Cyanogel-Induced Synthesis of RuPd Alloy Networks for High-Efficiency Formic Acid Oxidation

Abstract

For direct formic acid fuel cells (DFAFC), palladium (Pd)-based alloy catalysts with competitive morphology and elemental composition are essential to boost the performance of the formic acid oxidation reaction (FAOR) in the anode zone. Herein, we design and synthesize RuPdx alloy nano-network structures (ANs) via the facile wet-chemical reduction of Pd-Ru cyanogel (Pd_x [Ru(CN)₆]_y·aH₂O) as an effective electrocatalyst for the FAOR. The formation of Pd-Ru cyanogel depends on the facile coordination of K₂PdCl₄ and K₃ [Ru(CN)₆]. The unique structure of cyanogel ensures the presentation of a three-dimensional mesoporous morphology and the homogeneity of the elemental components. The as-prepared RuPd₃ ANs exhibit good electrocatalytic activity and stability for the FAOR. Notably, the RuPd₃ ANs achieve a mass-specific activity of 2068.4 mA mg⁻¹ in FAOR, which shows an improvement of approximately 16.9 times compared to Pd black. Such a competitive FAOR performance of RuPd₃ ANs can be attributed to the advantages of structure and composition, which facilitate the exposure of more active sites, accelerate mass/electron transfer rates, and promote gas escape from the catalyst layer, as well as enhance chemical stability.

1. Introduction

Direct Formic Acid Fuel Cell (DFAFC) has been identified as a promising energy storage device by virtue of its low cost, high security, low-temperature adaptability, and theoretically large energy density [1,2,3,4,5]. Formic acid (HCOOH), as a fuel substance of DFAFC, is a clean and renewable energy source and can be obtained through biomass conversion processes [6,7,8,9]. As the anode reaction in a DFAFC, the formic acid oxidation reaction (FAOR, HCOOH → CO₂ + H₂) performs an essential function in the energy density and charge/discharge cycle stability of the cell. Moreover, as a C1 molecular electrocatalytic reaction, FAOR has substantial research significance [10,11,12]. Pd-based materials are considered to be specific catalysts for the 2e⁻ transfer step of FAOR in DFAFC (Scheme 1), possessing a lower polarisation potential and a more rapid kinetic step compared to the indirect pathway of Pt-based nanomaterials [13,14,15]. However, they have the following pressing problems. (1) Commercial Pd-based catalysts are expensive, [16] which limits the large-scale application of fuel cells [17]. (2) The FAOR performance of commercial catalysts is not yet sufficient and has considerable opportunities for improvement [18]. Accordingly, the design of catalysts with low cost and high activity/stability is a high priority for researchers [19,20,21].

The current strategies for enhancing the activity/stability of FAOR in Pd-based metal catalysts focus on electronic and structural modulation [22,23,24]. Electronic modulation is commonly utilized to enhance the intrinsic activity of catalysts [25,26,27]. The alloying/metallic of Pd with other 3d transition metals can achieve both cost reduction and modulation of the valence electrons in the Pd 3d orbitals [28], thus, improving the ability of catalytic activity and anti-toxicity [29], Further, the alloying of Ru with Pd allows for superior resistance to CO_ads poisoning, while reducing the dosage of Pd metal. As a relatively inexpensive noble metal, Ru is only 1/5 the price of Pd. In previous research, alloying Ru with Pd has proven to be effective in improving these properties [30], because the -OH species generated on Ru can facilitate the oxidative removal of toxic intermediates, and the d-band centre of Pd will be shifted by Ru.

