A perspective on the promoting effect of Ir and Au on Pd toward the ethanol oxidation reaction in alkaline media

Abstract

There remain great challenges in developing highly efficient electrocatalysts with both high activity and good stability for the ethanol oxidation reaction in alkaline media. Herein, two architectures of tri-metallic PdIrAu/C electrocatalysts are designed and the promoting effect of Au and Ir on Pd toward the ethanol oxidation reaction (EOR) in alkaline media is investigated in detail. On the one hand, the tri-metallic Pd₇Au₇Ir/C electrocatalyst with a solid solution alloy architecture is less active relative to Pd₇Ir/C and Pd/C while the stabilizing effect of Au leads to both a higher activity and a lower degradation percentage after 3000 cycles of the accelerated degradation test (ADT) on Pd₇Au₇Ir/C than those on Pd₇Ir/C. On the other hand, the tri-metallic Pd₇Ir@(1/3Au)/C electrocatalyst with a near surface alloy architecture delivers a much higher activity with an improvement up to 50.4% compared to Pd₇Ir/C. It is speculated that for the tri-metallic Pd₇Ir@(1/3Au)/C electrocatalyst, certain Au atoms are well designed on surfaces to introduce an electronic modification, thus leading to an anti-poisoning effect and improving the EOR activity.

Keywords

fuel cells / catalysts / ethanol oxidation / alkaline media / solid solution alloy / near surface alloy

Introduction

Nowadays, the continuously intensified global warming as well as the environmental pollutions keep bringing strong impetus to explore advanced strategies to utilize clean and renewable energy, including hydrogen, methanol, ethanol, and so on, instead of fossil fuels [¹,²]. Among clean and renewable energy, ethanol is liquid, hence more facile for storage and transportation relative to hydrogen. Besides, ethanol is less toxic and has a higher energy density (8.01 kWh·kg⁻¹) compared to that of methanol (6.09 kWh·kg⁻¹). Fuel cells are a type of electrochemical devices that directly convert chemical energy into electrical energy while do not follow the principle of the Carnot efficiency, therefore resulting in a high energy efficiency [³–⁵]. Especially, an irreversible trend of electrification ever-increasingly promotes the development of fuel cells. Among various kinds of fuel cells, the direct ethanol fuel cells (DEFCs) distinguish themselves from others especially in the field of portable energy applications due to the above unique advantages. In addition, ethanol is carbon-neutral, and numerous researches on anion exchange membrane DEFCs (AEM DEFCs) deliver promising performances. It is worth mentioning that compared to proton exchange membrane DEFCs (PEM DEFCs), an alkaline media will significantly enhance electrode kinetics, thus leading to the employment of low cost anion exchange membrane and electrocatalysts in both electrodes [⁶–⁸]. More attractively, Pd-based electrocatalysts rather than Pt-based ones are more active toward the anodic ethanol oxidation reaction (EOR) in alkaline media [⁹,¹⁰], thus greatly reducing the cost of fuel cells and shedding light on the mass production of AEM DEFCs in the near future.

Although promising performances have been achieved, there still require great improvements in both the activity and stability of AEM DEFCs for commercialization. Moreover, ingenious designs on the EOR electrocatalysts are also required in order to totally break the C-C bond in ethanol molecule, thus enhancing the energy efficiency of AEM DEFCs. Indeed, many efficient EOR electrocatalysts have been developed via combining Pd with other metals, including transition metals and their oxides (Ni, Fe, Cu, etc.) [¹¹–¹³] or other noble metals (Ag, Ru, Rh, etc.) [¹⁴–¹⁶]. It is believed that for those electrocatalysts, an electronic effect, bi-functional effect or synergistic effect from modified metals or metal oxides contributes to the improved activity or stability toward the EOR in alkaline media. Transition metals and their oxides are usually combined with Pd because of their low cost and much higher abundance. However, from a perspective of the long-term operation, they tend to dissolve easily under frequent potential changes. Therefore, it is anticipated that the combination of Pd with other noble metals remains more desired anode electrocatalysts of AEM DEFCs toward the EOR in alkaline media.

