Enhancement of hydrogen evolution reaction kinetics in alkaline media by fast galvanic displacement of nickel with rhodium – from smooth surfaces to electrodeposited nickel foams

Energy-efficient hydrogen production is one of the key factors for advancing the hydrogen-based economy. Alkaline water electrolysis is the main route for the production of high-purity hydrogen, but further improvements of hydrogen evolution reaction (HER) catalysts are still needed. Industrial alkaline electrolysis relies on Ni-based catalysts, and here we describe a drastic improvement of HER activity of Ni in alkaline media using several model catalysts for HER obtained upon nickel surface modification in aqueous solution of rhodium salts, when a spontaneous deposition of rhodium takes place based on the chemical displacement reaction 3Ni + 2Rh3+ = 3Ni2+ + 2Rh. In the case of smooth Ni-poly electrodes, HER activity surpasses the activity of Pt-poly already after 30 s of exchange with Rh. SEM analysis showed that Rh is uniformly distributed, while surface roughness changes within 10%, agreeing with electrochemical measurements. Furthermore, XPS analysis has shown effective incorporation of Rh in the surface, while DFT calculations suggest that hydrogen binding is significantly weakened on the Rh-modified Ni surfaces. Such tuning of the hydrogen binding energy is seen as the main factor governing HER activity improvements. The same galvanic displacement protocols were employed for nickel foam electrodes and electrodeposited Ni on Ti mesh. In both cases, somewhat longer Rh exchange times are needed to obtain superior activities than for the smooth Ni surface, but up to 10 min. HER overpotential corresponding to −10 mA cm−2 for nickel foam and electrodeposited Ni electrodes, after modification with Rh, amounted to only −0.07 and −0.09 V, respectively. Thus, it is suggested that a fast spontaneous displacement of Ni with Rh could effectively boost HER in alkaline media with minor cost penalties compared to energy saving in the electrolysis process.

Nickel Foam Electrode with Low Catalyst Loading and High Performance for Alkaline Direct Alcohol Fuel Cells

Nickel foam has a unique three-dimensional (3-D) network structure that helps to effectively utilize catalysts and is often used as an electrode support material for alkaline direct alcohol fuel cells. In this chapter, first, the effect of nickel foam thickness on cell performance is explored. The results show that the thickness affects both mass transfer and electron conduction, and there is an optimal thickness. The thinner the nickel foam is, the better the conductivity is. However, the corresponding three-dimensional space becomes narrower, which results in a partial agglomeration of the catalyst and the hindrance of mass transfer. The cell performance of 0.6 mm nickel foam electrode is better than that of 0.3 and 1.0 mm. Secondly, to fully exert the catalytic function of the catalyst even at a lower loading, a mixed acid-etched nickel foam electrode with lower Pd loading (0.35 mg cm−2) is prepared then by a spontaneous deposition method. The maximum power density of the single alkaline direct ethanol fuel cell (ADEFC) can reach 30 mW cm−2, which is twice the performance of the hydrochloric acid treated nickel foam electrode. The performance improvement is attributed to the micro-holes produced by mixed acids etching, which enhances the roughness of the skeleton and improves the catalyst electrochemical active surface area.

Copper Hexacyanoferrate Thin Film Deposition and Its Application to a New Method for Diffusion Coefficient Measurement

The use of Prussian blue analogues (PBA) materials in electrochemical energy storage and harvesting has gained much interest, necessitating the further clarification of their electrochemical characteristics. However, there is no well-defined technique for manufacturing PBA-based microelectrochemical devices because the PBA film deposition method has not been well studied. In this study, we developed the following deposition method for growing copper hexacyanoferrate (CuHCFe) thin film: copper thin film is immersed into a potassium hexacyanoferrate solution, following which the redox reaction induces the spontaneous deposition of CuHCFe thin film on the copper thin film. The film grown via this method showed compatibility with conventional photolithography processes, and the micropattern of the CuHCFe thin film was successfully defined by a lift-off process. A microelectrochemical device based on the CuHCFe thin film was fabricated via micropatterning, and the sodium ion diffusivity in CuHCFe was measured. The presented thin film deposition method can deposit PBAs on any surface, including insulating substrates, and it can extend the utilization of PBA thin films to various applications.

Metal-Assisted Catalytic Etching (MACE) for Nanofabrication of Semiconductor Powders

Electroless etching of semiconductors has been elevated to an advanced micromachining process by the addition of a structured metal catalyst. Patterning of the catalyst by lithographic techniques facilitated the patterning of crystalline and polycrystalline wafer substrates. Galvanic deposition of metals on semiconductors has a natural tendency to produce nanoparticles rather than flat uniform films. This characteristic makes possible the etching of wafers and particles with arbitrary shape and size. While it has been widely recognized that spontaneous deposition of metal nanoparticles can be used in connection with etching to porosify wafers, it is also possible to produced nanostructured powders. Metal-assisted catalytic etching (MACE) can be controlled to produce (1) etch track pores with shapes and sizes closely related to the shape and size of the metal nanoparticle, (2) hierarchically porosified substrates exhibiting combinations of large etch track pores and mesopores, and (3) nanowires with either solid or mesoporous cores. This review discussed the mechanisms of porosification, processing advances, and the properties of the etch product with special emphasis on the etching of silicon powders.

Metal Assisted Catalytic Etching (MACE) for Nanofabrication of Semiconductor Powders

Electroless etching of semiconductors was elevated to an advanced micromachining process by the addition of a structured metal catalyst. Patterning of the catalyst by lithographic techniques facilitates the patterning of crystalline and polycrystalline wafer substrates. Galvanic deposition of metals on semiconductors has a natural tendency to produce nanoparticles rather than flat uniform films. This characteristic makes possible the etching of not only wafers but also particles with arbitrary shape. While it has been widely recognized that spontaneous deposition of metal nanoparticles can be used in connection with etching to porosify wafers, it is also possible to produced nanostructured powders. MACE can be controlled to produce (1) etch track pores with shapes and sizes closely related to the shape and size of the metal nanoparticle, (2) hierarchically porosified substrates exhibiting combinations of large etch track pores and mesopores, and (3) nanowires with either solid or mesoporous cores. This review discussed the mechanisms of porosification, processing advances and the properties of the etch product with special emphasis on the etching of silicon powders.

Hydrogen Evolution on Reduced Graphene Oxide-Supported PdAu Nanoparticles

Hydrogen evolution reaction (HER) was investigated on reduced graphene oxide (rGO)-supported Au and PdAu nanoparticles in acid solution. The graphene spread over glassy carbon (rGO/GC) was used as a support for the spontaneous deposition of Au and Pd. The resulting Au/rGO and PdAu/rGO electrodes were characterized using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) techniques. Phase AFM images have shown that the edges of the rGO sheets were active sites for the deposition of both Au and Pd. XPS analysis revealed that the atomic percentages of both Au and PdAu nanoparticles were slightly higher than 1%. The activity of the PdAu/rGO electrode for the HER was remarkably high, with the overpotential close to zero. HER activity was stable over a 3 h testing time, with a low Tafel slope of approx. −46 mV/dec achieved after prolonged hydrogen evolution at a constant potential.

Silica-confined Ru highly-dispersing on ZrO2 with enhanced activity and thermal stability in dichloroethane combustion

An efficient strategy (spontaneous deposition to enhance noble metal dispersity and core-shell confinement to inhibit the noble metal sintering) is presented to synthesize highly active and thermal stable Ru/ZrO2@SiO2 catalyst...