Metal‐Rich Chalcogenides for Electrocatalytic Hydrogen Evolution: Activity of Electrodes and Bulk Materials

The route for commercial photoelectrochemical water splitting: a review of large-area devices and key upscaling challenges

Green-hydrogen is considered a "key player" in the energy market for the upcoming decades. Among currently available hydrogen (H2) production processes, photoelectrochemical (PEC) water splitting has one of the lowest environmental impacts. However, it still presents prohibitively high production costs compared to more mature technologies, such as steam methane reforming. Therefore, the competitiveness of PEC water splitting must rely on its environmental and functional advantages, which are strongly linked to the reactor design, to the intrinsic properties of its components, and to their successful upscaling. This review gives special attention to the engineering aspects and categorizes PEC devices into four main types, according to the configuration of electrodes and strategies for gas separation: wired back-to-back, wireless back-to-back, wired side-by-side, and wired separated electrode membrane-free. Independently of the device architecture, the use of concentrated sunlight was found to be mandatory for achieving competitive green-H2 production. Additionally, feasible strategies for upscaling the key components of PEC devices, especially photoelectrodes, are urgently needed. In a pragmatic context, the way to move forward is to accept that PEC devices will operate close to their thermodynamic limits at large-scale, which requires a solid convergence between academics and industry. Research efforts must be redirected to: (i) build and demonstrate modular devices with a low-cost and highly recyclable embodiment; (ii) optimize thermal and power management; (iii) reduce ohmic losses; (iv) enhance the chemical stability towards a thousand hours; (v) couple solar concentrators with PEC devices; (vi) boost PEC-H2 production through the use of organic compounds; and (vii) reach consensual standardized methods for evaluating PEC devices, at both environmental and techno-economic levels. If these targets are not met in the next few years, the feasibility of PEC-H2 production and its acceptance by industry and by the general public will be seriously compromised.

Mechanochemical one-pot synthesis of heterostructured pentlandite-carbon composites for the hydrogen evolution reaction

We have utilized carbon sources as milling additives to enable a direct mechanochemical one-pot synthesis of Fe3Co3Ni3S8/carbon (Pn/C) materials using elemental reaction mixtures. The obtained Pn/C materials are thoroughly characterized and their carbon content could be adjusted up to 50 wt%. In addition to carbon black (CB) as an additive, Pn/C materials were produced using graphite, reduced graphene oxide (rGO), and carbon nanotubes (CNTs), which allows the overall physicochemical properties of materials for energy storage applications to be adjusted. By employing the Pn/C materials as electrocatalysts for the HER in a zero-gap proton exchange membrane (PEM) electrolyzer, we were able to reach a current density of 1 A cm-2 at a cell potential as low as 2.12 V using Pn, which was synthesized with 25 wt% CB. Furthermore, electrolysis at an applied current density of 1 A cm-2 for 100 h displays a stable performance, thus providing a sustainable synthesis procedure for potential future energy storage applications. Herein, we show that catalyst supports play an important role in the overall performance.

Developing electrochemical hydrogenation towards industrial application

Electrochemical hydrogenation reactions gained significant attention as a sustainable and efficient alternative to conventional thermocatalytic hydrogenations. This tutorial review provides a comprehensive overview of the basic principles, the practical application, and recent advances of electrochemical hydrogenation reactions, with a particular emphasis on the translation of these reactions from lab-scale to industrial applications. Giving an overview on the vast amount of conceivable organic substrates and tested catalysts, we highlight the challenges associated with upscaling electrochemical hydrogenations, such as mass transfer limitations and reactor design. Strategies and techniques for addressing these challenges are discussed, including the development of novel catalysts and the implementation of scalable and innovative cell concepts. We furthermore present an outlook on current challenges, future prospects, and research directions for achieving widespread industrial implementation of electrochemical hydrogenation reactions. This work aims to provide beginners as well as experienced electrochemists with a starting point into the potential future transformation of electrochemical hydrogenations from a laboratory curiosity to a viable technology for sustainable chemical synthesis on an industrial scale.

