Electrocatalytic synergism in Ni/Mo cathodes for hydrogen evolution in acid medium: a new model

Heterogeneous Structure of Ni-Mo Nanoalloys Decorated on MoOx for an Efficient Hydrogen Evolution Reaction Using Hydrogen Spillover

Herein, a heterogeneous structure of Ni-Mo catalyst comprising Ni4Mo nanoalloys decorated on a MoOx matrix via electrodeposition is introduced. This catalyst exhibits remarkable hydrogen evolution reaction (HER) activity across a range of pH conditions. The heterogeneous Ni-Mo catalyst showed low overpotentials only of 24 and 86, 21 and 60, and 37 and 168 mV to produce a current density of 10 and 100 mA cm-2 (η10 and η100) in alkaline, acidic, and neutral media, respectively, which represents one of the most active catalysts for the HER. The enhanced activity is attributed to the hydrogen spillover effect, where hydrogen atoms migrate between the Ni4Mo alloys and the MoOx matrix, forming hydrogen molybdenum bronze as additional active sites. Additionally, the Ni4Mo facilitated the water dissociation process, which helps the Volmer step in the alkaline/neutral HER. Through electrochemical analysis, in situ Raman spectroscopy, and density functional theory calculations, the fast HER mechanism is elucidated.

Metal-insulator-semiconductor photoelectrodes for enhanced photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting provides a scalable and integrated platform to harness renewable solar energy for green hydrogen production. The practical implementation of PEC systems hinges on addressing three critical challenges: enhancing energy conversion efficiency, ensuring long-term stability, and achieving economic viability. Metal-insulator-semiconductor (MIS) heterojunction photoelectrodes have gained significant attention over the last decade for their ability to efficiently segregate photogenerated carriers and mitigate corrosion-induced semiconductor degradation. This review discusses the structural composition and interfacial intricacies of MIS photoelectrodes tailored for PEC water splitting. The application of MIS heterostructures across various semiconductor light-absorbing layers, including traditional photovoltaic-grade semiconductors, metal oxides, and emerging materials, is presented first. Subsequently, this review elucidates the reaction mechanisms and respective merits of vacuum and non-vacuum deposition techniques in the fabrication of the insulator layers. In the context of the metal layers, this review extends beyond the conventional scope, not only by introducing metal-based cocatalysts, but also by exploring the latest advancements in molecular and single-atom catalysts integrated within MIS photoelectrodes. Furthermore, a systematic summary of carrier transfer mechanisms and interface design principles of MIS photoelectrodes is presented, which are pivotal for optimizing energy band alignment and enhancing solar-to-chemical conversion efficiency within the PEC system. Finally, this review explores innovative derivative configurations of MIS photoelectrodes, including back-illuminated MIS photoelectrodes, inverted MIS photoelectrodes, tandem MIS photoelectrodes, and monolithically integrated wireless MIS photoelectrodes. These novel architectures address the limitations of traditional MIS structures by effectively coupling different functional modules, minimizing optical and ohmic losses, and mitigating recombination losses.

Cathode Material Development in the Past Decade for H2 Production from Microbial Electrolysis Cells

Cathode materials are critical for microbial electrolysis cell (MEC) development and its contribution to achieving a circular hydrogen economy. There are numerous reports on the progress in MEC cathode development during the past decade, but a comprehensive review on the quantitative comparisons and critical assessments of these works is lacking. This Review summarizes and analyzes the published literature on MEC cathode and catalyst development in the past decade, providing an overview of new materials examined during this time period and quantitative analyses on system performance and trends in materials development. Collected data indicate that hybrid materials have become the most popular catalyst candidate while nickel materials also attract increasing interest and exploration. However, the dilemma between higher H₂ production rate and larger MEC volume remains and still requires more investigation of novel MEC cathode catalysts and configurations to offer a solution.

