption of *OH to form H\u003csub\u003e2\u003c/sub\u003eO) in the initial structure, which turns to the first step (adsorption of *OOH) when the strain passes 0.5% (1.0% for SWG-9). However, for the sites (SWG-5 and SWG-8) whose first step is the rate-limiting step in the initial structure, the energy barrier continues to increase with the increase of strain, making the catalytic activity worse. To understand this observation, we modulated the charge density variation before and after applying strain (\u003cstrong\u003eFigure S14\u003c/strong\u003e, the one for SWG is also moved to Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ed). As shown in the initial structure of SWG, the carbon sites generally have low charge density, which means more positive charges for the adsorption of *OH inhibiting the desorption to form H\u003csub\u003e2\u003c/sub\u003eO. When the strain is increased to 0.5%, the charge density increases and thus the adsorption of *OH is weakened, corresponding to the promoted catalytic performance. An apparent increase trend of the charge density is observed along with the stretching process for SWG, which refers to the weakening interaction of carbon sites and the negatively charged reaction intermediates, including *OH, and *OOH. The weak adsorption of *OOH makes the first reaction step as the rate-limiting step and increases the overpotential. Like SWG-5 and SWG-8 whose first step is the rate-limiting, overpotential of MVG-5 increases when the charge density increases by applying strain (Figure S12c and Figure S13). However, since the charge density changes very little, the effect on the overpotential of MVG-5 is almost negligible. The charge density around DVG-1 decreases slightly with the application of strain, leading to the enhancement of *OH adsorption, but the effect is minimal (Figure S12d). For other sites (MVG-6, DVG-3 and DVG-6), their overpotentials are higher than those for the aforementioned sites (MVG-5 and DVG-1) and could not become ideal active sites, although they decrease gradually with the application of strain (Figure S12e). Therefore, the applied strain affects the charge density distribution of SWG obviously, modifies the interaction with the reaction intermediates, alters the rate-determining steps and the energy barriers, and thus endows SWG with the highest catalytic activity. As a common defect type, the activation of Stone-Wales defects could boost the catalytic efficiency of defective carbon materials by waking up more inert sites, which opens a new way to developing C-MFCs.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv class=\"Section2\" id=\"Sec6\"\u003e\n \u003ch2\u003e2.4 Carbon powder catalyst with intrinsic strain\u003c/h2\u003e\n \u003cp\u003eTo demonstrate and investigate the aforementioned strain-induced catalytic activity for carbon materials of practical significance, we present here a spherical diameter engineering strategy, in which various PS@RG-O (PS: polystyrene, RG-O: reduced graphene oxide) carbon powder catalysts with different curvature-induced strain were prepared. Figure \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ea depicts the preparation of the PS@RG-O carbon catalysts. Briefly, the G-O dispersion was added dropwise into an aqueous dispersion of PS nanospheres to form the surface coating via the electrostatic and \u0026pi;-\u0026pi; / interactions (see details in \u003cstrong\u003eMethods\u003c/strong\u003e). Hydrazine hydrate (N\u003csub\u003e2\u003c/sub\u003eH\u003csub\u003e4\u003c/sub\u003e\u0026middot;H\u003csub\u003e2\u003c/sub\u003eO) was used to reduce the material to RG-O, which was verified by the decreased O1s intensity as shown in the X-ray photoelectron spectroscopy (XPS) surveys (\u003cstrong\u003eFigure S15\u003c/strong\u003e)\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e. Nitrogen was also observed in the spectra, suggesting the sample is an O/N co-doped RG-O material (\u003cstrong\u003eTable S2\u003c/strong\u003e). Note that the reduction of G-O at high temperature usually results in much purer graphene samples with less hetero-dopants, but it is not applicable in our experiment due to the thermal-decomposition of PS and thus structural collapse.