In terms of structural control, the architecture of the catalyst directly affects the exposition of the active site in the electrocatalytic reaction and the ability of the produced gas to escape [31,32], having a direct impact on the long-term stability and kinetic conversion efficiency of catalytic reactions, as well as causes the loss of mass transfer during DFAFC operation. Mesoporous-enriched three-dimensional (3D) materials typically have a large specific surface area and channels, which provides appreciable catalytically active sites and mass transport pathways in electrocatalytic reactions [33,34,35]. Moreover, large amounts of product gases are released during the DFAFC operation, whereas the 0 D structure does not facilitate the escape of CO₂ and H₂, further avoiding the blocking of oxygen by the product gas under high current density. Pd-based alloy catalyst layers with 3D porous structures exhibit greater competitiveness than 0D, 1D and 2D nanomaterials when operating in DFAFC.

The cyanogel-induced synthesis of nanoelectrocatalysts can achieve a combination of these two advantages. The cyanogel-induced synthesis of Pd-based alloy catalysts has been demonstrated in previous studies [36], due to the special coordination pattern of cyanogel that allows the Pd and the other 3d-transition (M) atoms (such as Fe [37], Co [38], Ni [39,40,41], Rh [42], Pt [43] and Cu [44], etc.) to occupy specific positions along the cyanogel skeleton, thus, enabling a predictable morphological and electronic structure during liquid phase reduction, resulting in better long-term stability in electrochemical tests. In addition, the conditions for the synthesis of cyanogel are in line with the concept of low carbon emissions and environmental friendliness. The strategy results in a stable colloidal structure at ambient temperature and pressure. The reduction process is rapid, usually taking only a few seconds to obtain a phase homogeneous alloy, and the high feed conversion rate of the cyanogel reduction reaction results in almost eliminating feedstock waste, as well as allowing for high throughput production.

In this work, for the first time, a series of yellow, orange and tawny Ru-Pd-based cyanogels are conveniently available by using potassium chloropalladite (K₂PdCl₄) and potassium hexacyanoruthenate (K₃[Ru(CN)₆]) as precursors. Notably, this is the first report on the usage of Ru-based cyano inorganic complexes for the synthesis of cyanogels. The molar ratio of Ru/Pd in the cyanogels can easily be adjusted by controlling the feeding ratios, and this feature of Pd_x[Ru(CN)₆]_y·aH₂O cyanogels facilitates the subsequent construction of alloy products with controllable compositions and adjustable properties. Ultimately, the RuPdx alloy was successfully reduced by NaBH₄. The RuPd₃ ANs exhibited superior catalytic activity and stability in the electrocatalytic FAOR (Scheme S1).

2. Results and Discussion

2.1. Physical Characterisation of Pd_x [Ru(CN)₆]_y·aH₂O Cyanogels

As observed, the stable Pd_x[Ru(CN)₆]_y·aH₂O cyanogels can be obtained at a series of feeding ratios of K₂PdCl₄ and K₃[Ru(CN)₆] from 1:1, 2:1, and 3:1 (Figure S1). According to the principle of cyanogel formation, the ruthenium cyanide ion ([Ru(CN)₆]³⁻) rapidly coordinates with chloropalladite ion ([PdCl₄]²⁻) and forms a Ru-CN-Pd bond. The aurantiacus Pd_x [Ru(CN)₆]_y·aH₂O cyanogel is completed in an instant. The RuPdx alloy networks were grown on a cyanogel skeleton by using an excess of fresh aqueous NaBH₄ solution as a reducing agent at room temperature.

The crystal structure of the prepared Pd_x[Ru(CN)₆]_y·aH₂O cyanogels were characterized by X-ray diffraction (XRD) tests (Figure 1a). The results show that Pd_x[Ru(CN)₆]_y·aH₂O cyanogels exhibit the amorphous peak shape of the classical Prussian-like structure in terms of crystal structure after freeze-drying into aero-cyanogels, which matches the peak positions of other Prussian-like structures reported in the previous research [45]. Furthermore, the series of peaks that are situated on 28.33, 40.49, 50.15, 58.60, 66.35 and 73.70 belong to the standard card KCl (JCPDS 97-015-4214), suggesting the by-product prefers to be KCl, rather than KCN, in the formation of the Pd_x[Ru(CN)₆]_y·aH₂O cyanogel:

$$y{K}_3\left[Ru{\left(CN\right)}_6\right]+x{K}_{2}PdC{l}_4+a{H}_{2}O⟶P{d}_x{\left[Ru{\left(CN\right)}_6\right]}_y·a{H}_{2}O+\left(2x+3y+n\right)KCl$$

