Juntao Lu

Alkaline polymer electrolyte fuel cells completely free from noble metal catalysts

In recent decades, fuel cell technology has been undergoing revolutionary developments, with fundamental progress being the replacement of electrolyte solutions with polymer electrolytes, making the device more compact in size and higher in power density. Nowadays, acidic polymer electrolytes, typically Nafion, are widely used. Despite great success, fuel cells based on acidic polyelectrolyte still depend heavily on noble metal catalysts, predominantly platinum (Pt), thus increasing the cost and hampering the widespread application of fuel cells. Here, we report a type of polymer electrolyte fuel cells (PEFC) employing a hydroxide ion-conductive polymer, quaternary ammonium polysulphone, as alkaline electrolyte and nonprecious metals, chromium-decorated nickel and silver, as the catalyst for the negative and positive electrodes, respectively. In addition to the development of a high-performance alkaline polymer electrolyte particularly suitable for fuel cells, key progress has been achieved in catalyst tailoring: The surface electronic structure of nickel has been tuned to suppress selectively the surface oxidative passivation with retained activity toward hydrogen oxidation. This report of a H2–O2 PEFC completely free from noble metal catalysts in both the positive and negative electrodes represents an important advancement in the research and development of fuel cells.

Activating Pd by Morphology Tailoring for Oxygen Reduction

Pd has been the focus of recent research for Pt-alternative catalysts for the oxygen reduction reaction (ORR). It has been found that upon appropriate modification of its electronic structure, the catalytic activity of Pd can become comparable to that of Pt. However, the structure-activity relationships of Pd catalysts have hitherto not been well studied or understood. In the present work, we report a new finding that there is a strong dependence of the activity of Pd toward the ORR on its morphology. By simply adjusting the precursor concentration in the electrochemical deposition of Pd, we are able to tailor the morphology of the deposited Pd from nanoparticles to nanorods. Surprisingly, the surface-specific activity of Pd nanorods (Pd-NRs) toward the ORR was found to be not only 10-fold higher than that of Pd nanoparticles (Pd-NPs), the conventional shape of electrocatalysts, but also comparable to that of Pt at operating potentials of fuel cell cathodes. The morphology-activity relationships of Pd-NRs were further studied through a combination of electrochemical experiments and density functional theory (DFT) calculations. As revealed by its characteristic profile for CO stripping, the morphology of Pd-NRs features the exposure of Pd(110) facets, which exhibit superior ORR activity. The underlying mechanism, indicated by DFT calculations, could be ascribed to the exceptionally weak interaction between an O adatom and a Pd(110) facet. This finding furthers our understanding of Pd catalysis and casts a new light on the relevant catalyst design criteria.

Designing Advanced Alkaline Polymer Electrolytes for Fuel Cell Applications

Although the polymer electrolyte fuel cell (PEFC) is a superior power source for electric vehicles, the high cost of this technology has served as the primary barrier to the large-scale commercialization. Over the last decade, researchers have pursued lower-cost next-generation materials for fuel cells, and alkaline polymer electrolytes (APEs) have emerged as an enabling material for platinum-free fuel cells. To fulfill the requirements of fuel cell applications, the APE must be as conductive and stable as its acidic counterpart, such as Nafion. This benchmark has proved challenging for APEs because the conductivity of OH(-) is intrinsically lower than that of H(+), and the stability of the cationic functional group in APEs, typically quaternary ammonia (-NR(3)(+)), is usually lower than that of the sulfonic functional group (-SO(3)(-)) in acidic polymer electrolytes. To improve the ionic conductivity, APEs are often designed to be of high ion-exchange capacity (IEC). This modification has caused unfavorable changes in the materials: these high IEC APEs absorb excessive amounts of water, leading to significant swelling and a decline in mechanical strength of the membrane. Cross-linking the polymer chains does not completely solve the problem because stable ionomer solutions would not be available for PEFC assembly. In this Account, we report our recent progress in the development of advanced APEs, which are highly resistant to swelling and show conductivities comparable with Nafion at typical temperatures for fuel-cell operation. We have proposed two strategies for improving the performance of APEs: self-cross-linking and self-aggregating designs. The self-cross-linking design builds on conventional cross-linking methods and works for APEs with high IEC. The self-aggregating design improves the effective mobility of OH(-) and boosts the ionic conductivity of APEs with low IEC. For APEs with high IEC, cross-linking is necessary to restrict the swelling of the membrane. In our self-cross-linking design, a short-range cross-linker, tertiary amino groups, is grafted onto the quaternary ammonia polysulfone (QAPS) so that the cross-linking process can only occur during membrane casting. Thus, we obtain both the stable ionomer solution and the cross-linked membrane. The self-cross-linked QAPS (xQAPS) possesses a tight-binding structure and is highly resistant to swelling: even at 80 °C, the membrane swells by less than 3%. For APEs with low IEC, the key is to design efficient OH(-) conducting channels. In our self-aggregating design, long alkyl side-chains are attached to the QAPS. Based on both the transmission electron microscopy (TEM) observations and the molecular dynamics (MD) simulations, these added hydrophobic groups effectively drive the microscopic phase separation of the hydrophilic and hydrophobic domains and produce enlarged and aggregated ionic channels. The ionic conductivity of the self-aggregated QAPS (aQAPS) is three-fold higher than that of the conventional QAPS and is comparable to that of Nafion at elevated temperatures (e.g., greater than 0.1 S/cm at 80 °C).