Opening the pathway towards a scalable electrochemical semi-hydrogenation of alkynols via earth-abundant metal chalcogenides

Electrosynthetic methods are crucial for a future sustainable transformation of the chemical industry. Being an integral part of many synthetic pathways, the electrification of hydrogenation reactions gained increasing interest in recent years. However, for the large-scale industrial application of electrochemical hydrogenations, low-resistance zero-gap electrolysers operating at high current densities and high substrate concentrations, ideally applying noble-metal-free catalyst systems, are required. Because of their conductivity, stability, and stoichiometric flexibility, transition metal sulfides of the pentlandite group have been thoroughly investigated as promising electrocatalysts for electrochemical applications but were not investigated for electrochemical hydrogenations of organic materials. An initial screening of a series of first row transition metal pentlandites revealed promising activity for the electrochemical hydrogenation of alkynols in water. The most active catalyst within the series was then incorporated into a zero-gap electrolyser enabling the hydrogenation of alkynols at current densities of up to 240 mA cm-2, Faraday efficiencies of up to 75%, and an alkene selectivity of up to 90%. In this scalable setup we demonstrate high stability of catalyst and electrode for at least 100 h. Altogether, we illustrate the successful integration of a sustainable catalyst into a scalable zero-gap electrolyser establishing electrosynthetic methods in an application-oriented manner.

Boosting the overall electrochemical water splitting performance of pentlandites through non-metallic heteroatom incorporation

We report on synthesis of the heterotrimetallic pentlandite-type material Fe₃Co₃Ni₃S₈ (FCNS) in presence of suitable phosphorus-(FCNSP) and nitrogen-(FCNSN) donors for the overall electrochemical water splitting. Throughout the experiments, a preferential incorporation of N into the FCNS-lattice is observed whereas the addition of phosphorus generally leads to metal-phosphate-FCNS composites. The obtained FCNSP, FCNSN, and FCNSNP facilitate the oxygen evolution reaction (OER) at 100 mAcm⁻² in 1.0M KOH with overpotentials of 479, 440, and 427 mV, respectively, outperforming the benchmark IrO₂ (564 mV) and commercial Ni metal powder (>600 mV). Likewise, FCNSN and FCNSNP reveal an improved performance toward the hydrogen evolution reaction (HER) in 0.5M H₂SO_4, outperforming the pristine FCNS. All materials revealed high stability and morphological robustness during OER and HER. Notably, DFT calculation suggests that N and P doping boost the OER activity of the pristine FCNS, whereas N doping enhances the HER activity.

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Fine Fe₃Co₃Ni₃S₈ (FCNS) nanoparticles were prepared through ball milling methods

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Subsequent regulated thermal treatment in inert ambientes produces N- and P-doped FCNS

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The obtained materials showed high activity toward electrochemical water splitting

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DFT calculation confirms that N and P doping boosts OER, whereas N enhances HER

Transition Metal Non-Oxides as Electrocatalysts: Advantages and Challenges

The identification of hydrogen as green fuel in the near future has stirred global realization toward a sustainable outlook and thus boosted extensive research in the field of water electrolysis focusing on the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR). A huge class of compounds consisting of transition metal-based nitrides, carbides, chalcogenides, phosphides, and borides, which can be collectively termed transition metal non-oxides (TMNOs), has emerged recently as an efficient class of electrocatalysts in terms of performance and longevity when compared to transition metal oxides (TMOs). Moreover, the superiority of TMNOs over TMOs to effectively catalyze not only OERs but also HERs and ORRs renders bifunctionality and even trifunctionality in some cases and therefore can replace conventional noble metal electrocatalysts. In this review, the crystal structure and phases of different classes of nanostructured TMNOs are extensively discussed, focusing on recent advances in design strategies by various regulatory synthetic routes, and hence diversified properties of TMNOs are identified to serve as next-generation bi/trifunctional electrocatalysts. The challenges and future perspectives of materials in the field of energy conversion and storage aiding toward a better hydrogen economy are also discussed in this review.

