Enhanced hydrogen production via coupled methanol oxidation reaction using Pt nanowires as bifunctional electrocatalysts

Chemical

Platinum acetylacetonate (Pt(acac)₂, ≥ 0.98), potassium hydroxide (KOH, ≥ 0.85), N,N-dimethylformamide (DMF, ≥ 0.99), potassium iodide (KI, ≥ 99.0%), PVP (K30), and Nafion (5.0 wt%) were procured from Sinopharm. Pt black electrocatalyst was sourced from Johnson Matthey.

Synthesis of Pt NWs

To the reaction vessel, 0.15 mmol of Pt(acac)₂, 300 mg of PVP, 0.75 mmol of KI, and 30 mL of DMF were added. CO was introduced until saturation, and the mixture was then reacted at 130 °C for 6 h. Subsequently, the product was immersed in KOH for 12 h to facilitate thorough rinsing and purification. The influence of reaction time, iodide ions, PVP, and CO on the morphology of the reaction products was systematically investigated through a series of experiments.

Electrochemical measurements

The HER and MOR activity of the electrocatalysts were initially evaluated using a three-electrode configuration. The procedure employs carbon rods, Hg/HgO electrodes, and working electrodes modified with Pt NWs. First, 2 mg of Pt NWs were mixed with 1 mL of water and isopropanol in a 4:1 volume ratio and sonicated for 30 min to obtain a uniform material ink. Subsequently, 4 μL of the ink was meticulously deposited onto the working electrode, followed by the addition of 4 μL of Nafion (0.05 wt%). All potentials were converted to the reversible hydrogen electrode (RHE): E_RHE = E_Hg/HgO + 0.095 + 0.059 × pH.

The electrochemical surface area (ECSA) of Pt NWs was determined using the hydrogen adsorption/desorption charge current as a reference, as given in:

Where Q_Pt denotes the average charge of the hydrogen adsorption/desorption region, Q_ref = 210 μC·cm^-2 and m_Pt represents the Pt loading mass. CO stripping voltammetry was conducted in a 1 mol·L^-1 KOH solution. Initially, CO was introduced into the solution and held at -1 V to ensure complete adsorption on the crystal surface. Excess CO in the electrolyte was then removed by purging with nitrogen gas. The CO stripping experiment was subsequently performed within the potential range of 0 V to 1.2 V.

Characterizations

Structural characterization was conducted using X-ray diffraction (XRD). Surface properties of the materials were assessed using X-ray photoelectron spectroscopy (XPS) with a PHI 5000 VersaProbe III. For transmission electron microscopy (TEM), assessments were conducted using TECNAI G2 F20 and HT7800.

Synthesis and characterization of Pt NWs

To synthesize the materials, a solution comprising Pt precursor, KI, and PVP was prepared in DMF. Subsequently, saturated CO was introduced, and the reaction was conducted at 130 °C for a specified duration to yield Pt NWs [Figure 1]. The morphology of Pt NWs was thoroughly examined through TEM. As illustrated in Figure 2A, these NWs display a uniform diameter of 2.39 nm [Supplementary Figure 1]. Moreover, the surface of Pt NWs is predominantly characterized by the Pt (200) and Pt (111) crystal planes [Figure 2B-E]^[37]. Additionally, the Pt NWs exhibit a polycrystalline architecture, with the corresponding diffraction rings clearly observable [Figure 2F]. To assess the uniformity of the synthesized material, we characterized the microscopic morphology of both individual NWs at various locations and NWs. As depicted in Supplementary Figure 2, the diameter of the same NW remains consistent across multiple locations, approximately 2.39 nm, with its surface predominantly exposing the Pt (200) and Pt (111) planes. Similarly, the diameters of different NWs are also approximately 2.39 nm, and the exposed crystal planes are uniformly aligned. These observations underscore the remarkable uniformity of the synthesized NWs. Concurrently, Pt NWs display distinct XRD peaks at 39.7°, 46.2°, 67.5°, and 81.3° (Pt#04-0802) [Figure 2G]. Simultaneously, the oxidation states of the surface elements in the Pt NWs were examined using XPS. The Pt 4f spectrum reveals peaks attributable to metallic Pt and Pt²⁺ [Figure 2H]^[38]. As a comparison, the morphology of commercial Pt black is characterized by typical nanoparticles, with the surface primarily composed of Pt (111) and Pt (200) crystal planes [Supplementary Figure 3].

Figure 1. Schematic representation of the synthesis of Pt NWs. NWs: Nanowires.

