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DOI: 10.1039/D4CC03008G (Communication) Chem. Commun., 2024, 60, 10672-10675

Solution-plasma interaction for synthesizing highly active Pt–Ni alloy oxygen reduction nanocatalysts for PEMFCs†

Shuxiu Yang , Yu Tang , Yawen Tan , Bona Fu , Mingyang Liu , Qingzhu Shu , Shuhong Liu and Hong Zhao *
School of Materials Science and Engineering, Dalian Jiaotong University, Dalian 116028, P. R. China. E-mail: Zhaohong@djtu.edu.cn

Received 20th June 2024 , Accepted 17th August 2024

First published on 20th August 2024

Abstract

A highly active PtNi-alloy catalyst (p-PtNi/KB) is synthesized using solution-plasma interaction. The plasma effect results in a substantial increase of active sites on the support surface. This facilitates the alloying and functionalization of supports, enabling achievement of enhanced catalytic activity and effective resolution of cost-related challenges in fuel cells.

Proton exchange membrane fuel cells (PEMFCs) are being widely recognized for their eco-friendly characteristics, high energy conversion efficiency, rapid start-up, and low operating temperature, attracting significant attention.1 To address the sluggish kinetics and high overpotential of the oxygen reduction reaction (ORR),2 it is imperative to employ efficient catalysts that significantly enhance the performance and efficiency of fuel cells.3,4 Current catalysts still heavily rely on expensive and scarce platinum (Pt) as the primary active component, attributed to its highest activity for ORR.5 The high cost, however, hinders the commercialization of fuel cells and other electrochemical energy technologies.6 Therefore, the development of oxygen reduction catalysts with low loading, high activity, and high stability is key to advancing the progress of fuel cells.

In recent years, the design of Pt-based alloys (Pt–M, M = transition metal element) with reduced Pt utilization and enhanced catalytic activity towards ORR has emerged as one of the most promising catalyst strategies.7,8 The increase in activity is attributed to the crucial interplay between electronic and geometric effects, which modify the positioning of the d-band center relative to platinum's Fermi level, thereby modulating the binding energy of oxygenated intermediates.9,10 Additionally, the textural properties of nanocatalysts, such as particle size, uniformity, and dispersion, are also critical factors for achieving enhanced ORR activity and durability. Reducing the size of nanoparticles (NPs) is advantageous for obtaining a higher effective surface area, which maximizes the number of active sites.11 Considering durability, the non-uniform distribution of nanoparticles may lead to instability due to Ostwald ripening caused by potential differences between dissolved metal ions and metal NPs.12,13

Glow discharge plasma, as a form of non-thermal plasma technology, demonstrates exceptional performance in material synthesis and surface modification due to its high reactivity and non-equilibrium characteristics.14,15 During discharge, a positively charged “sheath layer” forms on the material's surface due to the rapid electron migration rate, effectively managing particle size and preventing aggregation to ensure good dispersion and stability.16 Concurrently, plasma bombardment induces the generation of numerous defect sites on the support surface,17 thereby augmenting its electrochemical active surface area (ECSA), which facilitates the capture and immobilization of Pt NPs. In addition, the high-energy electrons generated during plasma processing serve as a substitute for conventional chemical reducing agents, avoiding by-products and secondary pollution.18 It also highlights the potential of non-thermal plasma to improve catalyst performance and production efficiency.