\u003c/p\u003e\n \u003cp\u003eBy changing the PS spheres with varied spherical diameter (400 nm, 1.3 \u0026micro;m, and 2.5 \u0026micro;m, \u003cstrong\u003eFigure S16\u003c/strong\u003e), the curvature-induced strain (0.38%, 0.12%, and 0.06%, respectively) of the RG-O on PS surface can be realized. The first two images in Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003eb show the 1.3 \u0026micro;m PS@RG-O spheres, which have a regular spherical shape with smooth surface and homogeneous graphene coating. The high-resolution transmission electron microscopy (HRTEM) image exhibits that the plane of RG-O coating is continuous along the curvy edge with shell thickness about\u0026thinsp;~\u0026thinsp;3 nm containing\u0026thinsp;~\u0026thinsp;8 graphene layers\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e, and the thickness of coating RG-O is approximately consistent with that of 400 nm and 2.5 \u0026micro;m PS@RG-O (\u003cstrong\u003eFigure S17\u003c/strong\u003e). The typical X-ray diffraction (XRD) patterns of n-PS@RG-O (n\u0026thinsp;=\u0026thinsp;400 nm, 1.3 \u0026micro;m, and 2.5 \u0026micro;m) show broad diffraction peaks at ~\u0026thinsp;10.8\u0026deg; and ~\u0026thinsp;20.0\u0026deg; (Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ec), and the Raman spectra show apparent D (1358 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and G (~\u0026thinsp;1594 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) bands for graphene (Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ed). These results further prove the successful preparation of RG-O coated PS-sphere samples\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e25\u003c/span\u003e 27\u003c/sup\u003e.\u003c/p\u003e\n \u003cp\u003eThe ORR performance of the n-PS@RG-O was evaluated by LSV measurements carried out on a rotating disk electrode (RDE) using a three-electrode system in an O\u003csub\u003e2\u003c/sub\u003e saturated 0.1 M KOH. The ORR activity of the n-PS@RG-O was found to be strongly correlated with the PS diameters. As shown in Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ee, the LSV measurements exhibit an increased trend for the diffusion-limiting current density with increasing curvature from n\u0026thinsp;=\u0026thinsp;2.5 \u0026micro;m to 500 nm. However, the surface area is also a key parameter to consider. In order to study the effect of curvature-induced strain on the ORR catalytic activity more convincingly, the electrochemical double-layer capacitance (C\u003csub\u003edl\u003c/sub\u003e), which is positively proportional to electrochemical active surface area (ECSA), was obtained from cyclic voltammetry (CV) curves in a non-faradic potential range (\u003cstrong\u003eFigures S18\u003c/strong\u003e and 5f)\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e. ECSA represents the effective area that the catalysts work in the catalytic process, as a factor positively related to electrochemical performance. After normalization of the current density j at 0.60V (\u003cem\u003evs\u003c/em\u003e. RHE) by ECSA, the activity of n-PS@RG-O still decrease distinctly as the diameter of PS spheres increases (Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003eg), suggesting that the difference in current density was not caused by the variation of the surface areas. Due to the inevitable defects in chemically reduced graphene oxide, the strain-induced enhancement of oxygen reduction activity of defective graphene could thus be expected. These results reflect that the increased curvature-induced strain of n-PS@RG-O is most probably one of the key reasons for the observed enhancement in the ORR performance.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"3. Conclusion","content":"\u003cp\u003eIn summary, we have performed a proof-of-concept study to explore the mechanical effects on pure carbon catalysts. Using HOPG as the model catalyst, we established a platform to apply strain to the HOPG continuously and collected the electrochemical signals simultaneously, thus realized the ORR activity characterization of HOPG lamina under a controllable/continuous strain deformation. For the first time, the correlation between the surface tensile strain (ε) of a graphitic carbon and its ORR activation effect was established experimentally and theoretically. It was found that there was a negligible negative correlation between tensile strain and ORR performance for a clean HOPG lamina in the strain range from \u0026minus;\u0026thinsp;1.0\u0026ndash;2.0% with only\u0026thinsp;~\u0026thinsp;13.0% improvement by applying ~-0.8% compressive strain. In the case of a defective HOPG lamina, however, a great current density boost of ~\u0026thinsp;35.0% was observed when ~\u0026thinsp;0.6% tensile strain was applied on the surface. Combined experimental and theoretical