Ultra-Low Pt Loading in PtCo Catalysts for the Hydrogen Oxidation Reaction: What Role Do Co Nanoparticles Play?
ABSTRACT: The effect of the nature of the catalyst on the performance and mechanism of the hydrogen oxidation reaction (HOR) is discussed for the first time in this work. HOR is an anodic reaction that takes place in anionic exchange membrane fuel cells (AEMFCs) and hydrogen pumps (HPs). Among the investigated catalysts, Pt exhibited the best performance in the HOR. However, the cost and the availability limit the usage. Co is incorporated as a co-catalyst due to its oxophylic nature. Five different PtCo catalysts with different Pt loading values were synthesized in order to decrease Pt loading. The catalytic activities and the reaction mechanism were studied via electrochemical techniques, and it was found that both features are a function of Pt loading; low-Pt-loading catalysts (Pt loading < 2.7%) led to a high half-wave potential in the hydrogen oxidation reaction, which is related to higher activation energy and an intermediate Tafel slope value, related to a mixed HOR mechanism. However, catalysts with moderate Pt loading (Pt loading > 3.1%) exhibited lower E1/2 than the other catalysts and exhibited a mechanism similar to that of commercial Pt catalysts. Our results demonstrate that Co plays an active role in the HOR, facilitating Hads desorption, which is the rate-determining step (RDS) in the mechanism of the HOR.
Project description:Operating fuel cells in alkaline environments permits the use of platinum-group-metal-free (PGM-free) catalysts and inexpensive bipolar plates, leading to significant cost reduction. Of the PGM-free catalysts explored, however, only a few nickel-based materials are active for catalyzing the hydrogen oxidation reaction (HOR) in alkali; moreover, these catalysts deactivate rapidly at high anode potentials owing to nickel hydroxide formation. Here we describe that a nickel-tungsten-copper (Ni<sub>5.2</sub>WCu<sub>2.2</sub>) ternary alloy showing HOR activity rivals Pt/C benchmark in alkaline electrolyte. Importantly, we achieved a high anode potential up to 0.3 V versus reversible hydrogen electrode on this catalyst with good operational stability over 20 h. The catalyst also displays excellent CO-tolerant ability that Pt/C catalyst lacks. Experimental and theoretical studies uncover that nickel, tungsten, and copper play in synergy to create a favorable alloying surface for optimized hydrogen and hydroxyl bindings, as well as for the improved oxidation resistance, which result in the HOR enhancement.
Project description:Hydroxide exchange membrane fuel cells offer possibility of adopting platinum-group-metal-free catalysts to negotiate sluggish oxygen reduction reaction. Unfortunately, the ultrafast hydrogen oxidation reaction (HOR) on platinum decreases at least two orders of magnitude by switching the electrolytes from acid to base, causing high platinum-group-metal loadings. Here we show that a nickel-molybdenum nanoalloy with tetragonal MoNi<sub>4</sub> phase can catalyze the HOR efficiently in alkaline electrolytes. The catalyst exhibits a high apparent exchange current density of 3.41 milliamperes per square centimeter and operates very stable, which is 1.4 times higher than that of state-of-the-art Pt/C catalyst. With this catalyst, we further demonstrate the capability to tolerate carbon monoxide poisoning. Marked HOR activity was also observed on similarly designed WNi<sub>4</sub> catalyst. We attribute this remarkable HOR reactivity to an alloy effect that enables optimum adsorption of hydrogen on nickel and hydroxyl on molybdenum (tungsten), which synergistically promotes the Volmer reaction.
Project description:Efficient electrocatalyst toward hydrogen evolution/oxidation reactions (HER/HOR) and oxygen reduction reaction (ORR) is desirable for water splitting, fuel cells, etc. Herein, we report an advanced platinum phosphide (PtP<sub>2</sub>) material with only 3.5 wt % Pt loading embedded in phosphorus and nitrogen dual-doped carbon (PNC) layer (PtP<sub>2</sub>@PNC). The obtained catalyst exhibits robust HER, HOR, and ORR performance. For the HER, a much low overpotential of 8 mV is required to achieve the current density of 10 mA cm<sup>-2</sup> compared with Pt/C (22 mV). For the HOR, its mass activity (MA) at an overpotential of 40 mV is 2.3-fold over that of the Pt/C catalyst. Interestingly, PtP<sub>2</sub>@PNC also shows exceptional ORR MA which is 2.6 times higher than that of Pt/C and has robust stability in alkaline solutions. Undoubtedly, this work reveals that PtP<sub>2</sub>@PNC can be employed as nanocatalysts with an impressive catalytic activity and stability for broad applications in electrocatalysis.
