Nano-folded gold electrocatalysts enhance the selectivity of carbon dioxide reduction
The local structure and geometry of catalytic interfaces can influence the selectivity of chemical reactions. Here, using a pre-strained polymer, we uniaxially compress a thin gold film to form a nano-folded catalyst. We observe two kinds of folds and can tune the ratio of loose to tight folds by varying the extent of pre-strain in the polymer. We characterize the nano-folded catalysts using x-ray diffraction, scanning, and transmission electron microscopy. We observe grain reorientation and coarsening in the nano-folded gold catalysts. Electroreduction of carbon dioxide with these nano-folded catalysts reveals an enhancement of Faradaic efficiency for carbon monoxide formation by a factor of about four. This result suggests that electrolyte mass transport limitations and an increase of the local pH in the tight folds of the catalyst outweigh the effects of alterations in grain characteristics. Together, our studies demonstrate that nano-folded geometries can significantly alter grain characteristics, mass transport, and catalytic selectivity.
<!>Details of the experiments and the simulations 1.1 Preparation of the nano-folded Au catalysts<!>X-ray diffraction measurements<!>Scanning transmission electron microscopy analysis<!>Sheet resistance measurement<!>Transfer Au films to arbitrary substrates<!>Electrochemical measurements<!>Carbon monoxide analysis<!>Surface roughness measurement<!>Mass Transport Model for CO2 electroreduction
<p>Electroreduction of carbon dioxide (CO2) to valuable carbon-rich products is a potential solution to end the anthropogenic carbon cycle. 1,2 However, slow kinetics and reduced control over product yield and selectivity have hindered widespread commercial viability. 3,4 Nanostructured catalysts offer the potential to address these limitations. 5 Indeed, a variety of nanoparticles, [6][7][8][9] thin films, [10][11][12][13][14] and nanoporous materials [15][16][17][18] have been explored, but there are still challenges with scalability, reproducibility, extreme reaction conditions, and cost. 6,10,12,15 Mechanical wrinkling of metallic thin films using pre-strained polystyrene (PS) substrates offers a reproducible strategy to create nanostructured interfaces. This approach is compatible with nanoparticles, 19 nanoporous, 20 thin films, 21,22 and twodimensional (2D) materials (e.g. graphene and MoS2), [23][24][25][26][27][28] and the morphology can be controlled using lithography. 24,25,29,30 Previous studies with wrinkled catalysts show improvement in different electrochemical reactions including hydrogen evolution reaction (HER), 25,27,28,30 glucose sensing, 31 and DNA detection. 32 For example, wrinkled platinum (Pt) arrays electrodes improved the performance of HER from ~70 mA/cm 2 to ~ 120 mA/cm 2 at 0.1 V as compared to conventional Pt on carbon. This improvement was attributed to weak adhesion of hydrogen (H2) gas bubbles, resulting in a lower overpotential.</p><p>Here, for the first time, we present evidence that a nano-folded gold (Au) catalyst can improve the selectivity of CO2 reduction. We utilized a pre-strained PS substrate to uniaxially compress and create a novel Au catalyst with a combination of loose (>200 nm) and tight (< 200 nm) nano-folds (Figure 1). Importantly, we can control the ratio of tight to loose folds by varying the magnitude of the pre-strain in the PS substrate. Also, we can release the nano-folded catalysts from the PS substrate and transfer them to alternate substrates or use them as freestanding catalysts. We characterized the nanofolded Au catalysts with Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), Scanning Transmission Electron Microscopy (STEM) and electron diffraction. XRD results suggest grain reorientation and STEM studies show grain coarsening in the nano-folded catalyst. Electrocatalytic studies indicate that the Faradaic efficiency (FECO) of the nano-folded Au catalyst is approximately four times higher than the flat Au catalyst. We attribute the enhanced selectivity to the nano-folds in the Au catalyst that hinder diffusion of the electrolytes leading to an increase of the local pH, which suppresses the HER. 15 Our approach to preparing nano-folded Au catalysts is based on previous studies with some modifications (see Section 1.1 in the Supporting Information). 21,33 Specifically, we thermally evaporated 60 nm of Au on pre-strained PS at a rate of approximately 0.5 Å/sec. After evaporation, we clamped and then heated the composite film above the glass transition temperature of PS to drive uniaxial shrinkage with tunable macroscopic prestrain as measured by the ratio of the final to the initial length of the film (Figure 1a).