In this work, we propose a strategy for the synthesis of highly active PtNi alloy electrocatalysts using solution-plasma interaction, and validate their applicability as cathode catalysts for PEMFCs. The reaction setup is illustrated in Fig. 1. The platinum wire electrode is connected to the positive electrode of the DC power supply as the anode. Three graphite rods with equal diameter and length are bundled together with a quartz capillary tube inserted in the middle, forming the cathode component. The graphite rod has oblique cut surfaces at both ends, with a slope angle of 10°. The distance between these surfaces gradually increases from top to bottom, forming a “—” shape at the upper end. The three “—” shapes form angles of 120° with each other. This design increases the radial distance between adjacent graphite rods at their tops, allowing for accelerated downward flow from top to bottom when driven by a peristaltic pump through quartz capillaries into a collector. When applying a certain voltage, a stable glow is generated between the platinum wire and the quartz capillary. The p-PtNi/KBs catalysts are synthesized via plasma-assisted methodology in the precursor dispersion solution. The catalysts are then mixed with D2020® perfluorosulfonic acid ionomer at an NC ratio of 0.7 and subject to ultrasonic treatment to form a homogeneous slurry. The slurry is then sprayed onto a proton exchange membrane (Gore, 12 μm) to prepare a catalyst-coated membrane (CCM) with an area of 25 cm2. The CCM is pressed together with two gas diffusion layers (TGP-H-060, Toray) that have a thickness of 190 μm to form the membrane electrode assembly (MEA). Single-cell testing is conducted using the Scribner 850e fuel cell test system.


image file: d4cc03008g-f1.tif
Fig. 1 Diagrammatic representation of a plasma-assisted apparatus for the synthesis of p-PtNi/KBs catalysts.

The microstructure of the Ketjenblack and PtNi alloy nanoparticles deposited on Ketjenblack with plasma effects is characterized using high-magnification transmission electron microscopy (TEM), as shown in Fig. 2. The comparative results depicted in Fig. 2(a) and (b) reveal that the pure carbon surface becomes rough under continuous plasma treatment, indicating that the plasma provides sufficient energy to break C–C bonds and generate more active sites on the carrier surface.19Fig. 2(c) and (d) shows the uniform distribution of metal NPs on Ketjenblack after plasma treatment by comparison, with inset depicting the size distribution histogram of metal NPs, yielding an average particle size of 4.72 nm. High-resolution TEM (HR-TEM) image of representative p-PtNi/KB, as shown in Fig. 2(f), reveals lattice fringes with a d-spacing of 0.22 nm, consistent with face-centered cubic (111) planes for Pt–Ni alloy NPs. Correspondingly, Fig. S1 (ESI†) displays the TEM images and Pt particles size distribution of p-Pt/KB and commercial Pt/C with d-spacing of 0.23 nm. Fig. 2(e) further confirms the crystal structure of catalysts with different Pt–Ni molar ratios through XRD characterization. Notably, distinct peaks corresponding to pure Pt or Ni are not observed in p-PtNi/KBs, and their 2θ values are situated between pure Pt and Ni. This observation suggests that incorporating smaller Ni atoms into the Pt lattice shortened the Pt–Pt bond length, resulting in a shift towards higher angles, indicative of the formation of an alloy phase between Ni and Pt.20 The high-angle annular dark-field (HAADF) image and energy-dispersive X-ray spectroscopy (EDS) elemental mapping further confirm the uniform dispersion of PtNi alloy nanoparticles on the support, which is clearly identified (Fig. 2(g)). This is attributed to the charge transfer between the electron-rich sites in the plasma generated by glow discharge and Pt4+ in chloroplatinic acid, as well as Ni2+ in nickel chloride, which then undergo high-temperature fusion to form strong bonding interactions that fix alloy particles on the support. From ICP-based component analysis, the actual Pt–Ni molar ratios of catalysts are slightly higher than material ratios, indicating a more complete reduction of the platinum precursor compared to the nickel precursor (Table S1, ESI†).


image file: d4cc03008g-f2.tif
Fig. 2 Structural characterizations of support and catalyst. (a) and (b) HR-TEM images of the support before (pure carbon) and after (p-pure carbon) through continuous plasma treatment. (c) and (d) TEM images of representative PtNi/KB (Pt[thin space (1/6-em)]:[thin space (1/6-em)]Ni = 1[thin space (1/6-em)]:[thin space (1/6-em)]1) before and after continuous plasma treatment, with the inset in (d) displaying the histogram of particle size distribution. (e) XRD patterns of catalysts with different Pt–Ni molar ratios. (f) HR-TEM image of p-PtNi/KB (Pt[thin space (1/6-em)]:[thin space (1/6-em)]Ni = 1[thin space (1/6-em)]:[thin space (1/6-em)]1), with the inset providing a magnified view of the red area. (g) HAADF-STEM-EDS mapping images of p-PtNi/KB (Pt[thin space (1/6-em)]:[thin space (1/6-em)]Ni = 1[thin space (1/6-em)]:[thin space (1/6-em)]1).

