Evaluation of carbon supported platinum–ruthenium nanoparticles for ammonia electro-oxidation: Combined fuel cell and electrochemical approach

Highlights

• Ammonia oxidation reaction investigated on carbon-supported PtRu alloys.

• Various Pt:Ru atomic ratios were studied: 100:0, 90:10, 70:30 and 50:50.

• Pt₉₀Ru₁₀/C catalyst is the most active in electrochemical and fuel cell experiments.

• Main products of ammonia oxidation are nitrogen, nitrate and nitrite.

• 10 at% of Ru improves the reaction by supplying OH_ads.

Abstract

Ammonia electro-oxidation reaction (AmER) was investigated by using conventional electrochemical experiments, direct ammonia fuel cell (DAFC) and galvanostatic electrolysis experiments. The working electrode/anodes were composed of carbon supported PtRu/C nanoparticles (NPs) with atomic Pt:Ru ratios of 100:0, 90:10, 70:30 and 50:50. The resulting nanoparticles ranged between 5.1 and 7.3 nm in size depending on the Ru content and were analyzed by XRD, TEM and synchrotron radiation photoelectron Spectroscopy (SRPES). Alloying Pt with Ru shifted AmER to the lower onset potentials compared to Pt/C. Among nanostructured PtRu/C electrocatalysts, the Pt₉₀Ru₁₀ composition showed the best activity and stability in the conventional electrochemical (cyclic voltammetry and chronoamperometry) experiments, DAFC and 8 h galvanostatic electrolysis. The concentration of nitrite and nitrate was doubled using PtRu/C 90:10 compared to Pt/C, because of excess of OH_ads species formed on Ru. The results show that the addition of small amount of Ru to Pt NPs improves the AmER due to additional formation of OH_ads that promote the reaction on alloyed PtRu nanoparticles.

Introduction

The ammonia electro-oxidation reaction (AmER) has been extensively studied in the recently years from the different point of view, such as environmental applications, e.g., wastewater treatment and ammonia sensors and for energy generation in Direct Ammonia Fuel Cells (DAFCs) [1], [2], [3], [4], [5], [6], [7], [8], [9]. DAFC offers an important alternative to the current energy generation systems as clean and carbon-free fuel for efficient power source [4], [10], [11]. Ammonia has low production cost, is easy to handle and to transport [2], [11], furthermore, the theoretical charge for ammonia oxidation to N₂ is 4.75 Ah g⁻¹ that is very close to theoretical charge of methanol, 5.02 Ah g⁻¹, in its oxidation to CO₂ [2]. Moreover, liquid ammonia has 70% more hydrogen content and 50% higher specific energy density than liquid hydrogen per unit volume and is a carbon-free chemical energy carrier [12].

In this context different electrocatalysts have been evaluated for ammonia electro-oxidation in alkaline media [2], [3], [4], [5], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]. Platinum is the most active catalyst for this process, however it suffers from deactivation by N_ads poisoning intermediate. The energy of N_ads on the catalyst surface is too high (−394 kJ mol⁻¹ [13]), to enable the N₂ formation [23], as a consequence these strongly adsorbed intermediates block surface active sites and prevent continuous oxidation of ammonia on deactivated Pt surfaces. According to the literature, the degree of N_ads coverage on Pt surface increases with anodic potential and reaches the highest value at the peak current density of ammonia electro-oxidation in voltammetry experiments [14], [15].

In order to overcome the deactivation of platinum by poisoning reaction intermediates and to increase the current density of AmER, Pt-binary electrocatalysts have been reported in the literature, such as PtIr [2], [5], [12], [15], [17], PtPd, PtSnO_x [2], [17], PtRh [2], [4], [5], PtNi [15] and PtRu [2], [11], [15]. It is well known that different factors, e.g., nanoparticle synthesis method, particle size, particle size distribution, surface and bulk structure, catalyst support, etc. play a role in electrocatalysis. In addition to these factors, the atomic ratio of Pt to the second metal influences AmER and the optimum performance varies depending on the second metal.

Among the reported bi-metallic catalysts, PtRu showed superior catalytic activity and stability towards AmER compared to monometallic Pt. For instance, Endo et al. [15] studied monometallic Pt and Ru and bi-metallic Pt₈₀Ru₂₀, and Pt₆₀Ru₄₀ catalysts prepared on carbon glass using thermal decomposition method. The authors showed that Pt₈₀Ru₂₀ had the highest current densities compared to other compositions. For Pt₈₀Ru₂₀, the onset potential was shifted by 100 mV to lower potential if compared to Pt, showing a synergetic effect between Pt and Ru. In the work by Vidal-Iglesias et al. [2], authors studied Pt_xRu_1−x (x = 100, 80, 50 and 20) unsupported nanoparticles synthesized using sodium borohydride in water/oil microemulsion method. They demonstrated using CV and CA experiments that the onset potential of AmER shifted about 100 mV to lower potential using Pt₈₀Ru₂₀ unsupported nanoparticles compared to Pt. In their study, Pt₅₀Ru₅₀ showed very low catalytic activity and Pt₂₀Ru₈₀ showed no activity at all [2].

Thus in both studies [2], [15], the unsupported Pt₈₀Ru₂₀ combination showed the optimum performance, however the practical fuel cell catalysts are often composed of nanoparticles dispersed on high surface area support, e.g., carbon black. Therefore, in the present work PtRu/C electrocatalysts with different Pt:Ru atomic ratios: 100:0, 90:10, 70:30 and 50:50 were synthesized using facile and easy to scale-up borohydride reduction method and applied for AmER. For the first time, PtRu catalysts were tested in the direct ammonia fuel cell (DAFC) experiments in combination with detailed oxidation product evaluation during 8-h galvanostatic electrolysis.