**3. Results and Discussion**

Figure 1 displays the SEM images of the pure Pd particles and the Pd–Pt bimetallic catalysts with various morphologies fabricated by PSWP modification with different frequencies in an aqueous solution containing a Pt precursor. The pure Pd particles were electrodeposited on the surface of the carbon paper (Figure 1a), and the average particle size was about 270 nm (Figure 1b). A large number of Pd–Pt particles with a relatively smooth surface were electrodeposited on the surface of the carbon paper, after the pure Pd particles were treated by the PSWP with a frequency of 90 Hz (Figure 1c,d). The size of the Pd–Pt particles was similar to that of the pure Pd particles (Figure 1d). When the modification frequency of the PSWP decreased to 50 Hz, the obtained Pd–Pt bimetallic catalysts showed a well-defined dendritic morphology (Figure 1e). A lot of long secondary dendrites grew on the trunk of the dendrite of the Pd–Pt bimetallic catalysts, and the short tertiary dendrites were also formed on the secondary dendrite arms (Figure 1e,f). As the frequency of the PSWP further reduced to 10 Hz, the Pd–Pt bimetallic catalysts showed an agglomerated morphology with a rough surface (Figure 1g,h). Obviously, the frequency of the PSWP had a significant effect on the morphology of the Pd–Pt bimetallic catalysts. This may be related to the process of adsorption/desorption of oxygen on the surface of Pd and Pt metals during PSWP modification [37]. The oxidation processes of the Pd and Pt surfaces were affected by the upper limit potential of the PSWP, and the dynamic adsorption/desorption process of oxygen on the Pd and Pt surfaces affected the dissolution of Pd and Pt. The lower limit potential of the PSWP was responsible for the deposition of Pd and Pt. When the frequency of the PSWP was high (90 Hz), the square-wave period of the Pd particle surface was relatively short, and thus the modification effect on the surface morphology of Pd particles was limited. As a result, many Pd–Pt particles with a relatively smooth surface were formed (Figure 1c,d), and their morphology was similar to that of pure Pd particles (Figure 1a,b). As the frequency of the PSWP decreased to 50 Hz, the square-wave period increased, resulting in a long period of the dissolution and deposition of metal atoms on the surface of Pd particles. The long-period deposition process caused the diffusion-limited growth of metal ions near the electrode surface. These metal ions tended to diffuse towards the tip of the electrode surface, which finally led to the formation of Pd–Pt bimetallic catalysts with a dendritic morphology (Figure 1e,f). When the frequency of the PSWP reduced to the smallest frequency, i.e., 10 Hz, the PSWP period was longer. The formed dendrites were dissolved, since the dissolution caused by the upper limit potential played a dominant role during the long square-wave period. Consequently, the Pd–Pt bimetallic catalysts displayed an agglomerated morphology with a rough surface (Figure 1g,h).

To further gain insight into the formation mechanism of the Pd–Pt bimetallic catalysts with a dendritic morphology, the surface morphology of the Pd–Pt bimetallic catalysts as a function of the modification time of the PSWP was investigated. Figure 2 shows the SEM images of the Pd–Pt bimetallic catalysts obtained by the PSWP modification with different times. As the modification time increased from 1 h to 4 h, the morphology of the modified Pd–Pt bimetallic catalysts evolved from the agglomerated particles (Figure 2a,b), leaf-like catalysts (Figure 2c,d) to dendritic catalysts (Figure 1e,f). During modification, in the early stage (modification time of 1 h) of the formation of the Pd–Pt bimetallic catalysts, the short modification time had limited effect on the morphology of the Pd–Pt bimetallic catalysts. Only the Pd–Pt bimetallic catalysts with an agglomerated morphology were observed (Figure 2a,b). When the modification time was extended to 2 h, the diffusion of metal ions near the electrode surface was limited, and the metal ions diffused toward the tip of the electrode surface during the deposition process. As a result, the leaf-like catalysts with a protruding texture were formed (Figure 2c,d). As the modification time was further extended to 4 h, the leaf-like catalysts were selectively dissolved, only leaving

the protruding texture part. Finally, the Pd–Pt bimetallic catalysts displayed a well-defined dendritic morphology (Figure 1e,f).

