Chinese Journal of Chemical Physics  2018, Vol. 31 Issue (5): 626-634

The article information

Jing Yang, Jie Wei, Wei Chen, Yan-xia Chen
杨晶, 魏杰, 陈微, 陈艳霞
Electrochemical Study on Hydrogen Evolution and ${\rm C}\rm{O}$$_\rm{2}$ Reduction on Pt Electrode in Acid Solutions with Different pH
不同pH酸性溶液中铂电极上氢析出和二氧化碳还原的电化学研究
Chinese Journal of Chemical Physics, 2018, 31(5): 626-634
化学物理学报, 2018, 31(5): 626-634
http://dx.doi.org/10.1063/1674-0068/31/cjcp1804057

Article history

Received on: April 1, 2018
Accepted on: May 15, 2018
Electrochemical Study on Hydrogen Evolution and ${\rm C}\rm{O}$$_\rm{2}$ Reduction on Pt Electrode in Acid Solutions with Different pH
Jing Yang, Jie Wei, Wei Chen, Yan-xia Chen     
Dated: Received on April 1, 2018; Accepted on May 15, 2018
Hefei National Laboratory for Physical Science at the Microscale and Department of Chemical Physics, University of Science and Technology of China, Hefei 230026, China
*Author to whom correspondence should be addressed. Yan-xia Chen, E-mail: yachen@ustc.edu.cn
Abstract: Hydrogen evolution reaction (HER) is the major cathodic reaction which competes ${\rm C}\rm{O}_\rm{2}$ reduction reaction (${\rm C}\rm{O}_\rm{2}$ RR) on Pt electrode. Molecular level understanding on how these two reactions interact with each other and what the key factors are of ${\rm C}\rm{O}_\rm{2}$ RR kinetics and selectivity will be of great help in optimizing electrolysers for ${\rm C}\rm{O}_\rm{2}$ reduction. In this work, we report our results of hydrogen evolution and ${\rm C}\rm{O}_\rm{2}$ reduction on Pt(111) and Pt film electrodes in ${\rm C}\rm{O}_\rm{2}$ saturated acid solution by cyclic voltammetry and infrared spectroscopy. In solution with pH > 2, the major process is HER and the interfacial pH increases abruptly during HER; ${\rm C}\rm{O}_\rm{ad}$ is the only adsorbed intermediate detected in ${\rm C}\rm{O}_\rm{2}$ reduction by infrared spectroscopy; the rate for ${\rm C}\rm{O}_\rm{ad}$ formation increases with the coverage of UPD-H and reaches maximum at the onset potential for HER; the decrease of ${\rm C}\rm{O}_\rm{ad}$ formation under HER is attributed to the available limited sites and the limited residence time for the reduction intermediate ($\rm{H}_\rm{ad}$), which is necessary for ${\rm C}\rm{O}_\rm{2}$ adsorption and reduction.
Key words: Hydrogen evolution reaction    ${\rm C}\rm{O}_\rm{2}$ Reduction    Pt(111)    Acidic solution    pH effect    Infrared spectroscopy    
Ⅰ. INTRODUCTION

Electrochemical reduction of ${\rm C}\rm{O}_\rm{2}$ (${\rm C}\rm{O}_\rm{2}$ RR) is considered to be a promising approach toward production of value-added chemicals or fuels powered by intermittent renewable energy sources [1, 2]. In this field, extensive studies have been carried out [3]. Earlier studies on ${\rm C}\rm{O}_\rm{2}$ RR on precious metals like Pt and Pd in NaH${\rm C}\rm{O}_\rm{3}$ solution revealed that the major cathodic current (> 99%) is from hydrogen evolution reaction (HER) [4]. In solution with pH=1, besides the adsorbed CO [5], small amount of formic acid, formaldehyde, and methanol are detected during ${\rm C}\rm{O}_\rm{2}$ reduction on Pt [6]. However, their current efficiency has not been quantitatively analyzed yet.

