- Email: [email protected]
oxidation reaction in alkaline media?
Nano Energy 62 (2019) 601–609
Contents lists available at ScienceDirect
Nano Energy journal homepage: www.elsevier.com/locate/nanoen
Full paper
Does the oxophilic effect serve the same role for hydrogen evolution/ oxidation reaction in alkaline media?
T
Lin-fan Shena, Bang-an Lua, Xi-ming Qua, Jin-yu Yea, Jun-ming Zhanga, Shu-hu Yina, Qi-hui Wuc, Rui-xiang Wanga, Shou-yu Shena, Tian Shengb,∗, Yan-xia Jianga,∗∗, Shi-gang Suna,∗∗∗ a
State Key Laboratory of Physical Chemistry of Solid Surfaces, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, PR China b College of Chemistry and Materials Science, Anhui Normal University, 241000, Wuhu, PR China c College of Mechanical and Energy Engineering, Jimei University, 361021, Xiamen, PR China
ARTICLE INFO
ABSTRACT
Keywords: Hydrogen evolution reaction Hydrogen oxidation reaction Oxophilic effect Electronic effect Pt-based alloy
Improving the slow kinetics of hydrogen evolution/oxidation reaction(HER/HOR) on Pt in the alkaline electrolyte is key to the development of water splitting and hydroxide exchange membrane fuel cells, which feature a potential cost advantage over their acid-operating counterparts. However, it is still unconfirmed whether adsorbed surface hydroxyl species (OHad) plays a significant role in determining HER/HOR activity. Moreover, the active sites should be different in the alkaline due to the sluggish reaction rate. In the present work, electrochemical tests have shown that for modified bulk Pt surface and Pt3Ni nanoalloy, HER rate is co-determined by the oxophilic effect and electronic effect, while the rate of HOR is associated with the electronic effect. Density functional theory (DFT) calculations reveal the fundamentally different HER and HOR mechanism of Pt-based nanoparticles, and the surface charge may account for such difference. Finally, the adsorption and oxidation of carbon monoxide (CO) as a novel descriptor are provided to predicate the activity of HER and HOR.
1. Introduction Hydrogen energy, as renewable energy sources, is the most likely fuel to solve global warming and energy security. The hydrogen oxidation reaction (HOR) primarily finds application in polymer electrolyte membrane fuel cell (PEMFC), while the hydrogen evolution reaction (HER) features in various electrolyzers such as water splitting [1–3]. Many materials are more stable in alkaline compared to in acid. Unfortunately, the HER/HOR kinetics on Pt is much slower in alkaline than in acid electrolytes [4]. Although previous studies, mainly focusing on the various effects of Pt-Had in combination with HER/HOR activity, have helped to improve alkaline HER/HOR activities of pure Pt, several key questions remain unanswered in the bimetallic system [5–8]. First, the controversy between hydrogen binding energy(HBE) and bifunctional mechanism: these two lines of research-studying the strength of the bond between adsorbed hydrogen intermediate (Had) and the metal to pinpoint the influence of electronic effect, and comparing Pt with more oxophilic metal or alloy that combination of Pt with more oxophilic components to measure the role of the surface
hydroxyl species (OHad)-have been crucial to understanding the reasons for the poor HER/HOR activity of Pt in alkaline electrolytes [9–12]. Furthermore, Koper et al. proposed the potential of zero free charge (pzfc) to understand the sluggish kinetics of HER and HOR in alkaline [13,14]. They found the pzfc of Pt(111) is more positive to HER/HOR potential, and the negative charge on Pt(111) during HER/HOR would increase energetic barrier of Volmer step, due to higher reorganization energy of water to transport hydroxyl ion (i.e. OH−) throughout the interfacial double layer region [15]. More importantly, HER/HOR are generally considered as a reaction determined by the Volmer step on Pt (111) with the value of symmetry factor is 0.5 in HER/HOR in alkaline media [16,17]. The opinion that HER and HOR on the Pt-based surface have the same active sites, which is available on Pt in acid media due to their very fast kinetic and high reversibility, should be strongly doubted in alkaline media where the kinetic of HER/HOR decrease 2 to 3 orders of magnitude. Notably, the addition of foreign metals may also influence the reversibility of HER/ HOR. Modifying the platinum surface with more oxophilic species - by decorating it with 3D transition metal (3D-TM = Ni, Co, Fe, Mn)
Corresponding author. Corresponding author. ∗∗∗ Corresponding author. E-mail addresses: [email protected] (T. Sheng), [email protected] (Y.-x. Jiang), [email protected] (S.-g. Sun). ∗
∗∗
https://doi.org/10.1016/j.nanoen.2019.05.045 Received 20 February 2019; Received in revised form 5 May 2019; Accepted 15 May 2019 Available online 24 May 2019 2211-2855/ © 2019 Elsevier Ltd. All rights reserved.
Nano Energy 62 (2019) 601–609
L.-f. Shen, et al.
hydroxides (3D-TM(OH)2) adislands or alloying it with nickel, for example, which are expected to provide sites for OHad - enhanced the HER activity, was controversial in HOR which found the surface Ni(OH)2 is hinder HOR activity [1,18–20]. Furthermore, the classical HER system of Pt-Ni do not show the superior activity in HOR as well as in HER: alloying with Ni in alkaline can increase HER activity by 10 times, whereas HOR can be increased slightly by 3 times [9,17]. Additionally, additive of Ni on Pt(111) can only improve HER activity without enhancing HOR activity [9,21]. However, it cannot explain the difference between the HER and HOR. Markovic et al. observed that adding alkali metal (AM+: K+, Li+ and Ba2+) cations into the alkaline electrolyte improved the HER rates, but impeded the HOR activity [21]. Recently, Jia et al. also found increasing the AM+ concentration in alkaline solution only promotes the HER, while varying the identity of AM+ affects both HER/HOR, and they ascribed this asymmetry to the modified hard-soft acid-base (HSAB) theory. They suggested AM+ as a Lewis hard acid that binds tightly to OH−(i.e. a Lewis hard base), but weakly to the nearly neutral OHad (i.e. a Lewis soft base). The unbalanced binding energy originating from the unbalanced charge between OH− and OHad drives the OHad desorption into the bulk, thereby the AM+ only boosting the Volmer step of HER [15,22]. The above observations also demonstrate such difference between HER and HOR. It seems like oxophilic sites and optimized M-Had sites do not have the same influence on HER and HOR. A full molecular understanding of this result will indeed enable the discovery of more active and costeffective catalysts for water splitting and fuel cells in alkaline. From a more fundamental point of view, HER/HOR is also an electrochemical reaction of fundamental scientific importance. However, much less fundamental work has been directed towards understanding both HER and HOR in alkaline solutions, although these two processes are of great concern for the development of alkaline electrolyzers and alkaline fuel cells (AFCs) systems. So far, there is no systematic study based on the HER/HOR activity of the model surface. Here, we use a rational design method to isolate the contributions from Had and OHad for the HER/HOR in alkaline environments, by successfully transferring the knowledge gained from polycrystalline Pt surfaces to Pt-based nanoparticle catalysts. First, by using bulk Pt surface decorating with different coverage Ni(OH)2 as model catalysts and the subsequence HER/HOR activity trends, we found the rate of the HER is controlled by both the Had and OHad while HOR is mainly determined by near-optimal Had. And the observed enhancement HOR activity should be attributed to the tuning Pt-Had, rather than the bifunctional mechanism. Density functional theory (DFT) calculations reveal the fundamentally different HER and HOR mechanism of Ptbased nanoparticles, and the surface charge may account for such difference. Finally, the adsorption and oxidation of CO as a novel descriptor are provided to predicate the activity of HER and HOR.
hydrogen underpotential desorption (Hupd) on catalysts [14]. The SEMEDS of different coverage of Ni(OH)2/Pt foil shows the presence of Ni. The increasing content of Ni can also be reflected by the Pt-Ni atomic ratio calculated by X-ray photoelectron spectroscopy (XPS) (Table S1) and CO stripping results (Fig. S5). The HER activity exhibited a monotonically increasing relationship with Ni(OH)2 coverage, whereas the positive role of Ni(OH)2 on HOR activity can only be observed on the surface with the coverage of Ni(OH)2 more than 15%. Previous literature suggested that such enhancement can be attributed to the bifunctional mechanism, rather than electronic effects. However, interesting changes of the electronic structure were observed in XPS, as shown in Fig. 1c, which was ignored in 3D-TM(OH)2/Pt system. After large adsorption 35% coverage of Ni(OH)2 clusters on its surface, the binding energy of Pt 4f5/2 decreased approximately 0.27eV compared with pure Pt foil, along with Pt-Had binding energy of Pt disk greatly decreased, resulting from the strong interaction between Pt and Ni (OH)2, and such weakening Pt-Had accordingly would facilitate HER/ HOR activity. By contrast, low coverage of surface Ni species (only 5%) has no obvious impact of Pt-Had energy (Fig. 1b and c). As such, the improvement of HOR activity to that of Pt disk should be ascribed to the Ni-induced weakening of the EPt-H, rather than the surface Ni(OH)2. In addition, the HER/HOR performance of Ni(OH)2 decorating Pt/C nanoparticles was also tested under the same condition with the same conclusion (Fig. S6). Electrochemical impedance spectroscopy (EIS) at the potential window of ± 0.05V (vs.RHE) were used to study the kinetics of HER/ HOR. The experimental admittance spectra were fitted by equivalent electric circuit (EEC) presented in Fig. S7 [24]. The main difference is observed in the charge transfer resistance values. The semicircle in high frequency (HF, Rct1) is ascribed to the mass transfer processes of the adsorbed Had, while the semicircle in low frequency (LF, Rct2) could be associated with the charge transfer process. On the Pt-based materials, the HER/HOR occurs on the surface covered by more strongly adsorbed Hupd that is a spectator during the HER/HOR. As such the Hupd attenuates the availability of Pt active sites for adsorption/desorption of H2 and Had. As shown in Table S2 the inverse of the Rct1 for Ni(OH)2/Pt disk show a clear enhancement of Hupd rates in alkaline media, as compared with Pt disk, suggesting that Ni can accelerate Had formation in alkaline. The Rct2 in HER region exhibits a shift towards apparently smaller resistance as the coverage of Ni(OH)2 increases, ranging from 770 Ω to 34.7 Ω, while Pt with 15% coverage of Ni(OH)2 has smallest Rct2 of 222 Ω in HOR region (Fig. 1e, S8). These observations further confirmed that HOR kinetics is not governed by the OHad on the surface, and the unbalance between HER and HOR. Since the role of Ni (OH)2 on HOR activity seems to be complicated and moderate coverage is required for high HOR activity, we therefore conclude that the observed improvement of HOR activity on Ni(OH)2/Pt disk in the previous work [9] mostly originates from the optimized electronic structure rather than the bifunctional effect. The similar conclusion is available on PtRu and PtNi alloy [12,17]. In contrast, the bifunctional mechanism in HER is confirmed by the monotonical relationship between HER activity and Ni(OH)2 coverage. Thereby revoking the bifunctional mechanism for HOR on that rests on the erroneous assignment of the CO stripping, together with the idea that HER and HOR have the same active sites.