The coordination was further verified by the Fourier transform infrared (FT-IR) spectra (Figure 1b). The sharp peak, stretching the vibration peaks of the cyanogroup, at 2096 cm⁻¹, indicates the formation of a Ru-CN-Pd bond, which further suggests that Ru is linked to Pd by a cyanogroup-bridging coordination [46]. The peaks at 3469 and 1610 cm⁻¹ belong to the hydroxyl stretching and bending vibrational peaks of crystallized water, respectively [44].

2.2. Physical Characterisation of As-Prepared RuPd₃ ANs, RuPd₂ and RuPd

The RuPd₃ ANs and contrast samples (RuPd₂ and RuPd) were obtained by a liquid–chemical reduction process, using fresh NaBH₄ solution. The atomic content of Pd and Ru was confirmed by energy dispersive spectroscopy (EDS, Figure S3), consisting with the inductively coupled plasma mass spectrometry (ICP-MS) test results (Ru/Pd = 51.1/48.9, 35.4/64.6 and 23.2/76.8). Moreover, the elemental ratio of RuPdx is close to the feeding ratio in the preparative process, indicating that the catalyst prepared by the cyanogel method can achieve a high reaction conversion rate, as well as effectively avoiding the waste of noble metals in the synthetic process. As characterised by XRD patterns (Figure 2a), the 2-Theta diffraction peaks of RuPd₃ ANs situated on 39.71, 45.62 and 67.43 are attributed to the (111), (200) and (220) facets, respectively, which are between Pd (JCPDS 97-006-4914) and Ru (JCPDS 97-004-1515) standard cards, suggesting the formation of RuPdx alloy. Compared to RuPd₂ and RuPd, the diffraction peaks show a clear leftward shift between the Ru and Pd JCPDS cards, as well as towards the Pd side, which further demonstrates that both Ru and Pd elemental ratios can be changed to form stable alloy phases rather than heterogeneous structures/interfaces. Clearly shown in Figure S2, the lattice parameter appeared on a straight fitted line based on a solid solution between Pd and Ru, in the light of Vegard’s Law: $d_{\mathrm{Ru}Pd}=x\cdot d_{Pd}+\left(1-x\right)·d_{Ru}$, confirming the well-defined alloy formation in RuPd₃ ANs.

Further, an XPS measurement (Figure 2b–d and Figure S4) was performed to investigate the changes in the surface properties of the catalysts under different element ratios. According to the XPS analysis (Figure 2b), the surface ratios of Ru for RuPd₃, RuPd₂ and RuPd are approximately 0.45, 0.30 and 0.24, respectively, which are consistent with the EDS and ICP results (

Table 1. The element analysis (Ru content) of RuPd, RuPd₂ and RuPd₃ Ans.

Sample Name	EDS Analysis	XPS Analysis	ICP Analysis
RuPd	0.47	0.45	0.51
RuPd2	0.30	0.30	0.35
RuPd3	0.12	0.24	0.23

), demonstrating that the cyanogel-induced synthesis of RuPdx alloys networks are all homogeneous physical phases rather than core-shell structures.

