Investigation into the activity of heteropolyacids towards the oxygen reduction reaction on PEMFC cathodes

Fundamental and Practical Aspects of Break-In/Conditioning of Proton Exchange Membrane Fuel Cells

Proton exchange membrane fuel cells (PEMFCs) have proven to be a promising power source for various applications ranging from portable devices to automotive and stationary power systems. The production of PEMFC involves numerous stages in the value chain, with each stage presenting unique challenges and opportunities to improve the overall performance and durability of the PEMFC stack. These include steps such as manufacturing the key components such as the platinum-based catalyst, processing these components into the membrane electrode assemblies (MEAs), and stacking the MEAs to ultimately produce a PEMFC stack. However, it is also known that the break-in or conditioning phase of the stack plays a crucial role in the final performance as well as durability. It involves several key phenomena such as hydration of the membrane, swelling of the ionomer, redistribution of the catalyst and the creation of suitable electrochemical interfaces - establishment of the triple phase boundary. These improve the proton conductivity, the mass transport of reactants and products, the catalytic activity of the electrode and thus the overall efficiency of the FC. The cruciality of break-in is demonstrated by the improvement in performance, which can even be over 50 % compared to the initial state. The state-of-the-art approach for the break-in of MEAs involves an electrochemical protocol, such as voltage cycling, using a PEMFC testing station. This method is time-consuming, equipment-intensive, and costly. Therefore, new, elegant, and cost-effective solutions are needed. Nevertheless, the primary aim is to achieve maximum/optimal performance so that it is fully operational and ready for the market. It is therefore essential to better understand and deconvolute these complex mechanisms taking place during break-in/conditioning. Strategies include controlled humidity and temperature cycling, novel electrode materials and other advanced break-in methods such as air braking, vacuum activation or steaming. In addition, it is critical to address the challenges associated with standardisation and quantification of protocols to enable interlaboratory comparisons to further advance the field.

Ionic self-assembly of metalloporphyrin/heteropolyacid on multi-wall carbon nanotubes with enhanced electrocatalytic activity toward oxygen reduction reaction

We report the synthesis of a series of hybrid electrocatalysts toward oxygen reduction reaction (ORR) by ionic self-assembly of positively charged Fe(III) meso-tetra([Formula: see text]-methyl-4-pyridyl)porphyrin (FeP) with negatively charged H3PMo[Formula: see text]O[Formula: see text] (PMo[Formula: see text] in ethanol solution under ambient conditions in the presence of suspended multi-wall carbon nanotubes (MWCNTs). Self-assembled FeP/PMo[Formula: see text] was well-dispersed on MWCNTs with a tunable loading from 26.3% to 55.6%. Interestingly, the hybrid electrocatalysts demonstrated a much higher ORR activity than individual PMo[Formula: see text] and FeP in acidic media. We speculate that this activity enhancement might originate from a possible synergy between PMo[Formula: see text] and FeP. This study may be extended to the design and synthesis of other types of hybrid electrocatalysts for applications in electrochemical energy conversion, storage and sensing.

Highly Active PdNi/RGO/Polyoxometalate Nanocomposite Electrocatalyst for Alcohol Oxidation

A PdNi/RGO/polyoxometalate nanocomposite has been successfully synthesized by a simple wet-chemical method. Characterizations such as transmission electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction analysis, and X-ray photoelectron spectroscopy are employed to verify the morphology, structure, and elemental composition of the as-prepared nanocomposite. Inspired by the fast-developing fuel cells, the electrochemical catalytic performance of the nanocomposite toward methanol and ethanol oxidation in alkaline media is further tested. Notably, the nanocomposite exhibits excellent catalytic activity and long-term stability toward alcohol electrooxidation compared with the PdNi/RGO and commercial Pd/C catalyst. Furthermore, the electrochemical results reveal that the prepared nanocomposite is attractive as a promising electrocatalyst for direct alcohol fuel cells, in which the phosphotungstic acid plays a crucial role in enhancing the electrocatalytic activities of the catalyst.

Polymer Electrolyte Fuel Cells Employing Heteropolyacids as Redox Mediators for Oxygen Reduction Reactions: Pt-Free Cathode Systems

In this study, the heteropolyacids of H3+xPVxMO12-xO40 (x = 0, 2, and 3) were applied as redox mediators for the oxygen reduction reaction in polymer electrolyte fuel cells, of which the cathode is free from the usage of noble metals such as Pt/C. In this system, the electrochemical reduction of heteropolyacid over the carbon cathode and the subsequent reoxidation of the partially reduced heteropolyacid by exposure to the dissolved oxygen in the regenerator are important processes for continuous power generation. Thus, the redox properties of catholytes containing these heteropolyacids were investigated in detail. The substitution quantity of V in the heteropolyacid affected the onset reduction potential as well as the reduction current density, resulting in a difference in cell performance. The chemical composition of heteropolyacid also had a significant impact on the reoxidation property. Among the three compounds, H6PV3Mo9O40 was the most suitable redox mediator. Furthermore, the pH of the catholyte was found to be the crucial factor in determining the reoxidation rate of partially reduced heteropolyacid as well as cell performance.

