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Hydrogen is a fuel with a potentially zero-carbon footprint viewed as a viable alternative to fossil fuels. It can be produced in a large scale via electrochemical water splitting using electricity derived from renewable sources, but this would require highly active, inexpensive, and stable hydrogen evolution reaction (HER) catalysts to replace the Pt benchmark. Transition-metal phosphides (TMPs) are potential Pt replacements owing to their generally high activity as well as versatility as HER catalysts for different pH media. This review summarizes the recent progress in the development of TMP HER electrocatalysts, focusing on the strategies that have been recently explored to tune the activity in acidic, neutral, and basic media. These strategies are the doping of TMPs with metal and nonmetal elements, fabrication of multimetallic phosphide phases, and construction of multicomponent heterostructures comprising TMPs and another component such as a different TMP or a metal oxide/hydroxide. The synthetic methods utilized to design the catalysts are also presented. Finally, the challenges still remaining and future research directions are discussed.

Coordination polymers (CPs) and metal-organic frameworks (MOFs) have emerged as versatile precursors for transition-metal phosphides catalysts. However, the controlled synthesis of CPs-derived bime...

The characterization of the active structure of water-splitting catalysts is crucial to evolve to a sustainable energy future based on hydrogen. Such information can only be obtained by operando methods. We present a diffuse reflectance UV-vis (DRUV) spectroelectrochemical study that allows tracking the changes of solid oxygen evolution catalysts under working conditions. The versatility of our approach is demonstrated on two cobalt-containing catalysts, Zn0.35Co0.65O and CoAl2O4. The changes the catalysts undergo during the oxygen evolution reaction can be tracked by probing the electronic structure using UV-vis spectroscopy. These findings are compared to ex situ analyses, which support the assignments of the structures stabilized under different potentials. Thus, structure–activity correlations can be proposed, and deeper insights into the catalytically active structures can be obtained.

Rational design of cost effective, high performance and stable hydrogen evolution reaction HER electrocatalysts in both acidic and alkaline media holds the key to the future hydrogen based economy. Herein, we introduce an effective approach of simultaneous non metal S and metal Fe or Mn doping of MoP to achieve excellent HER performance at different pH. The catalysts show remarkable overpotentials at 10 mA cm 2 of only 65 and 68 mV in 0.5 M H2SO4, and 50 and 51 mV in 1.0 M KOH, respectively, as well as much higher turnover frequencies compared to undoped MoP. Furthermore, the catalysts exhibit outstanding long term stability at a fixed current of 10 mA cm 2 for 40 h. The effects of both dopants, such as electronic structure modification and enhancement of the intrinsic activity, increase of the electrochemically active surface area, and formation of coordinatively unsaturated edge sites, act cooperatively to accelerate the HER at both pH media. Additionally, the presence of oxophilic Mn and Fe at the surface results in Mn or Fe oxide hydroxide species that promote the dissociation of water molecules in alkaline electrolyte. This work introduces a facile and effective design principle that could pave the way towards engineering highly active HER catalysts for a wide pH range.

This is a complete self-standing study on operando diffuse reflectance UV-vis spectroelectrochemistry for investigating oxygen evolution electrocatalysts.

International audience; To arrive to sustainable hydrogen-based energy solutions, the understanding of water-splitting catalysts plays the most crucial role. Herein, we combined state-of-the-art hypotheses on electrocatalytic active metal sites towards the oxygen evolution reaction (OER) to develop a highly efficient catalyst based on earth-abundant cobalt and zinc oxides. The precursor catalyst Zn 0.35 Co 0.65 O was synthesized via a fast microwave-assisted approach at low temperatures. Subsequent, it transformed in situ from the Wurtzite structure to the layered γ-Co(O)OH, while most of its zinc leaches out. This material shows outstanding catalytic performance and stability towards the OER in 1 M KOH (overpotential at 10 mA cm-2 η initial = 306 mV, η 98 h = 318 mV). By comparing the electrochemical results and ex situ analyses to today's literature, we were able to identify clear structure-activity correlations. Our findings suggest that coordinately unsaturated cobalt octahedra on the surface are indeed the active centers for the OER.

The hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) play key roles in the conversion of energy derived from renewable energy sources into chemical energy. Efficient, robust, and inexpensive electrocatalysts are necessary for driving these reactions at high rates at low overpotentials and minimize energetic losses. Recently, electrocatalysts derived from hybrid metal phosphonate compounds have shown high activity for the HER or OER. We review here the utilization of metal phosphonate coordination networks and metal-organic frameworks as precursors/templates for transition-metal phosphides, phosphates, or oxyhydroxides generated in situ in alkaline solutions, and their electrocatalytic performance in HER or OER.

A novel non-precious multiwalled carbon nanotubes (CNTs)—supported metal oxide electrocatalyst was developed for methanol electrooxidation in alkaline medium. The catalyst was fabricated by simultaneous electrodeposition of copper-cobalt-nickel ternary nanostructures (CuCoNi) on a glassy carbon electrode (GCE) modified with CNTs. The proposed electrode was characterized using X-ray diffraction (XRD), energy dispersive X-ray (EDX), and scanning electron microscopy (SEM). The electrochemical behavior and the electrocatalytic performance of the suggested electrode towards the oxidation of methanol were evaluated by cyclic voltammetry (CV), linear sweep voltammetry (LSV), and chronoamperometry (CA) in alkaline medium. Several parameters were investigated, e.g., deposition time, potential scan rate, etc. Compared to Cu, Co, or Ni mono electrocatalysts, the electrode based on ternary-metals exhibited superior electrocatalytic activity and stability towards methanol electrooxidation. For instance, CuCoNi@CNTs/GCE has shown at least 2.5 times electrocatalytic activity and stability compared to the mono eletrocatalysts. Moreover, the present study found that the optimized loading level is 1500 s of simultaneous electrodeposition. At this loading level, it was found that the relation between the Ip/ν1/2 function and scan rate gives the characteristic features of a catalytic process. The enhanced activity and stability of CuCoNi@CNTs/GCE was attributed to (i) a synergism between three metal oxides coexisting in the same structure; (ii) the presence of CNTs as a support for the metal oxides, that offers high surface area for the deposited tertiary alloy and suppresses the aggregation and sintering of the metals oxide with time; as well as (iii) the increase of the conductivity of the deposited semiconducting metal oxides.

