Interfacial Electron Transfer in Strategically Engineered Pt₃Rh/C Ultrafine Alloy Nanoparticle Catalysts Facilitates Exceptional Performance in Li-O₂ Batteries

Abstract

A major challenge for Li-O₂ batteries is the slow kinetics of oxygen reduction (ORR) and evolution (OER) reactions. This work presents a high-performance Pt₃Rh/C composite cathode where Pt-Rh nanoalloys are uniformly dispersed on 3D nanoporous carbon. The bimetallic architecture demonstrates significantly enhanced ORR/OER activity compared to conventional catalysts. Super P, with a large specific surface area and omnipresent pores with diverse size distribution, provided sufficient storage space for Li₂O₂ and facilitated transport channels for Li⁺ and O₂, while the highly conductive Pt₃Rh NPs optimized catalytic efficiency. XPS reveals a prominent electron transfer process between Pt and Rh; the Rh sites in Pt₃Rh/C alloy can effectively act as electron donors to improve the oxygen/lithium peroxide (O₂/Li₂O₂) redox chemistry in LOB. Therefore, the Pt₃Rh/C electrode shows the minimum overpotential (0.60 V) for efficient oxygen reduction and evolution under an upper-limit capacity of 2000 mAh g⁻¹. This work introduces a Pt₃Rh/C nanoalloy synthesis method that boosts Li-O₂ battery efficiency by accelerating oxygen reaction kinetics.

1. Introduction

Lithium-oxygen batteries (LOBs) are positioned as next-generation energy storage systems due to their ultra-high theoretical energy density (3500 Wh kg⁻¹), addressing critical demands of modern sustainable infrastructure [1,2,3]. Despite this promise, the practical deployment of LOBs encounters certain interrelated scientific bottlenecks: (i) intrinsically sluggish kinetics of oxygen reduction/evolution reactions (ORR/OER) at the cathode, (ii) inefficient charge/mass transport across the electrolyte-O₂-catalyst triple-phase interface, and (iii) progressive passivation caused by insulating Li₂O₂ deposition during cycling. These limitations collectively manifest as severe overpotential, limited specific capacity, and rapid capacity fading, as illustrated in Figure 1a(I) [4,5,6]. Overcoming kinetic limitations and developing bifunctional catalysts with high efficiency to accelerate ORR/OER kinetics are essential for LOB advancement.

In recent years, alloying Pt with 3d transition metals, such as PtRu, PtAu, PtIr, PdCu, etc, has proven to be an effective method for increasing the activity of cathode catalysts and reducing the cost of Pt through the ligand effect and the strain effect [7,8,9]. Nevertheless, practical implementation reveals three intrinsic limitations (Figure 1a(II)): (i) limited bulk electrical conductivity impeding interfacial charge transfer, (ii) prone to agglomeration resulting in the coverage of active sites, and (iii) prone to oxidation and degradation during cycling. These limitations collectively impair interfacial transformation kinetics, leading to suboptimal O₂/Li₂O₂ conversion efficiency relative to thermodynamic predictions. In the current study, various methods have been employed, such as the use of ordered [10] and high-entropy alloys [11], cladding layer strategies [12], and multicomponent doping [13] among others. Particle aggregation in simple alloy nanoparticles elevates discharge–charge overpotential in LOBs. Stability enhancement thus becomes pivotal for implementing transition metal alloys as oxygen cathode catalysts. Thus, the challenge persists in designing a Pt alloy catalyst that can inhibit particle agglomeration and oxidative degradation of bimetallic alloys while maintaining durability under operating conditions.