Moreover, changes in elemental composition can also play a role in the modulation of the electronic structure of the alloy phase, as was demonstrated by comparing the Ru and Pd elemental binding energies of RuPd₃ ANs, RuPd₂ and RuPd. For RuPd₃ ANs, two peaks appeared at the binding energy of 334.94 and 340.24 eV, which were ascribed as Pd 3d_5/2 and Pd 3d_3/2, respectively (Figure 2c), indicating the negative shift of 0.35 and 0.65 eV to a lower binding energy with respect to that of RuPd₂ and RuPd. It is implied by the reduction in binding energy that the electron donation from Ru to Pd and the d-band centre of Pd may be downward-shifted, which favours a weakening of the interaction between the Pd surface and the absorber, so as to boost the electrocatalytic activity in FAOR [28,47]. By contrast, the binding energy of Ru 3p is elevated, with the RuPd₃ ANs showing a 0.46 and 1.05 eV positive shift compared to RuPd₂ and RuPd, respectively (Figure 2d). In summary, with the increase in Pd content, the binding energy of Ru gradually increases, while Pd shows the opposite shift, further verifying that Ru has a significant modulating effect on the electronic structure of Pd and shows binding energy which is favourable to the electrocatalytic reaction of Pd-based materials at Ru/Pd = 1:3.

2.3. Microscopic Morphological Characterisation

The morphological and structural characterisation of as-prepared RuPd₃ ANs were achieved by a scanning electron microscope (SEM) and high-resolution transmission electron microscopy (HRTEM). The 3D porous structure of RuPd₃ ANs was confirmed by SEM (Figure S5). Confirmed by the magnified SEM (Figure 3a) and wide-ranged HRTEM images (Figure 3b), the nano-network structure incorporates abundant mesoporous channels, which provide more active sites for the FAOR as well as facilitate the escape of gases from the catalyst layer [45,48]. The surface area of the catalyst for RuPd₃ ANs was calculated by Brunauer–Emmett–Teller (BET) to be 245.78 m² g⁻¹ from the adsorption-desorption isotherm of N₂ (Figure S6a). The pore size distribution graph further confirms the porous structure of RuPd₃ ANs with a mesoporous pore size of approximately 4.8 nm (Figure S6b). As shown in Figure 3c, the further enlarged HRTEM image revealed clear lattice stripes, which further confirmed the well-crystallised quality of the RuPd₃ ANs. The lattice distance of RuPd₃ ANs measured by the HRTEM (Figure 3d) is 0.228 nm, which is between Pd (111) and Ru (111), consistent with results from XRD patterns. This further verified the Ru content to be approximately 25%. EDS elemental mapping shows a homogeneous distribution of Ru and Pd along the nano-network (Figure 3e), indirectly verifying the ordered arrangement of the RuPd₃ alloy formed on the basis of the cyanogel skeleton.