This paper is devoted to the investigation of the tolerance of glucose oxidation on NiO x /MnO x /GC binary catalyst to poisoning by halide ions. NiO x /MnO x /GC electrode was prepared by the consecutive electrodeposition of MnO x followed by the deposition of NiO x nanoparticles atop the previously deposited MnO x nanoparticles. The electrode was characterized electrochemically and morphologically. Interestingly, experimental results showed that the NiO x /MnO x /GC electrode shows a superior electrocatalytic activity compared with the NiO x /GC electrode and a behavior comparable to the NiO x /GC electrode regarding tolerance to poisoning by halides, even in the presence of high concentrations of Cl− and Br− up to 0.3 M. On the other hand, it was found that iodide ions have a significant effect on glucose electrooxidation under the prevailing operating conditions and even at concentrations lower than those of the other two halide ions.

This work addresses the electrocatalytic activity of a new catalyst composed of nickel and manganese binary oxides, prepared by electrodeposition, toward glucose electro-oxidation in alkaline medium. Cyclic voltammetry, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) were used to characterize those electrocatalysts. It has been found that the electrocatalytic activity critically depends on the loading level and the order of deposition of the two oxides; the NiOx/MnOx/GC electrode (MnOx deposited first) showed an excellent electrocatalytic activity and stability toward glucose oxidation compared to NiOx/GC, MnOx/GC or MnOx/NiOx/GC electrodes (NiOx deposited first). At the present work conditions, it has found that, the optimum loading level is 60 cycles of MnOx followed by 10 min deposition of nickel. At this loading level, it has been found that, the relation between Ip/v1/2 function and scan rate gives the characteristic feature of a catalytic process.

A novel binary electrocatalyst fabricated from manganese and nickel oxides nanoparticles (MnOx and NiOx), prepared by electrodeposition, is proposed as an anode for an amplified electrochemical oxidation of glucose in NaOH solutions. Cyclic voltammetry and scanning electron microscopy images were used to characterize these electrocatalysts. It has been found that the electrocatalytic activity critically depends on the order of the deposition of the two oxides. The NiOx/MnOx/GC electrode (MnOx deposited first) showed a superior electrocatalytic activity towards glucose oxidation compared to NiOx/GC, MnOx/GC or MnOx/NiOx/GC electrodes (NiOx deposited first). The extraordinary activity obtained at the NiOx/MnOx/GC electrode is attributed to the compilation of the better adsorption of glucose molecules on the MnOx sites and the increase of conductivity of the NiOx due to the increase of Ni3+ content. The results lead us to conclude that there is a synergism between the two oxides towards the electrooxidation of glucose.

The current study presents a comparison for the electro-oxidation of formic acid (FA), glucose (GL), and methanol (ME) at nickel oxide nanoparticles (NiOx) modified electrodes. The modification with NiOx was pursed onto a bare glassy carbon (GC) and Pt-modified (Pt/GC) electrodes electrochemically, and the catalytic activity was measured in 0.3 M NaOH. Cyclic voltammetry (CV), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDX) are all used to provide a concrete characterization of the prepared electrodes. A catalytic enhancement of GL oxidation (GLO) and ME oxidation (MEO) was observed at the NiOx-modified GC (NiOx/GC) electrode, while the same electrode did not show any activity towards FA oxidation (FAO), revealing that FAO is substrate dependent. On the other hand, assembling NiOx onto the Pt/GC electrode assisted in improving the catalytic activity of all reactions (GLO, MEO, and FAO). The catalytic enhancement observed at the NiOx/Pt/GC electrode for GLO, MEO, and FAO was not only confined in the large increase of the oxidation current but also in a negative shift in the onset potential of the oxidation reaction. We believe NiOx could successfully play an essential role in this catalytic enhancement, presumably via participation in these reactions in a way facilitating the charge transfer or providing the oxygen atmosphere necessary for promoting an oxidative removal for unwanted poisoning species.

Nickel oxide nanostructured modified platinum nanoparticles (PtNPs) supported in glassy carbon electrode (NiOx/Pt/GC) was used as an effective anode for formic acid (FA), methanol (ME) and ethanol (ET) electrooxidation in 0.3 M NaOH solution. GC surface was fabricated with NiOx nanostructured and Pt nanoparticles electrochemically. The modified electrodes were characterized using cyclic voltammetry (CV) and scanning electron microscopy (SEM). The catalytic improvement observed at NiOx/Pt/GC electrode for FAO, MEO, and ETO was not only confined in the large increase of the oxidation current but also in a negative shift in the onset potential of the oxidation reactions. The influence of temperature on the oxidation current was investigated and the apparent activation energy, E a , for each fuel was calculated at a specific potential. Furthermore, NiOx/Pt/GC electrode showed a satisfactory stability for FAO, MEO, and ETO in 0.3 M NaOH solution.