In 1990, Bocarsly et al. reported that in pyridine (Pyr)-containing acidic solution with pH close to the p$K_\rm{a}$ of Pyr (Pyr+$\rm{H}^{+}$=Pyr$\rm{H}^{+}$, p$K_\rm{a}$=5.3), the Faradaic yield for ${\rm C}\rm{H}_\rm{3}$OH generation from ${\rm C}\rm{O}_\rm{2}$ reduction on Pt can be up to 30% with HER as the main competitive reaction [7]. Since then, extensive studies on Pyr catalyzed ${\rm C}\rm{O}_\rm{2}$ RR have been carried out, several mechanisms have also been proposed [2, 8]. However, a few recent studies of ${\rm C}\rm{O}_\rm{2}$ RR on Pt electrodes questioned the catalytic activity of Pyr as well as the reproducibility of earlier results [9-11]. Since HER is the major competitive reaction during ${\rm C}\rm{O}_\rm{2}$ reduction on Pt with or without molecular electrocatalysts such as Pyr, understanding the electrochemical behavior, the effect of the existence of ${\rm C}\rm{O}_\rm{2}$ and its reaction intermediates on HER kinetics will be of great help: (ⅰ) to unravel the role of Pyr on ${\rm C}\rm{O}_\rm{2}$ RR, (ⅱ) to reveal the viability of using Pt for ${\rm C}\rm{O}_\rm{2}$ reduction, (ⅲ) to figure out the optimum conditions for ${\rm C}\rm{O}_\rm{2}$ RR electrolysers. We have systematically studied ${\rm C}\rm{O}_\rm{2}$ RR on Pt in acid solution with different pH, with or without Pyr and how it competes with HER. In this contribution, we report part of these results of HER on Pt(111) in ${\rm C}\rm{O}_\rm{2}$ saturated solution with pH close to the p$K_\rm{a}$ of Pyr.

Ⅱ. EXPERIMENTS

The electrolyte solutions with 0.1 mol/L NaCl$\rm{O}_\rm{4}$ + $x$ mol/L HCl$\rm{O}_\rm{4}$ (x=$10^{-1}$, $10^{-2}$, $10^{-3}$, $3$ × ${10^{-3}}$, $10^{-4}$, $10^{-5}$, and $5$ × ${10^{-6}}$ mol/L) were prepared using NaCl$\rm{O}_\rm{4}$ (99.99%, Suprapure, Sigma Aldrich) and HCl$\rm{O}_\rm{4}$ (70%, Suprapure, Sigma Aldrich) and ultra-pure water (18.2 M$\Omega$$\cdot$cm, from Mili Q water system). The pH values of the solutions were determined using a pH meter. Before each experiment, all solutions were purged with $\rm{N}_\rm{2}$ (99.999%, the Linde Group, China) for 20 min. ${\rm C}\rm{O}_\rm{2}$ saturated solution was ensured by further purging the solution with ${\rm C}\rm{O}_\rm{2}$ (99.99%, the Linde Group, China) for 15 min continuously during the measurements.

Pt(111) and thin Pt film deposited on the flat reflecting face of a hemi-cylindrical Si prism were used as the working electrodes. The preparation, pretreatment, and characterization of Pt(111) electrode were described in detail in Ref.[12]. The base CV for the Pt(111) electrode in 0.1 mol/L HCl$\rm{O}_\rm{4}$ is given in FIG. 1(a). It displays the well-defined features reported previously, confirming that the homemade Pt(111) is well-ordered and the cell system used in our study is clean. The Pt thin film electrode with a thickness of ca. 50 nm was deposited by electrolessplating, following a procedure described elsewhere [13]. The active surface area of the film electrode was ca. 3.7 c$\rm{m}^\rm{2}$ estimated from the charge for the oxidation of a saturated $\rm{H}_\rm{ad}$ layer formed in the potential region from 0.4 V to 0.05 V.

FIG. 1 Cyclic voltammogram of Pt (111) in (a) 0.1 mol/L HCl$\rm{O}_\rm{4}$ and (b) 0.1 mol/L NaCl$\rm{O}_\rm{4}$+ $5.0$ × $10^{-6}$ mol/L HCl$\rm{O}_\rm{4}$ with (circle) and without (star) ${\rm C}\rm{O}_\rm{2}$, scan rate: 50 mV/s, electrode rotation speed: 0 r/min.

A conventional two-compartment electrochemical cell was used for the electrochemical experiment, while the flow cell used for the infrared spectroscopic measurement under attenuated total reflection configuration is described in detail in Ref.[14]. A Pt foil (99.99%) and a reversible hydrogen electrode (RHE) were used as counter and reference electrodes respectively. The measurements with single crystalline electrodes were done under hanging meniscus rotating disk electrode configuration. The electrode rotation speed was controlled by a modulated rotator (Pine Instruments Company). The electrode potentials were controlled by a potentiostat (CHI700C, Shanghai ChenHua). All potentials are quoted against the RHE. When recording the $I-E$ curves, Ohmic compensation is done automatically by the potentiostat. All experiments were carried out at room temperature (ca. 25 ℃).