A feasibility analysis for alkaline membrane direct methanol fuel cell: thermodynamic disadvantages versus kinetic advantages

As the proton exchange membrane direct methanol fuel cell (PEMDMFC) faces sustaining obstacles, alkaline membrane direct methanol fuel cell (AMDMFC) is attracting increasing attention. Although some advantages may be expected, the feasibility of AMDMFC does not seem well verified. In this paper, thermodynamic disadvantages and kinetic advantages of AMDMFC are elucidated. In thermodynamic aspect, a large voltage loss due to the pH difference across the membrane is predicted by theoretical calculation; in kinetic aspect, besides the well-known superiority of alkaline media for oxygen reduction, experimental data show much higher anodic performance in carbonate/bicarbonate than in acid. In-situ FTIR measurements indicate that methanol can be fully oxidized to carbon dioxide in carbonate/bicarbonate as in sulfuric acid. Taking into account all the foreseeable advantageous and disadvantageous factors, AMDMFC is worth study, and an alkaline membrane stable at elevated temperatures is the prerequisite for a successful AMDMFC.

Constructing ionic highway in alkaline polymer electrolytes

Alkaline polymer electrolytes (APEs) are an emerging material that enables the use of nonprecious-metal catalysts in electrochemical energy technology, such as fuel cell and water electrolysis. Yet the OH− conduction in APE has been of much lower efficiency than the H+ conduction in its acidic counterpart (typically Nafion), leading to a large dissipative loss in energy conversion applications. Here we report that, by properly constructing ion-aggregating structures in APE, a OH− conducting highway can be built, such that the OH− conduction in APE becomes as efficient as the H+ conduction in Nafion (greater than 0.1 S cm−1 at 80 °C under moderate ion-exchange capacity 1.0 mmol g−1). The optimal approach to constructing such an ionic highway is first screened computationally using coarse-grained molecular dynamics (CGMD) simulations, and then implemented experimentally based on a quaternary ammonia polysulfone (QAPS) model system. The resulting ordered structure of ion assembly has been unambiguously revealed by both the theoretically calculated structure factor and experimental results of TEM and SAXS. These findings have not only furthered our understanding about the ionic channels in APE, but also provided a general strategy for the rational design of polymer electrolytes.

Pt–Ru catalyzed hydrogen oxidation in alkaline media: oxophilic effect or electronic effect?

A current challenge to alkaline polymer electrolyte fuel cells (APEFCs) is the unexpectedly sluggish kinetics of the hydrogen oxidation reaction (HOR). A recently proposed resolution is to enhance the oxophilicity of the catalyst, so as to remove the Had intermediate through the reaction with OHad, but this approach is questioned by other researchers. Here we report a clear and convincing test on this problem. By using PtRu/C as the HOR catalyst for the APEFC, the peak power density is boosted to 1.0 W cm−2, in comparison to 0.6 W cm−2 when using Pt/C as the anode catalyst. Such a remarkable improvement, however, can hardly be explained as an oxophilic effect, because, as monitored by CO stripping, reactive hydroxyl species can generate on certain sites of the Pt surface at more negative potentials than on the PtRu surface in KOH solution. Rather, the incorporation of Ru has posed an electronic effect on weakening the Pt–Had interaction, as revealed by the voltammetric behavior and from density-functional calculations, which thus benefits the oxidative desorption of Had, the rate determining step of HOR in alkaline media. These findings further our fundamental understanding of the HOR catalysis, and cast new light on the exploration of better catalysts for APEFCs.