In order to investigate the influence of plasma-modified supports on the catalytic performance of the PtNi alloy, further Raman characterization and FTIR spectroscopy analysis are conducted, as shown in Fig. 3(a) and (b). The Raman spectra exhibit two distinctive characteristic peaks, positioned at approximately 1350 cm−1 (D band) and 1590 cm−1 (G band), respectively corresponding to the disordered carbon structure and graphitic carbon structure of carbon materials. The intensity ratio of D to G (ID/IG) is directly proportional to the number of defects, indicating the degree of disorder in the carbon material. The peak of catalysts treated by plasma with different PtNi molar ratios has exhibited a 13 cm−1 shift towards a higher Raman shift compared to pure carbon. This observation suggests an alteration in the structural composition of carbon materials. One plausible explanation is that the active π bonds in C[double bond, length as m-dash]C dissociate due to plasma-induced attack.21 The ID/IG ratios of the PtNi alloy catalyst are much higher compared to pure carbon (1.02), with p-PtNi/KB (Pt[thin space (1/6-em)]:[thin space (1/6-em)]Ni = 1[thin space (1/6-em)]:[thin space (1/6-em)]1) exhibiting a more prominent increase in ID/IG ratio of 1.16. The aforementioned observation implies that plasma etching induces defects and vacancies within the carbon lattice, thereby promoting disorder. The heightened ID/IG ratios also indicate an expanded surface area and reactivity, consequently bolstering the catalytic activity of PtNi alloy catalysts.


image file: d4cc03008g-f3.tif
Fig. 3 Physical properties characterization of catalysts at different Pt–Ni molar ratios. (a) Raman spectra. (b) FTIR spectra.

Furthermore, in the FTIR spectrum (Fig. 3(b)), a significant enhancement in the vibration intensity of oxygen-containing functional groups such as C[double bond, length as m-dash]O, C–O, and COO–H/O–H is observed on the catalyst surface following plasma treatment. This augmentation is attributed to the generation of free radicals on the carbon surface due to plasma bombardment, which subsequently react with active atoms leading to abundant formation of oxygen-containing functional groups.22 Introducing oxygen-containing functional groups into defect sites helps enhance the hydrophilicity of carbon materials. Simultaneously, the improved surface disorder increases active sites for metal deposition, promoting the uniform dispersion and effective anchoring of PtNi alloy nanoparticles on the support. This leads to improved dispersion and interfacial interaction between metal nanoparticles and support, consequently adjusting the electronic properties of surface metal particles and influencing activity and stability of catalyst. Generally speaking, higher plasma voltage boosts energy, intensifying electrochemical corrosion and increasing oxygen-containing groups on the surface. The defect levels of PtNi catalysts with different molar ratios show little variation in this study, as the plasma-treated voltage is consistently 400 V.