Ⅲ. RESULTS AND DISCUSSION A. Cyclic voltammetric study on HER on Pt(111) in ${\rm C}\rm{O}_\rm{2}$ saturated electrolyte

FIG. 1(b) displays two representative CVs of Pt(111) in 0.1 mol/L NaCl$\rm{O}_\rm{4}$+$5.0$ × $10^{-6}$ mol/L HCl$\rm{O}_\rm{4}$ with and without saturated ${\rm C}\rm{O}_\rm{2}$ (pH=5.3) under stationary condition. For the CV without ${\rm C}\rm{O}_\rm{2}$, the small cathodic peak at ca. 0.6 V in the negative-going potential scan is the reductive desorption of O$\rm{H}_\rm{ad}$ through

$ \rm{Pt}-\rm{O}{{\rm{H}}_{\rm{ad}}}+{{\rm{H}}_{\rm{3}}}{{\rm{O}}^{\rm{ + }}}+{\rm{e}} \to {\rm{Pt}} + 2{{\rm{H}}_{\rm{2}}}{\rm{O}} $ (1)

which is followed by the reductive desorption of O$\rm{H}_\rm{ad}$ through

$ {\rm{Pt -\rm{O}}}{{\rm{H}}_{{\rm{ad}}}}+{\rm{ e}} \to {\rm{Pt}} + \rm{O}{{\rm{H}}^{\rm{ - }}} $ (2)

when the consumption of proton near the electrode surface is much faster than its supply by diffusion from the bulk solution. As a result, the pH near the electrode surface (denoted as p$\rm{H}^\rm{s}$ hereafter) becomes higher than 7, which explains why reaction (2) occurs at more negative potentials than that for reaction (1) [15]. As a result of high p$\rm{H}^\rm{s}$, the onset potential for under potential deposition (UPD) of H occurs only at E < 0.2 V. When the scan direction is reversed at -0.2 V, oxidation of UPD-H occurs immediately. After part of the $\rm{H}_\rm{ad}$ atoms are oxidized, the p$\rm{H}^\rm{s}$ decreases, and the rest of UPD-H is oxidized at higher potentials with a small peak at ca. 0.3 V as typical for those in acidic environments. Reaction induced change of the interfacial pH as well as its impact on the related electrode reaction in solution with low $\rm{H}^{+}$ or O$\rm{H}^{-}$ concentration and without buffer has been discussed thoroughly by our group previously [15-17].

From the CV recorded in ${\rm C}\rm{O}_\rm{2}$ saturated solution, we found that there is a pair of symmetric anodic and cathodic peaks in the potential region from 0.5 V to 0.8 V, which are from the adsorption and desorption of carbonate through

$ \text{Pt-C}{{\text{O}}_{\text{3,ad}}}\text{+}{{\text{H}}_{\text{3}}}{{\text{O}}^{\text{+}}}\text{+e}\rightleftharpoons \text{Pt+HCO}_{3}^{-}+{{\text{H}}_{\text{2}}}\text{O} $ (3)

as well confirmed by using infrared spectroscopy [18]. The good symmetry of the anodic and cathodic current wave for carbonate adsorption and desorption suggests that the kinetics for its adsorption and desorption is fast. By comparing the CV recorded in ${\rm C}\rm{O}_\rm{2}$ saturated solution with the CV given in FIG. 1(a), we see that the onset potential for bicarbonate adsorption is ca. 50 mV more negative than that for O$\rm{H}_\rm{ad}$ adsorption, whose adsorption will inhibit the O$\rm{H}_\rm{ad}$ adsorption, as similar to the case of acetate adsorption [19]. In contract to the case in ${\rm C}\rm{O}_\rm{2}$ free solution, in ${\rm C}\rm{O}_\rm{2}$ saturated solution, it is found that the current waves for H-UPD and for the oxidation of UPD-H are symmetric, which occur in the potential region as that for the case with pH=1 (on the RHE scale) (FIG. 1(a)). This indicates that in ${\rm C}\rm{O}_\rm{2}$ saturated solution, the p$\rm{H}^\rm{s}$ near Pt surface will not change significantly upon the UPD of H due to the buffer capability of ${\rm C}\rm{O}_\rm{2}$ even when the pH of the bulk solution is 5.3. When E is more negative than 0.1 V, HER through

