Study on electrochemical properties of Pt-Ni nanocatalysts with high index crystal facets

We prepared Pt-Ni high-index crystalline nanoparticles with various Pt-Ni molar ratios to study the microscopic morphology and electrochemical properties of these catalysts. The results demonstrated that when Pt: Ni is 1:1, the prepared catalyst has the most uniform morphology and the optimal catalytic activity, as evidenced by an electrochemical active area of 8.546 m2/g, the current density of 1.544 mA/cm2 for ethanol oxide and 1100 s steady state current density of 1.132 mA/cm2. Pt-Ni high-index crystalline nanocatalysts have such high catalytic efficiencies due to the abundance of step atoms and the doping of non-precious metal nickel.


Introduction
Nowadays, there is no doubt that fossil energy sources are facing an urgent situation of imminent depletion.We need to develop more rational, cleaner, and more efficient ways to utilize the existing energy sources to achieve the "dual carbon goal" of China.[1].Direct ethanol fuel cells (DEFC) are widely welcomed for their high efficiency and cleanliness, low environmental pollution, wide range of fuel sources, low operating temperature, independence from the Carnot cycle, and high energy conversion [2].In addition, Pt is the main material of traditional catalysts.Many researchers found that adjusting the size of metal nanoparticles, different morphologies, and exposing more nanoparticles to fully contact with the fuel can effectively improve the catalytic efficiency of catalysts [3].Among them, exposure to different crystalline surfaces of metal nanoparticles has a strong influence on catalytic performance.For example, Liu et al. [4] synthesized Pt nanoparticles wrapped with {730} high-index facets in the preparation process by controlling the voltage, current, and time, and performed catalytic tests on methanol.It was found that their catalytic performance was much higher than that of the catalyst wrapped with low-index facet surface nanocatalysts.Huang et al. [5] allow the {411} exposed facet to be preserved in concave polyhedral Pt nanocrystals using the amine coordination, which helps stabilize the low-coordinated Pt site.A study found the high density of atomic defects and steps exposed by the high-index facets (HIF) surface often serve as active sites to strengthen catalytic activity and stability of catalysts [6].However, the high-index facets themselves have very high energy density which makes the crystals lower in energy density and eventually grow to low-index facets [7].Pt-Ni nanocatalysts with high index crystalline facets (HIF) were produced by one-step doping of Pt with transition metal Ni using a hydrothermal method.By adjusting different Pt/Ni molar ratios (Pt: Ni=3:1, Pt: Ni=2:1, Pt: Ni=1:1, Pt: Ni=0.5:1) to produce the optimal performance Pt-Ni high-index facets surface nanocatalysts with reducing the catalyst cost, we can complete the procedure above.

Preparation of catalyst
Weighing a certain mass of glycine and PVP (K30) into deionized water and stirring them for 30 min, then we added different moles of NiCl 2 according to different molar ratios of Pt x Ni y to the above mixture and stirred for 30 minutes.The solution was transferred to the reaction kettle and kept at 200℃ for seven hours.Afterward, we collected the powder dried at 60℃ then the following catalysts were obtained: Pt HIFs, Pt: Ni=3:1, Pt: Ni=2:1, Pt: Ni=1:1, Pt: Ni=0.5:1.

X-ray diffraction characterization.
To understand the analysis of Ni doping on the crystal structure of Pt-Ni high-index facets nanocatalysts, Pt-Ni high index surface catalysts were subjected to XRD tests as shown in Figure 1.A comparison of the standard PDF cards for Pt and Ni reveals that the diffraction peaks all correspond to the standard diffraction peaks for Pt, and the diffraction peak appearing at 39.7°corresponds to Pt.The diffraction peaks at 39.7°, 46.2°, 67.5°, 81.3°, and 85.7° are attributed to (111), ( 200), ( 220), (311), and (222) crystal planes of Pt respectively.The diffraction peaks of the prepared catalysts are all between Pt and Ni and no Ni diffraction peaks were detected, indicating that Ni is not a single crystal of Ni form, but in the form of Pt-Ni alloy [8].The diffraction peaks of the catalysts doped with Ni are shifted to the large angle direction compared with the standard pattern of Pt, which is because the doping of Ni causes the lattice contraction of Pt [9].   Figure 3 shows the catalysts prepared by adjusting different molar ratios of Pt to Ni, which are Pt HIFs without Ni, Pt: Ni=3:1, Pt: Ni=2:1, Pt: Ni=1:1, Pt: Ni=0.5:1 catalyst, 200 nanoparticles were selected for particle size statistics, and it was found that the particle sizes were 41.73 nm, 39.03 nm, 38.26 nm, and 49.76 nm, respectively.The particle size of nanoparticles was found to increase and then decrease after Ni doping.When it is Pt: Ni=3:1, the catalyst nanoparticles are the same as those without Ni doping and the high-index facets are {720} and {310} mainly.When Pt: Ni=2:1, the surface of the nanoparticles is rough and fuzzy, and has some small protrusions.The measurement of the depression angle suggests that the exposed high-index facets are {410} and {720} mainly.