2.4. Electrochemical Analysis

Cyclic voltammetry tests of RuPd₃ ANs, RuPd₂, RuPd and Pd black were carried out in a 0.5 M H₂SO₄ aqueous solution to investigate their electrochemical properties (Figure S7). By calculating the oxygen reduction peak, the electrochemically active surface area (ECSA) of RuPd₃ ANs, RuPd₂, RuPd and Pd black can be estimated to be 8.1, 7.3, 3.7 and 6.6 m² g⁻¹, respectively (Figure 4a). The reduction peak potential on RuPd₃ ANs is negatively shifted by 56, 166 and 82 mV compared to that of RuPd₂, RuPd and Pd black catalysts, respectively, indicating that RuPd₃ ANs can provide –OH species at a lower potential, which is conducive to the 2 e⁻ transfer pathway of FAOR. It also provides further evidence that the elevated content of Ru/Pd = 1:3 is more conducive to the electrocatalytic FAOR. The electrocatalytic activities for the FAOR of these four catalysts were assessed by CV curves (Figure 4b,c) conducted in N₂-saturated 0.5 M H₂SO₄ + 0.5 M HCOOH at 50 mV s⁻¹. Compared to the Pd black, both the RuPd₃ ANs and RuPd₂ take on much larger mass-normalized peak current densities for FAOR. The improved FAOR activities of these catalysts could be ascribed to the modification of the electronic structure, which is the reduction in the electronic density of states near the Fermi level. The RuPd₃ ANs exhibit superior mass electrocatalytic activity with a peak current density of 2068.4 mA mg_Pd⁻¹, which was 2.9, 7.6 and 16.9 times higher than that of RuPd₂ (705.8 mA mg_Pd⁻¹), RuPd (277.3 mA mg_Pd⁻¹), and Pd black (122.2 mA mg_Pd⁻¹), respectively. The FAOR mass specific activity of RuPd₃ ANs is higher than most reported Pd-based electrocatalysts (Table S1), demonstrating that RuPd₃ ANs can achieve higher current densities at lower potentials. Moreover, compared to RuPd₂ and RuPd catalysts, RuPd₃ ANs have lower oxidation potentials for FAOR, with potential advantages of 77 and 222 mV, respectively. In the comparison of area specific activity, RuPd₃ ANs display 1.4 times the current density of Pd Black, as well as a negative shift of 82 mV of onset potential for FAOR, indicating that RuPd₃ ANs possess the best intrinsic activity. From the Tafel plots in Figure 4e, the RuPd₃ ANs present a higher output current density than RuPd₂, RuPd and Pd black at the same polarization overpotential, suggesting favourable charge transfer and promoted reaction kinetics of FAOR. The electrochemical impedance spectroscopy (EIS) of RuPd₃ ANs and Pd black (Figure S8) shows that RuPd₃ ANs exhibit a smaller semi-circular arc in the region of kinetic control (high-frequency zone) than that of Pd black, which indicates that RuPd₃ ANs have less charge transfer resistance (R_ct) than Pd black in acidic condition. The chronoamperometric measurements shown in Figure S9 were brought into effect in 0.5 M H₂SO₄ + 0.5 M HCOOH electrolyte at a ceaseless potential of 0.6 V (vs. RHE). After 6000 s, commercial Pd black decreased to 0.4 mA cm⁻², while the RuPd₃ ANs showed a comparative decrease of 40.13% to 15.1 mA cm⁻². To further investigate the electrochemical behaviour of RuPd₃ ANs and Pd black during FAOR, 200 cycles of an accelerated endurance test (ADTs) were executed in 0.5 M H₂SO₄ + 0.5 M HCOOH electrolyte. As shown in Figure S10a, with the increase in the CV cycles, the current density enhances slightly, and then gradually stabilizes after 60 cycles. Compared to the rapid decay of the Pd black counterpart (Figure S10b), RuPd₃ ANs exhibit better FAOR stability. Over the next 140 cycles of testing, both RuPd₃ ANs and Pd black all show varying degrees of attenuation. At 200th cycles, the mass-normalized activity of RuPd₃ ANs decays to 855.0 mA mg_Pd⁻¹ (41.35% of the current density in the first cycles), which is still 7.0 times higher than that of Pd black. The Pd black decays to 18.2 mA mg_Pd⁻¹ in the 200th cycle, which is only 14.9% of the current density in the first cycle. Notably, after 60 cycles of ADTs, the FAOR curve for Pd black showed two distinct oxidation peaks, suggesting an indirect pathway to electrocatalytic FAOR after the poisoning of Pd black, while that for RuPd₃ ANs maintained its initial electrochemical characteristics, suggesting a clear advantage of RuPd₃ ANs over Pd black in terms of their anti-CO toxicity performance. The morphology of the RuPd₃ ANs can be well maintained after ADTs testing. As shown in Figure S11, the morphology of RuPd₃ ANs was not significantly changed, and the mesoporous-enriched 3D structure did not collapse/agglomerate, benefiting from the structural stability derived from the robust cyanogel skeleton.

3. Materials and Methods

3.1. Electrochemical Analysis

Potassium hexacyanoruthenate (K₃[Ru(CN)₆]) was supplied by Alfa Aesar Co., Ltd. (Shanghai, China). Potassium chloropalladite (K₂PdCl₄) was purchased from D.B. Chemical Reagent Co., Ltd. (Shanghai, China). Formic acid (HCOOH), ethanol (C₂H₅OH), sulfuric acid (H₂SO₄) and sodium borohydride (NaBH₄) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Pd black was purchased from Johnson Matthey Corporation. All the reagents were not further purified.