Metal-organic framework (MOF)-derived plate-shaped CoS1.097 nanoparticles for an improved hydrogen evolution reaction

Metal-organic framework (MOF)-derived transition metal sulfides are viewed as reliable, cost-effective, and alternative hydrogen evolution reaction (HER)-efficient electrocatalysts. They have been used to replace platinum (and their alloys) for production of renewable energy carriers such as hydrogen. Progress towards development of non-precious transition-metal sulfides through different synthetic routes to obtain unique morphological nanostructures with enhanced HER activity is challenging. We introduced a transition-metal sulfide, cobalt sulfide (CoS_1.097), derived from a cobalt MOF [Co-BPY-DDE] by following facile, one-step solvothermal sulfurization. By varying the sulfurization temperature (from 140 °C to 180 °C) during the solvothermal method, three cobalt-sulfide products were obtained: CoS_1.097-140, CoS_1.097-160, and CoS_1.097-180, respectively. Temperature variation had a vital role in optimizing the HER activity of the electrocatalyst. Besides, notable plate-shaped cobalt sulfide nanoparticles (CoS_1.097-160) required overpotential of 163 mV to deliver a current density of 10 mA cm^-2 with a low Tafel slope of 53 mV dec^-1, thereby demonstrating faster reaction kinetics during the evolution of molecular hydrogen. Furthermore, 25 h of long-term stability of the electrocatalyst reflected its practical applicability in acidic media. CoS_1.097-160 had uniform plate-shaped morphology and large electrochemical active surface area, which contributed to enhanced electrochemical performance through water electrolysis.

Single source precursor route for the first graphene oxide-Pd6P nanocomposite: application in electrochemical hydrogen evolution reaction

For the first time, Pd₆P has been synthesised using a simple, straightforward and one-pot method i.e., thermolysis of a Pd(II) complex of a bidentate (P, N) organophosphorus ligand (anthracene-9-yl-CHN-CH₂CH₂-PPh₂). The electrocatalyst (obtained after grafting nanospheres of Pd₆P over layers of graphene oxide) shows high activity in electrochemical hydrogen evolution reactions (HER) with an overpotential of 133 mV to drive 10 mA cm^-2 of cathodic current density. The GO-Pd₆P nanocomposite is robust and effective for a continuous HER run for up to 16 hours.

[NiFe]-(Oxy)Sulfides Derived from NiFe2O4 for the Alkaline Hydrogen Evolution Reaction

The development of noble-metal-free electrocatalysts is regarded as a key factor for realizing industrial-scale hydrogen production powered by renewable energy sources. Inspired by nature, which uses Fe- and Ni-containing enzymes for efficient hydrogen generation, Fe/Ni-containing chalcogenides, such as oxides and sulfides, received increasing attention as promising electrocatalysts to produce hydrogen. We herein present a novel synthetic procedure for mixed Fe/Ni (oxy)sulfide materials by the controlled (partial) sulfidation of NiFe2O4 (NFO) nanoparticles in H2S-containing atmospheres. The variation in H2S concentration and the temperature allows for a precise control of stoichiometry and phase composition. The obtained sulfidized materials (NFS) catalyze the hydrogen evolution reaction (HER) with increased activity in comparison to NFO, up to −10 and −100 mA cm−2 at an overpotential of approx. 250 and 450 mV, respectively.

Magnetic NiFe2 O4 Nanoparticles Prepared via Non-Aqueous Microwave-Assisted Synthesis for Application in Electrocatalytic Water Oxidation

Phase‐pure spinel‐type magnetic nickel ferrite (NiFe₂O₄) nanocrystals in the size range of 4 to 11 nm were successfully synthesized by a fast and energy‐saving microwave‐assisted approach. Size and accessible surface areas can be tuned precisely by the reaction parameters. Our results highlight the correlation between size, degree of inversion, and magnetic characteristics of NiFe₂O₄ nanoparticles, which enables fine‐tuning of these parameters for a particular application without changing the elemental composition. Moreover, the application potential of the synthesized powders for the electrocatalytic oxygen evolution reaction in alkaline media was demonstrated, showing that a low degree of inversion is beneficial for the overall performance. The most active sample reaches an overpotential of 380 mV for water oxidation at 10 mA cm⁻² and 38.8 mA cm⁻² at 1.7 V vs. RHE, combined with a low Tafel slope of 63 mV dec⁻¹.

