Self-Supported Nanoporous Gold with Gradient Tin Oxide for Sustainable and Efficient Hydrogen Evolution in Neutral Media

Hydrogen evolution reaction (HER) in neutral medium suffers from slow kinetics as compared to that in alkaline or acidic conditions, owing to larger Ohmic loss and low proton concentration. Here we report that a self-supported nanoporous Au-SnOx (NPAu-SnOx) catalyst with gradient tin oxide surface could significantly enhance HER activity in neutral buffer solution (0.2 M PBS). The NP Au-SnOx catalyst exhibits a low onset overpotential of 38 mV and a small Tafel slope of 79 mV dec. The current density of 10 mA cm is manifested at an overpotential as low as 148 mV, representing the comparable performance of Pt/C catalyst. This high catalytic activity can retain at least 10 hours without any detectable decay. The superior HER activity is proposed to originate from the gradient SnOx structure and metal/oxide interfaces in nanoporous ligaments. Furthermore, the X-ray photoelectron spectroscopy reveals that the gradient oxide in the ligament is remarkably stable during long-term reaction.


Introduction
Hydrogen is an attractive and promising energy carrier toward the sustainable energy economy owing to its high energy density and environment-friendly production [1][2]. In many approaches to hydrogen production, the electrocatalytic hydrogen evolution reaction (HER) from water splitting is the most economical and effective route for the future hydrogen demands [3][4][5]. Owing to the appropriate hydrogen binding energy (HBE) that is located near the peak of a volcano plot (Exchange current density vs. HBE), Pt and its alloys are currently the most active HER catalysts and play a benchmark for HER electrocatalysts [6][7][8][9].
significance to develop HER catalysts in neutral media, which can power up the devices sailing in the water. However, HER proceeding in a neutral electrolyte sustains slower kinetics in comparison with that in acidic or alkaline media because of the low proton concentration and larger Ohmic loss [15]. To enhance reaction rates at neutral pH, water molecule must act as the reactants. As a result, much higher overpotential should be applied to reach the same level of the electric current density in neutral solution [16][17]. It was reported that several catalysts, including amorphous MoS x films [18], FeP nanoparticles [19][20], CoP composites [21][22] and CoS film [23] are active for the HER in neutral solutions. Most of these HER catalysts are in form of powders or colloids with low-dimensional or porous nanostructures and immobilized on current collectors by making use of a polymer binder (nafion or PTFE) [24][25][26]. Such complicated sample preparation process is a time-and cost-consuming process and makes them neither cost-effective nor efficient for practical application.
So far, it is still extremely challenging to improve HER catalytic activity and stability in neutral media [27][28]. The current density is far below than that in acidic or alkaline electrolyte, requiring a high overpotential (>200 mV) to attain the standard current density of 10 mA cm −2 . For instance, the HER current density recorded at −0.2 V vs. reversible hydrogen electrode (vs. RHE) for amorphous molybdenum sulfide on dealloyed nanoporous gold (NPG) is −5.7 mA/cm 2 in 0.5 M H 2 SO 4 . However, its current density is only 0.48 mA/cm 2 at the same overpotential in 0.2 M phosphate buffer solution (PBS, pH = 7) [29]. In fact, constructing metal/oxide interface is a promising and effective route to enhance HER performance for heterostructured catalysts, such as core-shell structure [30][31][32][33]. It was reported that strong interactions between the adsorbed H 2 O species and metal oxide surfaces may grant metal/oxide interfaces the ability to promote the water dissociation step in HER by optimizing hydrogen adsorption energy and facilitating the water species adsorption [1,34]. However, the metal oxides always suffer from the poor conductivity which suppresses charge transport and thereby efficiency of electrocatalysis [35][36]. Inspired by the molybdenum sulfide coated on the NPG [29,37], the strategy of designing metal oxide on nanoporous ligament can not only make use of highly conductive metallic skeleton and high mass transport efficiency in nanoporous structure, but also enhance the catalytic performance of metal oxide.