</p><p>Wrinkling of thin films using pre-strained polymers has been attributed to the mismatch of the bending energy of the Au and PS films, and wrinkling occurs when the pre-strain is above a critical strain,</p><p>, where E* 1 and E* 2 are the plane-strain moduli of the substrate and the thin film. 34 Assuming representative literature values for E* 2 and E* 1 , we estimate the critical strain to be 6 x 10 -4 , which is significantly below our pre-strain values of 30% and 70%. 33 In our studies, we investigated three pre-strain conditions 0 % (Au-0, Figure S1), 30% (Au-30, Figure 1b), and 70 % (Au-70, Figure 1c). Wrinkles formed at the lower values of pre-strain can be further compressed into folds at higher values of pre-strain. 30 We observed a greater fraction of loose (d > 200 nm) in Au-30 and a higher fraction of tight (d < 200 nm) folds at higher values of pre-strain in Au-70 (Figure 1). Specifically, we observed more than 40% of tight folds in Au-70 as compared to only about 3% of tight folds in Au-30 (Figure 1d and S2). We characterized the microstructure of the flat and nano-folded Au catalysts by XRD and observed that the intensity of (200), ( 220) and (311) planes increased with increasing pre-strain, and the peaks did not shift or broaden (Figure 2a). The XRD results suggest that the changes in the nano-folded samples are morphological; they introduce more diffraction planes and alter the grain structure with no significant changes in atomic displacement. Previously, it has been observed that the grain structures of polycrystalline Au films can be altered dramatically when the deformation exceeds the elastic limit of Au, 35 and Au nanostructures undergo plastic deformation when the compressive strain is larger than 5%. 36 We attribute the increased XRD intensities of Au-30 and Au-70 to preferred grain orientation, which was introduced by compression. 37 We performed two control experiments to investigate if the heating temperature and the orientation of the folds with respect to the x-ray beam would affect the intensity of the XRD patterns (Figure S3) and we did not observe any obvious changes in both cases.</p><p>In order to investigate the structural changes in the nano-folded catalyst, we prepared and characterized thinned (30 nm) of Au-0 and Au-70 using STEM. The STEM imaging and analysis were done in the convex regions of the folds, and we observed larger grains in the Au-70 (Figure 2c) as compared to Au-0 (Figure 2b) indicating grain coarsening in the nano-folded catalysts. This is consistent with previous studies on cyclic plastic deformation of Au thin films that indicate that mechanical deformation can alter grain energetics and drive grain coarsening. 35,36,38,39 Also, the electron diffraction analysis indicates that the compression did not introduce new crystal planes in Au-70 (Figure 2c).</p><p>The STEM diffraction patterns show a similar family of planes for Au-70, such as (111), ( 200), ( 220) and (311), as compared to Au-0, which also agrees with the XRD patterns. We measured the sheet resistance of the Au catalysts using the four-point probe measurement technique (Figure S4). We observed that within the statistical variation, the Au-0 and nano-folded Au catalysts had similar electrical sheet resistivity of 2.28 x 10 -8 (Au-30) and 2.10 x 10 -8 Ω/m (Au-70). These values are also similar to the bulk resistivity of Au at 300 K (2.25 x 10 -8 Ω/m) (Table 1). 40 The four-point measurements indicate that the compression did not compromise the electrical conductivity, which provides evidence that no significant defects such as cracks were introduced in the nano-folded Au films. Also, it is known that metal films with larger grains typically have lower resistivity, 41 and while we did observe some decrease in resistivity of Au-70, the decrease is within the margin of statistical variation. An interesting feature of our approach is that our nano-folded catalysts are permanently deformed, and we can even release it from the PS substrate and transfer it to arbitrary substrates, such as flat silicon wafers or even curved syringe needles (Figure 3). Specifically, after the transfer, we observed that the average width of Au-30 and Au-70 decreased and increased by approximately 25% and 11 % respectively while the height decreased by 29% and 8.7% (Table 1, Figure S6 and S7), and no deformation was induced in Au-0 (Figure S5). This observation can be rationalized by noting that the prestrain was high and there is a small extent of relaxation after release. For Au-30, we observed collapse of the roof of the wrinkles resulting in a decrease in both width and height, whereas for Au-70 we observed relaxation by spreading of the folds resulting in an increase in width and a decrease in height of the folds. We investigated the electrocatalytic activity in the CO2 reduction reaction using a home-built, gas-tight electrolysis cell (Figure S8 and S9). Details of the cell and electrocatalytic measurements are available elsewhere and in section 1.6 in the supporting information. 