Figure 2. (A) TEM image of Pt NWs; (B and C) A HRTEM image of Pt NWs; (D and E) Distribution of crystal plane spacing; (F) SAED data of Pt NWs; (G) XRD of Pt NWs; (H) XPS Pt 4f spectrum of Pt NWs. TEM: Transmission electron microscopy; NWs: nanowires; XRD: X-ray diffraction; XPS: X-ray photoelectron spectroscopy; HRTEM: high-resolution transmission electronmicroscopy; SAED: selected area electron diffraction.

To elucidate the formation process of Pt NWs, a series of comparative experiments was conducted. The role of iodide ions in the reaction was first investigated by analyzing the morphology of reaction products under varying iodide ion concentrations, while maintaining constant reaction time and temperature. As illustrated in Figure 3A, in the absence of iodide ions, the products predominantly comprised nanoparticles alongside a minor fraction of nanorods. Upon the addition of iodide ions (I:Pt = 1.25), the rod-like morphology began to prevail [Figure 3B]. Further increases in iodide ion concentration facilitated the gradual formation of NWs, achieving an ideal NW structure at I:Pt = 5 [Figure 3C and D]. These phenomena indicate the critical role of iodide ions in driving NW formation. Studies reveal that the roles of iodide ions can be categorized into two main aspects: first, they form Pt-I compounds with the Pt precursor, and their reduction kinetics differ from those of the Pt precursor, which induces the oriented growth of Pt materials; second, iodide ions act as capping agents, strongly adsorbing onto the Pt surface, thereby further promoting the directional growth of Pt. Meanwhile, the roles of CO and PVP in the synthesis of Pt NWs are systematically investigated. Under the optimized conditions for Pt NW synthesis, the absence of CO predominantly yields nanoparticles rather than NWs, underscoring the critical role of CO in guiding the formation of the desired NW morphology [Supplementary Figure 4A]. Similarly, the impact of PVP was examined. As shown in Supplementary Figure 4B, the absence of PVP also results in the formation of nanoparticles instead of NWs, highlighting the indispensable role of PVP in facilitating the synthesis of Pt NWs. The growth dynamics of Pt NWs are further explored under optimized conditions. After one hour of reaction, uniform Pt nanoparticles are predominant [Figure 3E]. As the reaction time extended, shorter NW structures emerged [Figure 3F and G], and after six hours, the products exhibited uniformly elongated NWs [Figure 3H]. Additionally, to assess the impact of extended reaction times on the morphology, the reaction time was increased to 24 h [Supplementary Figure 5]. The results showed that the morphology remained largely consistent with that observed at six hours. Therefore, to optimize the efficiency of Pt NW synthesis, six hours was selected as the optimal reaction time.

Figure 3. (A-D) TEM images of Pt nanostructures synthesized using 300 mg of PVP and saturated CO under varying iodide ion concentrations; (E-H) TEM images of reaction products obtained at different time intervals from the reaction between 300 mg of PVP, saturated CO and 124.5 mg of KI. TEM: Transmission electron microscopy; PVP: polyvinylpyrrolidone.

HER activity of Pt NWs

The electrocatalytic activity of Pt NWs for HER was first evaluated in a three-electrode configuration. This assessment was conducted in N₂-saturated 1 M KOH solution [Figure 4A]. Cyclic voltammetry (CV) measurements for Pt NWs, commercial Pt black and Pt/C reveal comparable electrochemical behavior [Figure 4B and Supplementary Figure 6A]. Notably, Pt NWs exhibit more pronounced electrochemical reduction peaks than Pt black within 0-0.5 V, indicating a higher active surface area for the former. Moreover, Pt NWs exhibit superior electrocatalytic performance compared to Pt black and Pt/C at equivalent current densities [Figure 4C and D, Supplementary Figure 6B]. The Tafel plots for the samples were derived by fitting the Linear Sweep Voltammetry (LSV) curves associated with their HER performance. The Tafel slope for Pt NWs was determined to be 48.8 mV·dec^-1, in contrast to 52.1 mV·dec^-1 for Pt black, underscoring the enhanced electrocatalytic activity of Pt NWs for HER [Figure 4E]. Furthermore, the stability of Pt NWs in alkaline electrolytes was assessed via chronoamperometry (CA). As depicted in Figure 4F, the current density for Pt NWs exhibits minimal degradation over time, outperforming commercial Pt black in maintaining HER activity. This observation underscores the superior stability of Pt NWs in facilitating the HER under alkaline conditions. The hydrogen production of Pt NWs at various voltages was quantitatively analyzed using chromatography. In nitrogen-saturated alkaline electrolytes, the hydrogen yield increases with decreasing applied voltages [Supplementary Figure 7].