Herein, we report a facile and effective method to fabricate a self-supported nanoporous Au-SnO x (NP Au-SnO x ) composite with gradient oxide on nanoporous Au skeleton surface for efficient hydrogen evolution by means of one-step chemical dealloying. The unique hybrid structureis able to produce hydrogen with high activity and remarkable stability in neutral media.

Fabrications of the NP Au-SnO x and NPG
Ag 65 Au 33 Sn 2 and Ag 65 Au 35 precursors were prepared by arc melting pure Ag, Au and Sn with a desired atomic ratio under argon atmosphere. After the verification of composition by energy dispersive X-ray spectroscopy (EDX), the alloy ingot was rolled at room temperature to get a ribbon with a thickness of 25 m. The NPAu-SnO x was fabricated by corroding Ag 65 Au 33 Sn 2 alloy ribbon in 69% HNO 3 solution at room temperature for 8 hours. Nanoporous gold (NPG) was also prepared by corroding Ag 65 Au 35 in the same condition for comparison. For further removing Sn, the SnO x -free nanoporous Au catalyst was dealloyed for a second time in a 6 M HCl solution for 12 hours. Those resulting catalysts were carefully rinsed with deionized water for several times to clean up the residual chemicals and used as work electrodes for electrochemical evaluations directly. The Pt/C ink was prepared by mixing 5 mg commercial Pt/C powder (20 wt%, Hesen, HPT020) into 40 μL Nafion (0.05%, Sigma Aldrich) solution containing isopropanol (30%) and water (70%). After a rigorous sonication of 30 min in ice water, 10 μL of the suspension was drop-cast onto a 3-mm-diameter glassy carbon electrode (loading mass: 0.283 mg cm -2 ) and was dried in air to form a uniform thin film for electrochemical characterizations.

Microstructural Characterizations
X-ray diffraction (XRD) measurements were carried out ona Rigaku DMAX/2400 X-ray diffractometer with Cu K α radiation. Scanning electron microscope (SEM) observations were performed with a fieldemission scanning electron microscope (JSM-7600F). A JEOL JEM-2100F transmission electron microscope (TEM) with an acceleration voltage of 200 kV was employed to characterize the structure of nanoporous catalysts. The surface atomic structure of the NP Au-SnO x and NPG were characterized by using spherical-aberration-corrected TEM in a scanning TEM (STEM) mode. X-ray photoelectron spectra (XPS) were obtained in AXIS-ULTRA DLD-600W XPS system with an Al K α (mono) anode.

Electrochemical Measurements and Product Analysis
All electrochemical measurements were performed on a CHI 760E electrochemical workstation with a three-electrode cell. A graphite rod was used as counter electrode. All potentials were measured vs. a saturated calomel (Hg/Hg 2 Cl 2 , SCE) reference electrode (saturated KCl, TJ Aida R0232) and converted to RHE reference scale using E (vs. RHE) = E (vs. SCE) + 0.0591 × pH + 0.244 V. The NP Au-SnO x and NPG electrodes after dealloying were used as directly as working electrodes for electrochemical measurements. While for the commercially available Pt/C catalysts, its ink with a concentration of 2 mg mL −1 was loaded onto glassy carbon electrode. The HER activities were evaluated by linear sweep voltammetry (LSV) at a scan rate of 5 mV/s in aqueous electrolytes of 0.2 M PBS, 0.5 M H 2 SO 4 and 0.1 M KOH. The electrolytes were purged with high pure N 2 for 30 min prior to measurements. All electrochemical tests were carried out at room temperature.