It has been long proved that Au has an obvious stabilizing effect on other noble metal electrocatalysts when being incorporated with for the oxygen reduction reaction (ORR). However, its promoting effect on ORR electrocatalytic activities is insignificant and sometimes, an improvement in the stability leads to a compromise on the activity [¹⁷,¹⁸].When being alloyed with Au, the free energy of alloy phases will be obviously reduced, thus helping alloy nanoparticles strongly resistant to dissolution or surface oxidation [¹⁹,²⁰]. Very recently, an anti-poisoning effect electronically induced by Au has been reported to promote the EOR activity in alkaline media through removing toxic intermediates from the active sites, and the bimetallic PdAu/C electrocatalyst with an optimized Pd:Au ratio of 1:1 demonstrated both a higher activity and a better stability toward the EOR in alkaline media [²¹]. Another noble metal of Ir will also benefit the EOR in alkaline media when being combined with Pd owing to the much more facile adsorption of OH_ads species on Ir at lower potentials, therefore facilitating the removal of ethoxy intermediate store lease more active sites on Pd surface for on-going EOR. Correspondingly, an efficient combination of Ir with Pd could lead to a representative feature with more negative onset potentials for the EOR in alkaline media than that of pure Pd/C [²²]. Likewise, there remains a less satisfying catalytic activity and stability for the EOR on PdIr bimetallic electrocatalysts in alkaline media.

Inspired by the above understanding, herein the possibility to design a highly efficient tri-metallic PdIrAu/C electrocatalyst was investigated in order to enhance the catalytic activity of the EOR in alkaline media. Two synthetic routes were employed to design tri-metallic PdIrAu/C either with a solid solution alloy architecture or with a near surface alloy architecture. The Pd₇Au₇Ir/C electrocatalyst with a solid solution alloy architecture and a Pd:Au:Ir atomic ratio of 7:7:1 was synthesized by using a co-reduction method. The PdAuIr/C electrocatalyst with a near surface alloy architecture was prepared via a Cu under potential deposition (UPD) and a subsequent Au³⁺ galvanic displacement, and an Au submonolayer was successfully formed on Pd₇Ir/C nanoparticles with a surface coverage of 1/3. Electrochemical measurements and physiochemical characterizations including cyclic voltammetry (CV), accelerated degradation test (ADT), X-ray diffraction (XRD), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS) were conducted for detailed analyses.

Experimental section

Chemicals and materials

All chemicals were analytical grade and used as received without any further treatment. The palladium chloride (PdCl₂), hydrogen hexachloroiridate hydrate (H₂IrCl₆·xH₂O), gold chloride trihydrate (HAuCl₄·3H₂O), copper sulfate (CuSO₄), sodium borohydride (NaBH₄) and potassium hydroxide (KOH) were purchased from Sigma-Aldrich while the hydrochloric acid (HCl), sulfuric acid (H₂SO₄), and ethanol (CH₃CH₂OH) from Sinopharm Co. Ltd., China. The catalyst support of carbon powders was received from E-TEK with the particle size located between 20 nm and 40 nm. The binder of 20 wt.% Nafion solution was bought from DuPont.

Synthesis of tri-metallic Pd₇Au₇Ir/C electrocatalyst

The tri-metallic PdAuIr/C electrocatalyst with a Pd:Au:Ir atomic ratio of 7:7:1 was synthesized using a co-reduction method as reported [²¹,²²] and for comparison, the Pd₇Ir/C and Pd/C electrocatalysts were prepared using the same method. In a typical synthesis of Pd₇Au₇Ir/C, carbon powders were first ultrasonically suspended in the metal precursor solution with a designated Pd:Au:Ir atomic ratio of 7:7:1 and kept stirring for 30 min. Then, an excessive amount of 2 wt.% NaBH₄ solution was added to the above-mixed suspension drop by drop under various stirring, and the mixture was kept stirring for more than 5 h until all the precursor ions were completely reduced. The precipitate was collected by filtration, washed by deionized water for several times, and dried at 60°C in an oven overnight. The Pd₇Ir/C and Pd/C were obtained using the same procedure and all the final products were guaranteed with a noble metal loading of 20%.