Figure 4. (A) A schematic representation of the three-electrode electrochemical testing configuration; (B) CV profile of the catalysts; (C) LSV curves of Pt NWs and Pt black; (D) Comparison of overpotentials across various materials; (E) Corresponding Tafel plots of Pt NWs and Pt black; (F) i-t curves for the electrocatalysts in HER. CV: Cyclic voltammetry; LSV: linear sweep voltammetry; NWs: nanowires; HER: hydrogen evolution reaction.

MOR activity of Pt NWs

The electrocatalytic performance of the catalysts toward MOR was systematically assessed by CV. Pt NWs exhibit an onset potential (E_onset) of 0.41 V, markedly lower than that of Pt black (0.48 V) and Pt/C (0.42 V), highlighting a more favorable MOR kinetics for Pt NWs [Figure 5A and Supplementary Figure 8]. The ECSA for each electrocatalyst was determined through the underpotential deposition of H⁺ on Pt surfaces [Supplementary Figure 9]. Pt NWs demonstrate an ECSA of 39.01 m² g^-1, substantially exceeding that of commercial Pt black (20.98 m² g^-1), which is attributed to the enhanced atomic exposure afforded by the 1D morphology of Pt NWs. As shown in Figure 5B, Pt NWs achieve a mass activity of 1.55 A mg^-1, approximately 2.6-fold greater than Pt black (0.59 A mg^-1), with a specific activity of 4.07 mA cm^-2, surpassing that of Pt black (2.61 mA cm^-2, Supplementary Figure 10). These findings underscore the capacity of the 1D architecture to elevate the catalytic efficacy of Pt-based materials. A comparative summary of MOR activity for various electrocatalysts from recent studies is presented in Table 1, clearly highlighting the superior performance of our 1D Pt NWs. Collectively, these results demonstrate the potential of 1D structuring to significantly enhance electrocatalytic activity. The stability of the electrocatalysts was evaluated using CA at 0.77 V. Both Pt NWs and Pt black exhibit an initial rapid decrease in current, a behavior attributed to the diffusion-controlled characteristics of the CA, in which current scales directly with reactant concentration^[51]. As the reaction progresses, methanol molecules on the electrode surface are consumed, resulting in a decrease in current. However, this drop is also influenced by the adsorption of CO* on the Pt surface, which can decelerate the rate of current density reduction. The current density decay for Pt NWs is significantly less pronounced than that for Pt black, indicating superior catalytic performance of Pt NWs in the methanol oxidation process compared to commercial Pt black [Figure 5C]. The crystalline phase and microstructural morphology of the samples were examined after stability testing. The results reveal that the crystalline phase retains the characteristic face-centered cubic (fcc) structure of Pt without any detectable alterations, and the NW morphology is fully preserved [Supplementary Figure 11].

Figure 5. (A) CV profiles of Pt NWs and Pt black in 1 M KOH containing 0.5 M CH₃OH; (B) Comparison of the catalytic activities of Pt NWs and Pt black; (C) CA stability curves of the catalysts; (D) CO stripping voltammetry profiles. CV: Cyclic voltammetry; NWs: nanowires.

CO stripping experiments were performed to elucidate the resistance of electrocatalysts to poisoning, a critical factor for MOR performance. CO, as an intermediate in the methanol oxidation process, readily adsorbs onto active sites during the reaction, resulting in catalytic activity attenuation. The E_onset for CO oxidation on Pt NWs is measured at 0.41 V, notably lower than the 0.56 V observed for commercial Pt black [Figure 5D]. These results further validate the improved anti-poisoning characteristics and overall catalytic efficiency of Pt NWs for MOR.

MOR-assisted water splitting

Building on the high activity of Pt NWs for both HER and MOR, we utilized Pt NWs as the anode and cathode to investigate methanol-assisted HER [Figure 6A]. LSV polarization curves were recorded for hydrogen production under various anode-cathode configurations and electrolyte conditions [Figure 6B]. In the absence of methanol, the Pt NW sample obtained 10 mA cm^-2 at 1.76 V; conversely, in the methanol-containing system, this current density was realized at a significantly lower voltage of 0.61 V. This finding demonstrates that integrating the methanol oxidation process can effectively reduce the total energy input necessary for water electrolysis. Moreover, when compared to the Pt black system, it was observed that Pt NWs require a lower voltage to achieve the targeted current density. This enhancement is primarily attributed to the 1D structural characteristics of Pt NWs, which optimizes the exposure of active sites and boosts the catalytic efficiency of Pt for both HER and MOR.

Figure 6. (A) A schematic representation of the two-electrode electrochemical testing configuration; (B) LSV curves for Pt NWs and Pt black. LSV: Linear sweep voltammetry; NWs: nanowires.