While Pt-Rh alloy catalysts have been traditionally prepared via multi-step methods such as co-impregnation, deposition, and homogeneous precipitation, this work introduces a paradigm-shifting single-step synthesis protocol [14,15,16,17,18]. By kinetically controlling the growth of Pt₃Rh nanoparticles on 3D nanoporous carbon substrates (Figure 1a(III)), we achieved precise atomic ratio tuning (3:1) while minimizing particle aggregation (average size: 3.19 nm). By controlling the doping ratio of Rh metal, we have regulated the Pt metal electrons and particle size, accelerating and enhancing the stability and activity of the catalyst. The Pt₃Rh/C catalyst in Li-O₂ batteries introduces three breakthroughs. (1) Stable Nanoalloy Architecture: Uniformly dispersed Pt₃Rh nanoalloys on 3D nanoporous carbon prevent agglomeration, enhancing mass transfer (vs. clustered Pt/C or Rh/C). (2) Superior Electrochemistry: Achieves record 4382 mAh g⁻¹ discharge capacity (400 mA g⁻¹) and 0.60 V overpotential. (3) Synergistic Electronics: Rh→Pt electron transfer optimizes d-band structure, modulating the electronic structure of Pt. This electronic reconfiguration confirms the formation of a stable Pt₃Rh/C alloy structure, wherein Rh acts as an electron donor to optimize catalytic activity. Pt₃Rh/C uniquely balances stability, activity, and cost, resolving Li-O₂ battery bottlenecks.

Our findings are a significant contribution to the advancement of energy storage technologies and open doors to the design and optimization of novel catalysts for various electrochemical applications.

2. Results and Discussion

Figure 1b illustrates the preparation process of the Pt₃Rh/C catalyst (see the experimental section). As a control, catalysts with varying proportions of Pt_xRh/C (Pt:Rh = 1:0, 7:1, 5:1, 1:1, and 0:1) were sequentially prepared by the same method. In simple terms, through a one-step reduction method, metal compounds are reduced to bimetallic Pt_xRh alloy nanoparticles that are uniformly dispersed on porous carbon carriers, thereby obtaining Pt_xRh/C products. The nanoporous carbon’s high surface area, adjustable porosity, and superior conductivity enhance metal precursor adsorption and enable efficient mass transfer, yielding uniformly dispersed Pt_xRh/C nanoalloys.

Product phases were characterized via powder X-ray diffraction (XRD) (Figure 1c), with Pt/C and Rh/C peak positions matching standard references (Pt: 87-0640; Rh: 87-0714), confirming successful synthesis. Particularly, the characteristic diffraction peaks of the Pt₃Rh/C alloy have slightly shifted compared with those of Pt/C and Rh/C. This arises from Rh atom incorporation into the Pt lattice during alloy formation, which reduces lattice constants and confirms the formation of a stable intermetallic structure [19]. To determine the actual loading of PtRh in the catalyst prepared by the above method, thermogravimetric analysis was carried out. As shown in Figure 1d, the Rh content in the Rh/C catalyst is 20%, and the Pt content in the Pt/C catalyst is 13.16%. The contents of PtRh in the three alloy catalysts of Pt₃Rh /C, Pt₅Rh/C, and Pt₇Rh/C were 15.96%, 12.29%, and 18.76%, respectively.

Catalyst morphologies were characterized via Transmission Electron Microscopy (TEM). Figure 1e–j confirms that nanoporous carbon architecture persists post-metal nanoalloy loading, with TEM revealing uniform particle distribution. This validates carbon substrate’s dual function: inhibiting nanoalloy agglomeration while enhancing catalytic stability. The average particle sizes of Pt/C and Rh/C are approximately 2.3 nm and 2.98 nm, respectively. The particle size of Pt₃Rh/C is approximately 3.19 nm, and it is composed entirely of short nanochains linked together, with a length of about 7 nm. The slight increase in the particle size of Pt₃Rh alloy nanoparticles is due to the osmotic rearrangement of Rh atoms with smaller radii during the growth of Pt nanoparticles, indicating that Rh has entered the Pt lattice to form a stable alloy structure [20].

X-ray photoelectron spectroscopy (XPS) was later used to analyze the elements and their valence states of the above catalysts. As shown in Figure 2a,b, the characteristic peaks of Pt 4f_7/2 and Pt 4f_5/2 in the XPS spectrum of Pt/C have two sub-peaks for each characteristic peak, which are located at the binding energies of 71.55/74.9 and 73.19/76.54 eV, respectively, representing Pt⁰ and Pt²⁺. The 3d_5/2 and 3d_3/2 orbitals of Rh/C contain two peaks, located at 307.8/311.9 and 309.87/313.97 eV, respectively, representing Ru⁰ and Rh³⁺. In the Pt₃Rh/C catalyst, the electron orbital energy spectrum peaks of Pt²⁺ and Rh³⁺ appear at similar positions, and a Pt 4d_5/2 signal peak appears at 315.45 eV. By comparison, it is found that the binding energy of Pt element is significantly lower than that of Pt/C, indicating that an electron transfers from Rh to Pt (Figure 2c). The incorporation of Rh into Pt modulates the electronic structure of the latter through directional electron transfer, as evidenced by XPS analysis. This electronic reconfiguration not only confirms the formation of a stable intermetallic Pt₃Rh/C alloy but also optimizes catalytic activity by tuning electronic structure of Pt [21].