studies indicate that the initial inert sites, the Stone-Wales defects, can be transformed into optimal active sites by applying tensile strain of ~\u0026thinsp;1.0%, which was realized by reducing the reaction energy barrier and altering the rate-determination steps. Finally, the relationship between the strain modulation and catalytic activity in carbon materials for ORR was authenticated by a spherical diameter engineering strategy with carbon powder catalysts, and an apparent improvement of the ORR activity was achieved with ~\u0026thinsp;0.4% surface strain in graphene. This work clarifies the intrinsic strain effects on the electrochemical catalytic performance for C-MFCs, which lays the foundation for mechanistic understanding of some complex catalytic reactions and triggers the invention of new concepts for carbon catalysis.\u003c/p\u003e "},{"header":"Methods","content":"\n\u003ch3\u003e1. Assembly Of The Bendable HOPG Configuration\u003c/h3\u003e\n\u003cp\u003eFigure 1a depicts the preparation of the bendable HOPG assembly. We firstly peeled off a piece of HOPG squared in 0.5 \u0026times; 0.5 cm with the mechanical peeling tape purchased from Minnesota Mining and Manufacturing company, and then stuck it on the sticky PC substrate, keeping the smooth HOPG side up. Then a copper wire was bonded to the edge of the HOPG surface using the conductive silver glue. Finally, a PI tape with a 2 mm diameter hole was pasted on the smooth surface of the HOPG to expose a fixed area.\u003c/p\u003e\n\u003ch3\u003e2. Preparation Of Polystyrene Spheres (PS)\u003c/h3\u003e\n\u003cp\u003eThe polystyrene spheres with three different sizes (PS-a: ~400 nm, PS-b: ~1.3 \u0026micro;m, PS-c: ~2.5 \u0026micro;m) were synthesized according to the reported procedure with some modifications\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e. In a typical synthesis of 400 nm PS spheres as an example, 50 mL of styrene was first washed thoroughly with 50 mL of 10 wt. % NaOH solution and deionized water successively to remove the stabilizer. Then, the washed styrene was added to a triple-neck, 500 mL round-bottomed flask with 200 mL water containing 0.25 mg of PVP. After bubbling with nitrogen for 15 min, the mixture was then fluxed at 95℃ under magnetic stirring for 30 min. Subsequently, 20 mL of aqueous solution containing 0.75 mg K\u003csub\u003e2\u003c/sub\u003eS\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003e was added quickly into the flask to initiate the polymerization of styrene. After stirring at 400 rpm for 24 h at 95℃ temperature, the mixture was cooled down. Then the obtained milk-like product was centrifuged three times with ethanol and lyophilized to obtain the PS nanospheres powder.\u003c/p\u003e\n\u003ch3\u003e3. Preparation Of PS@RG-O\u003c/h3\u003e\n\u003cp\u003eGraphene oxide was synthesized from natural graphite flakes by a modified Hummers method\u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e. The 300 mg PS powder was added into 10 mL deionized water and ultrasonicated for 30 min to form uniform colloidal dispersion, followed by dropping 5 mL 0.5M G-O aqueous dispersion. The mixture was then ultrasonicated for 30 min and the excess G-O was removed by centrifugation. Finally, the matted yellow powder harvested after lyophilization was reduced by 5mL hydrazine hydrate at 95℃ for 24 hours to convert into grey powder eventually.\u003c/p\u003e\n\u003ch3\u003e4. Materials Characterization\u003c/h3\u003e\n\u003cp\u003eThe morphologies of the HOPG and PS@RG-O were examined by optical microscope (RamanMicro 200), scanning electron microscope (SEM, Hitachi S8220), transmission electron microscope (TEM, FEI Tecnai G2 F20 U-TWIN), and aberration-corrected atomic-resolution high-angle annular dark field scanning transmission electron microscopy (AC-HAADF-STEM, probe-corrected JEOL ARM200F with an acceleration voltage of 80 kV). The structure of the HOPG and PS@RG-O was characterized by Raman spectrometer (Renishaw inVia Raman spectrometer) with a 514.5 nm excitation laser and X-ray diffractometer (Bruker D2 Phaser, Cu Kα radiation γ\u0026thinsp;=\u0026thinsp;1.54184 \u0026Aring;). The elemental composition of the PS@RG-O was measured by an X-ray photoelectron spectrometer (ESCALAB 250XI, VG Instruments, CA, USA) using a non-monochromatized Mg-Kα X-ray source. The deformation loading of HOPG was operated on horizontal stretching machine (HY-0230, Shanghai Hengyi).