Research on fuel cell technology is constantly gaining importance, while global emission requirements are becoming more and more restrictive. For environmentally neutral proton exchange membrane fuel cells (PEMFCs) to become a competitive technology, sustainable infrastructures need to be established. One of the main showstoppers is the utilization of the rare and therefore costly precious metal Pt as the key element in the electrocatalysis of hydrogen and oxygen. A huge amount of research is done on immensely reducing or even replacing Pt for future PEMFC technology. In this research update, the progress on oxygen reduction reaction catalysts in acidic media over the past two years is reviewed, with special attention to their durability.

This study analysed the influence of the codeposition of SiC particles with different sizes: 50 nm, 500 nm and 5 μm, and the type of bath agitation (stirring or ultrasonic) on the electrocrystallisation of nickel coatings. The composites matrix microstructure was analysed by means of SEM, EBSD and XRD, to evaluate the grain size, crystal orientation, and internal stresses and was benchmarked against pure nickel samples electrodeposited in equivalent conditions. The codeposition of nano- and microsize particles with an approximate content of 0.8 and 4 vol.%, respectively, caused only a minor grain refinement and did not vary the dominant < 100 > crystal orientation observed in pure Ni. The internal stress was, however, increased by particles codeposition, up to 104 MPa by nanoparticles and 57 MPa by microparticles, compared to the values observed in pure nickel (41 MPa). The higher codeposition rate (11 vol.%) obtained by the addition of submicron-size particles caused a change in the grain growth from columnar to equiaxial, resulting in deposits with a fully random crystal orientation and pronounced grain refinement. The internal stress was also increased by 800% compared to pure nickel. The ultrasound (US) agitation during the deposition caused grain refinement and a selective particle inclusion prompting a decrease in the content of the particles with the larger particles. The deposits produced under US agitation showed an increase in the internal stresses, with double values compared to stirring. The increase in the deposits microhardness, from 280 HV in pure Ni to 560 HV in Ni/SiC submicron-US, was linked to the microstructural changes and particles content. Graphical abstract

This work has explored the potential of using pulse reverse (PR) plating for increasing the deposited fraction of SiC nanoparticles. Two PR waveforms were selected, a short pulse (500 Hz) waveform and a newly modified and adapted pulsed sequence that equals the plating thickness to the particles’ diameter (50 nm) for the on-time and half-diameter during the anodic time. The pulse waveforms were designed with 4 and 10 A⋅dm⁻² as the average current density and cathodic peak current density, respectively. Direct current (DC) deposits at the same values were also produced as reference. In all cases, the codeposition of nano-SiC particles influenced the microstructure. The electroplating under DC 10 A⋅dm⁻² showed the strongest grain refinement and increased the content of the particles (up to 2% vol.) PR using high-frequency achieved a similar codeposition. The maximum particle incorporation was achieved by the proposed adapted pulse waveform, doubling the SiC content produced by other set-ups (up to 4% vol.); increasing the microhardness of the deposits to 400 HV, despite no grain refinement compared to the pure metal. From these results, it was observed a relationship between the influence of the plating method on the microstructure, the particle content, and the material's hardness.

Direct current electrodeposited Sn–Ni/TiO₂ nanostructured coatings were produced by embedding two different doped types of TiO₂ particles within the alloy matrix, a commercially available doped carbon-based and doped N,S-TiO₂ particles. The structural characteristics of the composite coatings have been correlated with the effect of loading, type of particles in the electrolytic bath, and the applied current density. Regardless of the type of doped particles TiO₂, increasing values of applied current density resulted in a reduction of the co-deposition percentage of TiO₂ particles and an increase of Tin content into the alloy matrix. The application of low current density values accompanied by a high load of particles in the bath led to the highest codeposition percentage (~3.25 wt.%) achieved in the case of embedding N,S-TiO₂ particles. X-ray diffraction data demonstrated that in composite coatings the incorporation of the different types of TiO₂ particles in the alloy metal matrix modified significantly the nano-crystalline structure in comparison with the pure coatings. The best photocatalytic behavior under visible irradiation was revealed for the composite coatings with the highest co-deposition percentage of doped N,S-TiO₂ particles, that also exhibited enhanced wear resistance and slightly reduced microhardness compared to pure ones.