Preparation of tri-metallic Pd₇Ir@(1/3Au)/C electrocatalyst

The Cu UPD coupled with Au³⁺ galvanic displacement was performed to deposit about 1/3 coverage of Au submonolayer on the as-synthesized Pd₇Ir/C nanoparticles to prepare tri-metallic Pd₇Ir@(1/3Au)/C electrocatalyst. A Pd₇Ir/C catalyst ink was first prepared by ultrasonically dispersing 10 mg powders in 2 mL ethanol and then 8 mL ink was pipetted onto the glassy carbon electrode (GCE, 0.1256 cm²). The metal loading was calculated to be 64 mg/cm² and the whole experiment is conducted in a tailor-made three-electrode cell, in which a saturated calomel electrode (SCE) was adopted as the reference electrode, platinum foil (1 cm²) as the counter electrode, and GCE as the working electrode. It is noted that different from the measurements of EOR in alkaline media, the deposition potential was normalized to the reversible hydrogen electrode (RHE). Specifically, the Pd₇Ir/C modified GCE was then transferred to the mixed solution of 50 mmol/L CuSO₄ and 50 mmol/L H₂SO₄ which was in advance deaerated by Ar for 30 min. Repeated potential scans were first applied on the GCE between 0.353 V to 0.763 V (vs. RHE) at a scan rate of 20 mV/s to remove surface residuals and ended with an anodic scan to clean all the deposited Cu atoms. Then, a linear cathodic sweep from 0.763 V to 0.558 V with the same scan rate was performed to deposit Cu on half of the surface of Pd₇Ir nanoparticles. The final potential was held for 20 s when the Cu-modified GCE was immediately transferred to an Ar-saturated solution of 50 mmol/L HAuCl₄ and 50 mmol/L H₂SO₄ for another 3 min. The tri-metallic Pd₇Ir@(1/3Au)/C was successfully prepared and the Au coverage was calculated to be 1/3 based on the fact that 50% of the surface is covered by Cu before displacement and 3 Cu atoms can only displace 2 Au³⁺ cations. A diluted Nafion solution was pipetted in the end on the GCE as the binder before further electrochemical tests.

Electrochemical measurements

Electrochemical tests including CV and ADT of the as-synthesized samples were conducted in a conventional three-electrode cell with potentiostat (MetrohmAutolabPGSTAT302N). A mercuric oxide electrode (Hg/HgO/KOH, MMO) with 1 mol/L KOH electrolyte inside served as the reference electrode, a platinum foil (1 cm²) as the counter electrode and the glassy carbon electrode (GCE) which had a surface area of 0.1256 cm²and was deposited with different electrocatalysts as the working electrode. Different electrocatalyst inks were prepared by ultrasonically dispersing 10 mg powders in the mixed solution of 1.98 mL ethanol and 0.02 mL Nafion. 8 mL ink was pipetted onto the GCE and the metal loading was set to be 64 mg/cm². It is noted that the GCEs with Pd₇Ir@(1/3Au)/C was directly applied to the test after the preparation.

Stabilized CV curves of the EOR on different electrocatalysts were obtained in a mixed solution of 1 mol/L KOH and 1 mol/L ethanol in the potential range from –0.926 V to 0.274 V (vs. MMO) at a scan rate of 50 mV/s. The ADT was performed on different electrocatalysts by applying repeated potential scans between –0.926 V and 0.274 V vs. MMO in 1 mol/L KOH for 40 and 3000 cycles, and the corresponding EOR performance was recorded in 1 mol/L KOH and 1 mol/L ethanol after different cycles of ADT.