42 From the potential dependent FECO plots (Figure 4a), we observed that Au-70 showed enhanced selectivity for CO formation at potentials between -0.5 V to -1.0 V achieving a maximum FECO of 77.9%, followed by Au-30 with 48.1% and Au-0 of 19.2% at -0.7V.</p><p>In order to investigate the trend in the enhancement of FECO, we plot the current density per geometric surface area (JCO, Figure 4b) and the current density per electrochemically active surface area (ECSA) (jCO, Figure 4c) of the Au catalysts. We estimated the ECSA (Figure S10) by measuring the electrochemical capacitance of the Au catalysts. Au-70 exhibited the highest JCO, of 3.04 mA/cm 2 at -0.7 V that is approximately two-fold higher than Au-30 and six-fold than Au-0. This observation is consistent with the results in Figure 4a, since the geometric surface area of all the Aucatalysts was the same. When accounting for the real surface area in the ECSA plots, at the same potential, Au-70 has only a 7% increase of jCO to 1.45 mA/cm 2 Au, as compared to Au-30, which is 1.34 mA/ cm 2 Au. Both are significantly higher than Au-0, which has a jCO of 0.51 mA/cm 2 Au. This result suggests that roughness does play a role in the performance of nano-folded catalysts, as expected, 15,17,18 but cannot explain the entire increase in jCO. The measurement also yields surface roughness factors, which indicates a roughness factor of 2.1 for Au-70, 1.3 for Au-30 and 1 for Au-0 (Table S1). Aside from the increase in surface area, we attribute the enhancement of the selectivity of Au-70 to a local pH effect. 15,17,18 We simulated the pH at the interfaces of the catalysts using the effective diffusivities of [CO2]aq, HCO3 -, CO3 2and OHand a previously reported mass transport model. [43][44][45] In the model, the effective diffusivity in porous media is given as 𝒟 9 = 𝑥𝒟, 𝑥 = => ? , where 𝑥 is the correlation factor, 𝒟 is the bulk diffusivity, 𝑝 is the porosity fraction, which is the ratio of the void space to total volume, 𝛿 is the constriction factor, which accounts for the difference in cross-sectional area normal to diffusion and 𝜏 is the tortuosity, which is the actual distance a molecule travels divided by the shortest distance between two points. 46,47 We assume that the value of 𝑥 is always less than 1 for a tightly folded catalyst owing its porous morphology. The model then suggests that the diffusivity factors (0 ≤ 𝑥 ≤ 1), which means that the effective diffusivities must always decrease (0 ≤ 𝒟 9 ≤ 𝐷). For the same nano-folded catalyst, if we assume that the structural factors 𝑝, 𝛿, 𝜏 are the same for all species [CO2]aq, HCO3 -, CO3 2-and OH -, then the effective diffusivity would decrease by the same extent. We can then calculate the pH change and plot it vs 𝑥 for 0.2≤ 𝑥 ≤ 1 (Figure S11). We observe that the pH increases with decreasing 𝑥 and the local pH begins to rise significantly when the diffusivities decrease by about 40% (𝑥 = 0.4). This mass transport model confirms that reducing diffusivities can create a high pH at the interface of the Au-catalysts. The plot also shows that the pH for Au-70 is higher than that for Au-30, and both are significantly higher than Au-0. From prior literature, it is known that a higher local pH at the catalyst interface suppresses the HER reaction, which would improve the FECO and agrees with our findings. 15,17,18 In summary, we have demonstrated a new strategy to enhance the selectivity of CO formation by utilizing nano-folded Au catalysts for CO2 reduction. The catalysts can be made in a straightforward manner using a pre-strained polymer substrate. Also, they can be transferred to other substrates which is important for flexible sensor or electrochemical devices. [48][49][50][51] STEM analysis indicates that this process drives grain coarsening in nano-folded catalysts, which suggests a facile approach to modify grain morphology the ultra-thin (~30 nm) Au films. Finally, we reveal that tight fold morphologies can reduce mass transport of the electrolytic species and create high local pH to reduce the activity of HER and consequently enhance CO selectivity. We anticipate that this strategy of creating folds in catalysts could be utilized as a general strategy to tune the selectivity of other electro-catalysts.</p><!><p>We rinsed and dried the shrinkable film (Grafix KSF6-C) with isopropanol (IPA) and air prior to any experiment. We deposited gold (99.99% Au) on top of the heat shrinkable film by thermal evaporation at a rate of 0.5 Å/sec. To prepare the folded Au catalysts, we cut and clamped the Au film on top of the silicon (Si) wafers, and heated it at 200 ℃ for 3 mins to form the nano-folded Au catalysts. We calculated the strain of the nano-folded catalysts using the following equation,</p><p>where 𝜀 %L is the strain that applied to the Au film, 𝐿 M is the original length, and 𝐿 is the length of the Au film after heating.</p><!><p>We obtained the x-ray diffraction patterns (XRD) using an X'Pert Pro (Philips) instrument with the Empyrean Cu LFF tube. We set the x-ray generator at 45 kV and 40 mA, and we scanned all the samples from 5 o to 100 o with a step size of 0.02 o and 10 seconds per step.