To make a comprehensive analysis of the structure–performance relationship for Pt/C, Rh/C, and Pt_xRh/C electrocatalyst, the electrocatalytic properties of the as-prepared electrocatalysts were evaluated in an LOB system. In deep discharge–charge process (Figure 3a–d), the LOB with Pt₃Rh/C cathode delivers an exceptionally high discharge specific capacity of 4382 mAh g⁻¹ at a current density of 400 mA g⁻¹, much superior to that of Pt/C (3274 mAh g⁻¹), Rh/C (3656 mAh g⁻¹), Pt₅Rh/C (3539 mAh g⁻¹), and Pt₇Rh/C (2537 mAh g⁻¹). Significantly, the Pt₃Rh/C cathode can maintain its higher capacity retentions and coulombic efficiencies compared to other counterparts even at a current density of 100 and 200 mA g⁻¹ (Figure 3g and Figure S1), which corroborates the prominent advantage of Pt₃Rh/C catalyst in enhancing electrochemical performance (Table S1). The durability of the catalyst was assessed by comparing capacity retention rates under high (400 mA g⁻¹) / low (100 mA g⁻¹) current density conditions. As demonstrated in Table S2, the Pt₃Rh/C catalyst exhibited an exceptional capacity retention of 73.6%, significantly outperforming other comparable catalysts. This result confirms its ability to substantially enhance the long-term cycling stability of lithium–oxygen (Li-O₂) batteries. Figure 3e,f depict the first discharge–charge voltage curves at a current density of 200 mA g⁻¹ under an upper-limit capacity of 2000 mAh g⁻¹. It is evident that the Pt₃Rh/C cathode displays the smallest discharge–charge overpotential (0.60 V), far lower than that of other candidates, suggesting more efficient kinetics for ORR/OER during Li₂O₂ formation/decomposition [22]. Figure 3h shows the platform voltages at different current discharges. The Pt₃Rh/C electrode always maintains a relatively high platform voltage, demonstrating more outstanding electrochemical performance. Figure 3i and Table S2 compare the battery capacity of Pt₃Rh/C and other representative published results [23,24,25,26,27,28]. These results further confirm the outstanding advantages of Pt₃Rh/C catalyst in enhancing electrochemical performance.

To identify the cathode surface condition, the cycled LOBs were disassembled and analyzed. It is clear that accumulative film-like products with a high packing density completely cover the Pt/C (Figure 4a–c) and Rh/C (Figure 4d–f) cathode after cycling. Since the attachment of those indissoluble products on cathode surface, catalytic active centers were gradually blocked, which hinders the transfer of electrons, lithium ions, and superoxide species at the triple-phase boundary of cathode-electrolyte-Li₂O₂ during subsequent cycling, triggering a restricted electrochemistry and aggravated side reactions and thus causing the failure of LOB cycling [29]. The Pt₃Rh/C cathode (Figure 4g–i) exhibits remarkable morphological stability across cycling, preserving its porous carbon framework while sustaining uniformly dispersed alloy nanoparticles. This structural resilience, confirmed through ex situ characterization after extended operation, underscores its superior passivation resistance relative to traditional catalysts. Figure 4j meticulously illustrates the variations in oxygen-containing intermediates throughout the charging and discharging processes. The Pt₃Rh/C electrode exhibits excellent reversibility, with the discharge products being entirely decomposed during charging, thereby significantly enhancing the battery’s cycle life.

3. Materials and Methods

3.1. Materials

All the chemical substances used in this study were purchased from Shanghai Aladdin Co., LTD. (Shanghai, China): sodium citrate (C₆H₅Na₃O₇), rhodium trichloride (RhCl₃), chloroplatinic acid (H₂PtCl₆), sodium borohydride (NaBH₄), and conductive carbon black (Super P). All reagents are of analytical grade and can be used without further purification.

