Rational Design of Nanosized Pt Immobilized on Biomass-Derived Porous Carbon for Enhanced Methanol Oxidation

Abstract

We present the rational design of nanosized Pt nanocrystals immobilized on biomass-derived porous carbon matrices (Pt/BPC) through a convenient and eco-friendly strategy using wheat flour as a sustainable precursor. Interestingly, the three-dimensional BPC conductive network with optimized pore geometry enables enhanced metal–support interaction through d-orbital electron coupling, while the nitrogen-rich carbon scaffold provides abundant nucleation sites for the growth of ultrasmall Pt and effectively prevents them from aggregation. Accordingly, the resultant Pt/BPC catalyst demonstrates exceptional methanol oxidation performance with a large electrochemical surface area, a high mass activity of 1232.5 mA mg⁻¹, and excellent long-term stability, representing significant improvements over conventional carbon (e.g., carbon black, carbon nanotube, graphene, etc.)-supported Pt catalysts.

1. Introduction

The escalating global energy crisis and environmental deterioration necessitate the rapid development of efficient energy conversion and storage technologies [1,2,3,4]. Among these, direct methanol fuel cells (DMFCs) have emerged as a promising power generation technology, owing to their high energy conversion efficiency, low operating temperature, ease of liquid fuel storage, and simple system design [5,6,7]. These advantages make DMFCs widely used in electric vehicles, military systems, and portable electronics [8,9]. However, the large-scale deployment of DMFCs remains constrained by the lack of commercially viable anode catalysts that simultaneously exhibit high activity and long-term stability [10,11,12].

As is well known, platinum (Pt) remains the most effective single-metal anode electrocatalyst for the methanol oxidation reaction (MOR) due to its unique d-band structure [13,14,15,16]. Among various catalysts, the bimetallic PtRu/C structure is also recognized as a superior candidate for MOR applications. However, the practical application of Pt-based catalysts in DMFCs has been hindered by their low utilization efficiency and prohibitively high costs per unit area [17,18,19,20,21]. To address these limitations, extensive research has focused on synthesizing Pt nanocrystals supported by carbonaceous matrices (e.g., carbon black, graphene, and carbon nanotubes), which could enhance the catalytic activity and Pt utilization [22,23,24]. Yet, conventional carbon supports predominantly rely on fossil fuel-derived precursors, commonly requiring energy-intensive and environmentally detrimental synthesis processes [25,26,27]. Moreover, most current carbonaceous supporting materials are chemically inert, which lack efficient surface anchoring sites for the nucleation and growth of small-sized Pt nanoparticles, such as the carbon black and carbon nanotubes [28,29,30,31].

Noteworthily, natural biomass offers a sustainable alternative due to its inherent carbon-rich composition, tunable microstructure, and renewable abundance [32,33,34]. These advantages have spurred the development of advanced functional carbon materials from low-cost biomasses [35,36,37]. The broaden resource could be repurposed into efficient carbon-based electrocatalysts, combining environmental sustainability with practical utility. In this respect, wheat flour, a globally abundant food crop with annual production exceeding 700 million tonnes, represents an attractive feedstock due to the significant waste generated during over-processing [38,39,40]. Our previous theoretical and experimental studies have revealed that nitrogen dopants in graphitic carbon frameworks can promote the formation and dispersion of noble metal nanoparticles with superior electrocatalytic properties [41,42]. Given this, the inherent protein content in wheat flour provides a natural nitrogen source for creating N-doped porous carbon matrices during the thermal treatment process, potentially enabling the surfactant-free synthesis of ultrafine Pt nanoparticles with a uniform dispersibility. To the best of our knowledge, the synthesis strategy for creating this porous hybrid architecture from wheat flour, along with its subsequent characterization, has never been reported before.

Herein, we have demonstrated a convenient and eco-friendly method for the stereoscopic assembly of nanosized Pt nanocrystals immobilized on biomass-derived porous carbon matrices (Pt/BPC) with the use of wheat flour as a sustainable precursor (Figure S1). The resulting Pt/BPC catalyst exhibits superior methanol oxidation performance in terms of a large electrochemical active surface area (ECSA), a high mass activity, and exceptional long-term stability in acidic media, which far surpass those of reference Pt catalysts supported by conventional carbon black, carbon nanotubes, and graphene matrices. These remarkable properties originate from the unique structural advantages of the Pt/BPC catalyst, including large specific surface area, well-developed hierarchical porosity, uniform Pt nanoparticle distribution, and excellent electrical conductivity.