Electrolysis for product analysis was performed in a gas-tight electrochemical cell with two compartments separated by a piece of anion exchange membrane (Nafion 212, DuPont). Cathodic compartment cell with working electrode was connected with the gas-sampling loop of a gas chromatograph (GC-9790II, FULI analyzing incorporations) for the continuous cycling. Gas-phase product was automatically sampled every 15 min for quantitative analysis. Argon (Xiangyun Gas Incorporations, 99.999%) was used as carrier gas during the measurement. The GC columns led directly to a thermal conductivity detector (TCD) to quantify H 2 . Gas concentration was determined by the peak areas calibrated from standard gas with different concentrations. Faradaic efficiency (FE) of H 2 was calculated by equation FE = 2nF/Q, n is the number of moles calculated from GC analysis, F refers to Faraday's constant (96485 C mol −1 ) and Q represents all the charge passed.

Surface Structure of Nanoporous Electrocatalysts
Representative SEM images in Fig. 1(a) and d show porous characters of the NP Au-SnO x and NPG, respectively. All the samples exhibit bicontinuous skeletons and the interconnected channels. Statistical distributions of nanopores/ligaments are estimated for the NPAu-SnO x (15.4 nm) and NPG (16.2 nm) (Fig. S1, Appendix A). Relatively smaller pore size in the NPAu-SnO x results from the sluggish diffusion of Au atoms, which is perturbed by spontaneous releasing and burying of tin atoms into the ligament throughout chemical dealloying [38][39]. The ligaments/pores size can be tuned by the dealloying time and temperatures [40][41]. The concentration of tin element is estimated as 7.3 wt.% based on EDX analysis (Fig. S2, Appendix A). The XRD pattern of the NP Au-SnO x in Fig. S3 identifies all the crystal planes corresponding to face-centered-cubic (fcc) Au. After close inspection of Au (111) diffraction peak, a small shoulder at the 2θ angle of 37.8°corresponds to the (200) reflection of SnO 2 . The XRD result indicates the formation of tin oxide in the ligaments during the dealloying in HNO 3 . Spontaneous formation of tin oxide results from the oxidization of tin atoms in nitric acid during the dealloying, and the detailed characterization will be described in the following section. The way of introducing metal/ oxide interface in nanoporous structure by adding small fractions of metal into precursor alloy is convenient and cost-effective.
The NP Au-SnO x catalyst was also examined using STEM characterizations. Fig. 2(a) shows the typical image of the ligament infrastructure for chemical composition evaluation; Fig. 2(b-d) shows the EDX mappings of Au, Sn and O elements, respectively. It is evident that Au and Sn atoms are homogeneously distributed along the ligaments. However, it is also shown that O atoms are enriched at the edge of ligaments in the overlay mapping ( Fig. 2(e)). As represented in Fig. 3(a), a dark interface is visible at the ligament edge in high resolution TEM (HRTEM) image, suggesting surface oxide with a thickness of 5~10 nm. More surface analysis requires the XPS characterization, which is capable of providing the detailed information of the oxidation states of tin. In order to reveal the composition evolution with the depth, the ligament can be etched layer by layer by using argon ion beam sputtering (Fig. S4). As shown in Fig. 3(b), a doublet at 495.7 and 487.3 eV is assigned to the 3d 3/2 and 3d 5/2 peaks of Sn (IV) [42]. The percentage of SnO 2 is estimated as 92% by calculating the integral area in the deconvoluted spectrum (Fig. S5). As the etching depth increases, the peak intensities decrease gradually and shift to lower energy. The 3d 3/2 peak at 494.9 and 3d 5/2 peak at 486.5 eV corresponding to Sn (II) can be detected in the depth of 5~20 nm. The paired shoulder peaks at 493.5 and 485.0 eV are related to metallic state (Sn 0 ) [43]. The atomic concentrations of Au, Sn and O atoms are shown in Fig. S6 based on quantitative analysis. The oxygen concentration decreases from~20% on the top surface to~5% in the depth of 5~20 nm, indicating the gradient distribution of tin oxide perpendicular to the ligament surface. The top surface oxide is mainly SnO 2 , and the subsurface is dominated by SnO in the internal ligament. This gradient SnO x structure seems highly stable in the nitric acid. More importantly, homogeneously distributed Sn atoms could create numerous Au/SnO x interfaces in the ligaments of nanoporous structure. This unique structure with metal/ oxide interfaces will enhance catalytic performance significantly, which has been demonstrated in Au/CeO 2 , Ni/NiO and Co/Co 3 O 4 materials [44][45][46].