Physiochemical characterizations

XRD patterns were acquired from a D8 ADVANCE Da Vinci Poly-functional X-ray diffractometer. Cu Ka radiation operating at 40 keV was used as the light source and all the tests were conducted at a scan rate of 0.025°/s from 10° to 90°. TEM images were obtained from a JEOL JEM-2100F field emission microscope at a magnification of 80 K. The Pd 3d XPS diagrams were achieved from a Shimadzu Kratos AXIS UltraDLD X-ray photoelectron spectroscopy.

Results and discussion

EOR performance of tri-metallic Pd₇Au₇Ir/C electrocatalyst

The EOR performance of tri-metallic Pd₇Au₇Ir/C electrocatalyst was examined by CV and compared with that of Pd/C and Pd₇Ir/C. The CV curves and the corresponding anodic scans of the EOR on Pd₇Au₇Ir/C, Pd₇Ir/C, and Pd/C are presented in Fig. 1. The stabilized CV curves were obtained in a mixed solution of 1 mol/L KOH and 1 mol/L ethanol between –0.926 V to 0.274 V (vs. MMO) at a scan rate of 50 mV/s. It is observed that pure Pd/C possesses a peak current density of 59.73 mA/cm² and an onset potential of approximate –0.58 V. While as expected, the Pd₇Ir/C electrocatalyst shows a higher peak current density of 63.89 mA/cm² and a much lower onset potential of –0.68 V owing to a facile adsorption of OH_ads species on Ir at lower potentials [²²–²⁴]. Unfortunately, it is found that the tri-metallic Pd₇Au₇Ir/C electrocatalyst is less active relative to both Pd₇Ir/C and Pd/C, and its peak current density is reduced to 48.94 mA/cm², 18% lower than that of Pd/C. Moreover, the onset potential of the EOR on Pd₇Au₇Ir/C is almost the same as that on Pd/C. The stability of the EOR on Pd₇Au₇Ir/C, Pd₇Ir/C, and Pd/C electrocatalysts was evaluated by the degradation percentage in the peak current density after ADT. The ADT was performed by potential sweeping from –0.926 V to 0.274 V vs. MMO in 1 mol/L KOH at a scan rate of 50 mV/s and the EOR curves after 40 and 3000 cycles of ADT tests are displayed in Fig. 2. Although the initial activity of the EOR on Pd₇Au₇Ir/C is lower than that on Pd₇Ir/C, the stabilizing effect of Au is well verified by a higher activity and a lower degradation percentage after 3000 cycles of ADT on Pd₇Au₇Ir/C than those on Pd₇Ir/C. The peak current densities of the EOR on Pd₇Ir/C after 40 and 3000 cycles of ADT tests are, respectively, 63.89 mA/cm² and 9.10 mA/cm², indicating a degradation percentage up to 85.8%. However, when being alloyed with Au, the peak current densities of the EOR on Pd₇Au₇Ir/C presents 48.94 mA/cm² after 40 cycles and 18.43 mA/cm² after 3000 cycles, corresponding to a degradation percentage of 62.3%. It is no doubt that Au stabilizes the lattice when being added, hence reducing the free energy to make nanoparticles harder to dissolute and oxidized [¹⁸–²¹]. Therefore, compared to Pd₇Ir/C, indeed, the addition of Au in the tri-metallic Pd₇Au₇Ir/C electrocatalyst greatly improves its stability. However, there also appears an undesired compromise on the activity of the EOR in alkaline media. Therefore, detailed physicochemical characterizations were performed to analyze this phenomenon.