\u003c/p\u003e\n\u003ch3\u003e5. Electrochemical Measurements\u003c/h3\u003e\n\u003cp\u003eAll the electrochemical measurements for HOPG were performed at room temperature on a CS350H potentiostat workstation using a designed polytetrafluoroethylene (PTFE) three-electrode cell in 0.1 M KOH electrolytes. A Pt wire, a saturated Ag/AgCl electrode, and the HOPG connected with a copper wire were served as the counter, reference, and working electrode, respectively. For PS@RG-O, the electrochemical measurements were performed at room temperature on Bio-logic VMP potentiostat workstation using a standard three-electrode cell with a graphite rod, a saturated Ag/AgCl electrode, a rotating disk electrode (RDE) with a glassy carbon (GC) disk (5.0 mm diameter) as the counter, reference and working electrode, respectively. To deposit the catalyst onto a GC disk electrode, 1.0 mg of PS@RG-O catalyst was ultrasonically dispersed in 245 \u0026micro;L water, 245 \u0026micro;L isopropanol, and 10 \u0026micro;L 5 wt% Nafion solution for 1 h to form a uniform catalyst ink. In order to ensure that the PS@RG-O catalysts of different particle sizes have the same active surface area, the specific volume is controlled (5.00 \u0026micro;L for 400 nm PS@RG-O, 16.25 \u0026micro;L for 1.3 \u0026micro;m PS@G-O, and 31.25 \u0026micro;L for 2.5 \u0026micro;m PS@RG-O) onto GC electrode. Before data collection, cyclic voltammetry (CV) curves were repeatedly recorded at 50 mV s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, until the signals stabilized. Then, linear sweep voltammograms (LSV) curves were measured at 10 mV s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The ORR polarization curves were corrected by subtracting the background current for the N\u003csub\u003e2\u003c/sub\u003e-saturated electrolyte. All electrochemical data were presented without IR compensation. All the potentials were referred to the reversible hydrogen electrode (RHE). E (RHE)\u0026thinsp;=\u0026thinsp;E (Ag/AgCl)\u0026thinsp;+\u0026thinsp;0.0591pH\u0026thinsp;+\u0026thinsp;0.197\u003c/p\u003e\n\u003ch3\u003e6. Digital Image Correlation (DIC) Method\u003c/h3\u003e\n\u003cp\u003eDigital Image Correlation (DIC) is a non-contact optical measurement method that uses random speckle sprayed on the surface of an object to accurately match corresponding points in the speckle images before and after deformation of the object, and measure deformation displacement and other data. It can also be used to analyze the mechanical properties of objects in the deformation process, and is mainly used in the measurements of full-field displacement, deformation, amplitude, mode and other information.\u003c/p\u003e\n\u003ch3\u003e7. Ncorr\u003c/h3\u003e\n\u003cp\u003eNcorr\u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e is an open source 2D digital image correlation MATLAB program to obtain displacement and strain fields within a region of interest (ROI) for a material sample undergoing deformation. DIC does this by taking small subsections of the reference image, called subsets, and determining their respective locations in the current configuration. The transformation of initial reference subset points to the current configuration is typically constrained to a linear, first order transformation as shown below:\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$${\\tilde{x}}_{{cur}_{i}}={x}_{{ref}_{i}}+{u}_{rc}+\\frac{\\partial u}{\\partial {x}_{rc}}\\left({x}_{{ref}_{i}}-{x}_{{ref}_{c}}\\right)+\\frac{\\partial u}{\\partial {y}_{rc}}\\left({y}_{{ref}_{j}}-{y}_{{ref}_{c}}\\right)$$\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$${\\tilde{y}}_{{cur}_{j}}={y}_{{ref}_{j}}+{v}_{rc}+\\frac{\\partial v}{\\partial {x}_{rc}}\\left({x}_{{ref}_{i}}-{x}_{{ref}_{c}}\\right)+\\frac{\\partial v}{\\partial {y}_{rc}}\\left({y}_{{ref}_{ϵSj}}-{y}_{{ref}_{c}}\\right) (i,j)\\in S$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(, {x}_{{ref}_{c}}\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({y}_{{ref}_{c}}\\)\u003c/span\u003e\u003c/span\u003e are the x and y coordinates of an initial reference subset point, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({x}_{{ref}_{c}}\\)\u003c/span\u003e\u003c/span\u003eand \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({y}_{{ref}_{c}}\\)\u003c/span\u003e\u003c/span\u003e are the x and y coordinates of the center of the initial reference subset, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\tilde{x}}_{{cur}_{i}}\\)\u003c/span\u003e\u003c/span\u003eand \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\tilde{y}}_{{cur}_{j}}\\)\u003c/span\u003e\u003c/span\u003e are the x and y coordinates of a final current subset point, (i,j) are indices used for the relative location of the subset points with respect to the center of the subset, as well as for correspondences between subset points in the current and reference configuration, and S is a set which contains all of the subset points. The subscript \"rc\" is used to signify that the transformation is from the reference to the current coordinate system.