To further reveal the surface composition of the NP Au-SnO x , cyclic voltammetry (CV) measurements are conducted by sweeping from 0.93 V to −0.07 V vs. RHE in 0.2 M PBS electrolyte. The CV curve shows a cathodic peak at 0.56 V, which indicates the reduction of SnO 2 to SnO (Fig. S7). On the other hand, the two anodic peaks stem from Sn oxidization to SnO (at 0.09 V) and its further oxidization into SnO 2 (at 0.42 V) [47][48]. The reproducible CV curves are obtained after 20 sweeping cycles, indicating the formation of a stable surface composition. Because the intensity of characteristic peak is proportional to the corresponding percentage of tin species [49-50], the percentage of Sn species can be estimated by integrating the redox peak, as depicted in Fig. S7b. The ratio of SnO 2 :SnO:Sn is about 91:2:7. Therefore, the majority of Sn species on the surface is SnO 2 , which is consistent with the XPS analysis (Fig. S5).

HER Performance of Nanoporous Catalysts
To evaluate the HER electrocatalytic performance of nanoporous electrocatalysts, self-supported NP Au-SnO x and NPG after dealloying can be directly used as electrodes for electrochemical measurements in a N 2purged 0.2 M PBS electrolyte (pH = 7.0). All electrochemical measurements were performed on a CHI 760E electrochemical workstation with a typical three-electrode cell (Fig. S8). Fig. 4(a) shows the HER polarization curves of normalized current densities vs. applied potential (iR corrected) plot for a representative NP Au-SnO x electrocatalyst, in combination with that of the NPG and commercially available Pt/C catalysts (20 wt%) for comparison. Different from the behavior of the NPG, the NP Au-SnO x exhibits remarkably enhanced HER activity with very low onset overpotential (η 0 ) of 38 mV vs. reversible hydrogen electrode (RHE), and the current density rapidly increases to 10 mA cm −2 at the overpotential (η 10 ) of 148 mV without electrode rotation or stirring the electrolyte. Both η 0 and η 10 values are lower than these of the NPG (η 0 = 110 mV and η 10 = 591 mV). These results demonstrate that introducing SnO x in the NPG can enhance HER activity effectively. Furthermore, the overpotential for the NP Au-SnO x at the current density of 40 mA cm −2 (475 mV) is only 73 mV higher than Pt/C (402 mV, Fig. 4(b)), indicating that the NP Au-SnO x possess comparable performance with Pt/C at high overpotential.
To explore the intrinsic mechanism of the high activity in the NP Au-SnO x and evaluate the contribution of SnO x on the surface, a SnO x -free nanoporous Au with identical porous structure was synthesized by performing one additional dealloying in 6 M HCl solution to remove the SnO x from the NP Au-SnO x surface. The CV measurement confirms that there is no redox peak corresponding to tin oxide after additional dealloying, indicating the successful removal of SnO x from the surface. As expected, the HER activity of the SnO x -free NP Au decreases significantly relative to the NP Au-SnO x , behaving the similar performance of the NPG. Based on the electrocatalysis results for both NPG and SnO x -free NP Au, it can be concluded that the exceptional HER activity observed in the NP Au-SnO x is due to the existence of gradient SnO x in nanoporous ligaments.