Single Co3O4 Nanocubes Electrocatalyzing the Oxygen Evolution Reaction: Nano-Impact Insights into Intrinsic Activity and Support Effects

Single-entity electrochemistry allows for assessing electrocatalytic activities of individual material entities such as nanoparticles (NPs). Thus, it becomes possible to consider intrinsic electrochemical properties of nanocatalysts when researching how activity relates to physical and structural material properties. Conversely, conventional electrochemical techniques provide a normalized sum current referring to a huge ensemble of NPs constituting, along with additives (e.g., binders), a complete catalyst-coated electrode. Accordingly, recording electrocatalytic responses of single NPs avoids interferences of ensemble effects and reduces the complexity of electrocatalytic processes, thus enabling detailed description and modelling. Herein, we present insights into the oxygen evolution catalysis at individual cubic Co₃O₄ NPs impacting microelectrodes of different support materials. Simulating diffusion at supported nanocubes, measured step current signals can be analyzed, providing edge lengths, corresponding size distributions, and interference-free turnover frequencies. The provided nano-impact investigation of (electro-)catalyst-support effects contradicts assumptions on a low number of highly active sites.

System Theoretical Study on the Effect of Variable Nonmetallic Doping on Improving Catalytic Activity of 2D-Ti3C2O2 for Hydrogen Evolution Reaction

2D MXenes have been found to be one of the most promising catalysts for hydrogen evolution reaction (HER) due to their excellent electronic conductivity, hydrophilic nature, porosity and stability. Nonmetallic (NM) element doping is an effective approach to enhance the HER catalytic performance. By using the density functional theory (DFT) method, we researched the effect of nonmetallic doping (different element types, variable doping concentrations) and optimal hydrogen absorption concentration on the surface of NM-Ti₃C₂O₂ for HER catalytic activity and stability. The calculation results show that doping nonmetallic elements can improve their HER catalytic properties; the P element dopants catalyst especially exhibits remarkable HER performance (∆GH = 0.008 eV when the P element doping concentration is 100% and the hydrogen absorption is 75%). The origin mechanism of the regulation of doping on stability and catalytic activity was analyzed by electronic structures. The results of this work proved that by controlling the doping elements and their concentrations we can tune the catalytic activity, which will accelerate the further research of HER catalysts.

Crossing the Valley of Death: From Fundamental to Applied Research in Electrolysis

The growing societal and political focus on the use of environmentally friendly technologies has led to an ever-increasing interest in electrolysis technologies in the scientific communities. This development is reflected by the plethora of candidate catalysts for the hydrogen and oxygen evolution reactions, as well as the CO₂ reduction reaction, reported in the literature. However, almost none of them entered the stage of application yet. Likewise, the reports on process engineering inadequately address the utilization of these catalysts, as well as electrode and cell concepts, that might be suitable for the market. Evidently, a closer collaboration between chemists and engineers from industry and academia is desirable to speed up the development of these disruptive technologies. Herein, we elucidate the critical parameters and highlight the necessary aspects to accelerate the development of industrially relevant catalysts capable of fulfilling the forthcoming challenges related to energy conversion and storage. The aim of this Perspective, composed by industrial and academic partners, is to critically question current undertakings and to encourage researchers to strike interdisciplinary research pathways.

Synergistic effect of S-bridged Fe-Ni group on hydrogen evolution for pentlandite

Pentlandite is reported to exhibit good catalytic activity in hydrogen evolution reaction (HER). Many studies have paid attention to metal catalysis of pentlandite. However, the nonmetal catalysis is not considered for HER. Here, we unravel one probable catalytic mechanism of pentlandite toward HER using density functional theory. In our study models, (001) and (100) surfaces are created because there are three types of S-bridged M-M groups on them. Our study reveals that (Fe-Ni)-S center has a moderate value of Gibbs free energy while the corresponding value for (Fe-Fe)-S or (Ni-Ni)-S center is largely positive or negative. In (Fe-Ni)-S group, Fe and Ni can regulate the antibonding state of S, and then balance adsorption and desorption of proton. In addition, an intrinsic electronic potential difference exists between Fe and Ni in (Fe-Ni)-S group, which may boost the charge transfer. Particularly, (Fe-Ni)-S groups are perpendicular to the surface, and four of them make up one closed loop in the surface. It is suggested that the behaviors of such configuration composed of reaction centers resemble edge sites along the layers of MoS₂ toward HER. This study provides a deep insight into the synergistic effect of S-bridged Fe-Ni groups and enables the modulation of electrocatalytic reaction of pentlandite toward HER.