3.2. Methods—Synthesis of RuPdx Alloy Networks

In a typical synthesis, K₃[Ru(CN)₆] solution (0.3 mL, 0.1 M) was added to K₂PdCl₄ (0.3, 0.6 and 0.9 mL, 0.1 M) solution in continuous oscillation. Concussion should stop immediately after mixing. Pd_x[Ru(CN)₆]_y·aH₂O cyanogel quickly formed in a matter of minutes. Then, the newly prepared excess NaBH₄ (5.0 mL, 0.2 M) was added to Pd_x[Ru(CN)₆]_y·aH₂O cyanogel. After two hours, the black RuPd, RuPd₂ and RuPd₃ ANs were obtained by ultrasonic pulverisation in a cell pulveriser for 4 min, followed by centrifugal washing several times.

3.3. Characterisation

High-resolution transmission electron microscopy (HRTEM) and HADDF-STEM images were obtained on the Talos F200x G2 microscope operated with an accelerating voltage of 200 kV (Wuhan, China). The scanning electron microscopy (SEM) was characterized by Hitachi S4800 scanning electron microscopy (Nanjing, China). EDS measurements were conducted on Oxford Instruments ULTIM MAX 170 (Nanjing, China). The concentrations of metals were then measured with a Thermo Scientific Plasma Quad inductively coupled plasma mass spectrometry (ICP-MS, (Nanjing, China) and EDS (Nanjing, China). XPS measurements were conducted on a Thermo VG Scientific ESCALAB 250 spectrometer with an Al Kα radiator (Zibo, China). The binding energy was calibrated by means of the C 1s peak energy of 284.6 eV. XRD analyses were performed on a Model D/max-rC X-ray diffractometer employing Kα radiation (λ = 0.15406 nm) (Nanjing, China). BET measurements were required on MicrotracBEL BELSORP-max (Zibo, China) with adsorptive of N₂. The apparatus temperature and adsorption temperature are 0 °C and 77 K, respectively.

3.4. Electrochemical Measurements

Electrochemical measurements were performed on a CHI 760E analyser (Chenhua Co. Shanghai, China) with a conventional three-electrode system. The glassy carbon electrode loaded with catalyst was used as the working electrode, the graphite rod as the counter electrode, and the saturated calomel electrode (SCE) as the reference electrode for the FAOR tests. In electrochemical testing, 4.0 mg of electrocatalysts was dispersed into a mixture of 0.7 mL of alcohol, 1.2 mL of deionized water and 0.1 mL of Nafion solution (5 wt%), and the above substances were mixed by sonication thoroughly for 30 min. Then, 10 μL of the acquired catalyst ink (2 mg mL⁻¹) was dropped onto the surface of the glass carbon electrode of 3.0 mm in diameter. Thus, the mass loading of the catalyst on the GC electrode is 0.285 mg cm⁻². The catalysts were pre-activated in a cyclic voltammetric window in 0.5 M H₂SO₄ solution before the ADTs were tested.

4. Conclusions

In summary, we have developed a new strategy for the construction of Pd_x[Ru(CN)₆]_y·aH₂O cyanogels, using K₃ [Ru(CN)₆] as metal salt precursors, by utilizing the principles of cyanogel synthesis, as well as RuPdx alloy network-like catalysts, which were further obtained at ambient temperature and pressure. During electrocatalytic FAOR testing, the RuPd₃ ANs catalyst demonstrated a competitive electrocatalytic mass/area specific activity, electron/proton transfer rates, and operational long-term stability in comparison to commercial Pd black catalyst and other element ratio component of RuPdx alloys (1:1 and 1:2). The competitive FAOR activity can be attributed to the advantage of a stable 3D nano-network structure, and the modulating effect of electronic effects due to the specific RuPd₃ component ratio. The RuPd₃ ANs show good prospects for the catalyst layer in DFAFC. In addition, this work provides guidance for the subsequent synthesis of cyanogels using ruthenium cyanide as the central ligand.