2. Results and discussion 2.1. HER/HOR on Pt disk modified by Ni(OH)2 First, various coverage of Ni(OH)2 clusters were deposited on Pt disk surfaces to explore their role on the HER/HOR kinetics, respectively. The HER/HOR performance of Pt disk was assessed on a rotating disk electrode (RDE) in 0.1 M NaOH as a baseline (Fig. 1d). These were followed by decorating with Ni(NO3)2 solution into the electrolyte applying potentials above 0.6V(see experiment section in supplementary materials) [9,23]. We used scanning electron microscopy (SEM) to investigate the microstructures of bare Pt foil and Pt foil modified by electrochemically deposited different coverage of Ni(OH)2 clusters, referred as Ni(OH)2/Pt foil. The image of Pt foil in Fig. S1 displays the presence of a few defects on the flat surface. Ni(OH)2 islands on Pt foil randomly distributed across the Pt foil with hemisphere-like shapes (Figs. S2–S4), which is in agreement with prior reports [9]. The surface coverage of Ni(OH)2 on Pt disk is estimated by the reduction of
2.2. Structure characterization of Pt3Ni nanoparticles To further clarify the electronic and oxophilic effects, we turned to examine it on octahedral Pt-based nanoparticles. The transmission electron microscopy (TEM) and high-angle annular dark-field scanning TEM (HAADF-STEM) images show that all of the samples consists of octahedral nanoparticles with an edge size of 4.2 ± 0.6 nm, similar to reported work(Fig. 2, S9,S11) [25,26]. High-resolution TEM (HRTEM) images taking from individual Pt/C, Pt3Ni/C and Acid-Pt3Ni/C octahedra showed an edge lattice spacing of 0.23 nm, 0.221 nm, and 602
Nano Energy 62 (2019) 601–609
L.-f. Shen, et al.
Fig. 1. (a) Schematic illustration of Pt disk to Ni(OH)2/Pt disk transformation via electrochemical deposition. (b) CVs of Pt disk with 0%, 5%, 15%, and 35% coverage of Ni collected in an Ar-saturated 0.1 M NaOH electrolyte at 100 mV s−1. (c) Pt 4f core-level XPS spectra and (d) HER/HOR polarization curves of Pt disk with 0%, 5%, 15% and 35% coverage of Ni collected in an H2-saturated 0.1 M NaOH. (e) EIS comparison on HER/HOR of 5% coverage of Ni on the Pt disk.
0.224 nm which is consistent with that expected for fcc Pt(111) and Pt3Ni(111) plane (Fig. 2b-c, S9-10). TEM energy-dispersive x-ray spectroscopy (TEM-EDS) mapping and composition line-scan profiles across octahedra for Pt3Ni/C and Acid-Pt3Ni/C show that all elements were distributed throughout the NCs with clear Pt skeleton, while in Acid-Pt3Ni/C, Ni clearly weakened (Fig. 2d–g, S12). The surface Ni of Pt3Ni/C, corresponding to the OHad sites, disappeared after washing by glacial acetic acid (Fig. S13), while the Ni inside the NPs can tune the electronic structure of the Pt from the alloying effect. However, the bulk composition remains unchanged, as revealed by inductively coupled plasma optical emission spectroscopy (ICP-OES) in Table S3 and SEMEDS (Fig. S14). Estimated by the change of charge of Hupd, the coverage of Ni on Pt3Ni/C is 5%, which is similar to the 5% coverage of Ni(OH)2 on the Pt disk. Since the influence of facet effect and size effect can be excluded, we can evaluate electronic effect by the comparison of Pt/C and Acid-Pt3Ni/C, and oxophilic effect by the difference between Pt3Ni/ C and Acid-Pt3Ni/C. The diffraction peaks position for the Pt3Ni/C has been shifted to
higher 2θ values by comparison with that of Pt/C, reflecting lattice contraction with Ni content, but similar with Acid-Pt3Ni/C (Fig. 2j). XPS spectra (Table S4 summarizes the results of peak deconvolution) reveal that the Pt 4f peaks of Pt3Ni/C shifts towards higher binding energies than Pt/C, which means a weaker Had at the surface of the catalyst. Similar Pt 4f binding energy was observed for Pt3Ni/C and Acid-Pt3Ni/C, demonstrating that they have identical electronic effects (Fig. 2h). There are oxidized Ni species on the Pt3Ni/C surface which can be used as OHad site. After acid washed, the signal of Ni became weaker and showed a peak assigned to the metallic Ni, which further confirms that the surface Ni had been removed during the acid wash process(Fig. 2i). The near-surface molar ratio of Pt: Ni, calculated into 2.85:1 and 2.94:1 for Pt3Ni/C and Acid-Pt3Ni/C, close to that measured from ICP-OES (Fig. S15) and SEM-EDS. To access electronic effect, we used CV to evaluate the HBE [12,27]. The adsorption/desorption behavior of Hupd on Pt3Ni/C and AcidPt3Ni/C is similar but weaker than that of Pt/C in 0.1 M NaOH solution (Fig. S16), consistent with XPS results. It is well known that CO 603
Nano Energy 62 (2019) 601–609
L.-f. Shen, et al.
Fig. 2. (a)Schematic illustration of Pt3Ni/C to Acid-Pt3Ni/C transformation via acid treatment. Representative HAADF‐STEM images of the (b) Pt3Ni/C and (c) AcidPt3Ni/C catalysts, the inset pictures are HRTEM images on these nanocrystals. Elemental mapping of (d) Pt3Ni/C and (e) Acid- Pt3Ni/C. TEM-EDS line-scanning profile across individual (f) Pt3Ni/C and (g) Acid-Pt3Ni/C octahedral nanocrystals. (h) Pt 4f and (i) Ni 2p core-level XPS spectra of Pt/C, Pt3Ni/C, and Acid-Pt3Ni/C catalysts, respectively. (j) XRD patterns for different electrocatalysts.
oxidation can be used to monitor the generation of reactive hydroxyl species [28]. A single sharp oxidation peak appears on the CO stripping curve of Pt/C, and distinct features on Pt3Ni/C are also noteworthy: multiple CO stripping peaks emerge and the onset oxidation potential shifts negatively about ∼200 mV vs. Pt/C, suggesting that the Ni(OH)2 formed on the surface of Pt3Ni/C are capable of activating water molecules and reactively remove CO adsorbed on neighboring Pt sites [29]. However, the CO stripping of Acid-Pt3Ni/C features a sharp peak accompanied by a pre-peak, resulting from the low-coordinated sites generated by Ni dissolution and subsurface dealloying, and the onset oxidation potential (0.39 V) appears near the potential of Pt/C (0.43 V). According to the above analysis, we briefly summarise the order of Had and OHad energetics for three catalysts: Pt-Had decreased in the order of Pt3Ni/C ≈ Acid-Pt3Ni/C < Pt/C, while the Pt-OHad following the sequence Pt/C < Acid-Pt3Ni/C < Pt3Ni/C. Hence, upon comparing three catalysts, it offers a unique opportunity to study the influences of bifunctional mechanism and electronic effect for HER and HOR with a genuinely isolate electronic effect and oxophlic effect.
of 8 increase in HER activity than Pt/C. These results imply that HER activity can be increased by the surface Ni via promoting OH adsorption. This information was equally accessible by comparing Tafel slopes of three catalysts while the Tafel slope can be used to deduce HER mechanism. Generally, the HER in alkaline media undergoes VolmerHeyrovsky or Volmer-Tafel [30]. The Tafel slope of Pt3Ni/C (54 mV dec−1) and Acid-Pt3Ni/C (71 mV dec−1) near the value of Heyrovsky step (39 mV dec−1), which is lower than Pt/C (Volmer step), revealing that the higher water dissociation efficiency induced by alloying with Ni, and the rate-determining Heyrovsky step can be further enhanced by the surface Ni(OH)2 [31,32]. Hence, Pt3Ni/C is a better catalyst than Acid-Pt3Ni/C for promoting HER, the HER rate of a Pt-based alloy decreases in the order Pt3Ni/C > Acid-Pt3Ni/C > Pt/C, attributed to the synergistic effect of optimized Pt sites and oxoplilic sites. However, the conclusion was different in the region of HOR. As stressed in the introduction, efficient anodic electrochemical energy conversion in AFCs is determined by the rate of the reaction for the HOR (H2+2OH−↔2H2O+2e−) at the catalyst–electrolyte interfaces. This anodic reaction proceeds through the adsorption of molecular hydrogen, which is strongly dependent on the availability of active sites and the nature of surface atoms. This process is then followed by H2 dissociation and the formation of hydrogen intermediates (metal–Had interaction), a process that governs the intrinsic activity of the HOR on metal surfaces. Fig. 3b shows the HOR polarization curves on Pt-based nanoparticles in 0.1 M NaOH. The H2 mass transfer was corrected according to the Koutecky-Levich equation to get the kinetic current (ik) [17,33]. Then, exchange current (i0) of HER/HOR was obtained by fitting ik into the Butler-Volmer equation (Eq (1)). The electrochemically active surface area (ECSA) normalized exchange current densities (j0) were used to evaluate the catalytic activity.
2.3. HER/HOR activity on Pt-Based nanoparticles Having established the morphology and chemical nature of the Ptbased NP catalysts, then we discuss the influence of the oxophilic effect and electronic effect on HER and HOR, respectively. The HER/HOR performance of Pt-based catalysts was tested by producing linear sweep voltammetry (LSV) curve in 0.1 M NaOH (pH 13.0) at room temperature. To make a quantitative activity comparison without the influence of hydrogen mass transport, we choose the overpotential of 0.05V (RHE) for comparison. As shown in Fig. 3a, the current density of AcidPt3Ni/C at −0.05 V is 6.2 mA cm−2, 5 times as active for the HER relative to Pt/C (1.3 mA cm−2), suggesting that significant enhancement of the HER performance can be achieved by weaker Had. Pt3Ni/C further increasing the current density to 8.3 mA cm−2, resulting in a factor
ik = i0(eαFη/RT-e(α−1)Fη/RT) 604
(1)
Nano Energy 62 (2019) 601–609
L.-f. Shen, et al.
Fig. 3. (a)HER polarization curves of Pt/C, Pt3Ni/C, and Acid-Pt3Ni/C. (b) HOR polarization curves of Pt/C, Pt3Ni/C, and Acid-Pt3Ni/C catalysts. (c) Tafel plots for HER/HOR on Pt/C, Pt3Ni/C, and Acid-Pt3Ni/C. (d) Histograms of current densities at the overpotential of 0.05V for Pt/C, Pt3Ni/C, and Acid-Pt3Ni/C, respectively. EIS of Pt/C, Pt3Ni/C and Acid-Pt3Ni/C in (e) HER, (f)HOR. All the polarization curves were recorded at a scan rate of 10 mV s−1 and a rotation rate of 1,600 rpm in 0.1 M H2-saturated NaOH.
