Ying Ling

Bottom-up Design of High-Performance Pt Electrocatalysts Supported on Carbon Nanotubes with Homogeneous Ionomer Distribution

Homogeneous ionomer networks are primarily important for the enhancement of fuel cell performance. Here, we report a bottom‐up design of platinum (Pt) electrocatalysts with homogeneous ionomer layers, in which Pt nanoparticles are deposited on carbon nanotubes (CNTs) after sequentially wrapping with polybenzimidazole (PBI) and end‐capped hyperbranched sulfonated macromolecules (E‐HBM) (CNT/PBI/E‐HBM/Pt). E‐HBM, which are proton conductive macromolecules, are homogeneously coated on CNTs owing to the base–acid interaction between PBI and E‐HBM. The durability and oxygen reduction reaction (ORR) results indicate that CNT/PBI/E‐HBM/Pt exhibits higher Pt stability and ORR activity compared with the electrocatalyst without E‐BHM. The Nafion ionomer‐free membrane electrode assembly (MEA) fabricated from CNT/PBI/E‐HBM/Pt (704 mW cm−2) shows higher power density than the MEA from CNT/PBI/Pt with Nafion ionomer (30 wt %, 603 mW cm−2) owing to the unique bottom‐up design resulting in high Pt utilization efficiency.

Highly methanol-tolerant platinum electrocatalyst derived from poly(vinylpoyrrolidone) coating.

The design and fabrication of a methanol-tolerant electrocatalyst is still one of the most important issues in direct methanol fuel cells (DMFCs). Here, we focus on the design of a cathodic electrocatalyst in DMFCs and describe a new methanol-tolerant electrocatalyst fabricated from poly(vinylpyrrolidone) (PVP) coating on platinum nanoparticles assisted by hydrogen bonding between PVP and polybenzimidazole (PBI). The PVP layer has a negligible effect on the oxygen reduction reaction (ORR) activity, while the methanol oxidation reaction is retarded by the PVP layer. The PVP-coated electrocatalyst shows higher ORR activity under various methanol concentrations in the electrolyte, suggesting that the PVP-coated electrocatalyst has a higher methanol tolerance. Also, the PVP-coated electrocatalyst loses only 14% of the electrochemical surface area after 5000 potential cycles from 0.6-1.0 V versus the reversible hydrogen electrode, indicating better Pt stability than non-coated (27%) and commercial (38%) electrocatalysts due to the unique sandwich structure formed by the PVP and PBI. The power density of the PVP-coated electrocatalyst is four to five times higher compared to non-coated and commercial electrocatalysts with 12 M methanol feeding to the anode side, respectively. PVP coating is important for the enhancement of Pt stability and methanol tolerance. This study offers a new method for preparing a low-cost and high-methanol-tolerant Pt electrocatalyst, and useful information for real DMFC application to eliminate the methanol crossover problem in the cathode side.

High Performance Palladium Supported on Nanoporous Carbon under Anhydrous Condition

Due to the high cost of polymer electrolyte fuel cells (PEFCs), replacing platinum (Pt) with some inexpensive metal was carried out. Here, we deposited palladium nanoparticles (Pd-NPs) on nanoporous carbon (NC) after wrapping by poly[2,2′-(2,6-pyridine)-5,5′-bibenzimidazole] (PyPBI) doped with phosphoric acid (PA) and the Pd-NPs size was successfully controlled by varying the weight ratio between Pd precursor and carbon support doped with PA. The membrane electrode assembly (MEA) fabricated from the optimized electrocatalyst with 0.05 mgPd cm−2 for both anode and cathode sides showed a power density of 76 mW cm−2 under 120 °C without any humidification, which was comparable to the commercial CB/Pt, 89 mW cm−2 with 0.45 mgPt cm−2 loaded in both anode and cathode. Meanwhile, the power density of hybrid MEA with 0.45 mgPt cm−2 in cathode and 0.05 mgPd cm−2 in anode reached 188 mW cm−2. The high performance of the Pt-free electrocatalyst was attributed to the porous structure enhancing the gas diffusion and the PyPBI-PA facilitating the proton conductivity in catalyst layer. Meanwhile, the durability of Pd electrocatalyst was enhanced by coating with acidic polymer. The newly fabricated Pt-free electrocatalyst is extremely promising for reducing the cost in the high-temperature PEFCs.