**Figure 1.** SEM images of pure Pd particles (**a**) and the obtained Pd–Pt bimetallic catalysts modified by the periodic square-wave potential (PSWP) with the frequencies of 90 Hz (**c**), 50 Hz (**e**), 10 Hz (**g**) for 4 h in a solution of 0.1 mM H2PtCl6 +1MH2SO4. (**b**,**d**,**f**,**h**) are the high-magnification SEM images of the corresponding catalysts.

Figure 3 displays the XRD spectra of the Pd–Pt catalysts. All the catalysts showed the distinct characteristic diffraction peaks at about 40.1◦, 46.6◦, 67.9◦, 81.7◦, and 86.7◦, which corresponded to the (111), (200), (220), (311), and (222) lattice planes of the Pd–Pt bimetallic catalysts, respectively (Figure 3a) [38]. This indicated that the obtained Pd–Pt bimetallic catalysts possessed a polycrystalline structure. Figure 3b shows the enlarged (111) peaks of the Pd–Pt bimetallic catalysts with a dendritic morphology. It was found that the (111) peak of the Pd–Pt bimetallic catalysts with a dendritic morphology shifted to the position between the (111) peaks of monometallic Pt (JCPDS 87–0640) and Pd (JCPDS 87–0638). This phenomenon was attributed to the substitution of Pd atoms with Pt in the lattice, which resulted in the expansion of the face-centered cubic lattice [38], indicating the successful formation of Pd–Pt alloy.

**Figure 2.** SEM images of the obtained Pd–Pt bimetallic catalysts modified by the PSWP with a frequency of 50 Hz for1h(**a**) and 2 h (**c**) in a solution of 0.1 mM H2PtCl6 +1MH2SO4. (**b**,**d**) are the high-magnification SEM images of the corresponding Pd–Pt bimetallic catalysts.

**Figure 3.** (**a**) X-ray diffraction (XRD) patterns of the obtained Pd–Pt bimetallic catalysts; (**b**) the enlarged XRD spectrum of the Pd–Pt bimetallic catalysts with a dendritic morphology in the 2θ range of 38−42◦.

To obtain the detailed structural information of the Pd–Pt bimetallic catalysts with a dendritic morphology, the TEM analysis was conducted. Figure 4a displays the TEM images of the obtained Pd–Pt bimetallic catalysts with a dendritic morphology. It was observed that the long nanothorns grew on the tip of the Pd–Pt dendrites. The length of the nanothorns was about 100 nm. This is coincident with the observed SEM results (Figure 1e,f). Figure 4b,c shows the elemental mapping images of the corresponding Pd–Pt dendrites. The Pd–Pt dendries were composed of Pd (Figure 4b) and Pt (Figure 4c) elements, and these elements were uniformly distributed on the surface of Pd–Pt dendrites. Figure 4d shows the high-resolution TEM (HRTEM) image of the Pd–Pt dendrites. The well-defined lattice fringes of the Pd–Pt dendrites were observed, indicating the high crystallinity of the Pd–Pt dendrites. The interplanar spacing of the Pd–Pt dendrites was 0.223 nm, which matched with the (111) facet of the Pd–Pt phase (confirmed by XRD; Figure 3) [6].

**Figure 4.** (**a**) TEM image of Pd–Pt dendrites. The elemental mapping images of Pd (**b**) and Pt (**c**) of Pd–Pt dendrites. (**d**) High-resolution TEM (HRTEM) image of Pd–Pt dendrites.

Figure 5 shows the CV profiles of the Pd–Pt bimetallic catalysts tested in a N2-saturated 0.5 M H2SO4 solution. All the CV curves displayed the similar voltammetric features to Pd–Pt polycrystalline. These CV curves exhibited three typical potential regions including the hydrogen adsorption/desorption (–0.20 V to 0.04 V (vs. SCE)), the electric double layer (0.04 V to 0.50 V (vs. SCE)), and the formation/reduction of Pt/Pd oxides (0.50 V to 1.20 V (vs. SCE)) [39]. The multiple peaks of the hydrogen adsorption/desorption of the Pd–Pt bimetallic catalysts indicated that the Pd–Pt bimetallic catalysts possessed a welldeveloped polycrystalline structure [40,41], which coincided with the XRD results. The electrochemically active surface area (ECSA) can be estimated by integrating a columbic charge associated with reduction peaks of Pd/Pt oxides at about 0.47 V after electric double layer correction, assuming that the charge required for the reduction of the Pd/Pt oxides monolayer was 424 μC cm−<sup>2</sup> [42]. The calculated ECSAs of PdPts–10 Hz, PdPts– 50 Hz (4 H), PdPts–90 Hz, PdPts–1 H, and PdPts–2 H were 34.43 m2 g−1, 28.30 m2 g−1, 33.96 m2 g<sup>−</sup>1, 14.15 m2 g−1, and 31.60 m2 g−1, respectively. The relatively small specific ECSA of the Pt−Pd bimetallic catalysts with a dendritic morphology can be ascribed to the large dendrite size. This is consistent with the observed SEM results (Figure 1).