Here, α is the symmetry factor, and η is the overpotential. Fig. S17 shows the disk geometric normalized kinetic current density (logarithmic plot) versus the potential and the Butler−Volmer fitted curves. The obtained j0 values for the three catalysts were shown in Table S5. The j0 of octahedral Pt/C is similar to that of Pt (111) and spherical Pt/C nanoparticles reported in the literature, indicating that the facet effect is insignificant here [17,34,35]. Compared to Pt/C, Pt3Ni/C and Acid-Pt3Ni/C show the faster current densities increased against the applied potential, together with the larger j0, indicating higher HOR activities. And the polarization curve of Pt3Ni/C is close to that of AcidPt3Ni/C, and no apparent enhancement as in HER region is observed. At 0.05V, Pt3Ni/C gains an improved factor of 2.9 to Pt/C, even a little smaller than Acid-Pt3Ni/C with a factor of 3.1. The superiority of Pt3Ni/C over Pt/C should be a proof of electronic effect, since the
decreased HOR activity of Pt3Ni/C towards Acid-Pt3Ni/C, demonstrating the much less promoting effect of OHad on HOR, which is consistency with our results from 5% Ni(OH)2/Pt disk. The Pt/C, Pt3Ni/C and Acid-Pt3Ni/C catalysts demonstrated a Tafel slope of 144 mVdec−1, 129 mVdec−1 and 113 mVdec−1, respectively, which is in accord with a Volmer rds. Moreover, it was suggested that the Tafel-Volmer pathway would be the reaction mechanism with Volmer step as the RDS based on the data fitting to the Butler-Volmer equation with α+β = 1 [6,16,36]. This finding suggests that tuning the HBEs of the catalysts is an efficient route to improve the HOR activity in alkaline electrolyte. We used EIS to further monitor kinetic information during HER and HOR (Fig. 3e-f). The same EIS measurements as on Ni(OH)2/Pt disk were made on Pt-based nanoparticles. The relevant electrochemical 605
Nano Energy 62 (2019) 601–609
L.-f. Shen, et al.
parameters are summarized in Table S6. The mass transfer resistance and charge transfer resistance of HER on Pt/C is equivalent to that of the HOR. Unlike the single metal Pt/C, a much smaller charge transfer resistance was observed for Pt3Ni/C in HER region, while Acid-Pt3Ni/C has the smallest HOR charge transfer resistance. After the second metal Ni was introduced (Pt3Ni/C and Acid-Pt3Ni/C), the Hupd in HF was also accelerated than the pure Pt/C. Correlating the different resistance sequences with the HER/HOR trends, we believe that the role of Ni on the HER is to activate water and weaken Pt-Had, while the action of Ni on the HOR is to weaken Pt-Had bond energetics. Since alloying Pt with Ni post two effects: optimal Pt sits and additional Ni sites on the surface, the HER improvement with increasing Ni coverage of Pt3Ni/C, and the HOR rate slightly decreased at the overpotential of 0.05V, suggesting that the oxophilic effect serve the different role for hydrogen evolution/ oxidation reaction in alkaline media. The i0 values, extracted from fitting Eq (2) to the data, agree well with those obtained from the Butler–Volmer equation.
i0 =
RT i RT 1 = F F R ct
energy barrier for breaking the OH–H bond in water is 0.61 eV on Pt (111) surface together with 0.80 eV on Pt3Ni (111)@Ptskin, and such a high energy barrier hinders the dissociation of water to Hads. Notably, Ni(OH)2 can provide the active sites for hydroxyl adsorption, and the followed ΔGH2O is reduced to 0.05 eV on the Ni(OH)2/Pt3Ni (111) interface, indicating the Ni(OH)2 nanoparticles are effective for cleaving HO–H bonds [37]. Subsequently, the produced Hads would combine to form H2 while the binding free energies of H* intermediates (ΔGH*) are applied as an activity descriptor for the hydrogen generation step (Tafel step or Heyrovsky step). On Pt3Ni(111)@Ptskin, the H* adsorption energy is −0.16 eV, closer to 0 eV than −0.38 eV on Pt(111) and −0.32 eV on Pt3Ni(111). The free energy profiles in HER (Fig. 4d) indicating that the activities are determined the Volmer and Heyrovsky steps together and the order is: Pt (111) (0.99 eV) < Pt3Ni (111) @ Ptskin (0.96 eV) < Pt3Ni (111) (0.78 eV). For HOR, the first step is H2 dissociation to produce two H* on the surface, and the second step is the Volmer step for H* dissociation into solution. It is obvious that the value of H* adsorption energy closer to zero indicates the higher activity [38]. The ΔGH* value is 0.16 eV for the Pt3Ni (111)@Ptskin catalyst sites, which is close to the optimal value (ΔGH* = 0 eV) and is highly reactive. And thus, Fig. 4e shows that the HOR activities are in the order of: Pt (111) (0.76 eV) < Pt3Ni (111) (0.64 eV) < Pt3Ni (111)@Ptskin (0.32 eV). Zeta potential can help to understand the different mechanism of HER and HOR (Fig. S20) [39]. In 0.1 M NaOH solution, the Pt3Ni/C had a zeta potential of −28.9 mV, indicating a strongly negative surface charge. After acid washed, the Acid-Pt3Ni/C showed a zeta potential of −25.6 mV. The slightly negative charge of Pt3Ni/C might originate from the Ni(OH)2 on the surface. Because the presence of OHad + e− ↔ OH−, the negative charge of Pt-Ni alloy are expected to increase in the order Acid-Pt3Ni/C < Pt3Ni/C, and as a result, the availability of the active sites for OH− adsorption/desorption is expected to decrease in the same order. The strongly negative surface charge on the Pt3Ni/C facilitate the OHad desorption into the bulk (Eq. (3)), thereby boosting the Volmer step, and thus increases the HER current, but only in one direction. It will impede the HOR by destabilizing the OHad. As a result, the presence of OHad improves HER but not the HOR, matching the selective HER activity enhancement of Acid-Pt3Ni/C and Pt3Ni/C with increasing Ni content. Namely, the unbalanced surface charge between OH− and catalysts drives the OHad desorption into the bulk. The
(2)
Noted that it can only be seen as a rigorous comparison while the kinetics by impedance spectroscopy depends on the morphology, particle size and crystal face. As shown in Fig. 3e-f, despite the different surface morphology between the Ni(OH)2/Pt disk and PtNi alloy, their identical HER/HOR activity order suggested that: (1)For HER, optimized Had and OHad sites can accelerate HER activity; (2) HOR is only determined by Pt sites. Moreover, the asymmetry of HER and HOR current densities concerning the reversible potential illustrates that the HER and HOR have the different reaction mechanism and intermediates, which can be reflected by the symmetry factor (α). However, α is only an apparent transfer coefficient, the real reason for this unbalanced is the rds of HER is Heyrovsky step, but Volmer step for HOR. 2.4. Different reaction mechanism and surface charge To shed more light on the difference between hydrogen evolution and oxidation on three surfaces, the key reaction steps in alkaline HER/ HOR were investigated by DFT calculations. As shown in Fig. 4 and Figs. S18–19, for HER in alkaline medium, the initial step is the Volmer step via water dissociation to produce H* and OH* on the surface. The
Fig. 4. Side views of (a)Pt(111), (b)Pt3Ni(111) and, (c)Pt3Ni(111)@Ptskin models. Blue: Pt; cyan: Ni. (The same colors were used in this work). (d)Free energy profiles for hydrogen evolution reactions in alkaline medium via Heyrovsky mechanisms on Pt(111), Pt3Ni(111) and, Pt3Ni(111)@Ptskin. (e)Free energy profiles for hydrogen oxidation reactions on Pt(111), Pt3Ni(111) and, Pt3Ni(111)@Ptskin. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
606
Nano Energy 62 (2019) 601–609
L.-f. Shen, et al.
Fig. 5. (a) Variation of the CO band with Es for Pt/C, Pt3Ni/C and Acid-Pt3Ni/C linear bounded CO in 0.1 M NaOH. (b) Correlation between HER activity at −0.05VRHE and the potential of COad oxidation obtained for the investigated set of particles. (c) Correlation between HOR activity at 0.05VRHE and the COad wavenumber obtained for the investigated set of particles. The error bars depicted the standard deviation between at least three different and independent measurements.
schematic illustration of the catalytic roles of surface charge in the alkaline HER/HOR kinetics was shown in Fig. S21. This observation also following previous studies that Li+ can only accelerate HER activity, while Li+ with a positive charge can carry OH− in the double layer [9,22]. However, the case of Pt/C is beyond our discussion, since there is no foreign metal to optimate Pt-Had energetics or to add oxophlic sites, which have great influence on HER/HOR processes. H2O + M + e−→ M-Hads + OH− (Volmer) −
−
H2O + M-Hads + e → H2 + M + OH 2M-Hads → 2 M + H2 (Tafel)
(Heyrovsky)
the COad has been fully oxidized. Two IR bands were recorded. A small but broad negative-going band near 1850 cm−1 that are assigned to IR absorption of bridge bonded CO (COB), the negative-going band around 2000 cm−1 is ascribed to IR absorption of linearly bonded CO (COL) at Es varied from 0.1V to 0.24V with an interval of 0.02V [40]. We focus on the COL because the main existence of CO adsorbed on the Pt is linearly bonded. The center of this band shifts to higher wavenumbers with the increase of Es wherein the data are linearly correlated as shown in Fig. 5a, which represents the electrochemical Stark shift rate. The turnaround of Stark coefficient of COL represents the onset oxidation of CO [41,42]. The onset oxidation of Pt/C, Pt3Ni/C and AcidPt3Ni/C is 0.2V, 0.16V, 0.18V. Generally, OHad can facilitate the oxidation of CO adsorption on Pt and lower the oxidation potential. Hence, Pt3Ni/C exhibits an early oxidation potential than Pt/C and Acid-Pt3Ni/ C. Correlating the oxidation of COad with the corresponding HER activities (Fig. 5b) reveals that for shaped Pt-Ni alloys, stronger OHad induces higher catalytic HER activities. In addition, weaker CO surface chemisorption energies increase the C-O binding energy in the CO molecule and increase their C-O stretching frequency and wavenumber. Alloying with Ni maintains a favorable downshift in the Pt d band, leading to a weaker d−π* interaction with the COad [29,43]. Therefore the Had adsorbed on three NPs can be estimated from in situ MS-FTIR spectroscopy by the wavenumbers of CO. Fig. 5a presents the wavenumbers of the stretching vibrations of linearly bonded CO, with Acid-Pt3Ni/C exhibiting the highest wavenumbers (2043 cm−1), followed by Pt3Ni/C (2037 cm−1), and then Pt/C (2033 cm−1). The wavenumber of COL for Pt3Ni/C and Acid-Pt3Ni/C are basically same but higher than Pt/C, which means the order of Pt-Had energetics is Pt3Ni/C ≈ Acid-Pt3Ni/C < Pt/C. The wavenumber with the corresponding HOR activities (Fig. 5c) reveals that weaker Had induces higher catalytic HOR activities. We conclude that Pt/C bind surface species too strong, resulting in overall lower HOR activities, while Pt3Ni/C and Acid-Pt3Ni/C exhibits a weaker, more optimal binding strength. Therefore, CO is a good descriptor for HER/HOR. Specifically, the strength of CO adsorption can predicate the HOR, the onset oxidation can predicate HER.