Project description:Efficient heterogeneous catalysis of hydrogen oxidation reaction (HOR) by platinum group metal (PGM)-free catalysts in proton-exchange membrane (PEM) fuel cells represents a significant challenge toward the development of a sustainable hydrogen economy. Here, we show that graphene acid (GA) can be used as an electrode scaffold for the noncovalent immobilization of a bioinspired nickel bis-diphosphine HOR catalyst. The highly functionalized structure of this material and optimization of the electrode-catalyst assembly sets new benchmark electrocatalytic performances for heterogeneous molecular HOR, with current densities above 30 mA cm-2 at 0.4 V versus reversible hydrogen electrode in acidic aqueous conditions and at room temperature. This study also shows the great potential of GA for catalyst loading improvement and porosity management within nanostructured electrodes toward achieving high current densities with a noble-metal free molecular catalyst.
Project description:The widely studied Pt/C catalyst for direct methanol fuel cells (DMFCs) suffers severe carbon corrosion under operation, which undermines the catalytic activity and durability. It is of great importance to develop a carbon-free support with co-catalytic functionality for improving both the activity and durability of Pt-based catalysts. The direct loading of Pt on the smooth surface of oxides may be difficult. Herein, the Cu assisted loading of Pt on CeO<sub>2</sub> is developed. Cu pre-coated CeO<sub>2</sub> was facilely synthesized and Pt was electrochemically deposited to fabricate the carbon-free PtCu/CeO<sub>2</sub> catalyst. The PtCu/CeO<sub>2</sub> catalyst has a mass activity up to 1.84 and 1.57 times higher than Pt/C towards methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR), respectively. Better durability is also confirmed by chronoamperometry and accelerated degradation tests. The strategy in this work would be greatly helpful for developing an efficient carbon-free support of Pt-based catalysts for applications in DMFCs.
Project description:The integration of polymer electrolyte membrane fuel cell (PEMFC) stack into vehicles necessitates the replacement of high-priced platinum (Pt)-based electrocatalyst, which contributes to about 45% of the cost of the stack. The implementation of high-performance and durable Pt metal-free catalyst for both oxygen reduction reaction (ORR) and hydrogen oxidation reaction (HOR) could significantly enable large-scale commercialization of fuel cell-powered vehicles. Towards this goal, a simple, scalable, single-step synthesis method was adopted to develop palladium-cobalt alloy supported on nitrogen-doped reduced graphene oxide (Pd<sub>3</sub>Co/NG) nanocomposite. Rotating ring-disk electrode (RRDE) studies for the electrochemical activity towards ORR indicates that ORR proceeds via nearly four-electron mechanism. Besides, the mass activity of Pd<sub>3</sub>Co/NG shows an enhancement of 1.6 times compared to that of Pd/NG. The full fuel cell measurements were carried out using Pd<sub>3</sub>Co/NG at the anode, cathode in conjunction with Pt/C and simultaneously at both anode and cathode. A maximum power density of 68?mW/cm<sup>2</sup> is accomplished from the simultaneous use of Pd<sub>3</sub>Co/NG as both anode and cathode electrocatalyst with individual loading of 0.5?mg/cm<sup>2</sup> at 60?°C without any backpressure. To the best of our knowledge, the present study is the first of its kind of a fully non-Pt based PEM full cell.