**Figure 5.** Cyclic voltammetry curves of the obtained Pd–Pt bimetallic catalysts tested in a 0.5 M H2SO4 solution at a scan rate of 50 mV s<sup>−</sup>1, normalized by the Pd–Pt mass.

To evaluate the catalytic activity of the Pd–Pt catalysts, the voltammetry tests of Pd– Pt catalysts towards FAOR were conducted. Figure 6 shows the voltammetric curves of the Pd–Pt bimetallic catalysts towards FAOR. Two well-defined current peaks P1 and P2 appeared at about 0.44 V (vs. SCE) and 0.78 V (vs. SCE) (Figure 6a), corresponding to the direct oxidation of formic acid to CO2 and oxidation of adsorbed CO generated by dehydration of formic acid, respectively [7,14]. The Pd–Pt bimetallic catalysts with a dendritic morphology (PdPt–50 Hz (4 H)) exhibited the highest mass activity (P1, 0.77 A mg<sup>−</sup>1) among the obtained Pd–Pt bimetallic catalysts towards FAOR and was almost 2.5 times that of the commercial Pd/C catalyst (0.31 A mg<sup>−</sup>1) [36]. Furthermore, the Pd–Pt bimetallic catalysts with a dendritic morphology also displayed a higher mass activity compared with the reported Pd-based electrocatalysts towards FAOR (Table 1). Moreover, there was still much room for improvement in the mass activity of this catalyst by further reducing its particle size. Figure 6b shows the voltammetric curves of the Pd–Pt bimetallic catalysts normalized by the ECSA of Pd–Pt catalysts towards FAOR. The specific activity of the Pd–Pt bimetallic catalysts with a dendritic morphology was also much larger than those of the other obtained Pd–Pt bimetallic catalysts. The enhanced specific activity of the Pd–Pt bimetallic catalysts with a dendritic morphology towards FAOR can be ascribed to the large number of unsaturated atoms at the edges of dendrites (Figure 4d). During the preparation of conventional electrodes, the powder catalyst had to be mixed with polymer binders and conductive agents into a catalyst ink, and then the catalyst ink was transferred to the surface of a current collector. The introduction of polymer binders increased the interfacial resistance between the catalyst and the current collector. Besides, the physical transfer of the catalyst caused the agglomeration of the catalyst particles and thus reduced its effective catalytic active sites. These resulted in undesirable side effects on the catalytic activity of the catalyst. On contrary, the Pd–Pt bimetallic catalysts with a dendritic morphology were directly grown on the surface of the carbon paper, and the entire electrochemical synthesis process of the electrode did not involve the use of the binders and the transfer process of the catalyst. Consequently, the direct growth of dendritic Pd–Pt catalysts on the surface of the carbon paper could effectively reduce the interfacial resistance of electrode

and maximize the utilization of the effective catalytic active sites of the catalyst, thereby improving the mass activity of the Pd–Pt bimetallic catalysts with a dendritic morphology. Therefore, the enhanced mass activity of the Pd–Pt catalysts with a dendritic morphology was not only attributed to the large number of atomic defects at the edges of dendrites, but also ascribed to the high utilization of active sites caused by the "clean" electrochemical preparation method. In addition, compared with commercial Pd/C catalysts, the electronic effects, caused by the proper downshift of the d-band center of Pd resulting from Pd alloying with Pt, contributed to accelerating the kinetic of FAOR, thus enhancing its catalytic activity [43,44].