(3) (4) (5)
From experimental results and DFT calculations, in alkaline media, the mechanism of HER and HOR at the Pt-based nanoparticles can be proposed as follows. HER reaction: The Ni(OH)2 clusters on the surface promoting the dissociation of water into Hads species. Hydrogen adsorption on the nearby vacant Pt sites while the negative charge originating from the Ni (OH)2 on the surface facilitate the OH− desorption into the bulk. Had atoms on the Pt surface combine with H atoms from another water molecule to form H2 molecules and OH− desorbs from the Ni(OH)2. The facile desorption of OH− from the surface of Ni(OH)2 at both Volmer and Heyrovsky steps promotes the HER kinetics, thus greatly decreasing the onset potential and Tafel slope at the Pt3Ni/C relative to that at the Pt/C and Acid-Pt3Ni/C. HOR reaction: H2 molecules adsorbed on the optimized Pt skin surface to form M-Hads species, due to the less negative charge on the surface of Pt skin, it is easier for OH− to near Pt skin surface, thus largely increasing the possibility of adsorbing new hydrogen intermediates. M-Hads species combine with OH− atoms from solution to form a water molecule. The facile adsorption of OH− ions from the solution promotes the HOR kinetics. 2.5. A novel descriptor of active sites As discussed above, Pt-based bimetallic catalysts have different active site towards HER and HOR. How to descript such active sites remains a great challenge. Herein, an experimental descriptor is developed to further correlate the chemical nature of the catalyst particle surfaces with HER and HOR activities. CO is sensitive to surface charge state, can monitor the OHad. In situ electrochemical FTIR spectroscopy can provide, at the molecular level, information on the species involved in electrochemical reactions, such as adsorption, desorption, and surface bonding. In -situ electrochemical FTIR spectroscopy of surfaceadsorbed CO (COad) was employed to infer the value of Pt-Had and PtOHad energetics during HER and HOR for three catalysts. Fig. S22 shows the MS-FTIR spectra for CO adsorbed on three catalysts in 0.1 M NaOH solution, and the reference spectrum was taken at 0.7 V where
3. Conclusion In summary, we perform a clear and convincing judgment that oxophilic effects serve different roles on HER and HOR in alkaline. That is, OHad can greatly facilitate HER activity via bifunctional effects, but does not serve a significant role in determining HOR activity. We clarify that the observed enhanced HOR activity on 3D-TM(OH)2/Pt in previous work mainly results from weakening Pt-Had binding energy, rather than so-called bifunctional effects. This conclusion was further proved by octahedral Pt-based nanoparticles. The further and deeper 607
Nano Energy 62 (2019) 601–609
L.-f. Shen, et al.
understanding of the catalyst surface was received by DFT calculations and Zeta potential, indicating that surface charge can account for different reaction mechanism of HER/HOR on Pt-based nanocatalysts. Considering all used methods, it became evident that optimal HER electrocatalysts should balance bifunctional effects and electronic effects while tailoring the electronic structure of electrocatalyst seems to be more effective for HOR. Furthermore, we conclude that HER and HOR may have different active sites. We propose CO as a novel descriptor to predicate the HER/HOR activity. These findings further promote our fundamental understanding of the HER/HOR catalysis and hold promise for significant improvements of rational design of electrocatalysts for HER/HOR.
[41] W. Chen, S.-G. Sun, Z.-Y. Zhou, et al., J. Phys. Chem. B 107 (2003) 9808–9812. [42] S. Shi-Gang, C. Ai-Cheng, J. Electroanal. Chem. 323 (1992) 319–328. [43] H. Igarashi, T. Fujino, Y. Zhu, et al., Phys. Chem. Chem. Phys. 3 (2001) 306–314. Lin-fan Shen is currently pursuing a M.S. degree at Xiamen University under the supervision of Prof. Shi-Gang Sun and Prof. Yan-Xia Jiang. Her research interest is electrocatalysts for electrochemical devices such as water splitting and fuel cells.
Acknowledgements This work was supported by the National Key Research and Development Program of China (2017YFA0206500), the National Natural Science Foundation of China (21773198, U1705253, 21621091 and U1705255) and the Natural Science Foundation of Anhui Province (1908085QB58).
Bang-an Lu obtained his M.S. in applied chemistry from Harbin Engineering University in 2012. He is currently pursuing a Ph.D. at Xiamen University under the supervision of Prof. Shi-Gang Sun and Prof. Na Tian. His research interests focus on tuning surface/nearsurface of Pt-based nanocatalysts nanocatalysts and evaluating their performance of electrocatalytic reactions in fuel cells.
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.nanoen.2019.05.045. References
Xi-ming Qu is currently pursuing his Ph.D. degree under the supervision of Prof. Yan-Xia Jiang. He received his Master's degree in Xiamen University 2016. His current research interests include the synthesis and electrocatalytic properties of non-noble metal based nanocrystals.
[1] J.N. Schwämmlein, B.M. Stühmeier, K. Wagenbauer, et al., J. Electrochem. Soc. 165 (2018) H229–H239. [2] K. Li, Y. Li, Y. Wang, et al., Energy Environ. Sci. 11 (2018) 1232–1239. [3] D. Liu, S. Lu, Y. Xue, et al., Nano Energy 59 (2019) 26–32. [4] W. Sheng, M. Myint, J.G. Chen, et al., Energy Environ. Sci. 6 (2013) 1509–1512. [5] D. Strmcnik, M. Uchimura, C. Wang, et al., Nat. Chem. 5 (2013) 300–306. [6] Y. Cong, B. Yi, Y. Song, Nano Energy 44 (2018) 288–303. [7] N. Danilovic, R. Subbaraman, D. Strmcnik, et al., Angew. Chem. 124 (2012) 12663–12666. [8] Z. Zhao, H. Liu, W. Gao, et al., J. Am. Chem. Soc. 140 (2018) 9046–9050. [9] R. Subbaraman, D. Tripkovic, D. Strmcnik, et al., Science 334 (2011) 1256–1260. [10] J. Durst, A. Siebel, C. Simon, et al., Energy Environ. Sci. 7 (2014) 2255–2260. [11] Z.-F. Ma, Q. Jia, Angew. Chem. Int. Ed. 56 (2017) 15594–15598. [12] Y. Wang, G. Wang, G. Li, et al., Energy Environ. Sci. 8 (2015) 177–181. [13] F.J. Sarabia, P. Sebastián-Pascual, M.T.M. Koper, et al., ACS Appl. Mater. Interfaces 11 (2019) 613–623. [14] I. Ledezma-Yanez, W.D.Z. Wallace, P. Sebastián-Pascual, et al., Nat. Energy 2 (2017) 17031. [15] Q. Jia, E. Liu, L. Jiao, et al., Curr. Opin. Electrochem. 12 (2018) 209–217. [16] P.J. Rheinländer, J. Herranz, J. Durst, et al., J. Electrochem. Soc. 161 (2014) F1448–F1457. [17] S. Lu, Z. Zhuang, J. Am. Chem. Soc. 139 (2017) 5156–5163. [18] R. Subbaraman, D. Tripkovic, K.-C. Chang, et al., Nat. Mater. 11 (2012) 550–557. [19] B. Sidhureddy, A.R. Thiruppathi, A. Chen, J. Electroanal. Chem. 794 (2017) 28–35. [20] J. Ohyama, T. Sato, Y. Yamamoto, et al., J. Am. Chem. Soc. 135 (2013) 8016–8021. [21] N. Danilovic, R. Subbaraman, D. Strmcnik, et al., Electrocatalysis 3 (2012) 221–229. [22] E. Liu, J. Li, L. Jiao, et al., J. Am. Chem. Soc. 141 (2019) 3232–3239. [23] X. Yu, J. Zhao, L.-R. Zheng, et al., ACS Energy Lett. 3 (2018) 237–244. [24] V.M. Nikolic, S.L. Maslovara, G.S. Tasic, et al., Appl. Catal. B Environ. 179 (2015) 88–94. [25] X. Huang, Z. Zhao, L. Cao, et al., Science 348 (2015) 1230–1234. [26] X. Huang, Z. Zhao, Y. Chen, et al., Energy Environ. Sci. 7 (2014) 2957–2962. [27] K. Elbert, J. Hu, Z. Ma, et al., ACS Catal. 5 (2015) 6764–6772. [28] L. Zhuang, J. Jin, H.D. Abruña, J. Am. Chem. Soc. 129 (2007) 11033–11035. [29] V. Beermann, M. Gocyla, S. Kuhl, et al., J. Am. Chem. Soc. 139 (2017) 16536–16547. [30] S.-Z. Qiao, Y. Zheng, Y. Jiao, et al., Angew. Chem. Int. Ed. 57 (2017) 7568–7579. [31] B.E. Conway, B.V. Tilak, Electrochim. Acta 47 (2002) 3571–3594. [32] Y. Zheng, Y. Jiao, M. Jaroniec, et al., Angew. Chem. Int. Ed. 54 (2015) 52–65. [33] B.-A. Lu, T. Sheng, N. Tian, et al., Nano Energy 33 (2017) 65–71. [34] J. Zheng, Y. Yan, B. Xu, J. Electrochem. Soc. 162 (2015) F1470–F1481. [35] W. Sheng, H.A. Gasteiger, Y. Shao-Horn, J. Electrochem. Soc. 157 (2010) B1529–B1536. [36] J. Zheng, W. Sheng, Z. Zhuang, et al., Sci. Adv. 2 (2016) e1501602. [37] P. Wang, X. Zhang, J. Zhang, et al., Nat. Commun. 8 (2017) 14580. [38] B. Zhang, J. Liu, J. Wang, et al., Nano Energy 37 (2017) 74–80. [39] S. Zhou, D. Huo, S. Goines, et al., J. Am. Chem. Soc. 140 (2018) 11898–11901. [40] Z.-C. Zhang, X.-C. Tian, B.-W. Zhang, et al., Nano Energy 34 (2017) 224–232.
Jin-yu Ye received his PhD degree from Department of chemistry of Xiamen University in 2016, and is currently an engineer of College of Chemistry and Chemical Engineering at Xiamen University. His current research interests include the surface electrochemistry, electrocatalysis, spectroelectrochemistry.
Jun-ming Zhang received his master degree in College of Chemistry and Pharmaceutical Sciences from Guangxi Normal University in 2016. He is currently pursuing his Ph.D. in physical chemistry under supervision ogf Prof. Yan-Xia Jiang at Xiamen University. His main research interests include controlling surface compositions of Pt-based nanocatalysts and their electrocatalytic performance towards hydrogen oxidation reaction and oxygen reduction reaction in PEMFC.
608
Nano Energy 62 (2019) 601–609
L.-f. Shen, et al. Shu-hu Yin received his B.S. degree in food science and engineering from Guangzhou University in 2016. He is currently a master student at Xiamen University under the supervision of Prof. Yan-Xia Jiang. His work focuses on Carbon-based electrocatalysts for fuel cell.
Tian Sheng received his B.S. degree from East China University of Science and Technology in 2011, Ph.D. degree from Queen's University Belfast in 2014, and is now an associate professor at Anhui Normal University. His research is to reveal heterogeneous catalytic reaction mechanisms by density functional theory calculations.
Qi-hui Wu obtained PhD degree in 2003 from Darmstadt University of Technology, Germany, then did postdoctorial researches at Georgia Institute of Technology and Ruhr Bochum University, respectively. In 2005, he joined Xiamen University as an Associate Professor, in 2007 he worked at Bonn University as an Alexander von Humboldt (AvH) Fellow, and 2008 at La Trobe University as a Senior Research Fellow, 2011 at City University of Hong Kong as a Research Fellow. Currently, He is a Professor of Materials Science at Jimei University. His research topics include surface science & engineering, and energy materials.
Yan-xia Jiang received her Ph.D. degree from the College of Chemistry, Jilin University in 1999. She then joined the faculty of the Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, and was promoted to associate professor and full professor in 2001 and 2007, respectively. Her current research interests include the surface electrochemistry, electrocatalysis, and spectroelectrochemistry.
Rui-xiang Wang received his M.S. degree from Guangxi Normal University in 2015. He is currently working toward the Ph.D. degree in Xiamen University and his current research focuses on synthesis and application of non-noble metal catalysts.
Shi-gang Sun obtained Doctorat d’Etat in 1986 from Université Pierre et Marie Curie (Paris VI), France, and is actually a professor of chemistry at the Department of Chemistry of Xiamen University, China. His research interests include electrocatalysis, electrochemical surface science, spectroelectrochemistry, and electrochemical energy conversion and storage. He has been elected member of Chinese Academy of Sciences, Fellow of Royal Society of Chemistry and Fellow of International Society of Electrochemistry.
Shou-yu Shen is a Ph.D. student at Xiamen University under the instruction of Prof. Shi-Gang Sun. He received his B.S. degree from School of Chemical and Material Engineering of Jiangnan University in June 2012. His current research mainly focuses on the research of Ni-Rich and Lithium-rich layered Oxide Cathode and insitu X-ray techniques.