2. Results and Discussion

The 3D porous nanostructure and micromorphology of the as-prepared Pt/BPC nanoarchitecture were carefully investigated by using FE-SEM and TEM. As shown in Figure 1a,b, FE-SEM analysis reveals a well-defined 3D configuration with extensive and interconnected through-holes ranging from several to tens of microns in lateral sizes. This rigid cellular carbon framework provides a hierarchical platform for the uniform deposition of Pt nanoparticles. In Figure 1c–f, high-magnification FE-SEM and TEM images confirm the homogeneous dispersion of ultrasmall Pt nanoparticles across the 3D linked configuration of the biomass-derived porous carbon matrix. The average diameter of these Pt nanocrystals seems about 4.3 nm, which presents a size comparable to Pt particles supported on the high-quality nanocarbon substrates [43,44]. High-resolution TEM image shown in Figure 1g further identifies distinct lattice fringes (d-spacing = 2.24 Å), corresponding to the (111) planes of face-centered cubic (fcc) Pt crystals. In addition, the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and corresponding element mapping images disclose the co-existence of multiple elements, including carbon, oxygen, nitrogen and platinum components (Figure 1h–l), which indicate the highly dispersed metal Pt nanocrystals on the surface of the porous carbonaceous nanoarchitecture.

The crystalline phases of the synthesized Pt/BPC nanoarchitecture were characterized by powder XRD, together with Pt/RGO, Pt/CNT, and Pt/C for comparison. As shown in Figure 2a, all these Pt-based hybrids exhibit characteristic diffraction peaks at 2θ = 39.8°, 46.3°, 67.6°, 81.2°, and 85.9°, indexed to the (111), (200), (220), (311), and (222) crystallographic planes of fcc platinum, respectively (JCPDS No. 04-0802). Notably, the Pt/BPC catalyst demonstrates significant peak broadening at the (111) plane compared to the reference materials, indicating much smaller Pt crystallite sizes. Quantitative analysis using the Scherrer equation reveals an average Pt crystallite size of 4.2 nm for Pt/BPC, consistent with the aforementioned TEM analysis result. The observed size reduction can be attributed to the highly interconnected porous structure of the biomass-derived carbon substrate with abundant N species, which effectively confines and stabilizes the Pt nanoparticles to ensure their small size while maintaining sufficient exposure of catalytically active crystal planes.

XPS technique was then employed to investigate the surface chemical composition and electronic states of the Pt/BPC nanocomposite. The XPS survey spectrum confirms the presence of C 1s, O 1s, N 1s and Pt 4f as the major peaks (Figure 2b), consistent with the results of energy-dispersive X-ray spectroscopy (EDX) (Figure S2). High-resolution C 1s spectrum in Figure 2c exhibits three characteristic peaks at 284.7, 286.1, 287.7 and 289.2 eV, corresponding to the C-C (sp² carbon), C-N, C-O-H (hydroxyl/epoxy), and C=O (carbonyl) functional groups, respectively. XPS analysis of the N 1s spectrum reveals three distinct N species with binding energies at 398.6 eV (pyridinic N), 400.2 eV (pyrrolic N), and 401.4 eV (graphitic N) (Figure 2d). Quantitative analysis indicates that pyridinic and pyrrolic N configurations dominate the nitrogen speciation, accounting for 88.78% of the total N content, which are particularly advantageous for anchoring and uniformly dispersing Pt catalytic sites. The O 1s spectrum in Figure 2e displays two characteristic oxygen species at 531.4 eV (C−O−H) and 532.6 eV (C−O), further confirming the presence of oxygen-containing functional groups in the BPC matrix. Moreover, Pt 4f spectrum (Figure 2f) displays four distinct peaks including metallic Pt⁰ (71.1 and 74.5 eV) and oxidized Pt species (71.8 and 76.5 eV). The coexistence of Pt⁰ and PtO_x species implies strong electronic interactions at the Pt-BPC interface, which may enhance catalytic activity through synergistic effects.

The Pt/BPC nanoarchitecture was next coated onto the glassy carbon electrode surface and tested as an anode catalyst for DMFCs, and the traditional Pt/RGO, Pt/CNT as well as Pt/C catalysts were also employed for comparison. The steady cyclic voltammetry (CV) curves of various catalysts recorded in N₂-purged 0.5 M H₂SO₄ solution at a scan rate of 50 mV s⁻¹ are shown in Figure 3a. These CV curves exhibit the typical peaks associated with hydrogen adsorption and desorption, as well as a plateau area caused by the formation/reduction of the surfaces of the Pt oxide. The ECSA value of the Pt/BPC electrode is determined to be 52.3 m² g⁻¹ based on the integral adsorption peak region of −0.1 to −0.2 V, which is larger than the values of the Pt/RGO (30.9 m² g⁻¹), Pt/CNT (34.8 m² g⁻¹), and Pt/C (24.9 m² g⁻¹) catalysts (Figure 3c and Table S1), indicating that the N-rich nanoporous BPC structure aids in exposing the catalytically active spots.