$ {\rm{2}}{{\rm{H}}_{\rm{3}}}{{\rm{O}}^{\rm{ + }}}+{\rm{ 2e}} \rightleftharpoons {{\rm{H}}_{\rm{2}}}+{\rm{ 2}}{{\rm{H}}_{\rm{2}}}{\rm{O}} $ (4)

occurs, it increases sharply with the negative shift in potential and reaches a plateau when E is negative than -0.05 V due to the small $\rm{H}^{+}$ concentration in the bulk solution with pH=5.3. When E is more negative than -0.1 V, the cathodic current increases again, which mainly comes from

$ \text{2}{{\text{H}}_{\text{2}}}\text{O+2e}\rightleftharpoons {{\text{H}}_{\text{2}}}\text{+2O}{{\text{H}}^{\text{-}}} $ (5)

Instead of ${\rm C}\rm{O}_\rm{2}$ reduction (for further evidence, see text below) [20, 21]. After scanning to -0.25 V and then reversing the potential scan, anodic current appears at E > 0.05 V, with an amplitude much higher than that for UPD-H during the cathodic scan in the same potential region. The extra current comes from $\rm{H}_\rm{2}$ oxidation, which is formed during previous negative-going scan down to -0.25 V.

In order to further confirm this, CVs with the same scan rate but under different electrode rotation speed or with different scan rate under stationary conditions are recorded, which are shown in FIG. 2 and FIG. 3. From FIG. 2, we see that with the increase of the electrode rotation speed, the current wave in the potential region from 0.05 V to 0.4 V decreases, only H-UPD current is observed when rotation speed is higher than 1600 r/min. Since the oxidation currents for ${\rm C}\rm{O}_\rm{2}$ reduction products, such as HCOOH, CO, and ${\rm C}\rm{H}_\rm{3}$OH are very small in the potential region with E < 0.4 V [14, 22], the anodic current observed in the potential region from 0.05 V to 0.4 V must come from the oxidation of $\rm{H}_\rm{2}$ formed at negative potential which has not been diffused away from the surface. This is further confirmed by the data given in FIG. 3, where we found that with the increase of potential scan rate the pseudo capacitance for bicarbonate adsorption does not change, while that in the H-UPD region decreases. At lower potential scan rate, more time is spent at HER potentials. As a result, more $\rm{H}_\rm{2}$ are produced (FIG. 3(b)), hence more $\rm{H}_\rm{2}$ are available near the electrode surface to be oxidized in the H-UPD potential region.

FIG. 2 Cyclic voltammogram of Pt(111) in 0.1 mol/L NaCl$\rm{O}_\rm{4}$+$5.0$ × $10^{-6}$ mol/L HCl$\rm{O}_\rm{4}$ with ${\rm C}\rm{O}_\rm{2}$ under different electrode rotation speed, scan rate: 50 mV/s. The inset shows the the measured (square) and estimated (circle) diffusion limiting current for HER at -0.1 V with $\rm{H}^{+}$ as reactant.
FIG. 3 (a) Cyclic voltammogram and (b) capacitance of Pt(111) in 0.1 mol/L NaCl$\rm{O}_\rm{4}$+$5.0$ × $10^{-6}$ mol/L HCl$\rm{O}_\rm{4}$ with ${\rm C}\rm{O}_\rm{2}$ under different potential scan rates, electrode rotation speed: 0 r/min.

Furthermore, we found that the diffusion limiting current for HER through reaction (4) is much larger than what is predicted by the Levich equation, Eq.(6) (inset in FIG. 2):

$ j_\textrm{L}=0.62nF{D_0}{^{2/3}}\nu^{-1/6}C_{0}{\omega}^{1/2} $ (6)

where n is the number of electrons, F is the Faraday constant, $D_\rm{0}$ is the diffusion coefficient, $\nu$ is the dynamic viscosity, $C_\rm{0}$ is the bulk concentration, and $\omega$ is the rotation speed.