Simultaneous enhancements in stability and CO tolerance of Pt electrocatalyst by double poly(vinyl pyrrolidone) coatings

CO poisoning and low durability of the anodic electrocatalyst is one of the obstacles restricting the practical application of direct methanol fuel cells (DMFCs). In this work, a highly CO-tolerant and durable platinum (Pt) electrocatalyst is fabricated by introducing double poly(vinyl pyrrolidone) (PVP) layers into the electrocatalyst, in which the first PVP layer is utilized to coat carbon black before Pt deposition and the second PVP layer is laid on the Pt nanoparticles. The first PVP layer on carbon black is favorable for higher Pt utilization efficiency attributed to the capping of micropores on carbon black. The second PVP layer on the Pt nanoparticles facilitates the removal of adsorbed CO species on the Pt nanoparticles and suppresses the Pt aggregation; meanwhile, the PVP layer on the Pt nanoparticles showed a negligible effect (7%) on the electrochemical surface area (ECSA) and methanol oxidation reaction (MOR). Thus, double PVP layers coated Pt electrocatalyst is potentially utilizable for real DMFC operation.

High Durability and Performance of a Platinum Electrocatalyst Supported on Sulfonated Macromolecules Coated Carbon Nanotubes

Low durability and fuel cell performance block the widespread application of polymer electrolyte fuel cells (PEFCs). Here, we deposit platinum nanoparticles on carbon nanotubes (CNTs) before coating by end‐capped hyperbranched sulfonated macromolecules (E‐HBM), which could provide sufficient proton conductivity in catalyst layer thanks to the presence of −SO3H groups. The fabricated electrocatalyst loses 43 % of initial electrochemical surface area (ECSA) after 200 000 potential cycles from 1.0 V to 1.5 V vs. RHE, which is 3 and 20 times higher than that of noncoated and commercial electrocatalysts, respectively. The mass and specific activities of newly fabricated electrocatalyst measured before and after durability test are 1.4 times higher than that of commercial carbon black/Pt. Nafion ionomer‐free membrane electrode assembly is fabricated from the newly designed electrocatalyst, and its fuel‐cell performance reaches 668 mW cm−2 with Pt loading of 0.1 mg cm−2, which is twice higher than that of commercial CB/Pt thanks to the homogeneous conductive macromolecules (E‐HBM) layers on CNTs acting as ionomer and facilitating the proton delivery in the catalyst layer.

Stable CO anti-poisoning and high durability of a Pt electrocatalyst supported on carbon nanotubes

Stable CO anti-poisoning and high durability of an anodic electrocatalyst are very important for direct methanol fuel cells (DMFCs). Here, we report a Pt electrocatalyst with stable CO tolerance and high durability, in which Pt nanoparticles were homogeneously deposited on poly(vinylpyrrolidone) wrapped carbon nanotubes and the Pt nanoparticles were further coated with poly(2,5-benzimidazole) (ABPBI) via in situ polymerization. Although the electrochemical surface area (ECSA) and methanol oxidation reaction (MOR) activity decreased by ∼13% after coating the ABPBI layer on the Pt nanoparticles, the durability was dramatically enhanced due to the presence of ABPBI, which decelerates the Pt migration and aggregation. Meanwhile, the introduction of ABPBI to the electrocatalyst results in high and stable CO tolerance. After a durability test, the CO oxidation peak of the ABPBI coated electrocatalyst was almost stable (only 12 mV shift) compared to those of commercial (96 mV) and non-coated (108 mV) electrocatalysts. Thus, ABPBI is of importance to improve the durability and maintain stable CO tolerance of an electrocatalyst.

SiO2 decoration dramatically enhanced the stability of PtRu electrocatalysts with undetectable deterioration in fuel cell performance

Prevention of Ru dissolution is essential for steady CO tolerance of anodic electrocatalysts in direct methanol fuel cells. Here, we demonstrate a facile way to stabilize Ru atoms by decorating commercial CB/PtRu with SiO2, which shows a six-fold higher stability and similar activity toward a methanol oxidation reaction leading to no discernible degradation in fuel cell performance compared to commercial CB/PtRu electrocatalysts. The higher stability and stable CO tolerance of SiO2-decorated electrocatalysts originate from the SiO2 coating, since Ru atoms are partially ionized during SiO2 decorating, resulting in difficulties in dissolution; while, in the case of commercial CB/PtRu, the dissolved Ru offers active sites for Pt coalescences and CO species resulting in the rapid decay of the electrochemical surface area and fuel cell performance. To the best of our knowledge, this is the first study about the stabilization of Ru atoms by SiO2. The highest stability is obtained for a PtRu electrocatalyst with negligible effect on the electrochemical properties.