HPO42- enhanced catalytic activity of N, S, B, and O-codoped carbon nanosphere-armored Co9S8 nanoparticles for organic pollutants degradation via peroxymonosulfate activation: critical roles of superoxide radical, singlet oxygen and electron transfer

In this study, we report a facile synthesis of a novel N, S, B, and O-codoped carbon nanosphere-armored Co9S8 nanoparticle composite (Co9S8@NSBOC) and its superior activation performance toward peroxymonosulfate (PMS) for methylene blue (MB) and ofloxacin degradation. The effects of various experimental parameters and the general applicability of the catalyst were investigated. Particularly, Co9S8@NSBOC exhibited high catalytic activity in a wide pH range of 3-12 and HPO42- exhibited a synergic catalytic effect with Co9S8@NSBOC in the degradation system. Radical quenching tests, EPR measurements and electrochemical analysis demonstrated that the degradation mechanism of pollutants in the Co9S8@NSBOC/PMS system included both radical and non-radical pathways, in which ˙O2-, 1O2 and electron transfer played dominant roles. Co2+, S2-, carbon defects, C[double bond, length as m-dash]O/C-O-C, pyridinic-N, graphitic-N, BC2O and C-S-C species on Co9S8@NSBOC, all contributed to PMS activation. The degradation pathways of MB and ofloxacin were proposed based on HPLC-MS/MS analysis of their degradation intermediates. This work not only presents a facile and practical synthetic method of cobalt sulfide-coupled multi-heteroatom-doped carbocatalysts, but also provides useful insights into their active sites and activation mechanisms toward PMS activation.

Sustainable and rapid preparation of nanosized Fe/Ni-pentlandite particles by mechanochemistry

In recent years, metal-rich sulfides of the pentlandite type (M9S8) have attracted considerable attention for energy storage applications. However, common synthetic routes towards pentlandites either involve energy intensive high temperature procedures or solvothermal methods with specialized precursors and non-sustainable organic solvents. Herein, we demonstrate that ball milling is a simple and efficient method to synthesize nanosized bimetallic pentlandite particles (Fe4.5Ni4.5S8, Pn) with an average size of ca. 250 nm in a single synthetic step from elemental- or sulfidic mixtures. We herein highlight the effects of the milling ball quantity, precursor types and milling time on the product quality. Along this line, Raman spectroscopy as well as temperature/pressure monitoring during the milling processes provide valuable insights into mechanistic differences between the mechanochemical Pn-formation. By employing the obtained Pn-nanosized particles as cathodic electrocatalysts for water splitting in a zero-gap PEM electrolyzer we provide a comprehensive path for a potential sustainable future process involving non-noble metal catalysts.

Enhancing the CO2 Electroreduction of Fe/Ni-Pentlandite Catalysts by S/Se Exchange

The electrochemical reduction of CO2 is an attractive strategy towards the mitigation of environmental pollution and production of bulk chemicals as well as fuels by renewables. The bimetallic sulfide Fe4.5 Ni4.5 S8 (pentlandite) was recently reported as a cheap and robust catalyst for electrochemical water splitting, as well as for CO2 reduction with a solvent-dependent product selectivity. Inspired by numerous reports on monometallic sulfoselenides and selenides revealing higher catalytic activity for the CO2 reduction reaction (CO2 RR) than their sulfide counterparts, the authors investigated the influence of stepwise S/Se exchange in seleno-pentlandites Fe4.5 Ni4.5 S8-Y SeY (Y=1-5) and their ability to act as CO2 reducing catalysts. It is demonstrated that the incorporation of higher equivalents of selenium favors the CO2 RR with Fe4.5 Ni4.5 S4 Se4 revealing the highest activity for CO formation. Under galvanostatic conditions in acetonitrile, Fe4.5 Ni4.5 S4 Se4 generates CO with a Faradaic Efficiency close to 100 % at applied current densities of -50 mA cm-2 and -100 mA cm-2 . This work offers insight into the tunability of the pentlandite based electrocatalysts for the CO2 reduction reaction.