A Nickel Coated Copper Substrate as a Hydrogen Evolution Catalyst

Replacing precious metals with low-cost metals is the best solution for large scale production. Copper is known for its excellent conductivity and thermal management applications. When it comes to hydrogen evolution reaction, it is highly unstable, especially in KOH solution. In this paper, we approached a simple method to reduce corrosion and improve the performance by depositing nickel-molybdenum oxide and nickel on copper substrates and the achieved tafel slopes of 115 mV/dec and 117 mV/dec at 10 mA/cm2. While at first, molybdenum oxide coated samples showed better performance after 100 cycles of stability tests, the onset potential rapidly changed. Cu-Ni, which was deposited using the electron gun evaporation (e-gun), has shown better performance with 0.28 V at 10 mA/cm2 and led to stability after 100 cycles. Our results show that when copper is alloyed with nickel, it acts as a promising hydrogen evolution reaction (HER) catalyst.

Towards Highly Efficient Chalcopyrite Photocathodes for Water Splitting: The Use of Cocatalysts beyond Pt

Solar radiation is a renewable and clean energy source used in photoelectrochemical cells (PEC) to produce hydrogen gas as a powerful alternative to carbon-based fuels. Semiconductors play a vital role in this approach, absorbing the incident solar photons and converting them into electrons and holes. The hydrogen evolution reaction (HER) occurs in the interface of the p-type semiconductor that works as a photocathode in the PEC. Cu-chalcopyrites such as Cu(In, Ga)(Se,S)₂ (CIGS) and CuIn(Se,S)₂ (CIS) present excellent semiconductor characteristics for this purpose, but drawbacks as charge recombination, deficient chemical stability, and slow charge transfer kinetics, demanding improvements like the use of n-type buffer layer, a protective layer, and a cocatalyst material. Concerning the last one, platinum (Pt) is the most efficient and stable material, but the high price due to its scarcity imposes the search for inexpensive and abundant alternative cocatalyst. The present Minireview highlighted the use of metal alloys, transition metal chalcogenides, and inorganic carbon-based nanostructures as efficient alternative cocatalysts for HER in PEC.

Challenging the Durability of Intermetallic Mo-Ni Compounds in the Hydrogen Evolution Reaction

Molybdenum-nickel materials are catalysts of industrial interest for the hydrogen evolution reaction (HER). Well-characterized surfaces of the single-phase intermetallic compounds Ni₇Mo₇, Ni₃Mo, and Ni₄Mo were subjected to accelerated durability tests (ADTs) and thorough characterization to unravel whether crystallographic ordering affects the activity. Their intrinsic instability leads to molybdenum leaching, resulting in higher specific surface areas and nickel-enriched surfaces. These are more prone to form Ni(OH)₂ layers, which leads to deactivation of the Mo-Ni materials. The crystal structure of the intermetallic compounds has, due to the intrinsic instability of the materials in alkaline media, no effect on the activity. Ni₇Mo₇, identified earlier as durable, proves to be highly unstable in the applied ADTs. The results show that the enhanced activity of unsupported bulk Mo-Ni electrodes can solely be ascribed to increased specific surface areas.

Enhancing the Effectiveness of Oxygen Evolution Reaction by Electrodeposition of Transition Metal Nanoparticles on Nickel Foam Material

Electrochemical oxygen evolution reaction (OER) activity was studied on nickel foam-based electrodes. The OER was investigated in 0.1 M NaOH solution at room temperature on as-received and Co- or Mo-modified Ni foam anodes. Corresponding values of charge-transfer resistance, exchange current-density for the OER and other electrochemical parameters for the examined Ni foam composites were recorded. The electrodeposition of Co or Mo on Ni foam base-materials resulted in a significant enhancement of the OER electrocatalytic activity. The quality and extent of Co, and Mo electrodeposition on Ni foam were characterized by means of scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) analysis.