The higher HER current is explained by the buffer capability of ${\rm C}\rm{O}_\rm{2}$ when the rate of the consumption of proton can be fast compensated by the dissociation of $\rm{H}_\rm{2}$${\rm C}\rm{O}_\rm{3}$. At E < -0.2 V, the HER current does not show obvious change with electrode rotation speed, which can be explained by the fact that water is the precursor for HER, the reaction is not affected by the mass transport since water concentration in the solution is ca. 55 mol/L. The pH of bulk solution is 5.3, only ca. 3 pH unit smaller than 8 for a basic solution, which explains why the plateau for HER through reaction (4) only covers a potential region of ca. 0.15 V (i.e., 3 × 0.059 V), agreeing well with the shift of the equilibrium potential for HER according to the Nernst equation. With the increase of potential scan rate, the slight positive shift in HOR current peak or negative shift in the HER current wave (FIG. 3) is not due to the uncompensated Ohmic resistance, since the overall current is small and Ohmic compensation is automatically applied during the data recording. Instead, we think it originates from low proton concentration which renders the local p$\rm{H}^\rm{s}$ change being more pronounced, i.e., the abrupt change of local p$\rm{H}^\rm{s}$ induced by fast consumption or production of $\rm{H}^{+}$ cannot be compensated by the diffusion. This is further confirmed by the comparison of the CVs recorded in solution with different pH (FIG. 4). From FIG. 4 we see that with the decrease of solution pH, the current for both HER and HOR at ca. 0 V increases, and the anodic and cathodic current peaks become more symmetric. Note that the shift of the current wave for carbonate adsorption is due to the pH induced change of concentration of bicarbonate precursors. The decay of HER current at E < -0.05 V is due to the mass transport limit of $\rm{H}^{+}$. All the data discussed above indicate that the characteristic current features at E < 0.4 V either as a function of potential scan rate, electrode rotation speed or solution pH agree well with the typical behavior of pH effect on HER and HOR, while the current features for ${\rm C}\rm{O}_\rm{2}$ reduction are not obvious at all.

FIG. 4 Cyclic voltammogram of Pt(111) in (a) ${\rm C}\rm{O}_\rm{2}$ and (b) $\rm{H}_\rm{2}$ saturated 0.1 mol/L NaCl$\rm{O}_\rm{4}$+$x$ mol/L HCl$\rm{O}_\rm{4}$ (x=$10^{-1}$, $10^{-2}$, $10^{-3}$, $3$ × ${10^{-3}}$, $10^{-4}$, and $10^{-5}$ mol/L) of different pH as indicated in the figure, scan rate: 50 mV/s, electrode rotation speed for (a) 0 r/min and (b) 1600 r/min, respectively.

In solutions with low pH ($\leq$1), it is well confirmed by infrared spectroscopy that ${\rm C}\rm{O}_\rm{ad}$ will be formed at potentials where HER or UPD-H occurs [5, 23-26]. In order to check whether ${\rm C}\rm{O}_\rm{ad}$ or other intermediates may be formed under present condition with high local pH, which may affect the HER or HOR kinetics, a preliminary study was carried out by holding the potential in the HER potential region for 2 min, then examining the CV with and without such holding treatment (FIG. 5). We found that the CVs recorded with and without such holding are nearly the same, indicating that both the HER and HOR activities are not affected by any possible intermediates formed during ${\rm C}\rm{O}_\rm{2}$ reduction. Slightly higher current at ca. 0.7 V which is superimposed on the current wave for carbonate adsorption is probably due to the oxidation of ${\rm C}\rm{O}_\rm{ad}$ on Pt(111). This will be further discussed in Section Ⅲ. B. The contribution of all simultaneous reactions in our system, such as HER, HOR, and ${\rm C}\rm{O}_\rm{2}$ RR, is unclear when only considering the current data. However, such question can be solved by combination of other in situ electrochemical techniques like differential electrochemical mass spectrometry (DEMS).

FIG. 5 Cyclic voltammogram of Pt(111) in ${\rm C}\rm{O}_\rm{2}$ saturated 0.1 mol/L NaCl$\rm{O}_\rm{4}$+$x$ mol/L HCl$\rm{O}_\rm{4}$ (x=$10^{-2}$, $10^{-3}$, and $10^{-4}$ mol/L) of different pH (a) pH=2, (b) pH=3, (c) pH=4, with (star) and without (circle) holding at -0.1 V for 2 min, scan rate: 50 mV/s, electrode rotation speed: 0 r/min.

The results discussed in this section can be summarized as follows: (ⅰ) in solutions with pH > 2, the interfacial pH increases abruptly during HER, which cannot be buffered by ${\rm C}\rm{O}_\rm{2}$; (ⅱ) as a result, HER with $\rm{H}^{+}$ as reactant occurs at lower overpotentials; while at higher overpotentials, HER with $\rm{H}_\rm{2}$O as reactant occurs; (ⅲ) carbonate adsorption is observed in the potential region from 0.55 V to 0.8 V; (ⅳ) the kinetics of both HER and HOR are not affected by the adsorbed intermediates which is formed by ${\rm C}\rm{O}_\rm{2}$ reduction; (ⅴ) the good symmetry of the anodic and cathodic current waves for carbonate adsorption and desorption suggests that the kinetics for its adsorption and desorption is fast.