3.2. Electrocatalysts’ Synthesis

Weigh 80 mg of sodium citrate and place it in a beaker. Add 200 mL of distilled water and stir until fully dissolved. Introduce a specific volume of RhCl₃ solution (19.22 mM) and H₂PtCl₆ solution (64.77 mM), then subject the mixture to ultrasonic stirring until uniformly dispersed. Subsequently, add 80 mg of Super P and continue with ultrasonic dispersion until an even consistency is achieved. Dissolve another specified amount of NaBH₄ in 10 mL of ice-cold water, then quickly transfer this solution into the aforementioned mixture while accelerating the stirring for 5 min. Following this initial agitation, maintain constant-speed stirring for an additional 24 h to ensure complete reaction completion. The resulting product is then washed, filtered by suction, and dried in an oven at 60 °C for 12 h to yield the Pt₃Rh/C catalyst.

Following the outlined procedure, catalysts with varying proportions of Pt_xRh/C (Pt:Rh = 1:0, 7:1, 5:1, 1:1, and 0:1) were sequentially prepared. Notably, the ratio of each component within the catalyst was meticulously adhered to according to a molar ratio of C:PtRh = 9:1.

3.3. Characterization Techniques

3.3.1. Structural and Chemical Analysis

In this paper, the composition and crystal structure of the samples were characterized by X-ray diffraction (XRD). The instrument used was the D/max-γ B automatic X-ray diffractometer. The scanning angle was a wide-angle scanning test ranging from 10° to 90°, the scanning rate was 4° min⁻¹, and the scanning angle resolution was 0.05°. The morphology of the samples was observed by Transmission Electron Microscopy (TEM). The instrument used was the ultra-high resolution (HUR) type JEM-2100 transmission electron microscope with an acceleration voltage of 150 kV. The content of precious metals in the carbon-supported catalyst was analyzed by thermogravimetric analysis (TG), and the experiment was conducted at 800 °C under oxygen conditions. The elemental composition, electronic structure, and valence state distribution of the samples were analyzed by X-ray photoelectron spectroscopy (XPS), and the instrument model used was PHI 5700 ESCA.

3.3.2. Li-O₂ Cell Assembly and Testing

The catalyst prepared above was mixed with the binder Polyvinylidene fluoride (PVDF) in a mass ratio of 9:1. An appropriate amount of N-Methylpyrrolidone (NMP) was added and ultrasonically dispersed until uniform. It was coated on nickel foam disks (1.4 × 1.4 cm) and vacuum-dried at 120 °C for 12 h to obtain electrode sheets.

Lithium–oxygen batteries were assembled using the 2025 type button battery case. The battery is tightly assembled and is carried out in an argon atmosphere glove box (O₂ < 0.01 ppm, H₂O < 0.01 ppm). The negative electrode adopts a lithium sheet with a diameter of 16 mm, the separator uses a glass fiber separator, and the electrolyte is 1 M Bis(trifluoromethane)sulfonimide lithium salt (LiTFSI) in Triethylene Glycol Dimethyl Ether (TEGDME). The battery assembly process is as follows: place the negative electrode shell flat and put it in the gasket, lithium sheet, and separator in sequence. Add 200 μL of electrolyte until the separator is completely wet. Then place the air electrode and nickel foam current collector directly opposite each other, cover the positive electrode shell, and complete the packaging with a button battery sealing machine at a pressure of 500 psi.

The charge and discharge test (GCD) of lithium–oxygen batteries was conducted on the NEWARE BTS battery test system.

4. Conclusions

In summary, we report a high-performance Pt₃Rh/C oxygen cathode with ultrafine nanoalloys uniformly dispersed on nanoporous carbon. The substrate’s hierarchical porosity enhances mass transfer, while porous confinement minimizes agglomeration, maximizing active site exposure. Compared to Pt/C and Rh/C, Pt₃Rh/C exhibits superior LOB performance: higher first-discharge capacity (4382 mAh/g⁻¹ at a current density of 400 mA/g⁻¹), lower overpotential (0.6 V), and improved rate performance. XPS confirms electron transfer from Rh to Pt in Pt₃Rh/C, optimizing electronic structure for enhanced Li₂O₂ redox stability and doubling active sites. This yields superior ORR/OER activity, offering a scalable alloy design for high-energy batteries and energy systems.