Project description:Since trace amounts of CO in H<sub>2</sub> gas produced by steam reforming of methane causes severe poisoning of Pt-based catalysts in polymer electrolyte membrane fuel cells (PEMFCs), research has been mainly devoted to exploring CO-tolerant catalysts. To test the electrochemical property of CO-tolerant catalysts, chronoamperometry is widely used under a CO/H<sub>2</sub> mixture gas atmosphere as an essential method. However, in most cases of catalysts with high CO tolerance, the conventional chronoamperometry has difficulty in showing the apparent performance difference. In this study, we propose a facile and precise test protocol to evaluate the CO tolerance via a combination of short-term chronoamperometry and a hydrogen oxidation reaction (HOR) test. The degree of CO poisoning is systematically controlled by changing the CO adsorption time. The HOR polarization curve is then measured and compared with that measured without CO adsorption. When the electrochemical properties of PtRu alloy catalysts with different atomic ratios of Pt to Ru are investigated, contrary to conventional chronoamperometry, these catalysts exhibit significant differences in their CO tolerance at certain CO adsorption times. The present work will facilitate the development of catalysts with extremely high CO tolerance and provide insights into the improvement of electrochemical methods.
Project description:The latest progress in alkaline anion-exchange membranes has led to the expectation that less costly catalysts than those of the platinum-group metals may be used in anion-exchange membrane fuel cell devices. In this work, we compare structural properties and the catalytic activity for the hydrogen-oxidation reaction (HOR) for carbon-supported nanoparticles of Ni, Ni<sub>3</sub>Co, Ni<sub>3</sub>Cu, and Ni<sub>3</sub>Fe, synthesized by chemical and solvothermal reduction of metal precursors. The catalysts are well dispersed on the carbon support, with particle diameter in the order of 10 nm, and covered by a layer of oxides and hydroxides. The activity for the HOR was assessed by voltammetry in hydrogen-saturated aqueous solutions of 0.1 mol dm<sup>-1</sup> KOH. A substantial activation by potential cycling of the pristine catalysts synthesized by solvothermal reduction is necessary before these become active for the HOR; in situ Raman spectroscopy shows that after activation the surface of the Ni/C, Ni<sub>3</sub>Fe, and Ni<sub>3</sub>Co catalysts is fully reduced at 0 V, whereas the surface of the Ni<sub>3</sub>Cu catalyst is not. The activation procedure had a smaller but negative impact on the catalysts synthesized by chemical reduction. After activation, the exchange-current densities normalized with respect to the ECSA (electrochemically active surface area) were approximately independent of composition but relatively high compared to catalysts of larger particle diameter.
Project description:Understanding how pH affects the activity of hydrogen oxidation reaction (HOR) and hydrogen evolution reaction (HER) is key to developing active, stable, and affordable HOR/HER catalysts for hydroxide exchange membrane fuel cells and electrolyzers. A common linear correlation between hydrogen binding energy (HBE) and pH is observed for four supported platinum-group metal catalysts (Pt/C, Ir/C, Pd/C, and Rh/C) over a broad pH range (0 to 13), suggesting that the pH dependence of HBE is metal-independent. A universal correlation between exchange current density and HBE is also observed on the four metals, indicating that they may share the same elementary steps and rate-determining steps and that the HBE is the dominant descriptor for HOR/HER activities. The onset potential of CO stripping on the four metals decreases with pH, indicating a stronger OH adsorption, which provides evidence against the promoting effect of adsorbed OH on HOR/HER.
Project description:Single-atom catalysts provide an effective approach to reduce the amount of precious metals meanwhile maintain their catalytic activity. However, the sluggish activity of the catalysts for alkaline water dissociation has hampered advances in highly efficient hydrogen production. Herein, we develop a single-atom platinum immobilized NiO/Ni heterostructure (Pt<sub>SA</sub>-NiO/Ni) as an alkaline hydrogen evolution catalyst. It is found that Pt single atom coupled with NiO/Ni heterostructure enables the tunable binding abilities of hydroxyl ions (OH*) and hydrogen (H*), which efficiently tailors the water dissociation energy and promotes the H* conversion for accelerating alkaline hydrogen evolution reaction. A further enhancement is achieved by constructing Pt<sub>SA</sub>-NiO/Ni nanosheets on Ag nanowires to form a hierarchical three-dimensional morphology. Consequently, the fabricated Pt<sub>SA</sub>-NiO/Ni catalyst displays high alkaline hydrogen evolution performances with a quite high mass activity of 20.6 A mg<sup>-1</sup> for Pt at the overpotential of 100 mV, significantly outperforming the reported catalysts.