\u003c/p\u003e \u003cp\u003eThe full field strains are calculated by Green-Lagrangian strain in Ncorr, which is obtained by using the four displacement gradients as shown below:\u003c/p\u003e \u003cp\u003e \u003cspan class=\"InlineEquation\"\u003e \u003cspan class=\"mathinline\"\u003e\\({E}_{xx}=\\frac{1}{2}(2\\frac{\\partial u}{\\partial x}+{\\left(\\frac{\\partial u}{\\partial x}\\right)}^{2}+{\\left(\\frac{\\partial v}{\\partial x}\\right)}^{2}\\)\u003c/span\u003e \u003c/span\u003e(2)\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\)\u003c/span\u003e\u003c/span\u003e\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$${E}_{xy}=\\frac{1}{2}(\\frac{\\partial u}{\\partial y}+\\frac{\\partial v}{\\partial x}+\\frac{\\partial u}{\\partial x}\\frac{\\partial u}{\\partial y}+\\frac{\\partial v}{\\partial x}\\frac{\\partial v}{\\partial y})$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e3\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ3\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ3\" name=\"EquationSource\"\u003e\n$${E}_{yy}=\\frac{1}{2}(2\\frac{\\partial v}{\\partial y}+{\\left(\\frac{\\partial u}{\\partial y}\\right)}^{2}+{\\left(\\frac{\\partial v}{\\partial y}\\right)}^{2})$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e4\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\n\u003ch3\u003e8. Computational Details\u003c/h3\u003e\n\u003cp\u003eThe density functional theory (DFT) calculations were performed with Vienna Ab initio Simulation Package (VASP) using a plane-wave basis set\u003csup\u003e\u003cspan additionalcitationids=\"CR32\" citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e. Plane-augmented wave (PAW) pseudopotential is used to describe the valence electrons-nuclei interactions\u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e,\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e, while the electronic exchange and correlation effects are demonstrated within the generalized gradient approximation (GGA) as introduced by Perdew et al.\u003csup\u003e\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e A 4\u0026times;1\u0026times;1 grid centered at the gamma (Γ) point is used as k-point sampling of the Brillioun zone using Monkhorst Pack Scheme. All calculations are nono-spin polarized and the cut off energy was set as 450 eV. Structure optimization was carried out until the force converges to 0.02 eV/\u0026Aring; and the energy converges to 10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e eV. A series of graphene-based model structures with different kinds of defect were built, e.g. mono-vacancy, di-vacancy, and Stone-Wales. The unit cell sizes for strain-free periodical graphene nanobelt with zigzag and armchair edge were 9.85 \u0026Aring; \u0026times; 24 \u0026Aring; \u0026times; 18 \u0026Aring; and 8.60 \u0026Aring;\u0026times;24 \u0026Aring;\u0026times;18 \u0026Aring;, respectively. The periodicity of all structures is along the a-axis, and the strain was realized by changing the size of the a-axis in the unit cell. A strain range of 0.0\u0026ndash;2.0% was studied.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe data that support the findings of this study are included in the published article (and its Supplementary Information) or available from the corresponding authors on reasonable request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe thank the financial support from the National key R\u0026amp;D Program of China (2021YFA1202802), Natural Science Foundation of China (Grants No. 12102098, 52073020), China Postdoctoral Science Foundation (Grant No. 2020M680479, 2021M690801), the Chinese Academy of Sciences, and the Australian Research Council (ARC, DP 190103881, FL 190100126 and CE230100032).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eB. L. and S.X. contributed equally to this work. B.W. conceived the idea. B.W. and L.D. directed the project. B.L. mainly performed experiments while S.X. and L.Z. did DFT calculations. X.L., J.X., H.L., Z.Y., Y.G., Q.Z., S.Z., B.Z. and Z.X. contributed to the experimental and theoretical data analysis such as electrochemical measurements, strain distribution, TEM, and others. B.W., L.D. and B.L. wrote the manuscript with input from all authors.