Analyses of physicochemical properties of Pd₇Au₇Ir/C

Figure 3 compares the XRD patterns of Pd/C, Pd₇Ir/C and Pd₇Au₇Ir/C electrocatalysts and magnified (111) peaks are shown for a clear observation. The position of (111), (200), (220), (311), and (222) planes are well marked, and the first board peak is in correspondence with graphite (002) facet from the carbon support. The characteristic peaks around 39.40°, 45.81°, 66.80°, 80.40° and 84.78° refer to Pd (111), (200), (220), (311) and (222) planes, and Ir (111), (200), (220), (311) and (222) are, respectively, indexed to the peaks at 40.67°, 47.31°, 69.15°, 83.43°, 88.05°. Au (111), (200), (220), (311), and (222) planes are identified as the peaks at 38.18°, 44.39°, 64.58°, 77.55°, and 81.72°. According to magnified (111) peaks, the peak of Pd₇Ir/C shifts to a higher value compared to Pd/C, indicating the formation of a homogeneous PdIr alloy phase. When Au is added, the (111) peak of Pd₇Au₇Ir/C shifts to a lower 2q value relative to that of Pd₇Ir/C, representing the formation of PdIrAu solid solution alloy phase, in which Au has a higher content than Ir.

The typical TEM images and the corresponding histograms of particle size distribution for Pd/C, Pd₇Ir/C, and Pd₇Au₇Ir/C are shown in Fig. 4. It is clearly seen that all the nanoparticles are evenly distributed on the carbon powders, and by randomly choosing 150 nanoparticles, the average particle sizes for Pd/C, Pd₇Ir/C, and Pd₇Au₇Ir/C are statistically calculated to be 4.41 nm, 4.48 nm, and 4.49 nm respectively. It is thus concluded that particle size has almost no influence on the activities between Pd₇Ir/C and Pd₇Au₇Ir/C.

The Pd 3d XPS spectra of Pd/C, Pd₇Ir/C, and Pd₇Au₇Ir/C are exhibited in Fig. 5 in which the black dash line represents the standard biding energy for the doublet peak of the Pd 3d electrons. As can be observed in Fig. 5, the XPS spectra of the three samples display a doublet consisting of a high (Pd 3d_3/2) and a low energy band (Pd 3d_5/2). The peak positions of Pd₇Ir/C at 340.8 eV and 335.5 eV is almost identical to that of Pd/C at 340.9 eV and 335.6 eV, demonstrating that the addition of Ir leads to no electronic modification on Pd. Therefore, the improvement of the EOR activity induced by Ir in Pd₇Ir/C can be ascribed to the promoting effect of a facile OH adsorption to help remove ethoxy intermediates. Meanwhile, when Au is added to Pd₇Ir/C, the peak positions of Pd₇Au₇Ir/C shift obviously to a higher energy band at 341.0 eV and 335.7 eV, indicating an influence of electronic modification from Au, as well as the formation of PdAuIr solid solution alloy phase, which is in consistence with the XRD result [²¹].

Although a better stability induced by an Au addition has been verified for the tri-metallic Pd₇Au₇Ir/C electrocatalyst relative to bi-metallic Pd₇Ir/C, there remains a compromise in its initial catalytic activity toward the EOR in alkaline media. According to physicochemical characterizations, it is presumed that a proper surface atomic ratio among Pd, Ir, and Au should be obtained by further optimization, for Pd plays the major role in the catalytic activity toward EOR and the relative content of Ir and Au should be appropriately modified. Consequently, three aspects should be paid more attentions to in the following design: ① substantial Pd atoms should be released on surfaces as EOR active sites, ② enough Ir atoms close to Pd atoms are indeed required to provide OH_ads species at lower potentials and ③ certain Au atoms should be well designed on surfaces to introduce an electronic modification, thus leading to an anti-poisoning effect and higher catalytic activity. Undoubtedly, innovative techniques to precisely control surface Au are thus desired and a trail on modification of Pd₇Ir/C nanoparticles by Cu UPD coupled with Au³⁺ galvanic displacement has been carried out.