Polyoxometalate-reduced graphene oxide hybrid catalyst: synthesis, structure, and electrochemical properties

The deposition of polyoxometalate (POM) on chemically reduced graphene oxide sheets was carried out through electron transfer interaction and electrostatic interaction between POM and graphene sheets to make a heterogeneous catalyst in aqueous media. Well dispersed individual phosphomolybdic acid (PMo) clusters were observed by electron microscopy and atomic force microscopy measurements. The interaction between polyoxometalate and the graphene sheet was confirmed by using various spectroscopic methods such as FT-IR, UV-vis, and Raman. The UV-visible, IR, and cyclic voltammetry results revealed alteration of the electronic structure of deposited PMo as a result of strong interaction with the graphene oxide surface. Electrochemical properties of the PMo-rGO catalyst were investigated in an aqueous acidic electrolyte. The hybrid catalyst showed enhanced electro-oxidation of nitrite compared with pure homogeneous PMo and rGO.

Three-dimensional tracking and visualization of hundreds of Pt-Co fuel cell nanocatalysts during electrochemical aging

We present an electron tomography method that allows for the identification of hundreds of electrocatalyst nanoparticles with one-to-one correspondence before and after electrochemical aging. This method allows us to track, in three-dimensions, the trajectories and morphologies of each Pt-Co nanocatalyst on a fuel cell carbon support. In conjunction with the use of atomic-scale electron energy loss spectroscopic imaging, our experiment enables the correlation of performance degradation of the catalyst with changes in particle/interparticle morphologies, particle-support interactions, and the near-surface chemical composition. We found that aging of the catalysts under normal fuel cell operating conditions (potential scans from +0.6 to +1.0 V for 30,000 cycles) gives rise to coarsening of the nanoparticles, mainly through coalescence, which in turn leads to the loss of performance. The observed coalescence events were found to be the result of nanoparticle migration on the carbon support during potential cycling. This method provides detailed insights into how nanocatalyst degradation occurs in proton exchange membrane fuel cells (PEMFCs) and suggests that minimization of particle movement can potentially slow down the coarsening of the particles and the corresponding performance degradation.

Functionalization and post-functionalization: a step towards polyoxometalate-based materials

Polyoxometalates (POMs) have remarkable properties and a great deal of potential to meet contemporary societal demands regarding health, environment, energy and information technologies. However, implementation of POMs in various functional architectures, devices or materials requires a processing step. Most developments have considered the exchange of POM counterions in an electrostatically driven approach: immobilization of POMs on electrodes and other surfaces including oxides, embedding in polymers, incorporation into Layer-by-Layer assemblies or Langmuir-Blodgett films and hierarchical self-assembly of surfactant-encapsulated POMs have thus been thoroughly investigated. Meanwhile, the field of organic-inorganic POM hybrids has expanded and offers the opportunity to explore the covalent approach for the organization or immobilization of POMs. In this critical review, we focus on the use of POM hybrids in selected fields of applications such as catalysis, energy conversion and molecular nanosciences and we endeavor to discuss the impact of the covalent approach compared to the electrostatic one. The synthesis of organic-inorganic POM hybrids starting from bare POMs, that is the direct functionalization of POMs, is well documented and reliable and efficient synthetic procedures are available. However, as the complexity of the targeted functional system increases a multi-step strategy relying on the post-functionalization of preformed hybrid POM platforms could prove more appealing. In the second part of this review, we thus survey the synthetic methodologies of post-functionalization of POMs and critically discuss the opportunities it offers compared to direct functionalization.

Self assembled 12-tungstophosphoric acid-silica mesoporous nanocomposites as proton exchange membranes for direct alcohol fuel cells

A highly ordered inorganic electrolyte based on 12-tungstophosphoric acid (H(3)PW(12)O(40), abbreviated as HPW or PWA)-silica mesoporous nanocomposite was synthesized through a facile one-step self-assembly between the positively charged silica precursor and negatively charged PW(12)O(40)(3-) species. The self-assembled HPW-silica nanocomposites were characterized by small-angle XRD, TEM, nitrogen adsorption-desorption isotherms, ion exchange capacity, proton conductivity and solid-state (31)P NMR. The results show that highly ordered and uniform nanoarrays with long-range order are formed when the HPW content in the nanocomposites is equal to or lower than 25 wt%. The mesoporous structures/textures were clearly presented, with nanochannels of 3.2-3.5 nm in diameter. The (31)P NMR results indicates that there are (≡SiOH(2)(+))(H(2)PW(12)O(40)(-)) species in the HPW-silica nanocomposites. A HPW-silica (25/75 w/o) nanocomposite gave an activation energy of 13.0 kJ mol(-1) and proton conductivity of 0.076 S cm(-1) at 100 °C and 100 RH%, and an activation energy of 26.1 kJ mol(-1) and proton conductivity of 0.05 S cm(-1) at 200 °C with no external humidification. A fuel cell based on a 165 μm thick HPW-silica nanocomposite membrane achieved a maximum power output of 128.5 and 112.0 mW cm(-2) for methanol and ethanol fuels, respectively, at 200 °C. The high proton conductivity and good performance demonstrate the excellent water retention capability and great potential of the highly ordered HPW-silica mesoporous nanocomposites as high-temperature proton exchange membranes for direct alcohol fuel cells (DAFCs).