The Synergistic Effects of Alloying on the Performance and Stability of Co3Mo and Co7Mo6 for the Electrocatalytic Hydrogen Evolution Reaction

Metal alloys have become a ubiquitous choice as catalysts for electrochemical hydrogen evolution in alkaline media. However, scarce and expensive Pt remains the key electrocatalyst in acidic electrolytes, making the search for earth-abundant and cheaper alternatives important. Herein, we present a facile and efficient synthetic route towards polycrystalline Co3Mo and Co7Mo6 alloys. The single-phased nature of the alloys is confirmed by X-ray diffraction and electron microscopy. When electrochemically tested, they achieve competitively low overpotentials of 115 mV (Co3Mo) and 160 mV (Co7Mo6) at 10 mA cm−2 in 0.5 M H2SO4, and 120 mV (Co3Mo) and 160 mV (Co7Mo6) at 10 mA cm−2 in 1 M KOH. Both alloys outperform Co and Mo metals, which showed significantly higher overpotentials and lower current densities when tested under identical conditions, confirming the synergistic effect of the alloying. However, the low overpotential in Co3Mo comes at the price of stability. It rapidly becomes inactive when tested under applied potential bias. On the other hand, Co7Mo6 retains the current density over time without evidence of current decay. The findings demonstrate that even in free-standing form and without nanostructuring, polycrystalline bimetallic electrocatalysts could challenge the dominance of Pt in acidic media if ways for improving their stability were found.

Hydrogen Evolution Reaction on Iridium-Modified Nickel Foam Surfaces

This work reports on cathodic hydrogen evolution reaction (HER), studied on Ir-activated nickel foam materials, prepared through spontaneous and electrodeposition methods (examined in 0.1 M NaOH electrolyte). Both Ir modifications of Ni foam caused substantial improvement of the HER kinetics, as compared with those recorded for as-received and surface-activated nickel foam materials. Electrochemical examinations were conducted through AC impedance spectroscopy and quasi-steady-state cathodic polarization experiments. Significance of catalytic nature of Ir deposit and employed deposition methodology on the HER behavior of such-obtained Ni foam/Ir composites were discussed in detail by means of SEM/EDX spectroscopy analysis.

NiMoFe and NiMoFeP as Complementary Electrocatalysts for Efficient Overall Water Splitting and Their Application in PV-Electrolysis with STH 12.3

Complementary water splitting electrocatalysts used simultaneously in the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) can simplify water splitting systems. Herein, earth-abundant NiMoFe (NMF) and phosphorized NiMoFeP (NMFP) are synthesized as complementary overall water splitting (OWS) catalysts. First, NMF is tested as both the HER and OER promoter, which exhibits low overpotentials of 68 (HER) and 337 mV (OER). A quaternary NMFP is then prepared by simple phosphorization of NMF, which shows a much lower OER overpotential of 286 mV. The enhanced OER activity is attributed to the unique surface/core structure of NMFP. The surface phosphate acts as a proton transport mediator and expedites the rate-determining step. With the application of OER potential, the NMFP surface is composed of Ni(OH)₂ and FeOOH, active sites for OER, but the inner core consists of Ni, Mo, and Fe metals, serving as a conductive electron pathway. OWS with NMF-NMFP requires an applied voltage of 1.452 V to generate 10 mA cm^-2 , which is one of the lowest values among OWS results with transition-metal-based electrocatalysts. Furthermore, the catalysts are combined with tandem perovskite solar cells for photovoltaic (PV)-electrolysis, producing a high solar-to-hydrogen (STH) conversion efficiency of 12.3%.

Enhancing Durability and Photoelectrochemical Performance of the Earth Abundant Ni-Mo/TiO2 /CdS/CIGS Photocathode under Various pH Conditions

Cu(In,Ga)(S,Se)₂ (CIGS) is a promising photocathode material owing to its high absorption coefficient, adjustable band gap, and suitable band edge for the hydrogen evolution reaction (HER). However, most CIGS photocathodes have suffered from instability in applications that require a wide range of pH conditions and have utilized noble metal HER catalysts to achieve a high performance. Thus, improving the durability of the CIGS photocathode under various pH conditions and developing a cost-effective non-noble metal catalyst are critical issues in the photoelectrochemical (PEC) application of this promising photocathode material. Here, we catalyze the CIGS photocathode with Ni-Mo as a non-noble metal to enhance the PEC efficiency, and we employ atomically grown TiO₂ to passivate the CdS/CIGS surface and improve the stability under a wide range of pH conditions. Our Ni-Mo alloy exhibits the best HER catalytic activity among reported earth-abundant HER catalysts in both acidic and alkaline solutions. The Ni-Mo/CdS/CIGS photocathode yields an onset potential of 0.5 V (vs. RHE) and a short-circuit photocurrent density as high as 15-25 mA cm^-2 under various pH conditions ranging from 0.4 to 14, which is highly comparable to that of Pt/CdS/CIGS. Furthermore, the passivation of CdS/CIGS with a thin TiO₂ layer, obtained by atomic layer deposition, effectively prevents the photocorrosion of CdS and the dissolution of the Mo back contact, which are the main causes of the degradation of the photocathode. The optimized Ni-Mo/TiO₂ /CdS/CIGS photocathode produces a stable photocurrent density at 0 V_RHE for 100 minutes except under strong alkaline conditions. The current work presents a very useful method to improve the efficiency and durability of the CIGS photocathodes with an earth-abundant metal catalyst, which completely replaces Pt.