EOR activity of tri-metallic Pd₇Ir@(1/3Au)/C electrocatalyst

Figure 6 shows the Cu UPD curve on Pd₇Ir/C in 50 mmol/L H₂SO₄ and 50 mmol/L HAuCl₄ at a scan rate of 20 mV/s. It is seen that Cu can be repeatedly deposited (cathodic scan) and removed (anodic scan) from the surface of Pd₇Ir/C nanoparticles and the whole process is at a higher potential region than the deposition potential of Cu bulk at 0.32 V (vs. RHE). A single cathodic scan from 0.761 V to 0.498 V (vs. RHE) was finally applied on Pd₇Ir/C to deposit a Cu submonolayer with a 50% surface coverage of the nanoparticles. The Au submonolayer is then formed via a galvanic displacement with only a 1/3 surface coverage due to the 50% surface coverage of Cu and different chemical valences between Au³⁺ and Cu²⁺. The overall mass of deposited Au can be calculated from the total charge in the deposited region marked as the gray area in Fig. 6. Specifically, the total charge of the gray area in Fig. 6 is 6.423 × 10⁻⁴ C, which can be converted to the mass of Au deposited on the GCE as 0.438 mg by using the value of the elementary charge (q₀ = 1.6 × 10⁻¹⁹ C) and the Avogadro’s number (NA= 6.02 × 10²³ mol⁻¹).

The EOR curve of the tri-metallic Pd₇Ir@(1/3Au)/C electrocatalyst was examined and normalized to the same noble metal loading as the reference samples according to the result of the above calculation. Figure 7 presents stabilized CV curves and anodic scans of the EOR on Pd/C, Pd₇Ir/C, and Pd₇Ir@(1/3Au)/C in 1 mol/L KOH+ 1 mol/L ethanol. The potential ranges from –0.926 V to 0.274 V (vs. MMO) and the scan rate is also 50 mV/s. It is attractively found that the Pd₇Ir@(1/3Au)/C presents a much higher activity with a peak current density of 96.09 mA/cm², corresponding to an improvement up to 50.4% compared to Pd₇Ir/C, indicating an obvious promoting effect form Au addition. In addition, Pd₇Ir@(1/3Au)/C also possesses a comparable value of onset potential at approximate –0.68 V (vs. MMO) as Pd₇Ir/C does. This notable enhancement of activity for Pd₇Ir@(1/3Au)/C clearly indicates the importance of surface composition for the design of the EOR electrocatalysts as the previous analyses.

Conclusions

In this paper, the promoting effect of two noble metals, Au and Ir on the EOR on Pd in alkaline media was investigated in detail via designing tri-metallic PdIrAu/C electrocatalyst either with a solid solution alloy architecture or with a near surface alloy architecture. The results demonstrated that in both architectures, Ir could greatly benefit the EOR in alkaline media owing to a facile adsorption of OH_ads species at lower potentials, which facilitated to remove ethoxy intermediates for on-going EOR. For the tri-metallic Pd₇Au₇Ir/C electrocatalyst with a solid solution alloy architecture, Pd₇Au₇Ir/C presented both a higher activity and a lower degradation percentage than that of Pd₇Ir/C after 3000 cycles of ADT tests. However, there existed a compromise in the initial activity of the EOR on Pd₇Au₇Ir/C relative to Pd₇Ir/C, which might be due to the improper surface atomic ratios among Pd, Ir, and Au. It is thus proposed that when synthesizing the tri-metallic PdIrAu/C electrocatalyst, certain Au atoms should be well designed on surfaces to introduce an electronic modification, which will lead to an anti-poisoning effect and improve the EOR activity. As proposed, the tri-metallic Pd₇Ir@(1/3Au)/C electrocatalyst with a near surface alloy architecture presented a much higher activity with a peak current density of 96.09 mA/cm², corresponding to an improvement of up to 50.4% compared to Pd₇Ir/C. It is believed that the perspective on the promoting effect of Au and Ir on the EOR on Pd in alkaline media points out a new strategy to design highly efficiently EOR catalysts with both high activity and good stability.

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