PtRu nanoparticles embedded in nitrogen doped carbon with highly stable CO tolerance and durability

As is well known, the lower durability and sluggish methanol oxidation reaction (MOR) of PtRu alloy electrocatalyst blocks the commercialization of direct methanol fuel cells (DMFCs). Here, we design a new PtRu electrocatalyst, with highly stable CO tolerance and durability, in which the PtRu nanoparticles are embedded in nitrogen doped carbon layers derived from carbonization of poly(vinyl pyrrolidone). The newly fabricated electrocatalyst exhibits no loss in electrochemical surface area (ECSA) and MOR activity after potential cycling from 0.6-1.0 V versus reversible hydrogen electrode, while commercial CB/PtRu retains only 50% of its initial ECSA. Meanwhile, due to the same protective layers, the Ru dissolution is decelerated, resulting in stable CO tolerance. Methanol oxidation reaction (MOR) testing indicates that the activity of newly fabricated electrocatalyst is two times higher than that of commercial CB/PtRu, and the fuel cell performance of the embedded PtRu electrocatalyst was comparable to that of commercial CB/PtRu. The embedded PtRu electrocatalyst is applicable in real DMFC operation. This study offers important and useful information for the design and fabrication of durable and CO tolerant electrocatalysts.

Fabrication of stable and well-connected proton path in catalyst layer for high temperature polymer electrolyte fuel cells

It is of importance to establish stable and well‐connected proton path in the catalyst layer to promote the fuel cell performance. Here, we describe a novel method to fabricate stable and efficient proton path for high temperature polymer electrolyte fuel cells (HT‐PEFCs), in which the ionic liquid is doped into the platinum electrocatalyst. The electrochemical results depict that ionic liquid doped electrocatalyst exhibits comparable electrochemical surface area (ESA) and enhanced durability indicating that ionic liquid negligibly affects the hydrogen adsorption/desorption process and protects the electrocatalyst from carbon corrosion. Interestingly, the catalyzing activity toward oxygen reduction reaction (ORR) of Pt electrocatalyst is boosted after doping with ionic liquid mainly due to the modified electronic structures of Pt atoms induced by nitrogen atoms from ionic liquid resulting in weak interaction between Pt atoms and intermediates. The fuel cell performance of ionic liquid doped electrocatalyst is much improved ascribed to the homogeneously dispersed ionic liquid on the surface of Pt electrocatalyst facilitating the fabrication of triple phase boundaries (TPBs) as a result of efficient proton conduction in the catalyst layer. The fuel cell performance only decreases 10 % after 100,000 potential cycles from 1.0 to 1.5 V versus RHE suggesting that ionic liquid forms a stable proton path in the catalyst layer. Thus, the ionic liquid doped Pt electrocatalyst is applicable for the real HT‐PEFC operation.

Constructing successive active sites for metal-free electrocatalyst with boosted electrocatalytic activities toward hydrogen evolution and oxygen reduction reactions

As well known, heteroatom doped carbon material served as metal‐free electrocatalyst for hydrogen evolution (HER) and oxygen reduction reactions (ORR) particularly relies on the heteroatom doping level. Here, we report a 3D porous nitrogen doped carbon (PNC) electrocatalyst with superior HER and ORR electrocatalytic activities derived from the calcination of the discarded cigarette butt and dicyandiamide possessing a high nitrogen content (20 at%). PNC electrocatalyst only demands 143 mV versus RHE to achieve 10 mA cm−2 ascribed to the high nitrogen percentile in PNC electrocatalyst favorable for constructing successive active centers contributing to efficiently catalyzing HER. Ignorable degradation in HER activity observed after 10000 potential cycles as well as undetectable decay in HER performance recorded after 12 h operation indicate high stability of PNC electrocatalyst. Meanwhile, half‐wave potential of PNC electrocatalyst reaches 0.81 V versus RHE in 1 M KOH electrolyte. Additionally, stable ORR activity attained after 1000 potential cycles is indicative of high stability. A comparably high zinc‐air battery performance with maximum power density of 66 mW cm−2 is achieved by PNC electrocatalyst. This study highlights the importance of nitrogen doping level in metal‐free electrocatalyst for boosting HER and ORR activities.