Farooq
Sher
*a,
Imane
Ziani
abc,
Nawar K.
Al-Shara
d,
Alexander
Chupin
e,
Nađa
Horo
cf,
Bohong
Wang
g,
Saba
Rahman
h,
Bilal
Fareed
ci and
Monica R.
Nemţanu
j
aDepartment of Engineering, School of Science and Technology, Nottingham Trent University, Nottingham NG11 8NS, UK. E-mail: Farooq.Sher@ntu.ac.uk; Tel: +44 (0) 115 84 86679
bDepartment of Chemistry, Laboratory of Applied Chemistry and Environment, Faculty of Sciences, Mohammed First University, Oujda 60000, Morocco
cInternational Society of Engineering Science and Technology, Nottingham, UK
dDepartment of Chemical and Environmental Engineering, University of Nottingham, University Park, Nottingham NG7 2RD, UK
ePeoples' Friendship University of Russia (RUDN University), Moscow 117198, Russia
fDepartment of Chemistry, Faculty of Science, University of Sarajevo, Sarajevo 71000, Bosnia and Herzegovina
gNational and Local Joint Engineering Research Center of Harbor Oil and Gas Storage and Transportation Technology, Zhejiang Key Laboratory of Petrochemical Environmental Pollution Control, Zhejiang Ocean University, No. 1 Haida South Road, 316022, Zhoushan, P. R. China
hChemical and Biomedical Engineering, Pennsylvania State University, Pennsylvania PA 16801, USA
iDepartment of Chemical Engineering, Pakistan Institute of Engineering and Applied Sciences, Nilore, 45650, Islamabad, Pakistan
jElectron Accelerators Laboratory, National Institute for Laser, Plasma and Radiation Physics, 409 Atomiştilor Street, Bucharest-Măgurele, Romania
First published on 29th August 2024
In addressing global energy demands, the focus on hydrogen gas production from renewable sources intensifies. This research review investigates hydrogen production via steam splitting using eutectic molten hydroxide (NaOH–KOH%) electrolysis, a promising solution for escalating energy needs. A pivotal aspect involves developing a novel reference electrode for eutectic molten hydroxide, enveloping Ni/Ni(OH)2 with an alumina or mullite tube ionic membrane. The mullite-covered electrode proves stable and reusable from 225 to 300 °C, showcasing a novel advancement in electrochemical stability. Compared to silver and platinum quasi-reference electrodes, the designed reference electrode demonstrates superior stability and efficacy in controlling the platinum working electrode, marking a significant innovation. Moreover, an intriguing cyclic voltammetry study examines different working electrodes, including Ni, Pt, Ag, Mo, and stainless steel (SS) in eutectic molten hydroxide at different temperature conditions. The observed reduction potential for hydrogen evolution follows the order: Ni > Pt > Ag > SS > Mo, corroborated by chronoamperometry, underscoring the reliability of the findings. In the pursuit of high-temperature eutectic molten hydroxide electrolysis to split steam into hydrogen fuel, cathodes of nickel, platinum, and stainless steel are deployed alongside stainless steel and graphite anodes. Operating within the temperature range of 225 to 300 °C and applying voltages ranging from 1.5 to 2.5 V, stainless steel as an anode yields impressive current efficiencies at 300 °C: 90.5, 80 and 68.6% for nickel, stainless steel, and platinum cathodes, respectively. This study positions steam splitting via molten hydroxides as a promising alternative for hydrogen production, poised for integration with renewable energy sources, marking a transformative step in sustainable energy practices.