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAdditional information\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSupplementary information\u003c/strong\u003e The\u0026nbsp;Supplementary Material is available from the online version.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCorrespondence and requests for materials\u003c/strong\u003e should be addressed to B.W.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eHu, C., Paul, R., Dai, Q. \u0026amp; Dai, L. Carbon-based metal-free electrocatalysts: from oxygen reduction to multifunctional electrocatalysis. Chem. Soc. 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Lett. \u003cb\u003e77\u003c/b\u003e, 3865\u0026ndash;3868 (1996).\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":false,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Mechanical effects, Catalysis, Carbon, Strain engineering, Mechanoelectrochemistry","lastPublishedDoi":"10.21203/rs.3.rs-2295214/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-2295214/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe mechanical effects on carbon-based metal-free catalysts (C-MFCs) have rarely been explored although the C-MFCs have attracted worldwide interest as alternatives to the noble metal catalysts. Stress is everywhere, but a specialized study is strongly limited because the stress usually intermingles with other structural variables, including the dopants, defects, and interfaces in catalysis. Herein, we report a proof-of-concept study by establishing a platform to apply strain to a highly oriented pyrolytic graphite (HOPG) lamina continuously and collecting the electrochemical signals simultaneously. For the first time, the correlation between the surface strain of a graphitic carbon and its oxygen reduction reaction (ORR) activation effect is established. Results show that the in-plane and edge carbon sites in HOPG could not be further activated by applying tensile strain, but when the in-plane defects were involved in the structure, a strong and repeatable dependence of the catalytic activity on the tensile strain was observed, wherein\u0026thinsp;~\u0026thinsp;35.0% improvement in ORR current density was realized by applying\u0026thinsp;~\u0026thinsp;0.6% tensile strain. The density function theory (DFT) simulation shows that appropriate strain on the specific defect can optimize the adsorption of reaction intermediates, and the Stone-Wales defect on graphene correlates with the mechanical effect. Moreover, the effect was further authenticated by preparing a powdered graphene-based catalyst with varied strain-involved, which showed an apparent improvement of the ORR activity with ~\u0026thinsp;0.4% surface strain. This work clarifies some basic principles of strain effects on graphitic carbon\u0026rsquo;s catalytic activities towards ORR, and may lay the foundation for developing carbon-based mechanoelectrocatalysis.\u003c/p\u003e","manuscriptTitle":"Intrinsic Mechanical Effects on the Activation of Carbon Catalysts","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2022-12-06 15:47:08","doi":"10.21203/rs.3.rs-2295214/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-communications","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"NCOMMS","sideBox":"Learn more about [Nature Communications](http://www.nature.com/ncomms/)","snPcode":"","submissionUrl":"https://mts-ncomms.nature.com/","title":"Nature Communications","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Communications","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"a01ad3eb-381b-40e4-b7ff-3e1376f08f63","owner":[],"postedDate":"December 6th, 2022","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":17463607,"name":"Physical sciences/Energy science and technology/Fuel cells"},{"id":17463608,"name":"Physical sciences/Energy science and technology/Energy storage/Batteries"},{"id":17463609,"name":"Physical sciences/Materials science/Materials for energy and catalysis/Fuel cells"}],"tags":[],"updatedAt":"2024-08-22T22:05:14+00:00","versionOfRecord":[],"versionCreatedAt":"2022-12-06 15:47:08","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-2295214","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-2295214","identity":"rs-2295214","version":["v1"]},"buildId":"WrCJVZZCHTDjtuVLN7oU0","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}