B. Infrared spectroscopic study on the interface Pt/${\rm C}\rm{O}_\rm{2}$ saturated electrolyte and its impact on HER

In order to get more insights into whether ${\rm C}\rm{O}_\rm{2}$ reduction also occurs during HER and how the adsorbed intermediates formed from ${\rm C}\rm{O}_\rm{2}$ RR affect the HER kinetics, we have carried out electrochemical in-situ infrared spectroscopic measurements of the Pt interface under attenuated total reflection configuration (ATR-FTIRS). FIG. 6 displays the cyclic voltammogram of Pt film in $\rm{N}_\rm{2}$ or ${\rm C}\rm{O}_\rm{2}$ saturated 0.1 mol/L NaCl$\rm{O}_\rm{4}$+ $5.0$ × $10^{-6}$ mol/L HCl$\rm{O}_\rm{4}$. Note that in order to get IR spectra with good quality, the potential scan rate is limited to 10 mV/s. As similar to the case for Pt (111) in $\rm{N}_\rm{2}$ saturated solution, due to the low proton concentration as well as lacking of buffer capability, the current wave for H-UPD and its oxidative removal is displaced by ca. 0.4 V. In contrast, in ${\rm C}\rm{O}_\rm{2}$ saturated solution, due to the buffer capability of ${\rm C}\rm{O}_\rm{2}$, the potential for H-UPD and its oxidation removal is only shifted toward negative values by ca. 0.2 V. The anodic current wave in ${\rm C}\rm{O}_\rm{2}$ saturated solution is much broader than that in $\rm{N}_\rm{2}$ saturated solution, which is probably due to the superimposition of $\rm{H}_\rm{2}$ oxidation. Because of the slow scan rate as well as stationary condition without stirring, the contribution of the current wave from HER and HOR is very obvious. As a result, the current wave for carbonate adsorption/desorption in the potential region from 0.5 V to 0.8 V becomes less obvious.

FIG. 6 Cyclic voltammogram of thin Pt film electrode in 0.1 mol/L NaCl$\rm{O}_\rm{4}$+$5.0$ × $10^{-6}$ mol/L HCl$\rm{O}_\rm{4}$ with (circle) and without (star) ${\rm C}\rm{O}_\rm{2}$, scan rate: 10 mV/s.

FIG. 7(a) displays the IR spectra of Pt interface at some selected potentials in $\rm{N}_\rm{2}$ saturated solution recorded simultaneously with the CV given in FIG. 7. From FIG. 7(a) we see that there is barely no spectral features at E > 0 V. When E is below 0 V, only the bending (1645 c$\rm{m}^{-1}$) and stretching mode (3000-3600 c$\rm{m^{-1}}$) [27] of water are observed, whose band intensity increases with decreasing the electrode potential. Such water species are assigned to water molecules which form hydrogen bonds with UPD-H atoms adsorbed on Pt surface, its band intensity displays roughly a linear relationship with the coverage of UPD-H atoms, similar to previous observations in solution with pH=1 [28, 29].

FIG. 7 Selected IR spectra at Pt film electrode in $\rm{N}_\rm{2}$ (a) and ${\rm C}\rm{O}_\rm{2}$ (b, c) saturated 0.1 mol/L NaCl$\rm{O}_\rm{4}$+$5.0$ × $10^{-6}$ mol/L HCl$\rm{O}_\rm{4}$ recorded during cyclic voltammetric potential scan, scan rate: 10 mV/s