TEM characterization of the catalyst.
We continue to increase the molar amount of Ni doping.When Pt: Ni=1:1, it can be found that the catalyst surface has more protrusions at this time, but still maintains the shape of a concave cube and the main high-index facets are {310}, {520}, {720}, and {830}.Then we continue to increase the doping of Ni.When Pt: Ni=0.5:1, the nanoparticles can hardly maintain the concave cube morphology, and the particle size begins to increase.The analysis shows that the addition of Ni 2+ can significantly affect the morphology of Pt-Ni alloy.When more Ni is added, the reduction rate and the growth rate of crystals are faster, which will cause the nanoparticles' excessive growth, and the final shape cannot maintain the concave cube shape.Because doping of Ni in the catalyst will change the electronic state of Pt and expose more Pt active sites [10].The oxidation current densities of Pt-Ni catalysts in ethanolic sulfuric acid solution were finally measured to be 3.510 mA/cm 2 , 3.963 mA/cm 2 , 4.544 mA/cm 2 , and 3.722 mA/cm 2 , respectively.It shows that the doping of Ni element in Pt high-index facets nanocatalyst (Pt HIF) can improve the oxidation current density.This is due to the existence of lots of steps and kink atoms as active sites on the surface of catalysts for the catalytic oxidation of ethanol.

I-T curve characterization.
Further exploring the stability of catalysts with the time of 1100 s in Figure 4(c).we can see current density gradually decreases before stabilizing at 1000 s, which is due to the high ethanol concentration in the vicinity of the electrode and the thin diffusion layer.When the ratio is 1:1, the maximum value is 1.132 mA/cm 2 which is about 4.7 times that of Pt HIF, indicating that the doping of Ni can improve high-index facets catalyst stability.

Conclusion
PtNi nanocatalysts with high-index facets were prepared through a one-step hydrothermal simple process and short reaction cycle.It was found by microscopic characterization that the lattice constant of the catalyst doped with Ni became smaller, and the diffraction angle shifted to a large angle, indicating that a Pt-Ni catalyst was synthesized.Microstructure analysis by TEM shows that the formed PtNi alloy has high-index facets which mainly include {310}, {520}, {720}, and {830}.The best catalytic activity showed when Pt: Ni in the catalyst is 1:1, its electrochemical activity specific surface area is 8.546 m 2 /g, the oxidation peak current density for ethanol is 4.544 mA/cm 2 , and the steady-state current density at 1100s is 1.132 mA/cm 2 .The above results indicate that the synthesized PtNi nanocatalysts with high-index facets can greatly promote catalytic performance after doping with Ni.

Challenge and Outlook
Although Pt-Ni high-index facets nanocatalysts have been used in this paper to prepare high-activity Pt-Ni electrocatalysts in one step by hydrothermal method, there are still many areas that need further improvement and exploration: (1) When studying different Pt-based transition metal catalysts, nanoparticles with similar structures should be precisely controlled to exclude the influence of surface structural factors on the catalytic performance.(2) Different transition metals and different molar ratios will expose different crystal faces of Pt.At present, the effect of defect sites generated during the electrochemical activation of transition metal-doped alloy catalysts with high-index facets on the reaction has not been thoroughly studied.Further exploration and search for more stable structures and transition metal content ratios are needed for researchers to obtain optimal catalytic activity.

Figure 2
shows the PtNi high-index facets nanocatalysts prepared with Pt: Ni=1:1 (all transmission directions are the [100] direction).From Figure2(c), the concave cube can be measured.The concave angles of PtNi are 15.7°, 16.8°, 16.9°, 17.4°, 17.7°, 20.3°, 20.9°, and 22.0° (clockwise), compared with the standard high-index facets concave angle.We can see that the main exposed high-index facets in PtNi nanoparticles are {310}, {520}, {720}, and {830} planes.EDS mapping in Figure2(d)shows that the element distribution of Pt and Ni can be seen, indicating the doping of Ni is very successful.Figure2(e) shows the model diagrams of these four high-index facets which clearly show the steps and kinked atoms on the high-index facets surface.

Figure 2 .
Figure 2. PtNi high-index facets nanocatalyst: (a) (b) is the TEM image; (c) is the HTEM image of the concave angle measurement of the concave surface; (d) is the high-resolution dark-field scanning image and the element faces of Pt, Ni and O Scanning diagram; (e) are four typical model diagrams.

Figure 3 . 1 Figure 4 .
Figure 3. TEM images of nanocatalysts: (a) (b) Pt HIFs; (c) (d) Pt: Ni=3:1; (e) (f) Pt: Ni=2:1; (g) (h) Pt: Ni=1:1; (i) (j) Pt: Ni=0.5:1.3.2.Catalytic performance analysis 3.2.1.Electrochemically active specific surface area.From the calculation results in Figure 4(a), doping Ni can form a containing oxide layer during the preparation process, and this containing oxide layer can cover the surface of the Pt nanocatalyst.The covering will improve the proton and electron conductivity of Pt-Ni catalysts, as well as the H + adsorption and desorption of Pt-Ni high index surface catalysts.When Pt: Ni=3:1, the measured electrochemical active area reaches 4.181 m 2 /g.When the amount of Ni continues to increase, the measured value is 4.902 m 2 /g and the ratio is 2:1.Up to 1:1, the measured value is 8.546 m 2 /g.By comparison, the addition of Ni makes the catalyst have higher catalytic activity in the catalytic ethanol process, which may closely match the high density of step atoms on the surface.(a)