A CuNi bimetallic cathode with nanostructured copper array for enhanced hydrodechlorination of trichloroethylene (TCE)

To address the challenges of low hydrodechlorination efficiency by non-noble metals, a CuNi bimetallic cathode with nanostructured copper array film was fabricated for effective electrochemical dechlorination of trichloroethylene (TCE) in aqueous solution. The CuNi bimetallic cathodes were prepared by a simple one-step electrodeposition of copper onto the Ni foam substrate, with various electrodeposition time of 5/10/15/20 min. The optimum electrodeposition time was 10 min when copper was coated as a uniform nanosheet array on the nickel foam substrate surface. This cathode exhibited the highest TCE removal, which was twice higher compared to that of the nickel foam cathode. At the same passed charge of 1080C, TCE removal increased from 33.9 ± 3.3% to 99.7 ± 0.1% with the increasing operation current from 5 to 20 mA cm^-2, while the normalized energy consumption decreased from 15.1 ± 1.0 to 2.6 ± 0.01 kWh log^-1 m^-3. The decreased normalized energy consumption at a higher current density was due to the much higher removal efficiency at a higher current. These results suggest that CuNi cathodes prepared by simple electrodeposition method represent a promising and cost-effective approach for enhanced electrochemical dechlorination.

Probing Transition-Metal Silicides as PGM-Free Catalysts for Hydrogen Oxidation and Evolution in Acidic Medium

In this experimental study, we investigate various transition-metal silicides as platinum-group-metal-(PGM)-free electrocatalysts for the hydrogen oxidation reaction (HOR), and for the hydrogen evolution reaction (HER) in acidic environment for the first time. Using cyclic voltammetry in 0.1 M HClO₄, we first demonstrate that the tested materials exhibit sufficient stability against dissolution in the relevant potential window. Further, we determine the HOR and HER activities for Mo, W, Ta, Ni and Mo-Ni silicides in rotating disk electrode experiments. In conclusion, for the HOR only Ni₂Si shows limited activity, and the HER activity of the investigated silicides is considerably lower compared to other PGM-free HER catalysts reported in the literature.

Electrocatalytic activity of porous Ni–Fe–Mo–C–LaNi5 sintered electrodes for hydrogen evolution reaction in alkaline solution

The hydrogen evolution reaction (HER) was studied in 6 M KOH solution at temperatures ranging between 303 K and 353 K on a porous Ni–Fe–Mo–C–LaNi₅ electrode.

Ni–reduced graphene oxide composite cathodes with new hierarchical morphologies for electrocatalytic hydrogen generation in alkaline media

Ni–reduced graphene oxide (rGO) composite cathodes were successfully prepared by composite electrodeposition under supergravity fields.

Advances and recent trends in heterogeneous photo(electro)-catalysis for solar fuels and chemicals