In the ${\rm C}\rm{O}_\rm{2}$ saturated solution, again, the positive-pointing OH stretching appears in the H-UPD potential region, while that for the water bending is not obvious. This is probably due to the appearance of the negative-pointing carbonate band at ca. 1545 c$\rm{m}^{-1}$ [30, 31], which appears at ca. 0.7 V, whose band intensity increases with further negative shift in potential and reaches the maximum at ca.0.4 V, in good agreement with the indications given by CV (FIG. 1 and FIG. 6). This confirms that the carbonate is desorbed at E < 0.5 V. Besides the water band, two positive bands at 1770 and 1989 c$\rm{m}^{-1}$ appear when E is below 0.2 V. These bands are assigned to adsorbed CO in multiply-bonded (${\rm C}\rm{O}_\rm{M}$) and linearly-bonded (${\rm C}\rm{O}_\rm{L}$) configuration, respectively [27]. In the reverse scan from -0.4 V to 0 V, both the band intensity and the peak frequencies of the CO band increase further with the positive shift in electrode potential. At higher potentials, the band intensity for ${\rm C}\rm{O}_\rm{L}$ increases while that for ${\rm C}\rm{O}_\rm{M}$ decreases, indicating there is a transfer of ${\rm C}\rm{O}_\rm{M}$ to ${\rm C}\rm{O}_\rm{L}$. At E above 0.4 V the intensity of both bands decreases again and finally disappears when E is above 0.8 V, which indicates that ${\rm C}\rm{O}_\rm{ad}$ is oxidized at higher potentials. The IR data reveal that in solution with pH=5, ${\rm C}\rm{O}_\rm{ad}$ can be formed from ${\rm C}\rm{O}_\rm{2}$ reduction in the potential region where H-UPD or HER occurs, similar to the case with pH=1 [5]. Besides the ${\rm C}\rm{O}_\rm{ad}$, no other adsorbed intermediates are detected by ATR-FTIRS in the potential region where H-UPD or HER occurs.

In order to follow the kinetics for ${\rm C}\rm{O}_\rm{ad}$ formation on Pt film electrode during H-UPD or HER, IR spectra were recorded with current transients upon stepping the electrode potential from 0.9 V to 0.3 V, 0 V, or -0.3 V, and after holding at that potential for 30 s the potential is step back to 0.9 V again. The current transients are displayed in FIG. 8 and the time-dependent IR spectra are shown in FIG. 9. From FIG. 8 we see that right after stepping the potential to 0.3 V or 0 V, only current for double layer charging was observed, and it decreases fast to zero. When stepping to -0.3 V, there is a large cathodic current right after the potential step, and it decreases fast with reaction time and reaches steady state at ca. 20 s after the potential step. From the simultaneously recorded IR spectra, we found that the rate for ${\rm C}\rm{O}_\rm{ad}$ build up is the slowest at 0.3 V and the fastest at 0 V. So far, it is not clear whether the amount of adsorbed H plays a role for such a difference, or it is just due to the change of thermodynamic driving force for ${\rm C}\rm{O}_\rm{2}$ RR. For the case of stepping to -0.3 V, more ${\rm C}\rm{O}_\rm{M}$ is formed than that for ${\rm C}\rm{O}_\rm{L}$, the IR band intensity of the ${\rm C}\rm{O}_\rm{L}$ and ${\rm C}\rm{O}_\rm{M}$ bands first increases with reaction time up to 20 s, then it does not increase anymore, although the overall ${\rm C}\rm{O}_\rm{ad}$ surface coverage is below 0.15 ML [5]. This coincides with the fact that the current decreases with reaction time and it reaches steady state also at ca. 20 s after potential step. Further studies are necessary to figure out whether this is because ${\rm C}\rm{O}_\rm{2}$ RR to ${\rm C}\rm{O}_\rm{ad}$ needs special active sites which are fully occupied by ${\rm C}\rm{O}_\rm{ad}$ or HER. The limited ${\rm C}\rm{O}_\rm{ad}$ formation under HER is probably due to dynamic turnover of $\rm{H}_\rm{ad}$ on the surface which limits the available sites and residence time for the reduction intermediates ($\rm{H}_\rm{ad}$) necessary for ${\rm C}\rm{O}_\rm{2}$ adsorption and reduction.

FIG. 8 Current transients for HER and ${\rm C}\rm{O}_\rm{2}$ RR on Pt film electrode after stepping the potential from 0.9 V to 0.3 V (square), 0 V (circle), and -0.3 V (triangle).
FIG. 9 Selected IR spectra at Pt film electrode in ${\rm C}\rm{O}_\rm{2}$ saturated 0.1 mol/L NaCl$\rm{O}_\rm{4}$+$5.0$ × $10^{-6}$ mol/L HCl$\rm{O}_\rm{4}$ recorded during stepping the potential from 0.9 V to 0.3 V (a), 0 V (b), and -0.3 V (c).