Tafel analysis is performed with an attempt to gain insights into the HER kinetics. The linear portions of the Tafel plots were fitted to the Tafel equation ( ¼ a þ blog j j j) and yielded the slope (b). As shown in Fig. 4(c), the NP Au-SnO x exhibits a Tafel slope of 79 mV/dec (η = 36-90 mV), and is very close to the value of Pt/C (76 mV/dec). However, the NPG displays a Tafel slope of 124 mV/dec, implying that the rate-limiting step is the Volmer step (120 mV/dec). A smaller Tafel slope for the NP Au-SnO x indicates better catalytic performance relative to the NPG. The geometric exchange current density (J 0 ) of the NP Au-SnO x electrode that is calculated from the Tafel plot data is 0.34 mA cm −2 , which is over 4 times higher than that of the NPG (0.08 mA cm −2 ). The large exchange current density of the NP Au-SnO x indicates higher intrinsic electrocatalytic activity than the NPG for HER. These electrocatalytic results are summarized in Tab. S1, and additional data from recent investigations on some representative HER electro catlysts in neutral medium are also included for comparison.
To quantitatively demonstrate the enhanced HER activities of the NP Au-SnO x electrodes, the specific activities are calculated by dividing the current at the overpotential of 200 mV by the electrochemical active surface areas (ECSA) that are evaluated by the double-layer capacitance (C dl ) measurements. According to the CV curve in a potential window of −0.2 V to −0.4 V (vs. SCE) at various scan rates, the C dl values are estimated by the linear slope in the plot of capacitive current density at −0.3 V against scan rate (Fig. S9). The ECSAs are further calculated using a constant factor of ≈17 μF/cm 2 corresponding to Au planar surface [51]. As shown in Fig. S9d, the specific activity of the NP Au-SnO x at the overpotential of 200 mV reaches 3.3 μA cm −2 , almost three times as high as that of the NPG (1.2 μA cm −2 ). This evidence demonstrates that the high activity of the NP Au-SnO x derived from the intrinsic highly activity per site, which may stem from the high activity of metal/oxide interfaces and good conductivity of Au skeleton.
Notably, the HER polarization curves for three catalysts in neutral electrolyte can be clearly divided into two stages: the first one is a slowly increasing stage of current density with applied potential from the onset potential to a transition potential E t (Pt/C: −0.3 V vs. RHE, NP Au-SnO x : −0.4 V vs. RHE, NPG: −0.5 V vs. RHE); the second stage corresponds to a sharp increase of current density beyond the transition potential. In neutral conditions, the HER on a transition metal surface is expected to proceed in either Volmer-Heyrovsky or Volmer-Tafel mechanisms [52]:  [14,[55][56]. Therefore, it requires high overpotential in the second stage.
It is reported recently that metal/oxide heterostructures, such as Ni/NiO heterostructure [45] and Co/ Co 3 O 4 core-shells [46,57], are able to promote the water dissociation step in alkaline media because of strong interactions between the adsorbed H 2 O species and oxide surfaces. The OH − could preferentially attach to metal oxide at the heterojunctions for the strong electrostatic attraction between metal ions to OH − ; meanwhile, H + prefers to adsorb at the neighboring site of metal. The strong interfacial interaction between metal and oxide, particularly the interface confinement effect in supported nanostructures, has been suggested to provide and stabilize highly active sites for molecular activation [30]. On the other hand, density functional theory (DFT) calculations suggested that the metal/oxide heterostructure possesses a lower water dissociation free energy and a more optimal HBE than the counterparts [58][59].
Weng et al. reported that the metal/oxide interfaces between Ni and CeO 2 not only facilitates the dissociation of water molecules but also modifies the HBE to the same level of Pt [60]. Therefore, there is a plausible scenario that SnO x oxide in NP Au-SnO x surface would promote the HER catalysis in a similar manner, which could make the significant enhancement of HER activity. As schematically depicted in Fig. 5, we propose that the mechanisms involved in the HER activity with and without SnO x . It is clear that water adsorption at the SnO x /Au heterojunction sites is strengthened due to concerted interaction of O atoms with SnO x domains and H atoms with Au ( Fig. 5(b)) [34]. However, without SnO x on the surface, only H atoms can adsorb on the metal surface. Water adsorption is then followed by water dissociation and hydrogen adsorption (H ad ) on the nearby vacant Au sites. Finally, two H ad atoms on the metal surface recombine to form H 2 (H 2 desorption step)and OH − desorbs from the SnO x domains, followed by adsorption of another water molecule on the same site [61].
