**1. Introduction**

Noble metals are widely used in various important catalytic or electrocatalytic reactions in the fields of energy conversion/storage as well as water pollution such as hydrogen production [1], reduction of oxygen [2], oxidation of organic or inorganic small molecule [3], fuel cells [4], and degradation of organic dyes in water [5]. Among these important applications, fuel cells have been receiving extensive attention due to their ability to directly convert the chemical energy of small-molecule fuel oxidation into electricity [6]. Direct formic acid fuel cells (DFAFCs) are believed to be a promising power generation system owing to their reasonable power density, high electromotive force, and limited fuel crossover [7,8]. As a key component of DFAFCs, electrocatalysts towards formic acid oxidation reaction (FAOR) play a vital role in the development of DFAFCs. Recently, noble metal palladium (Pd) has drawn intensive attention due to its high catalytic activity for FAOR and better resistance to CO poisoning than Pt [9–12]. However, the Pd catalyst is prone to dissolution in acidic media during catalytic reactions, hampering its commercial application [13]. In addition, there is still much room for improvement in the catalytic activity of the Pd catalyst. Therefore, it is particularly eager to develop a highly active and durable catalyst towards FAOR.

**Citation:** Liu, J.; Li, F.; Zhong, C.; Hu, W. Clean Electrochemical Synthesis of Pd–Pt Bimetallic Dendrites with High Electrocatalytic Performance for the Oxidation of Formic Acid. *Materials* **2022**, *15*, 1554. https:// doi.org/10.3390/ma15041554

Academic Editor: Enrico Negro

Received: 4 January 2022 Accepted: 16 February 2022 Published: 18 February 2022

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It is widely accepted the oxidation of formic acid involves two mechanisms, i.e., the direct oxidation pathway (HCOOH → CO2 + 2H+ + 2e−) and the CO oxidation pathway (HCOOH → COads + H2O → CO2 + 2H<sup>+</sup> + 2e−) [14]. In the direct oxidation pathway, HCOOH molecules directly dehydrogenate to form CO2 via one or more active intermediates. In the CO oxidation pathway, HCOOH molecules dehydrate to produce CO which depends on the applied potential. The generated CO may be further oxidized to CO2 or poison the catalyst. Up to date, several strategies have been developed to improve the electrocatalytic properties of Pd. The previous literature has reported that alloys of Pd with other metals (such as Cu [9], Pt [15], Co [16], Ag [17], Au [18], Sn [19], Ni [20], Rh [21], Zn [22], Pb [23], Cr [24], and Ir [25]) can not only enhance catalytic activity, but also improve the corrosion resistance of Pd. In particular in Pd–Pt alloys, the electronegativities and bulk Wigner–Seitz radii of Pt and Pd are similar [26]. The alloying of Pt and Pd will produce a synergistic electronic effect [27]. This effect favors formic acid oxidation via the direct oxidation pathway. Additionally, Pt also exhibits high stability in acidic media due to its chemically inert property [28].

It is generally accepted the morphology of catalysts plays a crucial role in enhancing the catalytic activity. A great deal of work has focused on the synthesis of bimetallic Pt–Pd catalysts with various morphologies. For instance, Guo et al. [15] synthesized threedimensional (3D) dendritic Pt-on-Pd bimetals on graphene sheets via a facile wet-chemical approach and found that Pt–Pd bimetallic nanodendrites/graphene hybrids exhibited high catalytic activity for the oxidation of methanol. Yuan et al. [29] prepared Pd–Pt random alloy nanocubes in an aqueous solution containing KBr, polyvinyl pyrrolidone, and sodium lauryl sulfate with PdCl2 and K2PtCl6 as precursors. Zhang et al. [30] reported different shapes of Pd–Pt alloys by a solvothermal process, such as flowers, dendrites, bars, cubes, and concave cubes, and concluded that the obtained Pd–Pt alloys had an enhanced catalytic activity and CO tolerance towards FAOR. Lu et al. [31] fabricated a reduced graphene oxide/Pt–Pd alloy nanocubes by a facile hydrothermal method. However, the synthesis of most Pd–Pt catalysts with various morphologies commonly uses organic additives, thus requiring additional removal processes of additives. Otherwise, incomplete removal of organic additives limits the active sites of the catalyst and thus negatively affects its performance. Additionally, during conventional electrode preparation, the powder catalysts have to be transferred to the surface of the electrode, which inevitably requires the use of conductive additives and binders. The use of conductive additives and binders will lead to a reduction in the active sites of the catalyst. Therefore, it is highly desired to develop a facile and surfactant-free route to synthesize Pd–Pt nanocatalysts. The electrochemical synthesis method is considered to be an effective catalyst preparation method because of its simplicity, low cost, easy operation, and high purity of the product [32]. However, conventional electrochemical synthesis techniques have limited control parameters, making it difficult to effectively control the morphology of the catalyst. The combination of different modes of electrochemical technologies can effectively expand the range of its control parameters and is an effective method to realize the control of the catalyst morphology. For example, Tian et al. [33,34] synthesized monometallic Pt and Pd tetrahexahedral nanocrystals by the electrochemical technology, which showed a high electrocatalytic activity for the oxidation of formic acid and ethanol. The dendritic Pt and highly dispersed Pd particles also synthesized by a similar electrochemical method and displayed a high electrocatalytic activity [35,36]. However, there are few reports on the synthesis of morphology-controlled Pd–Pt bimetallic catalysts via a clean electrochemical approach. The research on the effect of electrochemical parameters on the structure and morphology of Pd–Pt bimetallic catalysts is very limited. The intrinsic relationship between the microstructure of Pd–Pt bimetallic catalysts and its macroscopic catalytic performance also needs to be further illustrated.

Herein, Pd–Pt bimetallic catalysts with a dendritic morphology were in situ synthesized on the surface of a carbon paper by a facile and clean electrochemical method. Pure Pd particles were firstly electrodeposited on the surface of the carbon paper, and then Pd–Pt bimetallic catalysts with a dendritic morphology were obtained by the periodic

square-wave potential (PSWP) treatment of pure Pd particles in an aqueous solution of 0.5 M H2SO4 and 5 mM PdCl2. The effects of the frequency and treatment time of the PSWP on the morphology of the bimetallic Pd–Pt catalysts were systematically investigated. The obtained Pd–Pt bimetallic catalyst with a dendritic morphology displayed an outstanding catalytic activity (0.77 A mg<sup>−</sup>1) and a high stability towards FAOR.