609
Contents lists available at ScienceDirect
Nano Energy journal homepage: www.elsevier.com/locate/nanoen
Full paper
Does the oxophilic effect serve the same role for hydrogen evolution/ oxidation reaction in alkaline media?
T
Lin-fan Shena, Bang-an Lua, Xi-ming Qua, Jin-yu Yea, Jun-ming Zhanga, Shu-hu Yina, Qi-hui Wuc, Rui-xiang Wanga, Shou-yu Shena, Tian Shengb,∗, Yan-xia Jianga,∗∗, Shi-gang Suna,∗∗∗ a
State Key Laboratory of Physical Chemistry of Solid Surfaces, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, PR China b College of Chemistry and Materials Science, Anhui Normal University, 241000, Wuhu, PR China c College of Mechanical and Energy Engineering, Jimei University, 361021, Xiamen, PR China
ARTICLE INFO
ABSTRACT
Keywords: Hydrogen evolution reaction Hydrogen oxidation reaction Oxophilic effect Electronic effect Pt-based alloy
Improving the slow kinetics of hydrogen evolution/oxidation reaction(HER/HOR) on Pt in the alkaline electrolyte is key to the development of water splitting and hydroxide exchange membrane fuel cells, which feature a potential cost advantage over their acid-operating counterparts. However, it is still unconfirmed whether adsorbed surface hydroxyl species (OHad) plays a significant role in determining HER/HOR activity. Moreover, the active sites should be different in the alkaline due to the sluggish reaction rate. In the present work, electrochemical tests have shown that for modified bulk Pt surface and Pt3Ni nanoalloy, HER rate is co-determined by the oxophilic effect and electronic effect, while the rate of HOR is associated with the electronic effect. Density functional theory (DFT) calculations reveal the fundamentally different HER and HOR mechanism of Pt-based nanoparticles, and the surface charge may account for such difference. Finally, the adsorption and oxidation of carbon monoxide (CO) as a novel descriptor are provided to predicate the activity of HER and HOR.
1. Introduction Hydrogen energy, as renewable energy sources, is the most likely fuel to solve global warming and energy security. The hydrogen oxidation reaction (HOR) primarily finds application in polymer electrolyte membrane fuel cell (PEMFC), while the hydrogen evolution reaction (HER) features in various electrolyzers such as water splitting [1–3]. Many materials are more stable in alkaline compared to in acid. Unfortunately, the HER/HOR kinetics on Pt is much slower in alkaline than in acid electrolytes [4]. Although previous studies, mainly focusing on the various effects of Pt-Had in combination with HER/HOR activity, have helped to improve alkaline HER/HOR activities of pure Pt, several key questions remain unanswered in the bimetallic system [5–8]. First, the controversy between hydrogen binding energy(HBE) and bifunctional mechanism: these two lines of research-studying the strength of the bond between adsorbed hydrogen intermediate (Had) and the metal to pinpoint the influence of electronic effect, and comparing Pt with more oxophilic metal or alloy that combination of Pt with more oxophilic components to measure the role of the surface
hydroxyl species (OHad)-have been crucial to understanding the reasons for the poor HER/HOR activity of Pt in alkaline electrolytes [9–12]. Furthermore, Koper et al. proposed the potential of zero free charge (pzfc) to understand the sluggish kinetics of HER and HOR in alkaline [13,14]. They found the pzfc of Pt(111) is more positive to HER/HOR potential, and the negative charge on Pt(111) during HER/HOR would increase energetic barrier of Volmer step, due to higher reorganization energy of water to transport hydroxyl ion (i.e. OH−) throughout the interfacial double layer region [15]. More importantly, HER/HOR are generally considered as a reaction determined by the Volmer step on Pt (111) with the value of symmetry factor is 0.5 in HER/HOR in alkaline media [16,17]. The opinion that HER and HOR on the Pt-based surface have the same active sites, which is available on Pt in acid media due to their very fast kinetic and high reversibility, should be strongly doubted in alkaline media where the kinetic of HER/HOR decrease 2 to 3 orders of magnitude. Notably, the addition of foreign metals may also influence the reversibility of HER/ HOR. Modifying the platinum surface with more oxophilic species - by decorating it with 3D transition metal (3D-TM = Ni, Co, Fe, Mn)
Corresponding author. Corresponding author. ∗∗∗ Corresponding author. E-mail addresses: [email protected] (T. Sheng), [email protected] (Y.-x. Jiang), [email protected] (S.-g. Sun). ∗
∗∗
https://doi.org/10.1016/j.nanoen.2019.05.045 Received 20 February 2019; Received in revised form 5 May 2019; Accepted 15 May 2019 Available online 24 May 2019 2211-2855/ © 2019 Elsevier Ltd. All rights reserved.
Nano Energy 62 (2019) 601–609
L.-f. Shen, et al.
hydroxides (3D-TM(OH)2) adislands or alloying it with nickel, for example, which are expected to provide sites for OHad - enhanced the HER activity, was controversial in HOR which found the surface Ni(OH)2 is hinder HOR activity [1,18–20]. Furthermore, the classical HER system of Pt-Ni do not show the superior activity in HOR as well as in HER: alloying with Ni in alkaline can increase HER activity by 10 times, whereas HOR can be increased slightly by 3 times [9,17]. Additionally, additive of Ni on Pt(111) can only improve HER activity without enhancing HOR activity [9,21]. However, it cannot explain the difference between the HER and HOR. Markovic et al. observed that adding alkali metal (AM+: K+, Li+ and Ba2+) cations into the alkaline electrolyte improved the HER rates, but impeded the HOR activity [21]. Recently, Jia et al. also found increasing the AM+ concentration in alkaline solution only promotes the HER, while varying the identity of AM+ affects both HER/HOR, and they ascribed this asymmetry to the modified hard-soft acid-base (HSAB) theory. They suggested AM+ as a Lewis hard acid that binds tightly to OH−(i.e. a Lewis hard base), but weakly to the nearly neutral OHad (i.e. a Lewis soft base). The unbalanced binding energy originating from the unbalanced charge between OH− and OHad drives the OHad desorption into the bulk, thereby the AM+ only boosting the Volmer step of HER [15,22]. The above observations also demonstrate such difference between HER and HOR. It seems like oxophilic sites and optimized M-Had sites do not have the same influence on HER and HOR. A full molecular understanding of this result will indeed enable the discovery of more active and costeffective catalysts for water splitting and fuel cells in alkaline. From a more fundamental point of view, HER/HOR is also an electrochemical reaction of fundamental scientific importance. However, much less fundamental work has been directed towards understanding both HER and HOR in alkaline solutions, although these two processes are of great concern for the development of alkaline electrolyzers and alkaline fuel cells (AFCs) systems. So far, there is no systematic study based on the HER/HOR activity of the model surface. Here, we use a rational design method to isolate the contributions from Had and OHad for the HER/HOR in alkaline environments, by successfully transferring the knowledge gained from polycrystalline Pt surfaces to Pt-based nanoparticle catalysts. First, by using bulk Pt surface decorating with different coverage Ni(OH)2 as model catalysts and the subsequence HER/HOR activity trends, we found the rate of the HER is controlled by both the Had and OHad while HOR is mainly determined by near-optimal Had. And the observed enhancement HOR activity should be attributed to the tuning Pt-Had, rather than the bifunctional mechanism. Density functional theory (DFT) calculations reveal the fundamentally different HER and HOR mechanism of Ptbased nanoparticles, and the surface charge may account for such difference. Finally, the adsorption and oxidation of CO as a novel descriptor are provided to predicate the activity of HER and HOR.
hydrogen underpotential desorption (Hupd) on catalysts [14]. The SEMEDS of different coverage of Ni(OH)2/Pt foil shows the presence of Ni. The increasing content of Ni can also be reflected by the Pt-Ni atomic ratio calculated by X-ray photoelectron spectroscopy (XPS) (Table S1) and CO stripping results (Fig. S5). The HER activity exhibited a monotonically increasing relationship with Ni(OH)2 coverage, whereas the positive role of Ni(OH)2 on HOR activity can only be observed on the surface with the coverage of Ni(OH)2 more than 15%. Previous literature suggested that such enhancement can be attributed to the bifunctional mechanism, rather than electronic effects. However, interesting changes of the electronic structure were observed in XPS, as shown in Fig. 1c, which was ignored in 3D-TM(OH)2/Pt system. After large adsorption 35% coverage of Ni(OH)2 clusters on its surface, the binding energy of Pt 4f5/2 decreased approximately 0.27eV compared with pure Pt foil, along with Pt-Had binding energy of Pt disk greatly decreased, resulting from the strong interaction between Pt and Ni (OH)2, and such weakening Pt-Had accordingly would facilitate HER/ HOR activity. By contrast, low coverage of surface Ni species (only 5%) has no obvious impact of Pt-Had energy (Fig. 1b and c). As such, the improvement of HOR activity to that of Pt disk should be ascribed to the Ni-induced weakening of the EPt-H, rather than the surface Ni(OH)2. In addition, the HER/HOR performance of Ni(OH)2 decorating Pt/C nanoparticles was also tested under the same condition with the same conclusion (Fig. S6). Electrochemical impedance spectroscopy (EIS) at the potential window of ± 0.05V (vs.RHE) were used to study the kinetics of HER/ HOR. The experimental admittance spectra were fitted by equivalent electric circuit (EEC) presented in Fig. S7 [24]. The main difference is observed in the charge transfer resistance values. The semicircle in high frequency (HF, Rct1) is ascribed to the mass transfer processes of the adsorbed Had, while the semicircle in low frequency (LF, Rct2) could be associated with the charge transfer process. On the Pt-based materials, the HER/HOR occurs on the surface covered by more strongly adsorbed Hupd that is a spectator during the HER/HOR. As such the Hupd attenuates the availability of Pt active sites for adsorption/desorption of H2 and Had. As shown in Table S2 the inverse of the Rct1 for Ni(OH)2/Pt disk show a clear enhancement of Hupd rates in alkaline media, as compared with Pt disk, suggesting that Ni can accelerate Had formation in alkaline. The Rct2 in HER region exhibits a shift towards apparently smaller resistance as the coverage of Ni(OH)2 increases, ranging from 770 Ω to 34.7 Ω, while Pt with 15% coverage of Ni(OH)2 has smallest Rct2 of 222 Ω in HOR region (Fig. 1e, S8). These observations further confirmed that HOR kinetics is not governed by the OHad on the surface, and the unbalance between HER and HOR. Since the role of Ni (OH)2 on HOR activity seems to be complicated and moderate coverage is required for high HOR activity, we therefore conclude that the observed improvement of HOR activity on Ni(OH)2/Pt disk in the previous work [9] mostly originates from the optimized electronic structure rather than the bifunctional effect. The similar conclusion is available on PtRu and PtNi alloy [12,17]. In contrast, the bifunctional mechanism in HER is confirmed by the monotonical relationship between HER activity and Ni(OH)2 coverage. Thereby revoking the bifunctional mechanism for HOR on that rests on the erroneous assignment of the CO stripping, together with the idea that HER and HOR have the same active sites.