The key issue for the HER in neutral media is the high Ohmic loss and the interfacial charge-transfer resistance between electrode and electrolyte. In order to clarify the distinct electrochemical behaviors of the NP Au-SnO x and NPG electrodes, their electrochemical impedance spectroscopy (EIS) analysis is further conducted in a frequency range from 1000 kHz to 10 mHz under hydrogen evolution voltage of −0.15 V vs. RHE (corresponds to the potential at 10 mA cm −2 ) [14], as shown in Fig. 4(d). The Nyquist plots of the NPG and NP Au-SnO x electrodes display two semicircles in high-and low-frequency ranges, which indicate the presence of two time constants related to HER kinetics. In order to fit the impedance spectra, a serial model containing two constant phase elements was used to analyze the EIS spectra (inset of Fig. S10), wherein R s is the intrinsic electrode and electrolyte resistance, R p is the resistance of nanopores, R ct is the charge transfer resistance; C p is the constant phase elements corresponding to the capacitance of nanopores, and C d is the constant phase elements corresponding to the double-layer capacitance [62][63]. These parameters of R s , R p and R ct are summarized in Fig. S10. Measurement errors in fitting impedance data were ≤6.8% for NP Au-SnOx, and ≤7.4% for NPG, respectively. Owing to their similar conductive Au skeleton and pore size (Fig. S1), both the NPG and NP Au-SnO x electrodes almost have the same R s and R p values. In contrast, the NP Au-SnO x possesses a low R ct value of 5.5 Ω, much smaller than that of the NPG (47.8 Ω). The lower R ct value indicates a faster hydrogen reaction rate and enhanced HER activity [14,46,62]. Therefore, the enhanced HER performance of NP Au-SnOx was likely due to the low R ct on the electrode surface caused by the gradient SnO x oxide. Meanwhile, the self-supported nanoporous structure facilitates the mass transportation and the highly conductive metallic skeleton grants the fast electron transfer process.

The Stability of Nanoporous Catalysts
To evaluate the catalytic durability, 10 hours electrolysis is performed at −0.4 V vs. RHE without iR correction in N 2 -saturated 0.2 M PBS solution, as shown in Fig. 6(a). The current density for both the NP Au-SnO x and NPG can maintain stable within 10 hours without abrupt decay. Furthermore, the HER polarization curve of the NP Au-SnO x after 10 hours is similar to the original one prior to long-term stability test (Fig. 6(b)).The current density only shows a slight decrease in the first stage (before −0.4 V) and exhibits a weak enhancement in the second stage (after −0.4 V), suggesting its high durability. In order to investigate the stability of SnO x in long-term electrolysis, the NP Au-SnO x after catalysis evaluation was further detected by the XPS (Fig. 6(c)). The spectrum of Sn 3d shows almost the same after 10 hours electrolysis, indicating the identical chemical states of the oxide in surface as before. This may result from the gradient oxide structure on the ligament surface, that possesses higher stability under electrochemical catalytic condition compared to the nanoparticle or core-shell structure [64][65]. Another possibility is that neutral electrolyte is less corrosive and has negligible effect on electrode material. So metal/oxide interfaces in the ligament can remain stable during long-term electrochemical test. The HER polarization curves are also tested in 0.5 M H 2 SO 4 and 0.1 M KOH, and the NP Au-SnO x also represents higher activity in comparison with the NPG in acidic and alkaline electrolytes (Fig. S11). However, the catalytic activity of the NP Au-SnO x in alkaline environment decreases quickly after 200 continuous cycling linear sweep voltammetry (LSV), indicating the degeneration of catalytic activities (Fig. S12). To further verify whether metal oxides were reduced during HER