Pt Skin on AuCu Intermetallic Substrate: A Strategy to Maximize Pt Utilization for Fuel Cells

The dependence on Pt catalysts has been a major issue of proton-exchange membrane (PEM) fuel cells. Strategies to maximize the Pt utilization in catalysts include two main approaches: to put Pt atoms only at the catalyst surface and to further enhance the surface-specific catalytic activity (SA) of Pt. Thus far there has been no practical design that combines these two features into one single catalyst. Here we report a combined computational and experimental study on the design and implementation of Pt-skin catalysts with significantly improved SA toward the oxygen reduction reaction (ORR). Through screening, using density functional theory (DFT) calculations, a Pt-skin structure on AuCu(111) substrate, consisting of 1.5 monolayers of Pt, is found to have an appropriately weakened oxygen affinity, in comparison to that on Pt(111), which would be ideal for ORR catalysis. Such a structure is then realized by substituting the Cu atoms in three surface layers of AuCu intermetallic nanoparticles (AuCu iNPs) with Pt. The resulting Pt-skinned catalyst (denoted as Pt(S)AuCu iNPs) has been characterized in depth using synchrotron XRD, XPS, HRTEM, and HAADF-STEM/EDX, such that the Pt-skin structure is unambiguously identified. The thickness of the Pt skin was determined to be less than two atomic layers. Finally the catalytic activity of Pt(S)AuCu iNPs toward the ORR was measured via rotating disk electrode (RDE) voltammetry through which it was established that the SA was more than 2 times that of a commercial Pt/C catalyst. Taking into account the ultralow Pt loading in Pt(S)AuCu iNPs, the mass-specific catalytic activity (MA) was determined to be 0.56 A/mg(Pt)@0.9 V, a value that is well beyond the DOE 2017 target for ORR catalysts (0.44 A/mg(Pt)@0.9 V). These findings provide a strategic design and a realizable approach to high-performance and Pt-efficient catalysts for fuel cells.

A strategy for disentangling the conductivity–stability dilemma in alkaline polymer electrolytes

Alkaline polymer electrolytes (APEs) are a new class of polyelectrolytes enabling the use of nonprecious metal catalysts in electrochemical devices, such as fuel cells and water electrolyzers. However, the current development of APEs is facing a severe difficulty, the conductivity–stability dilemma. Specifically, to acquire high ionic conductivity, the polymer backbone has to be grafted with enough cationic functional groups, typically quaternary ammonium (–NR3+), but such a modification in structure has damaged the chemical inertness of the polymer backbone and induced degradation in an alkaline environment. Here we demonstrate a strategy for disentangling such a dilemma. To alleviate the damage to the polymer backbone, we reduce the grafting degree (GD) of functional groups, but design two cations on each grafted functional group so as to retain sufficient ion concentration. Such a seemingly simple change in structure has brought a notable effect in performance: not only can both high ionic conductivity and much improved chemical stability be achieved, but also the intermolecular interaction between polymer chains has thus been enhanced, rendering the resulting APE membrane much stronger in mechanical strength and highly anti-swelling in water even at 80 °C.

First implementation of alkaline polymer electrolyte water electrolysis working only with pure water

A new type of water electrolysis is implemented using an alkaline polymer electrolyte (APE) and non-precious metal catalysts, and working only with pure water. The membrane–electrode assembly (MEA) is fabricated by sandwiching a self-crosslinking quaternary ammonia polysulfone (xQAPS) membrane between a Ni–Fe anode and a Ni–Mo cathode, both impregnated with xQAPS ionomer. Such an initial prototype of APE water electrolysis has exhibited decent performance comparable to that of the well-developed alkaline water electrolyzer.

Highly Stable Alkaline Polymer Electrolyte Based on a Poly(ether ether ketone) Backbone

Alkaline polymer electrolyte fuel cells (APEFCs) promise the use of nonprecious metal catalysts and thus have attracted much research attention in the recent decade. Among the challenges of developing practical APEFC technology, the chemical stability of alkaline polymer electrolytes (APEs) seems to be rather difficult. Research found that, upon attachment of a cationic functional group, an originally stable polymer backbone, such as polysulfone (PSF), would degrade in an alkaline environment. In the present work, we try to employ poly(ether ether ketone) (PEEK), a very inert engineering plastic, as the backbone of APEs. The PEEK is functionalized with both a sulfonic acid (SA) group and a quaternary ammonia (QA) group, with the latter as the majority amount. Ionic cross-linking between SA and QA has rendered the thus-obtained membrane (xQAPEEK) with high mechanical strength and low swelling degree. More importantly, the xQAPEEK membrane exhibits outstanding stability in a 1 mol/L KOH solution at 80 °C for a test period of 30 days: the total weight loss of xQAPEEK is only 6 wt %, in comparison to a large degradation of quaternary ammonia PSF (more than 40 wt %) under the same conditions. Our findings not only have demonstrated an effective approach to preparing PEEK-based APE but also cast a new light on the development of highly stable APEs for fuel-cell application.