2. Results and discussion 2.1. HER/HOR on Pt disk modified by Ni(OH)2 First, various coverage of Ni(OH)2 clusters were deposited on Pt disk surfaces to explore their role on the HER/HOR kinetics, respectively. The HER/HOR performance of Pt disk was assessed on a rotating disk electrode (RDE) in 0.1 M NaOH as a baseline (Fig. 1d). These were followed by decorating with Ni(NO3)2 solution into the electrolyte applying potentials above 0.6V(see experiment section in supplementary materials) [9,23]. We used scanning electron microscopy (SEM) to investigate the microstructures of bare Pt foil and Pt foil modified by electrochemically deposited different coverage of Ni(OH)2 clusters, referred as Ni(OH)2/Pt foil. The image of Pt foil in Fig. S1 displays the presence of a few defects on the flat surface. Ni(OH)2 islands on Pt foil randomly distributed across the Pt foil with hemisphere-like shapes (Figs. S2–S4), which is in agreement with prior reports [9]. The surface coverage of Ni(OH)2 on Pt disk is estimated by the reduction of
2.2. Structure characterization of Pt3Ni nanoparticles To further clarify the electronic and oxophilic effects, we turned to examine it on octahedral Pt-based nanoparticles. The transmission electron microscopy (TEM) and high-angle annular dark-field scanning TEM (HAADF-STEM) images show that all of the samples consists of octahedral nanoparticles with an edge size of 4.2 ± 0.6 nm, similar to reported work(Fig. 2, S9,S11) [25,26]. High-resolution TEM (HRTEM) images taking from individual Pt/C, Pt3Ni/C and Acid-Pt3Ni/C octahedra showed an edge lattice spacing of 0.23 nm, 0.221 nm, and 602
Nano Energy 62 (2019) 601–609
L.-f. Shen, et al.
Fig. 1. (a) Schematic illustration of Pt disk to Ni(OH)2/Pt disk transformation via electrochemical deposition. (b) CVs of Pt disk with 0%, 5%, 15%, and 35% coverage of Ni collected in an Ar-saturated 0.1 M NaOH electrolyte at 100 mV s−1. (c) Pt 4f core-level XPS spectra and (d) HER/HOR polarization curves of Pt disk with 0%, 5%, 15% and 35% coverage of Ni collected in an H2-saturated 0.1 M NaOH. (e) EIS comparison on HER/HOR of 5% coverage of Ni on the Pt disk.
0.224 nm which is consistent with that expected for fcc Pt(111) and Pt3Ni(111) plane (Fig. 2b-c, S9-10). TEM energy-dispersive x-ray spectroscopy (TEM-EDS) mapping and composition line-scan profiles across octahedra for Pt3Ni/C and Acid-Pt3Ni/C show that all elements were distributed throughout the NCs with clear Pt skeleton, while in Acid-Pt3Ni/C, Ni clearly weakened (Fig. 2d–g, S12). The surface Ni of Pt3Ni/C, corresponding to the OHad sites, disappeared after washing by glacial acetic acid (Fig. S13), while the Ni inside the NPs can tune the electronic structure of the Pt from the alloying effect. However, the bulk composition remains unchanged, as revealed by inductively coupled plasma optical emission spectroscopy (ICP-OES) in Table S3 and SEMEDS (Fig. S14). Estimated by the change of charge of Hupd, the coverage of Ni on Pt3Ni/C is 5%, which is similar to the 5% coverage of Ni(OH)2 on the Pt disk. Since the influence of facet effect and size effect can be excluded, we can evaluate electronic effect by the comparison of Pt/C and Acid-Pt3Ni/C, and oxophilic effect by the difference between Pt3Ni/ C and Acid-Pt3Ni/C. The diffraction peaks position for the Pt3Ni/C has been shifted to
higher 2θ values by comparison with that of Pt/C, reflecting lattice contraction with Ni content, but similar with Acid-Pt3Ni/C (Fig. 2j). XPS spectra (Table S4 summarizes the results of peak deconvolution) reveal that the Pt 4f peaks of Pt3Ni/C shifts towards higher binding energies than Pt/C, which means a weaker Had at the surface of the catalyst. Similar Pt 4f binding energy was observed for Pt3Ni/C and Acid-Pt3Ni/C, demonstrating that they have identical electronic effects (Fig. 2h). There are oxidized Ni species on the Pt3Ni/C surface which can be used as OHad site. After acid washed, the signal of Ni became weaker and showed a peak assigned to the metallic Ni, which further confirms that the surface Ni had been removed during the acid wash process(Fig. 2i). The near-surface molar ratio of Pt: Ni, calculated into 2.85:1 and 2.94:1 for Pt3Ni/C and Acid-Pt3Ni/C, close to that measured from ICP-OES (Fig. S15) and SEM-EDS. To access electronic effect, we used CV to evaluate the HBE [12,27]. The adsorption/desorption behavior of Hupd on Pt3Ni/C and AcidPt3Ni/C is similar but weaker than that of Pt/C in 0.1 M NaOH solution (Fig. S16), consistent with XPS results. It is well known that CO 603
Nano Energy 62 (2019) 601–609
L.-f. Shen, et al.
Fig. 2. (a)Schematic illustration of Pt3Ni/C to Acid-Pt3Ni/C transformation via acid treatment. Representative HAADF‐STEM images of the (b) Pt3Ni/C and (c) AcidPt3Ni/C catalysts, the inset pictures are HRTEM images on these nanocrystals. Elemental mapping of (d) Pt3Ni/C and (e) Acid- Pt3Ni/C. TEM-EDS line-scanning profile across individual (f) Pt3Ni/C and (g) Acid-Pt3Ni/C octahedral nanocrystals. (h) Pt 4f and (i) Ni 2p core-level XPS spectra of Pt/C, Pt3Ni/C, and Acid-Pt3Ni/C catalysts, respectively. (j) XRD patterns for different electrocatalysts.
oxidation can be used to monitor the generation of reactive hydroxyl species [28]. A single sharp oxidation peak appears on the CO stripping curve of Pt/C, and distinct features on Pt3Ni/C are also noteworthy: multiple CO stripping peaks emerge and the onset oxidation potential shifts negatively about ∼200 mV vs. Pt/C, suggesting that the Ni(OH)2 formed on the surface of Pt3Ni/C are capable of activating water molecules and reactively remove CO adsorbed on neighboring Pt sites [29]. However, the CO stripping of Acid-Pt3Ni/C features a sharp peak accompanied by a pre-peak, resulting from the low-coordinated sites generated by Ni dissolution and subsurface dealloying, and the onset oxidation potential (0.39 V) appears near the potential of Pt/C (0.43 V). According to the above analysis, we briefly summarise the order of Had and OHad energetics for three catalysts: Pt-Had decreased in the order of Pt3Ni/C ≈ Acid-Pt3Ni/C < Pt/C, while the Pt-OHad following the sequence Pt/C < Acid-Pt3Ni/C < Pt3Ni/C. Hence, upon comparing three catalysts, it offers a unique opportunity to study the influences of bifunctional mechanism and electronic effect for HER and HOR with a genuinely isolate electronic effect and oxophlic effect.
of 8 increase in HER activity than Pt/C. These results imply that HER activity can be increased by the surface Ni via promoting OH adsorption. This information was equally accessible by comparing Tafel slopes of three catalysts while the Tafel slope can be used to deduce HER mechanism. Generally, the HER in alkaline media undergoes VolmerHeyrovsky or Volmer-Tafel [30]. The Tafel slope of Pt3Ni/C (54 mV dec−1) and Acid-Pt3Ni/C (71 mV dec−1) near the value of Heyrovsky step (39 mV dec−1), which is lower than Pt/C (Volmer step), revealing that the higher water dissociation efficiency induced by alloying with Ni, and the rate-determining Heyrovsky step can be further enhanced by the surface Ni(OH)2 [31,32]. Hence, Pt3Ni/C is a better catalyst than Acid-Pt3Ni/C for promoting HER, the HER rate of a Pt-based alloy decreases in the order Pt3Ni/C > Acid-Pt3Ni/C > Pt/C, attributed to the synergistic effect of optimized Pt sites and oxoplilic sites. However, the conclusion was different in the region of HOR. As stressed in the introduction, efficient anodic electrochemical energy conversion in AFCs is determined by the rate of the reaction for the HOR (H2+2OH−↔2H2O+2e−) at the catalyst–electrolyte interfaces. This anodic reaction proceeds through the adsorption of molecular hydrogen, which is strongly dependent on the availability of active sites and the nature of surface atoms. This process is then followed by H2 dissociation and the formation of hydrogen intermediates (metal–Had interaction), a process that governs the intrinsic activity of the HOR on metal surfaces. Fig. 3b shows the HOR polarization curves on Pt-based nanoparticles in 0.1 M NaOH. The H2 mass transfer was corrected according to the Koutecky-Levich equation to get the kinetic current (ik) [17,33]. Then, exchange current (i0) of HER/HOR was obtained by fitting ik into the Butler-Volmer equation (Eq (1)). The electrochemically active surface area (ECSA) normalized exchange current densities (j0) were used to evaluate the catalytic activity.
2.3. HER/HOR activity on Pt-Based nanoparticles Having established the morphology and chemical nature of the Ptbased NP catalysts, then we discuss the influence of the oxophilic effect and electronic effect on HER and HOR, respectively. The HER/HOR performance of Pt-based catalysts was tested by producing linear sweep voltammetry (LSV) curve in 0.1 M NaOH (pH 13.0) at room temperature. To make a quantitative activity comparison without the influence of hydrogen mass transport, we choose the overpotential of 0.05V (RHE) for comparison. As shown in Fig. 3a, the current density of AcidPt3Ni/C at −0.05 V is 6.2 mA cm−2, 5 times as active for the HER relative to Pt/C (1.3 mA cm−2), suggesting that significant enhancement of the HER performance can be achieved by weaker Had. Pt3Ni/C further increasing the current density to 8.3 mA cm−2, resulting in a factor
ik = i0(eαFη/RT-e(α−1)Fη/RT) 604
(1)
Nano Energy 62 (2019) 601–609
L.-f. Shen, et al.
Fig. 3. (a)HER polarization curves of Pt/C, Pt3Ni/C, and Acid-Pt3Ni/C. (b) HOR polarization curves of Pt/C, Pt3Ni/C, and Acid-Pt3Ni/C catalysts. (c) Tafel plots for HER/HOR on Pt/C, Pt3Ni/C, and Acid-Pt3Ni/C. (d) Histograms of current densities at the overpotential of 0.05V for Pt/C, Pt3Ni/C, and Acid-Pt3Ni/C, respectively. EIS of Pt/C, Pt3Ni/C and Acid-Pt3Ni/C in (e) HER, (f)HOR. All the polarization curves were recorded at a scan rate of 10 mV s−1 and a rotation rate of 1,600 rpm in 0.1 M H2-saturated NaOH.