process, gas products were analyzed by gas chromatography (GC) to determine the corresponding Faradaic efficiency (FE). Electrolysis was performed in a gas-tight electrochemical cell with two compartments separated by a piece of anion exchange membrane. Gas-phase product from one identical sample was automatically sampled in 15 min for quantitative analysis from −0.2 V to −0.7 V vs. RHE. Fig. 6(d) shows the H 2 FE at various potentials in N 2 -saturated 0.2 M PBS. For both the NPG and NP Au-SnO x catalysts, the FE of hydrogen reaches above 90%. If metal oxides were reduced to metal and make the significant contribution to the current, the FE of the NP Au-SnO x would be less than that of the NPG. However, there is no difference between the FEs for the NP Au-SnO x and the NPG within the range of −0.2 V to −0.7 V vs. RHE. Therefore, it can be concluded that metal oxides with gradient structure in the ligament would not be reduced during HER process, indicating its remarkable stability.

Conclusions
In summary, the addition of SnO x into nanoporous gold is identified to be highly active and durable for the HER under neutral media. The NP Au-SnO x features a low onset overpotential of 38 mV and a small Tafel slope of 79 mV dec −1 , and displays the current density of 10 mA cm −2 at an overpotential as low as 148 mV in neutral electrolyte, which is comparable to that of the Pt/C catalyst. Relative to the NPG, the improved catalytic performance of NP Au-SnO x results from the gradient SnO x structure and metal/oxide interfaces. Moreover, gradient oxide in the ligament provides structural stability for long-term reaction. The idea and concept delivered in this work can be extended toward nanoporous transition metals with low cost, such as Ni and Mo alloys. The insights in this study may serve as a principle to design high performance HER catalysts for water splitting in neutral media in the future. Intensity (a.u) Figure S4: XPS survey spectra of the NP Au-SnO x surface and at the depth of 5~20 nm Figure S5: Sn 3d XPS spectrum of the NP Au-SnO x surface. The spectrum is deconvoluted to calculate the percentage of SnO 2 by peak area  Figure S8: A typical three-electrode cell for HER studies, in which a graphite rod and an SCE reference electrode (saturated KCl) employed as the counter electrode and the reference electrode, respectively. Self-supported NP Au-SnO x and NPG after dealloying were used as work electrodes directly Figure S9: Electrochemical capacitance measurements for the estimation of electrochemical active surface areas (ECSAs) of the NPG and NP Au-SnO x electrodes. (a, b) Cyclic voltammograms in the potential window for the NPG and NP Au-SnO x electrodes at various scan rates, respectively. (c) The differences in current density at −0.3 V vs. SCE as a linear function of scan rate with the slope of double-layer capacitance, C dl . (d) Comparison of specific activity for the NPG and NP Au-SnO x electrodes. Here the specific activity is calculated by dividing the current density at the overpotential of 200 mV by the ECSAs that are evaluated by the double-layer capacitance (C dl ) measurements Figure S10: Comparison of R s , R p and R ct for both the NPG and NP Au-SnO x . Inset: The electrical equivalent circuit used for fitting EIS spectra, where R s is the intrinsic electrode and electrolyte resistance, R p is the resistance of nanopores, R ct is the charge transfer resistance; C p is the constant phase elements corresponding to the capacitance of nanopores, and C d is the constant phase elements corresponding to the double-layer capacitance  06 -This work Note: GC, glassy carbon electrode; FTO, fluorine-doped tin oxide electrode; CC, carbon cloth; OP represents onset overpotential; η-j represents the overpotential at a given current density j (mA cm -2 ); TS represents the Tafel slope; J 0 represents the exchange current density.