In the context of a future renewable energy system based on hydrogen storage as energy-dense liquid alcohols co-synthesized from recycled CO2, this article reviews advances in photocatalysis and photoelectrocatalysis that exploit solar (photonic) primary energy in relevant endergonic processes, viz., H2 generation by water splitting, bio-oxygenate photoreforming, and artificial photosynthesis (CO2 reduction). Attainment of the efficiency (>10%) mandated for viable techno-economics (USD 2.00-4.00 per kg H2) and implementation on a global scale hinges on the development of photo(electro)catalysts and co-catalysts composed of earth-abundant elements offering visible-light-driven charge separation and surface redox chemistry in high quantum yield, while retaining the chemical and photo-stability typical of titanium dioxide, a ubiquitous oxide semiconductor and performance "benchmark". The dye-sensitized TiO2 solar cell and multi-junction Si are key "voltage-biasing" components in hybrid photovoltaic/photoelectrochemical (PV/PEC) devices that currently lead the field in performance. Prospects and limitations of visible-absorbing particulates, e.g., nanotextured crystalline α-Fe2O3, g-C3N4, and TiO2 sensitized by C/N-based dopants, multilayer composites, and plasmonic metals, are also considered. An interesting trend in water splitting is towards hydrogen peroxide as a solar fuel and value-added green reagent. Fundamental and technical hurdles impeding the advance towards pre-commercial solar fuels demonstration units are considered.

Benchmarking hydrogen evolving reaction and oxygen evolving reaction electrocatalysts for solar water splitting devices

Objective comparisons of electrocatalyst activity and stability using standard methods under identical conditions are necessary to evaluate the viability of existing electrocatalysts for integration into solar-fuel devices as well as to help inform the development of new catalytic systems. Herein, we use a standard protocol as a primary screen for evaluating the activity, short-term (2 h) stability, and electrochemically active surface area (ECSA) of 18 electrocatalysts for the hydrogen evolution reaction (HER) and 26 electrocatalysts for the oxygen evolution reaction (OER) under conditions relevant to an integrated solar water-splitting device in aqueous acidic or alkaline solution. Our primary figure of merit is the overpotential necessary to achieve a magnitude current density of 10 mA cm(-2) per geometric area, the approximate current density expected for a 10% efficient solar-to-fuels conversion device under 1 sun illumination. The specific activity per ECSA of each material is also reported. Among HER catalysts, several could operate at 10 mA cm(-2) with overpotentials <0.1 V in acidic and/or alkaline solutions. Among OER catalysts in acidic solution, no non-noble metal based materials showed promising activity and stability, whereas in alkaline solution many OER catalysts performed with similar activity achieving 10 mA cm(-2) current densities at overpotentials of ~0.33-0.5 V. Most OER catalysts showed comparable or better specific activity per ECSA when compared to Ir and Ru catalysts in alkaline solutions, while most HER catalysts showed much lower specific activity than Pt in both acidic and alkaline solutions. For select catalysts, additional secondary screening measurements were conducted including Faradaic efficiency and extended stability measurements.

The effect of surface modification by reduced graphene oxide on the electrocatalytic activity of nickel towards the hydrogen evolution reaction

The RGO-modified Ni electrode exhibited outstanding activity towards HER. DFT calculations indicate that H atoms, formed upon H₂O discharge on Ni, spill onto the RGO, enabling continuous clearance of Ni-active sites.

Mo doped porous Ni–Cu alloy as cathode for hydrogen evolution reaction in alkaline solution

Molybdenum doped porous Ni–Cu cathodes are conveniently prepared by powder metallurgy in this work.

Bioelectrochemical hydrogen production with hydrogenophilic dechlorinating bacteria as electrocatalytic agents

Hydrogenophilic dechlorinating bacteria were shown to catalyze H(2) production by proton reduction, with electrodes serving as electron donors, either in the presence or in the absence of a redox mediator. In the presence of methyl viologen, Desulfitobacterium- and Dehalococcoides-enriched cultures produced H(2) at rates as high as 12.4 μeq/mgVSS (volatile suspended solids)/d, with the cathode set at -450 mV vs. the standard hydrogen electrode (SHE), hence very close to the reversible H(+)/H(2) potential value of -414 mV at pH 7. Notably, the Desulfitobacterium-enriched culture was capable of catalyzing H(2) production without mediators at cathode potentials lower than -700 mV. At -750 mV, the H(2) production rate with Desulfitobacterium spp. was 13.5 μeq/mgVSS/d (or 16 μeq/cm(2)/d), nearly four times higher than that of the abiotic controls. Overall, this study suggests the possibility of employing dechlorinating bacteria as hydrogen catalysts in new energy technologies such as microbial electrolysis cells.