</p><!><p>Data for STEM images were collected at 300 kV on an FEI Titan Themis CryoS/TEM operating in microprobe STEM mode with an aperture semi-angle of about 3 mrad. We collected a diffraction pattern at every scanning point using the Electron Microscope Pixel Array Detector (EMPAD) developed at Cornell University, 1 resulting in a 4D (x, y, kx, ky) dataset.</p><p>To generate the false-colored images, the diffraction patterns were turned into cepstrums 2 and then ran through Principal Components Analysis (PCA). The cepstrum is the Fourier transform of the log of the diffraction pattern. This transform emphasizes the periodicity in the diffraction pattern and is less sensitive to intensity fluctuations from dynamical diffraction such as thickness fringes or bend contours that cause nonuniform illumination in the diffraction pattern. The false-colored images are color composited from the loadings of the PCA. Similar colored regions indicate similar loadings, thus similar cepstrums, and ultimately similar grains. The colors themselves were arbitrarily chosen and have no inherent meaning.</p><!><p>We measured the sheet resistance of the Au films using the four-point probe method with a Keithley 2400 multimeter. Electric current (I) was flowed through the outer two probes, and the corresponding potential difference (V) was measured across the inner two probes.</p><!><p>We prepared a two-phase water/toluene solution. The toluene dissolves the PS substrates, and the Au film is suspended at the water/toluene interface. We removed and added new toluene every few hours over two days. To transfer the Au films to arbitrary substrates, we removed the toluene, lifted the Au films, and transferred them to the receiver substrate. The films could also be wrapped around a needle by rolling.</p><!><p>We measured the electrocatalytic properties of the Au catalysts using a home-built gastight electrolysis cell and an Autolab 302 potentiostat (Metrohm). We used a Hg/HgSO4 electrode (Hach) and a Pt mesh (VWR) as the reference and counter electrode respectively, and a solution of 0.1 M KHCO3 (Sigma Aldrich, ≥ 99.95%) was used as the electrolyte. CO2 was bubbled through a glass frit to the cathode compartment at a constant rate of 20 sccm and purged for 10 minutes prior to each measurement. To prepare the working electrode, we assembled the Au films with Ag wire (Alfa Assar) and carbon tape (Nisshin EM), and we covered with Ag wire and carbon tape with Kapton tape (Micronova) to avoid any background signal from the Ag wire. The cathode and anode compartments were separated with an AHO anion exchange membrane (Selemion Inc.), which minimizes the possible transfer of chemical species (e.g. Pt dissolved from the counter electrode, products produced from the cathode) across the cell. We converted all potentials in this work to the RHE scale by E (vs RHE) = E (vs Hg/HgSO4) + 0.64 V + 0.0591 × pH, where the pH for the 0.1 M KHCO3 electrolyte saturated by CO2 is 6.8. The gas-phase products were measured online using a gas chromatography-mass spectrometry instrument (GC-MS-QP2010SE, Shimadzu).</p><!><p>We quantified the amount of carbon monoxide (CO) using the GC-MS with a Plot-Q column (Restek). Helium (He) gas was the carrier gas to the GC-MS, and we calibrated the GC-MS using a custom gas mix (Airgas). We sampled the gas samples every ten minutes and six total measurements per value of the potential. We calculated the faradaic efficiency of CO (𝐹𝐸% QR ) using the following equation,</p><p>In addition, we calculated the partial current densities of CO (𝐽 QR ) using the following equation,</p><p>where 𝑝𝑝𝑚 QR is the concentration of CO formation, flow rate = 20 sccm, 𝑛 is the number of electrons transferred to CO2 to CO (𝑛 = 2 for this reaction), 𝑝 ^ = 1.013 bar, 𝑇 is 273.15 K, 𝑅 is gas constant, and 𝐹 is the Faraday's constant. The 𝑔𝑒𝑜𝑚𝑒𝑡𝑟𝑖𝑐 𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑎𝑟𝑒𝑎 in our studies was 0.25 cm 2</p><!><p>We calculated the surface roughness of the Au films by measuring the double-layer capacitance from 0 to -0.20 V (vs. RHE) at various scan rates in 0.</p><!><p>We developed a mass transport model for the Au electrocatalysts based on a previously published model with some modifications. 3,4 We simulated the local pH at the Au surfaces using Matlab 2018a and the parameters listed in Table S2 and S3. The general equation of the effective diffusivity is 𝒟 9 = 𝑥𝒟, 𝑋 = ℰ v > ? , where 𝒟 is the bulk diffusivity, ℰ c is the porosity, 𝛿 is the constriction factor, and 𝜏 is the tortuosity. For instance, we simulated 𝑥 = 0.4, and we estimated the 𝒟 9 (Table S3) for every electrolytic species ([CO2]aq, HCO3 -, CO3 2-, OH -).       Table S2. Electrochemical reactions and corresponding parameters considered for the mass transport model. Table S3. Concentration and diffusion coefficients of chemical species in the bulk electrolyte. 3,4</p>