Subsequently, the catalytic performance and long-term durability of plasma-induced PtNi alloys are evaluated using a half cell with a rotating disk electrode (RDE). The linear sweep voltammetry (LSV) results of Fig. 4(a) show that the p-PtNi/KB (Pt[thin space (1/6-em)]:[thin space (1/6-em)]Ni = 1[thin space (1/6-em)]:[thin space (1/6-em)]1) in 0.1 M HClO4 solution exhibits excellent ORR half-wave potential (E1/2) with a better value of 0.908 V, surpassing commercial Pt/C catalysts (E1/2 = 0.872 V), indicating its higher catalytic activity in acidic environments. Meanwhile, the performance advantage of plasma-synthesized PtNi alloy catalysts was further confirmed by a control group without plasma treatment. The Tafel slope of p-PtNi/KB (Pt[thin space (1/6-em)]:[thin space (1/6-em)]Ni = 1[thin space (1/6-em)]:[thin space (1/6-em)]1) is observed to be smaller with a value of 75.43 mV dec−1 in Fig. 4(b), revealing an enhancement in its ORR kinetics. The ECSA, determined from the cyclic voltammetry (CV) curves of catalysts with different Pt–Ni molar ratios (Fig. S2, ESI†), exhibits an increasing trend with higher Ni content, as illustrated in Fig. S3 (ESI†). It is noteworthy that a decreasing trend in half-wave potential of PtNi/KB (Pt[thin space (1/6-em)]:[thin space (1/6-em)]Ni = 1[thin space (1/6-em)]:[thin space (1/6-em)]2) was observed during the ORR activity test, which can be attributed to substantial dissolution of nickel experienced by PtNi NPs in an acidic medium.20 The mass activity at 0.9 V (vs. RHE) of p-PtNi/KB (Pt[thin space (1/6-em)]:[thin space (1/6-em)]Ni = 1[thin space (1/6-em)]:[thin space (1/6-em)]1) displays an approximate 3.6-fold enhancement compared to the commercial Pt/C (Fig. S4, ESI†), also suggesting better ORR performance. The electrochemical activity results are in accordance with the findings reported by Carpenter et al.,23 which determined that the composition of PtNi 1[thin space (1/6-em)]:[thin space (1/6-em)]1 was more electrochemically active. The XPS spectra of Pt 4f (Fig. S5, ESI†) reveals a distinct shift in the binding energy of the main Pt0 4f7/2 peaks for catalysts with different Pt–Ni molar ratios compared to commercial Pt/C (71.01 eV). This positive shift is primarily attributed to the electronic effects resulting from alloying Pt and Ni, particularly observed in p-PtNi/KB (Pt[thin space (1/6-em)]:[thin space (1/6-em)]Ni = 1[thin space (1/6-em)]:[thin space (1/6-em)]1), where it further enhances electron transfer and metal-support interaction between PtNi NPs and support material. The electrochemical stability of catalysts is assessed by conducting an accelerated durability test, with p-PtNi/KB (Pt[thin space (1/6-em)]:[thin space (1/6-em)]Ni = 1[thin space (1/6-em)]:[thin space (1/6-em)]1) serving as a representative example, as depicted in Fig. 4(c) and (d). After 10 k cycles of CV in a 0.1 M HClO4 solution, ECSA only decreases by 17% and the half-wave potential shifts towards lower potentials by only 16 mV. In comparison, ECSA of commercial Pt/C decreases by 38% and the half-wave potential shifts to lower potentials by 33 mV, clearly demonstrating superior stability characteristics of p-Pt/KB (Pt[thin space (1/6-em)]:[thin space (1/6-em)]Ni = 1[thin space (1/6-em)]:[thin space (1/6-em)]1) under the same test conditions.


image file: d4cc03008g-f4.tif
Fig. 4 Electrocatalytic characteristics and PEMFC performance measurements for p-PtNi/KBs catalysts of different Pt–Ni molar ratios and commercial Pt/C. (a) LSV curves for the ORR in O2-saturated 0.1 M HClO4 solution (at a scan rate of 10 mV s−1 and rotation speed of 1600 rpm). (b) Tafel plots of different samples. (c) and (d) Comparisons of accelerated durability tests between p-PtNi/KB (Pt[thin space (1/6-em)]:[thin space (1/6-em)]Ni = 1[thin space (1/6-em)]:[thin space (1/6-em)]1) and commercial Pt/C after 10k cycles. (e) Polarization and power density curves of H2–air fuel cells. (f) Stability of H2–air fuel cells operating at a current density of 1000 mA cm−2.