An interesting phenomenon we would like to point out here is that although the IR band intensities of the ${\rm C}\rm{O}_\rm{ad}$ bands do not show obvious increase further with reaction time at ca. 20 s after the potential is stepped to -0.3 V, the peak frequencies of the ${\rm C}\rm{O}_\rm{ad}$ bands still display a continuous increase with reaction time (FIG. 10). Similar increase of CO stretching frequency in the H-UPD potential region was observed before, but it was accompanied by an increase of the band intensity. The latter is explained by the adsorbed H-induced CO migration and ${\rm C}\rm{O}_\rm{ad}$ islands formation. The enhanced dipole-dipole coupling between adsorbed CO at the neighboring sites is suggested to be the origin for the increase of both ${\rm C}\rm{O}_\rm{ad}$ band intensity and peak frequency [32]. However, in the HER region, as observed in present study only the increase of peak frequency is obvious, while that of CO band intensity is not. Our preliminary explanation for this is that under HER condition, CO also diffuses along Pt surface until it finds the proper sites. The electronic effect of adsorbed H in the neighboring sites may lead to less electrons transfer from Pt to the anti-bonding orbital of CO bond. As a result, the CO stretching frequency increases. A schematic illustration of surface adsorption of CO is given in FIG. 11 for better understanding. Further studies with DFT calculations are underway to verify this.

FIG. 10 The integral band intensity (a, b) and peak frequency (c, d) of the IR spectra ${\rm C}\rm{O}_\rm{M}$ (a, c) and ${\rm C}\rm{O}_\rm{L}$ (b, d) at Pt film electrode in ${\rm C}\rm{O}_\rm{2}$ saturated 0.1 mol/L NaCl$\rm{O}_\rm{4}$ + $5.0$ × $10^{-6}$ mol/L HCl$\rm{O}_\rm{4}$ recorded during stepping the potential from 0.9 V to 0.3 V (square), 0 V (star), and -0.3 V (triangle).
FIG. 11 Schematic illustration of reaction at Pt electeode in ${\rm C}\rm{O}_\rm{2}$ saturated solution in different potential regions.
Ⅳ. CONCLUSION

The competition of hydrogen evolution reaction and ${\rm C}\rm{O}_\rm{2}$ reduction on Pt electrode is investigated by cyclic voltammetry and infrared spectroscopy. We found that in solution with pH > 2, the interfacial pH increases abruptly during HER. As a result, HER with $\rm{H}^{+}$ as reactant occurs at lower overpotentials, while at higher overpotentials, HER with $\rm{H}_\rm{2}$O as reactant occurs. ${\rm C}\rm{O}_\rm{ad}$ can be formed by ${\rm C}\rm{O}_\rm{2}$ reduction on Pt at potentials where UPD-H or HER occurs. The rate for ${\rm C}\rm{O}_\rm{ad}$ formation increases with the coverage of UPD-H and reaches its maximum at the onset potential for HER. The decrease of ${\rm C}\rm{O}_\rm{ad}$ formation under HER is attributed to the limited sites available and limited residence time for the reduction intermediates ($\rm{H}_\rm{ad}$) necessary for ${\rm C}\rm{O}_\rm{2}$ adsorption and reduction. Furthermore, we found that under HER condition, the peak frequency for ${\rm C}\rm{O}_\rm{ad}$ increases continuously with reaction time, while its band intensity does not. The diffusion of ${\rm C}\rm{O}_\rm{ad}$ on Pt surface introduced by the dynamic turnover of $\rm{H}_\rm{ad}$ is suggested to be the origin for such change.

Ⅴ. ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (No.21473175 and No.21273215) and the Ministry of Science and Technology of China (No.2015CB932301).

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不同pH酸性溶液中铂电极上氢析出和二氧化碳还原的电化学研究
杨晶, 魏杰, 陈微, 陈艳霞     
中国科学技术大学化学物理系,合肥微尺度物质科学国家研究中心,合肥 230026
摘要: 本文利用循环伏安法和电化学原位红外光谱的联用,研究了Pt(111)和Pt膜电极在CO$_2$饱和的酸性溶液中氢析出和CO$_2$还原的竞争.发现:(ⅰ)在pH$>$2的溶液中,主要反应是氢析出,界面pH值随着氢析出突然增加;(ⅱ)通过红外光谱检测,CO$_{\rm ad}$是CO$_2$还原过程中唯一的吸附中间体;(ⅲ)CO$_{\rm ad}$生成速率随着欠电位沉积氢(UPD-H)覆盖的增加而增大,并在氢析出的起始电位达到最大值;(ⅳ)在氢析出时,CO$_{\rm ad}$的减少与CO$_2$吸附和还原所必需的的中间产物(H$_{\rm ad}$)有限的可用位点和停留时间相关.
关键词: 氢析出    二氧化碳还原    Pt(111)    酸性溶液    pH效应    红外光谱