Here, α is the symmetry factor, and η is the overpotential. Fig. S17 shows the disk geometric normalized kinetic current density (logarithmic plot) versus the potential and the Butler−Volmer fitted curves. The obtained j0 values for the three catalysts were shown in Table S5. The j0 of octahedral Pt/C is similar to that of Pt (111) and spherical Pt/C nanoparticles reported in the literature, indicating that the facet effect is insignificant here [17,34,35]. Compared to Pt/C, Pt3Ni/C and Acid-Pt3Ni/C show the faster current densities increased against the applied potential, together with the larger j0, indicating higher HOR activities. And the polarization curve of Pt3Ni/C is close to that of AcidPt3Ni/C, and no apparent enhancement as in HER region is observed. At 0.05V, Pt3Ni/C gains an improved factor of 2.9 to Pt/C, even a little smaller than Acid-Pt3Ni/C with a factor of 3.1. The superiority of Pt3Ni/C over Pt/C should be a proof of electronic effect, since the
decreased HOR activity of Pt3Ni/C towards Acid-Pt3Ni/C, demonstrating the much less promoting effect of OHad on HOR, which is consistency with our results from 5% Ni(OH)2/Pt disk. The Pt/C, Pt3Ni/C and Acid-Pt3Ni/C catalysts demonstrated a Tafel slope of 144 mVdec−1, 129 mVdec−1 and 113 mVdec−1, respectively, which is in accord with a Volmer rds. Moreover, it was suggested that the Tafel-Volmer pathway would be the reaction mechanism with Volmer step as the RDS based on the data fitting to the Butler-Volmer equation with α+β = 1 [6,16,36]. This finding suggests that tuning the HBEs of the catalysts is an efficient route to improve the HOR activity in alkaline electrolyte. We used EIS to further monitor kinetic information during HER and HOR (Fig. 3e-f). The same EIS measurements as on Ni(OH)2/Pt disk were made on Pt-based nanoparticles. The relevant electrochemical 605
Nano Energy 62 (2019) 601–609
L.-f. Shen, et al.
parameters are summarized in Table S6. The mass transfer resistance and charge transfer resistance of HER on Pt/C is equivalent to that of the HOR. Unlike the single metal Pt/C, a much smaller charge transfer resistance was observed for Pt3Ni/C in HER region, while Acid-Pt3Ni/C has the smallest HOR charge transfer resistance. After the second metal Ni was introduced (Pt3Ni/C and Acid-Pt3Ni/C), the Hupd in HF was also accelerated than the pure Pt/C. Correlating the different resistance sequences with the HER/HOR trends, we believe that the role of Ni on the HER is to activate water and weaken Pt-Had, while the action of Ni on the HOR is to weaken Pt-Had bond energetics. Since alloying Pt with Ni post two effects: optimal Pt sits and additional Ni sites on the surface, the HER improvement with increasing Ni coverage of Pt3Ni/C, and the HOR rate slightly decreased at the overpotential of 0.05V, suggesting that the oxophilic effect serve the different role for hydrogen evolution/ oxidation reaction in alkaline media. The i0 values, extracted from fitting Eq (2) to the data, agree well with those obtained from the Butler–Volmer equation.
i0 =
RT i RT 1 = F F R ct
energy barrier for breaking the OH–H bond in water is 0.61 eV on Pt (111) surface together with 0.80 eV on Pt3Ni (111)@Ptskin, and such a high energy barrier hinders the dissociation of water to Hads. Notably, Ni(OH)2 can provide the active sites for hydroxyl adsorption, and the followed ΔGH2O is reduced to 0.05 eV on the Ni(OH)2/Pt3Ni (111) interface, indicating the Ni(OH)2 nanoparticles are effective for cleaving HO–H bonds [37]. Subsequently, the produced Hads would combine to form H2 while the binding free energies of H* intermediates (ΔGH*) are applied as an activity descriptor for the hydrogen generation step (Tafel step or Heyrovsky step). On Pt3Ni(111)@Ptskin, the H* adsorption energy is −0.16 eV, closer to 0 eV than −0.38 eV on Pt(111) and −0.32 eV on Pt3Ni(111). The free energy profiles in HER (Fig. 4d) indicating that the activities are determined the Volmer and Heyrovsky steps together and the order is: Pt (111) (0.99 eV) < Pt3Ni (111) @ Ptskin (0.96 eV) < Pt3Ni (111) (0.78 eV). For HOR, the first step is H2 dissociation to produce two H* on the surface, and the second step is the Volmer step for H* dissociation into solution. It is obvious that the value of H* adsorption energy closer to zero indicates the higher activity [38]. The ΔGH* value is 0.16 eV for the Pt3Ni (111)@Ptskin catalyst sites, which is close to the optimal value (ΔGH* = 0 eV) and is highly reactive. And thus, Fig. 4e shows that the HOR activities are in the order of: Pt (111) (0.76 eV) < Pt3Ni (111) (0.64 eV) < Pt3Ni (111)@Ptskin (0.32 eV). Zeta potential can help to understand the different mechanism of HER and HOR (Fig. S20) [39]. In 0.1 M NaOH solution, the Pt3Ni/C had a zeta potential of −28.9 mV, indicating a strongly negative surface charge. After acid washed, the Acid-Pt3Ni/C showed a zeta potential of −25.6 mV. The slightly negative charge of Pt3Ni/C might originate from the Ni(OH)2 on the surface. Because the presence of OHad + e− ↔ OH−, the negative charge of Pt-Ni alloy are expected to increase in the order Acid-Pt3Ni/C < Pt3Ni/C, and as a result, the availability of the active sites for OH− adsorption/desorption is expected to decrease in the same order. The strongly negative surface charge on the Pt3Ni/C facilitate the OHad desorption into the bulk (Eq. (3)), thereby boosting the Volmer step, and thus increases the HER current, but only in one direction. It will impede the HOR by destabilizing the OHad. As a result, the presence of OHad improves HER but not the HOR, matching the selective HER activity enhancement of Acid-Pt3Ni/C and Pt3Ni/C with increasing Ni content. Namely, the unbalanced surface charge between OH− and catalysts drives the OHad desorption into the bulk. The
(2)
Noted that it can only be seen as a rigorous comparison while the kinetics by impedance spectroscopy depends on the morphology, particle size and crystal face. As shown in Fig. 3e-f, despite the different surface morphology between the Ni(OH)2/Pt disk and PtNi alloy, their identical HER/HOR activity order suggested that: (1)For HER, optimized Had and OHad sites can accelerate HER activity; (2) HOR is only determined by Pt sites. Moreover, the asymmetry of HER and HOR current densities concerning the reversible potential illustrates that the HER and HOR have the different reaction mechanism and intermediates, which can be reflected by the symmetry factor (α). However, α is only an apparent transfer coefficient, the real reason for this unbalanced is the rds of HER is Heyrovsky step, but Volmer step for HOR. 2.4. Different reaction mechanism and surface charge To shed more light on the difference between hydrogen evolution and oxidation on three surfaces, the key reaction steps in alkaline HER/ HOR were investigated by DFT calculations. As shown in Fig. 4 and Figs. S18–19, for HER in alkaline medium, the initial step is the Volmer step via water dissociation to produce H* and OH* on the surface. The
Fig. 4. Side views of (a)Pt(111), (b)Pt3Ni(111) and, (c)Pt3Ni(111)@Ptskin models. Blue: Pt; cyan: Ni. (The same colors were used in this work). (d)Free energy profiles for hydrogen evolution reactions in alkaline medium via Heyrovsky mechanisms on Pt(111), Pt3Ni(111) and, Pt3Ni(111)@Ptskin. (e)Free energy profiles for hydrogen oxidation reactions on Pt(111), Pt3Ni(111) and, Pt3Ni(111)@Ptskin. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
606
Nano Energy 62 (2019) 601–609
L.-f. Shen, et al.
Fig. 5. (a) Variation of the CO band with Es for Pt/C, Pt3Ni/C and Acid-Pt3Ni/C linear bounded CO in 0.1 M NaOH. (b) Correlation between HER activity at −0.05VRHE and the potential of COad oxidation obtained for the investigated set of particles. (c) Correlation between HOR activity at 0.05VRHE and the COad wavenumber obtained for the investigated set of particles. The error bars depicted the standard deviation between at least three different and independent measurements.
schematic illustration of the catalytic roles of surface charge in the alkaline HER/HOR kinetics was shown in Fig. S21. This observation also following previous studies that Li+ can only accelerate HER activity, while Li+ with a positive charge can carry OH− in the double layer [9,22]. However, the case of Pt/C is beyond our discussion, since there is no foreign metal to optimate Pt-Had energetics or to add oxophlic sites, which have great influence on HER/HOR processes. H2O + M + e−→ M-Hads + OH− (Volmer) −
−
H2O + M-Hads + e → H2 + M + OH 2M-Hads → 2 M + H2 (Tafel)
(Heyrovsky)
the COad has been fully oxidized. Two IR bands were recorded. A small but broad negative-going band near 1850 cm−1 that are assigned to IR absorption of bridge bonded CO (COB), the negative-going band around 2000 cm−1 is ascribed to IR absorption of linearly bonded CO (COL) at Es varied from 0.1V to 0.24V with an interval of 0.02V [40]. We focus on the COL because the main existence of CO adsorbed on the Pt is linearly bonded. The center of this band shifts to higher wavenumbers with the increase of Es wherein the data are linearly correlated as shown in Fig. 5a, which represents the electrochemical Stark shift rate. The turnaround of Stark coefficient of COL represents the onset oxidation of CO [41,42]. The onset oxidation of Pt/C, Pt3Ni/C and AcidPt3Ni/C is 0.2V, 0.16V, 0.18V. Generally, OHad can facilitate the oxidation of CO adsorption on Pt and lower the oxidation potential. Hence, Pt3Ni/C exhibits an early oxidation potential than Pt/C and Acid-Pt3Ni/ C. Correlating the oxidation of COad with the corresponding HER activities (Fig. 5b) reveals that for shaped Pt-Ni alloys, stronger OHad induces higher catalytic HER activities. In addition, weaker CO surface chemisorption energies increase the C-O binding energy in the CO molecule and increase their C-O stretching frequency and wavenumber. Alloying with Ni maintains a favorable downshift in the Pt d band, leading to a weaker d−π* interaction with the COad [29,43]. Therefore the Had adsorbed on three NPs can be estimated from in situ MS-FTIR spectroscopy by the wavenumbers of CO. Fig. 5a presents the wavenumbers of the stretching vibrations of linearly bonded CO, with Acid-Pt3Ni/C exhibiting the highest wavenumbers (2043 cm−1), followed by Pt3Ni/C (2037 cm−1), and then Pt/C (2033 cm−1). The wavenumber of COL for Pt3Ni/C and Acid-Pt3Ni/C are basically same but higher than Pt/C, which means the order of Pt-Had energetics is Pt3Ni/C ≈ Acid-Pt3Ni/C < Pt/C. The wavenumber with the corresponding HOR activities (Fig. 5c) reveals that weaker Had induces higher catalytic HOR activities. We conclude that Pt/C bind surface species too strong, resulting in overall lower HOR activities, while Pt3Ni/C and Acid-Pt3Ni/C exhibits a weaker, more optimal binding strength. Therefore, CO is a good descriptor for HER/HOR. Specifically, the strength of CO adsorption can predicate the HOR, the onset oxidation can predicate HER.