To validate the ORR performance ascertained on a three-electrode system, p-PtNi/KB (Pt[thin space (1/6-em)]:[thin space (1/6-em)]Ni = 1[thin space (1/6-em)]:[thin space (1/6-em)]1) is utilized as the cathode catalyst material in a H2–air PEMFC and subjected to comparative analysis against commercial Pt/C. The current density exhibits a nearly linear increase as the potential decreases during the polarization scan at 80 °C, as depicted in Fig. 4(e). In the low current density domain, similar electrochemical activities are observed; however, as the current density increases from 500 mA cm−2 to 2500 mA cm−2, the power density of the p-PtNi/KB (Pt[thin space (1/6-em)]:[thin space (1/6-em)]Ni = 1[thin space (1/6-em)]:[thin space (1/6-em)]1) consistently exceeds the commercial Pt/C. Particularly, in the high-current density domain (HCD), its power density is significantly increased. This enhancement is attributed to the porous structure and larger specific surface area of the carbon support in p-PtNi/KB (Fig. S6, ESI†), which facilitates gas–liquid transport and effectively enhances mass transfer efficiency in PEMFCs. The power density reaches a value of 1147 mW cm−2 at 2500 mA cm−2, representing an improvement of 11.1% compared to commercial Pt/C. The electrochemical activity and mass transfer performance of p-PtNi/KB are prominently demonstrated under high current densities, highlighting its superior characteristics. In terms of durability testing, p-PtNi/KB shows excellent stability under a constant current density of 1000 mA cm−2, with only a 2.32% voltage decay after at least 100 hours, as shown in Fig. 4(f), further confirming the potential of p-PtNi/KB as a cathode catalyst for PEMFCs.

p-PtNi/KB, as a cathode catalyst, exhibits excellent electrochemical performance in PEMFCs, mainly due to four key advantages: (1) the bombardment by charged particles helps enhance the dispersion on the catalyst surface, optimizing the uniform distribution of platinum sites. This not only increases active site density but also maximizes platinum utilization, ensuring sustained high activity throughout catalysis. (2) The incorporation of smaller Ni atoms induces a reduction in the Pt–Pt bond length, alloying modifies the lattice parameters of platinum, moderate strain facilitates the creation of favorable reaction sites, optimizes reactant adsorption and desorption processes, and ultimately enhances catalytic reaction rates through synergistic effects. (3) Plasma effect induces defects on the surface of support and grafts oxygen-containing functional groups, creating numerous active sites. These sites offer strong chemical adsorption for anchoring Pt NPs on the carrier while promoting electronic coupling between them, boosting catalytic efficiency in electronic transport. (4) The mesoporous structure of p-PtNi/KBs provides abundant space for the formation of three-phase active regions. This structure facilitates the immobilization of catalyst and promotes rapid diffusion of oxygen on the electrode surface, reducing energy barriers during oxygen transport and ultimately enhancing the kinetic performance of oxygen reduction.

In summary, a high-activity and durable p-PtNi/KB catalyst has been successfully prepared using solution-plasma interaction. The results indicate that plasma is an efficient means to achieve alloying and functionalization of support. The p-PtNi/KB synthesized by plasma induction exhibits the highest ORR activity at a 1[thin space (1/6-em)]:[thin space (1/6-em)]1 molar ratio of Pt to Ni. This catalyst exhibited merely a modest decline of 17% in ECSA and a shift of only 16 mV in half-wave potential after 10k cycles of CV, revealing higher ORR activity and durability. In addition, when used as the cathode catalyst for a PEMFC, p-PtNi/KB exhibits a power density of 1147 mW cm−2 at a current density of 2500 mA cm−2, representing an increase of 11.1% compared to commercial Pt/C. At a current density of 1000 mA cm−2, the decay rate of working voltage is only 2.32% after continuous operation for at least 100 hours, indicating significant potential application of p-PtNi/KB as a fuel cell catalyst. These findings provide practical strategies for reducing costs and promoting the commercialization process in fuel cells.

We gratefully acknowledge the National Natural Science Foundation of China (22075035), the Education Department Foundation of Liaoning Province (LJKFZ20220204) and the Science and technology Foundation of Dalian City (2022JJ11CG005).

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4cc03008g
This journal is © The Royal Society of Chemistry 2024