Additionally, the aforesaid electrodes were put through methanol oxidation tests in a 0.5 M H₂SO₄ and 1 M methanol solution. Notably, the addition of methanol to the acidic medium results in two distinct current peaks at around 0.6 and 0.5 V in the CV curves (Figure 3b), which are attributed to the electrooxidation of the methanol molecules CO byproduct, respectively. The Pt/BPC electrode shows significantly higher mass/specific activities (1232.5 mA mg⁻¹/2.33 mA cm⁻²) than those of the Pt/RGO (478.1 mA mg⁻¹/1.54 mA cm⁻²), Pt/CNT (394.2 mA mg⁻¹/1.12 mA cm⁻²), and Pt/C (160.5 mA mg⁻¹/0.64 mA cm⁻²) catalysts (Figure 3c–d), firmly confirming that using 3D BPC material as the catalytic carrier can effectively improve the catalytic performance. The Pt/BPC displays competitive mass activity comparing to those of other Pt-based electrodes [45,46,47,48,49]. Linear sweep voltammetry (LSV) was employed to assess the onset potentials for methanol oxidation across different catalysts. As shown in Figure 3e, the Pt/BPC electrode exhibits a relative lower overpotential to generate the same current density (e.g., 100 mA mg⁻¹) in comparison to those of other traditional catalysts. The corresponding Tafel slopes of the Pt/BPC electrode, Pt/RGO, Pt/CNT, Pt/C are 55.3, 192.6, 227.3, and 227.7 mV dec⁻¹, respectively (Figure 3e), demonstrating a significantly enhanced reaction kinetics for Pt/BPC. The improvement in catalytic efficiency can be attributed to the optimal Pt nanoparticle dispersion on the 3D hierarchical carbon support, which could provide numerous catalytic centers and meanwhile facilitate the transport of external reactants into the internal active sites. The reduced activation barrier confirms the superior electrocatalytic performance of Pt/BPC toward methanol oxidation, consistent with the enhanced activity observed in CV measurements.

The long-term durability of DMFC catalysts is also a paramount factor that influences their commercial application. The longevity of the aforementioned electrocatalysts was first evaluated by conducting chronoamperometric studies at a constant voltage of 0.5 V (vs. SCE). As shown in Figure 4a, the Pt/BPC electrode exhibits a relatively high and stable oxidation current when compared to other control catalysts over a time period of 5000 s, proving its superior catalytic stability because of the strong interfacial interaction between Pt nanocrystals and N-enriched BPC matrix, which not only encourages the quick oxidative elimination of the intermediate products but also effectively prevents the accumulation or abscission of Pt nanoparticles. As for the chronopotentiometric tests (Figure 4b), it is observed that the electrode potential continues to increase in order to maintain the output current value, and there is a sudden increase in electrode potential until the complete loss of catalytic activity. Among these diverse electrodes, the Pt/BPC catalyst sustains with a low potential for the longest duration of ~1500 s, indicating its superior anti-poisoning ability towards CO species.

The AC impedance spectrum of the Pt/BPC electrode was also acquired in order to examine its electron conductivity, which is believed to be a contributing reason to its extraordinary electrocatalytic capacity. As seen from Figure 4c–d, the Nyquist plots of these studied electrodes reveal clearly defined semiarcs in the high-frequency domain, which are commonly employed to evaluate the charge-transfer resistances. Based on the fitting findings obtained using a typical equivalent circuit (Figure 4e), it is discovered that the Pt/BPC catalyst has the lowest charge-transfer resistance (11.9 Ω) when compared to the Pt/RGO (21.3 Ω), Pt/CNT (100.7 Ω), and Pt/C (1929.4 Ω) catalysts (Table S2). Furthermore, from Figure 4f, we systematically assess the ECSA, mass activity, Tafel slope, specific activity, and charge-transfer resistance of the Pt/BPC nanoarchitecture with those of above Pt/RGO, Pt/CNT, and Pt/C electrocatalysts, which validates that Pt/BPC is the best “pentagonal warrior”.