(3) (4) (5)
From experimental results and DFT calculations, in alkaline media, the mechanism of HER and HOR at the Pt-based nanoparticles can be proposed as follows. HER reaction: The Ni(OH)2 clusters on the surface promoting the dissociation of water into Hads species. Hydrogen adsorption on the nearby vacant Pt sites while the negative charge originating from the Ni (OH)2 on the surface facilitate the OH− desorption into the bulk. Had atoms on the Pt surface combine with H atoms from another water molecule to form H2 molecules and OH− desorbs from the Ni(OH)2. The facile desorption of OH− from the surface of Ni(OH)2 at both Volmer and Heyrovsky steps promotes the HER kinetics, thus greatly decreasing the onset potential and Tafel slope at the Pt3Ni/C relative to that at the Pt/C and Acid-Pt3Ni/C. HOR reaction: H2 molecules adsorbed on the optimized Pt skin surface to form M-Hads species, due to the less negative charge on the surface of Pt skin, it is easier for OH− to near Pt skin surface, thus largely increasing the possibility of adsorbing new hydrogen intermediates. M-Hads species combine with OH− atoms from solution to form a water molecule. The facile adsorption of OH− ions from the solution promotes the HOR kinetics. 2.5. A novel descriptor of active sites As discussed above, Pt-based bimetallic catalysts have different active site towards HER and HOR. How to descript such active sites remains a great challenge. Herein, an experimental descriptor is developed to further correlate the chemical nature of the catalyst particle surfaces with HER and HOR activities. CO is sensitive to surface charge state, can monitor the OHad. In situ electrochemical FTIR spectroscopy can provide, at the molecular level, information on the species involved in electrochemical reactions, such as adsorption, desorption, and surface bonding. In -situ electrochemical FTIR spectroscopy of surfaceadsorbed CO (COad) was employed to infer the value of Pt-Had and PtOHad energetics during HER and HOR for three catalysts. Fig. S22 shows the MS-FTIR spectra for CO adsorbed on three catalysts in 0.1 M NaOH solution, and the reference spectrum was taken at 0.7 V where
3. Conclusion In summary, we perform a clear and convincing judgment that oxophilic effects serve different roles on HER and HOR in alkaline. That is, OHad can greatly facilitate HER activity via bifunctional effects, but does not serve a significant role in determining HOR activity. We clarify that the observed enhanced HOR activity on 3D-TM(OH)2/Pt in previous work mainly results from weakening Pt-Had binding energy, rather than so-called bifunctional effects. This conclusion was further proved by octahedral Pt-based nanoparticles. The further and deeper 607
Nano Energy 62 (2019) 601–609
L.-f. Shen, et al.
understanding of the catalyst surface was received by DFT calculations and Zeta potential, indicating that surface charge can account for different reaction mechanism of HER/HOR on Pt-based nanocatalysts. Considering all used methods, it became evident that optimal HER electrocatalysts should balance bifunctional effects and electronic effects while tailoring the electronic structure of electrocatalyst seems to be more effective for HOR. Furthermore, we conclude that HER and HOR may have different active sites. We propose CO as a novel descriptor to predicate the HER/HOR activity. These findings further promote our fundamental understanding of the HER/HOR catalysis and hold promise for significant improvements of rational design of electrocatalysts for HER/HOR.
[41] W. Chen, S.-G. Sun, Z.-Y. Zhou, et al., J. Phys. Chem. B 107 (2003) 9808–9812. [42] S. Shi-Gang, C. Ai-Cheng, J. Electroanal. Chem. 323 (1992) 319–328. [43] H. Igarashi, T. Fujino, Y. Zhu, et al., Phys. Chem. Chem. Phys. 3 (2001) 306–314. Lin-fan Shen is currently pursuing a M.S. degree at Xiamen University under the supervision of Prof. Shi-Gang Sun and Prof. Yan-Xia Jiang. Her research interest is electrocatalysts for electrochemical devices such as water splitting and fuel cells.
Acknowledgements This work was supported by the National Key Research and Development Program of China (2017YFA0206500), the National Natural Science Foundation of China (21773198, U1705253, 21621091 and U1705255) and the Natural Science Foundation of Anhui Province (1908085QB58).
Bang-an Lu obtained his M.S. in applied chemistry from Harbin Engineering University in 2012. He is currently pursuing a Ph.D. at Xiamen University under the supervision of Prof. Shi-Gang Sun and Prof. Na Tian. His research interests focus on tuning surface/nearsurface of Pt-based nanocatalysts nanocatalysts and evaluating their performance of electrocatalytic reactions in fuel cells.
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.nanoen.2019.05.045. References
Xi-ming Qu is currently pursuing his Ph.D. degree under the supervision of Prof. Yan-Xia Jiang. He received his Master's degree in Xiamen University 2016. His current research interests include the synthesis and electrocatalytic properties of non-noble metal based nanocrystals.
[1] J.N. Schwämmlein, B.M. Stühmeier, K. Wagenbauer, et al., J. Electrochem. Soc. 165 (2018) H229–H239. [2] K. Li, Y. Li, Y. Wang, et al., Energy Environ. Sci. 11 (2018) 1232–1239. [3] D. Liu, S. Lu, Y. Xue, et al., Nano Energy 59 (2019) 26–32. [4] W. Sheng, M. Myint, J.G. Chen, et al., Energy Environ. Sci. 6 (2013) 1509–1512. [5] D. Strmcnik, M. Uchimura, C. Wang, et al., Nat. Chem. 5 (2013) 300–306. [6] Y. Cong, B. Yi, Y. Song, Nano Energy 44 (2018) 288–303. [7] N. Danilovic, R. Subbaraman, D. Strmcnik, et al., Angew. Chem. 124 (2012) 12663–12666. [8] Z. Zhao, H. Liu, W. Gao, et al., J. Am. Chem. Soc. 140 (2018) 9046–9050. [9] R. Subbaraman, D. Tripkovic, D. Strmcnik, et al., Science 334 (2011) 1256–1260. [10] J. Durst, A. Siebel, C. Simon, et al., Energy Environ. Sci. 7 (2014) 2255–2260. [11] Z.-F. Ma, Q. Jia, Angew. Chem. Int. Ed. 56 (2017) 15594–15598. [12] Y. Wang, G. Wang, G. Li, et al., Energy Environ. Sci. 8 (2015) 177–181. [13] F.J. Sarabia, P. Sebastián-Pascual, M.T.M. Koper, et al., ACS Appl. Mater. Interfaces 11 (2019) 613–623. [14] I. Ledezma-Yanez, W.D.Z. Wallace, P. Sebastián-Pascual, et al., Nat. Energy 2 (2017) 17031. [15] Q. Jia, E. Liu, L. Jiao, et al., Curr. Opin. Electrochem. 12 (2018) 209–217. [16] P.J. Rheinländer, J. Herranz, J. Durst, et al., J. Electrochem. Soc. 161 (2014) F1448–F1457. [17] S. Lu, Z. Zhuang, J. Am. Chem. Soc. 139 (2017) 5156–5163. [18] R. Subbaraman, D. Tripkovic, K.-C. Chang, et al., Nat. Mater. 11 (2012) 550–557. [19] B. Sidhureddy, A.R. Thiruppathi, A. Chen, J. Electroanal. Chem. 794 (2017) 28–35. [20] J. Ohyama, T. Sato, Y. Yamamoto, et al., J. Am. Chem. Soc. 135 (2013) 8016–8021. [21] N. Danilovic, R. Subbaraman, D. Strmcnik, et al., Electrocatalysis 3 (2012) 221–229. [22] E. Liu, J. Li, L. Jiao, et al., J. Am. Chem. Soc. 141 (2019) 3232–3239. [23] X. Yu, J. Zhao, L.-R. Zheng, et al., ACS Energy Lett. 3 (2018) 237–244. [24] V.M. Nikolic, S.L. Maslovara, G.S. Tasic, et al., Appl. Catal. B Environ. 179 (2015) 88–94. [25] X. Huang, Z. Zhao, L. Cao, et al., Science 348 (2015) 1230–1234. [26] X. Huang, Z. Zhao, Y. Chen, et al., Energy Environ. Sci. 7 (2014) 2957–2962. [27] K. Elbert, J. Hu, Z. Ma, et al., ACS Catal. 5 (2015) 6764–6772. [28] L. Zhuang, J. Jin, H.D. Abruña, J. Am. Chem. Soc. 129 (2007) 11033–11035. [29] V. Beermann, M. Gocyla, S. Kuhl, et al., J. Am. Chem. Soc. 139 (2017) 16536–16547. [30] S.-Z. Qiao, Y. Zheng, Y. Jiao, et al., Angew. Chem. Int. Ed. 57 (2017) 7568–7579. [31] B.E. Conway, B.V. Tilak, Electrochim. Acta 47 (2002) 3571–3594. [32] Y. Zheng, Y. Jiao, M. Jaroniec, et al., Angew. Chem. Int. Ed. 54 (2015) 52–65. [33] B.-A. Lu, T. Sheng, N. Tian, et al., Nano Energy 33 (2017) 65–71. [34] J. Zheng, Y. Yan, B. Xu, J. Electrochem. Soc. 162 (2015) F1470–F1481. [35] W. Sheng, H.A. Gasteiger, Y. Shao-Horn, J. Electrochem. Soc. 157 (2010) B1529–B1536. [36] J. Zheng, W. Sheng, Z. Zhuang, et al., Sci. Adv. 2 (2016) e1501602. [37] P. Wang, X. Zhang, J. Zhang, et al., Nat. Commun. 8 (2017) 14580. [38] B. Zhang, J. Liu, J. Wang, et al., Nano Energy 37 (2017) 74–80. [39] S. Zhou, D. Huo, S. Goines, et al., J. Am. Chem. Soc. 140 (2018) 11898–11901. [40] Z.-C. Zhang, X.-C. Tian, B.-W. Zhang, et al., Nano Energy 34 (2017) 224–232.
Jin-yu Ye received his PhD degree from Department of chemistry of Xiamen University in 2016, and is currently an engineer of College of Chemistry and Chemical Engineering at Xiamen University. His current research interests include the surface electrochemistry, electrocatalysis, spectroelectrochemistry.
Jun-ming Zhang received his master degree in College of Chemistry and Pharmaceutical Sciences from Guangxi Normal University in 2016. He is currently pursuing his Ph.D. in physical chemistry under supervision ogf Prof. Yan-Xia Jiang at Xiamen University. His main research interests include controlling surface compositions of Pt-based nanocatalysts and their electrocatalytic performance towards hydrogen oxidation reaction and oxygen reduction reaction in PEMFC.
608
Nano Energy 62 (2019) 601–609
L.-f. Shen, et al. Shu-hu Yin received his B.S. degree in food science and engineering from Guangzhou University in 2016. He is currently a master student at Xiamen University under the supervision of Prof. Yan-Xia Jiang. His work focuses on Carbon-based electrocatalysts for fuel cell.
Tian Sheng received his B.S. degree from East China University of Science and Technology in 2011, Ph.D. degree from Queen's University Belfast in 2014, and is now an associate professor at Anhui Normal University. His research is to reveal heterogeneous catalytic reaction mechanisms by density functional theory calculations.
Qi-hui Wu obtained PhD degree in 2003 from Darmstadt University of Technology, Germany, then did postdoctorial researches at Georgia Institute of Technology and Ruhr Bochum University, respectively. In 2005, he joined Xiamen University as an Associate Professor, in 2007 he worked at Bonn University as an Alexander von Humboldt (AvH) Fellow, and 2008 at La Trobe University as a Senior Research Fellow, 2011 at City University of Hong Kong as a Research Fellow. Currently, He is a Professor of Materials Science at Jimei University. His research topics include surface science & engineering, and energy materials.
Yan-xia Jiang received her Ph.D. degree from the College of Chemistry, Jilin University in 1999. She then joined the faculty of the Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, and was promoted to associate professor and full professor in 2001 and 2007, respectively. Her current research interests include the surface electrochemistry, electrocatalysis, and spectroelectrochemistry.
Rui-xiang Wang received his M.S. degree from Guangxi Normal University in 2015. He is currently working toward the Ph.D. degree in Xiamen University and his current research focuses on synthesis and application of non-noble metal catalysts.
Shi-gang Sun obtained Doctorat d’Etat in 1986 from Université Pierre et Marie Curie (Paris VI), France, and is actually a professor of chemistry at the Department of Chemistry of Xiamen University, China. His research interests include electrocatalysis, electrochemical surface science, spectroelectrochemistry, and electrochemical energy conversion and storage. He has been elected member of Chinese Academy of Sciences, Fellow of Royal Society of Chemistry and Fellow of International Society of Electrochemistry.
Shou-yu Shen is a Ph.D. student at Xiamen University under the instruction of Prof. Shi-Gang Sun. He received his B.S. degree from School of Chemical and Material Engineering of Jiangnan University in June 2012. His current research mainly focuses on the research of Ni-Rich and Lithium-rich layered Oxide Cathode and insitu X-ray techniques.
609