**Figure 6.** Voltammetric curves of the obtained Pd–Pt bimetallic catalysts in an aqueous solution of 0.5 M H2SO4 and 0.5 M HCOOH at a scan rate of 50 mV s<sup>−</sup>1, normalized by the Pd–Pt mass (**a**) and the Pd–Pt electrochemically active surface area (ECSA) (**b**).

**Table 1.** Comparison of the mass activity of the Pd–Pt bimetallic catalysts with a dendritic morphology prepared in this work with those of Pd-based electrocatalysts towards formic acid oxidation reaction (FAOR).


To assess the stability of the Pd–Pt catalysts, the chronoamperometric testing of the Pd–Pt bimetallic catalysts towards FAOR was conducted. Figure 7 shows the chronoamperometric curves at 0.15 V (vs. SCE) for 3000 s. In the initial stage, a rapid drop of the

current density was associated with electric double-layer charging [7]. Subsequently, a slow decay of the current density was observed, which was associated with surface poisoning by intermediates [7]. The current density of the Pd–Pt bimetallic catalysts with a dendritic morphology for a period of 3000 s was 0.08 A mg−1, which was higher than those of PdPts–10 Hz (0.04 A mg−1), PdPts–90 Hz (0.04 A mg−1), PdPts–1 H (0.02 A mg−1), and PdPts–2 H (0.03 A mg<sup>−</sup>1), even four times that of the commercial Pd/C catalyst reported in the literature (about 0.02 A mg−1) [36]. This indicated that the Pd–Pt bimetallic catalysts with a dendritic morphology possessed an outstanding catalytic activity and a high stability towards FAOR in an acid medium. Therefore, it is possible that the dendritic Pd–Pt catalyst directly electrodeposited on a carbon paper can be applied in direct formic acid fuel cells as a fuel cell anode, methanol fuel cells, and the degradation of organic dyes in water [5,54,55]. Additionally, since the catalyst can be continuously electrodeposited on the surface of a conductive substrate when the conductive substrate (such as carbon paper) is used as a "conveyor belt", the electrochemical synthesis method in this work can realize the continuous industrial production of catalytic electrodes.

**Figure 7.** Chronoamperometry curves of the obtained Pd–Pt bimetallic catalysts recorded in an aqueous solution of 0.5 M H2SO4 and 0.5 M HCOOH at 0.15 V (vs. saturated calomel electrode (SCE)) for 3000 s.

#### **4. Conclusions**

Pd–Pt catalysts with a dendritic morphology were in situ grown on the surface of a carbon paper via a facile and "green" two-step electrochemical method. The frequency of the PSWP had a significant effect on the morphology of the Pd–Pt bimetallic catalysts. Additionally, as the modification time increased, the morphology of the Pd–Pt bimetallic catalysts evolved from the agglomerated particles, leaf-like catalysts to dendritic catalysts. The obtained dendritic Pd–Pt catalysts displayed an outstanding catalytic activity (0.77 A mg−1) and a high stability towards FAOR. The improved catalytic activity of the Pd–Pt catalysts with a dendritic morphology can be ascribed to the high utilization of its active site and the improved specific activity related to its rough dendritic morphology. The dendritic Pd–Pt catalyst directly electrodeposited on a carbon paper possesses great potential to be applied in direct formic acid fuel cells as a fuel cell anode methanol fuel cells, and the degradation of organic dyes in water.

**Author Contributions:** Conceptualization, J.L. and F.L.; methodology, F.L.; investigation, J.L. and F.L.; data curation, J.L. and F.L.; writing—original draft preparation, J.L. and F.L.; writing—review and editing, J.L. and C.Z.; supervision, C.Z. and W.H.; project administration, C.Z. and W.H.; funding acquisition, J.L., C.Z. and W.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the National Natural Science Foundation of China (Nos. 51771134 and 51801134), National Natural Science Foundation for Distinguished Young Scholar (52125404), Tianjin Natural Science Foundation for Distinguished Young Scholar (18JCJQJC46500), "131" First Level Innovative Talents Training Project in Tianjin, Tianjin Natural Science Foundation (20JCQNJC01130), National Natural Science Foundation of China and Guangdong Province (U1601216), and the National Youth Talent Support Program.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data are contained within the article.

**Conflicts of Interest:** The authors declare no conflict of interest.