3. Experimental Section

3.1. Preparation of Biomass-Derived Porous Carbon (BPC)

First, 2 g of wheat flour and 1 g of solid potassium hydroxide (KOH, Sinopharm Chemical Reagent Co., Ltd, Shanghai, China) were sequentially combined with 10 mL of ethanol (Nanjing Chemical Reagent Co., Ltd, Nanjing, China). The mixture was ground rigorously in an agate mortar for 45 min to ensure intimate contact between precursors, yielding a homogeneous flour/KOH composite after ethanol evaporation. Subsequently, the composite was pyrolyzed under a N₂ atmosphere at 700 °C for 2.5 h (heating rate: 5 °C min⁻¹). The resulting carbonized material was then thoroughly washed with diluted hydrochloric acid (Nanjing Chemical Reagent Co., Ltd, Nanjing, China) to remove residual inorganic species, followed by vacuum drying at 50 °C to obtain the resulting 3D BPC nanoframeworks.

3.2. Preparation of the Pt/BPC Nanoarchitecture

The derived BPC powder (10 mg) was ultrasonically dispersed in a mixed solvent of deionized water (40 mL, self made) and ethylene glycol (40 mL, Nanjing Chemical Reagent Co., Ltd, Nanjing, China) to form a homogeneous suspension. Afterwards, a 0.1 M K₂PtCl₄ aqueous solution (0.128 mL, Shanghai Aladdin Biochemical Technology Co., Ltd, Shanghai, China) was introduced dropwise into the suspension under vigorous magnetic stirring for 1 h. The mixture was then subjected to solvothermal treatment at 120 °C for 10 h in a Teflon-lined autoclave (Xi’an Changyi Instrument and Equipment Co., Ltd, Xian, China), enabling the controlled self-assembly of ultrafine Pt nanoparticles onto the BPC matrix. After cooling to ambient temperature, the product was collected by centrifugation (12,000 rpm, 10 min), repeatedly washed with ethanol/water (v/v = 1:1), and lyophilized to yield the target Pt/BPC nanoarchitecture. For benchmarking, analogous procedures were applied to synthesize Pt nanoparticles supported on commercial carbon black (Vulcan XC-72R from Cabot, Boston, MA, USA), multiwalled carbon nanotubes (Chengdu Organic Chemicals CO., Ltd, Chengdu, China), and reduced graphene oxide (XFNANO Materials Tech Co., Ltd, Nanjing, China), designated as Pt/C, Pt/CNT, and Pt/RGO, respectively. The schematic synthesis procedure of Pt/BPC is shown in Figure 5.

3.3. Characterizations

The Pt/BPC nanoarchitecture was systematically investigated using field emission scanning electron microscopy (FE-SEM, Hitachi Regulus 8100, HITACHI, Tokyo, Japan) and high-resolution transmission electron microscopy (HRTEM, FEI Tecnai F20, FEI, Houston, TX, USA) to elucidate its morphological and structural features. The phase composition and crystallinity of the Pt/BPC nanoarchitecture were analyzed by powder X-ray diffraction (XRD, Rigaku Ultima IV, Rigaku, Japan) with Cu-Kα radiation (λ = 1.5406 Å). The chemical states of surface elements were probed via X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250Xi, Thermo Fisher, Waltham, MA, USA) with Al-Kα excitation.

3.4. Electrochemical Measurements

The electrochemical MOR activities of the as-prepared samples were tested using a CHI 760E electrochemical workstation (CH Instruments Ins, Shanghai, China). In addition, we established a specific three-electrode testing system, and the working electrodes were prepared following a protocol adapted from the previous work [50,51], involving drop-casting of the catalyst ink (2 mg mL⁻¹) onto the polished glassy carbon electrode. The electrocatalytic MOR properties were assessed through cyclic voltammetry (CV), linear sweep voltammogram (LSV), chronoamperometry, chronopotentiometry, and electrochemical impedance spectroscopy (EIS) in a mixed electrolyte of 0.5 M H₂SO₄ and 1 M methanol solution.

4. Conclusions

In summary, a facile and eco-friendly approach is developed for the fabrication of nanosized Pt nanocrystals immobilized on wheat flour-derived 3D N-rich porous carbon matrices via a combined thermal treatment and solvothermal assembly process. The resulting Pt/BPC nanoarchitecture, featuring a hierarchical porous network, abundant N species, uniform Pt dispersion, and superior electrical conductivity, demonstrates exceptional MOR performance with a large ECSA value, high mass/specific activities and reliable long-term durability, which significantly outperforms conventional Pt/RGO, Pt/CNT, and Pt/C catalysts. This work not only provides a promising approach for designing advanced electrocatalytic materials with low manufacturing costs but also opens up new avenues for developing biomass-derived porous carbon-based heterostructures in sustainable energy storage and conversion applications.