The sources of hydrogen, ranging from non-renewable ones like hydrocarbons to renewables such as hydropower, solar, wind, and biomass, offer diverse options for production.4 Terlouw et al.5 emphasize the possibility of substantial reductions in harmful emissions by generating hydrogen from renewable sources. The operational measure of the hydrogen production setup is outlined as one kilogram of hydrogen at 80 bar pressure, guaranteeing a purity surpassing 99.9%. This metric is derived from an average reference flow of 10 tons of production and storage of hydrogen per day.6 Electrolysis, leveraging water as a pristine and environmentally friendly energy source, stands as the foremost method for hydrogen generation, offering exceptional purity in its output, viable for applications ranging from small-scale to large-scale operations. However, challenges arise as electrolysis may contribute to CO2 emissions if non-renewable energy resources power the necessary electricity generation. Additional advancements are required to enable the commercialization of this clean energy approach for hydrogen gas production as fuel, necessitating materials capable of withstanding challenging operating conditions within electrolysis cells.7 Addressing the imperative to mitigate global warming necessitates an alternative fuel that is affordable, user-friendly, clean, and emits minimal greenhouse gases. Hydrogen emerges as a promising alternative, aligning with the criteria for an ideal fuel due to its potential benefits.8
Hydrogen functions as an energy carrier with high efficiency, emitting minimal to zero emissions during utilization, thus playing a pivotal role in combating global warming. Despite its environmental advantages, challenges persist, particularly in ensuring safety throughout production, storage, and transportation.9 Understanding key metrics such as the hydrogen concentration (measured at 15% during storage conditions at 20 MPa pressure and room temperature), global warming potential (GWP), nitrogen oxide emissions, and acidification potential is crucial for evaluating its environmental impact. The GWP is equivalent to 0.058 kg CO2 eq. per kW per h, with nitrogen oxide emissions measured at 7.04 × 105 NO2 eq. per kW per h and acidification potential at 3.63 × 104 kg SO2 eq. per kW per h. These metrics provide valuable insights into hydrogen's broader implications and guide efforts toward sustainable energy practices.9 In fuel cells, hydrogen reacts with oxygen, yielding energy and producing only water as a by-product. Unlike readily available fuels like coal and hydrocarbons, hydrogen exists as a compound in water, fossil hydrocarbons, and biomass products.10
Addressing economic concerns within the hydrogen fuel industry, efficient and affordable storage and distribution remain pivotal.11 Whether produced in small-scale local plants or large-scale central plants, hydrogen gas can be stored and transported as gaseous hydrogen, liquid hydrogen, or metal hydrides.12 The transportation of hydrogen involves three main routes. First, pipelines similar to those used for natural gas provide a convenient means of delivery, constructed from materials like low-carbon steel, aluminium, or brass.13 Second, large-scale storage tanks, akin to those for natural gas, facilitate storage. Gaseous hydrogen can also be stored in cylinders for small-scale operations, often at high pressures ranging from 20 MPa to 80 MPa for industrial use.13 Liquid hydrogen (LH2), with a low density of 70.8 kg/cm3 and a boiling point of −252.3 °C, requires extremely low temperatures for storage.14 Cryostats are employed for this purpose, although the associated conversion and maintenance costs make this method more suitable for aircraft than road vehicles. Finally, certain metal alloys offer an alternative for storing hydrogen.15 These alloys absorb hydrogen gas reversibly, forming metal hydrides, presenting a unique approach to hydrogen storage.
Embarking on a journey at the intersection of innovation and sustainability, this research review delves into the realm of hydrogen gas production through a lens of unprecedented possibilities. Driven by concerns over environmental impact and the ever-increasing demand for clean energy, the focus shifts towards the electrochemical process of splitting steam for hydrogen production via eutectic molten hydroxide electrolysis. This exploration is not merely a scientific pursuit; it is a quest to redefine our energy landscape. Imagine a novel reference electrode, a stable companion crafted from the fusion of Ni/Ni(OH)2 and an ionic membrane. Picture the efficient working electrodes Ni, Pt, Mo, Ag, and stainless steel each revealing its unique prowess in the intricate process of hydrogen evolution. This review aspires to unravel the mysteries of cyclic voltammetry, chronoamperometry, and two-electrode electrolysis within the dynamic environment of eutectic molten hydroxide, where process variables such as applied voltage, electrode material, and molten salt temperature shape the narrative of clean energy production. From the physical measurement of hydrogen gas using water displacement to the theoretical calculations echoing Dalton's law for gases. Through meticulous exploration and theoretical contemplation, this review sets out to redefine the boundaries of hydrogen gas production, laying the groundwork for a sustainable energy future. This review transcends the ordinary, unlocking the secrets that propel us toward a cleaner, brighter tomorrow.
As we embark on an in-depth exploration detailed in Table 1, a critical analysis unfolds, shedding light on nuanced operating conditions, efficiency benchmarks, persistent challenges, and noteworthy advancements. Notably, Eutectic Molten Hydroxide Electrolysis stands out, offering compelling advantages in energy efficiency and cost-effectiveness, presenting a viable option for hydrogen production. Beyond these merits, it showcases the ability to convert CO2 into hydrocarbons, presenting a sustainable solution for fuel production. However, this method is not without limitations, involving reduced production rates, specific electrode material demands, increased production expenses, decreased durability, and heightened energy consumption. The ensuing sections delve into a comprehensive study of the novelty and performance of Eutectic Molten Hydroxide in the intricate landscape of hydrogen production methods.
Electrolysis method | Operating conditions | Efficiency | Challenges | Noteworthy advancements | Reference |
---|---|---|---|---|---|
Alkaline electrolysis | Immersing two electrodes in a liquid alkaline electrolyte (e.g., 20–30% KOH), typically below 100 °C | Up to 70% | Diaphragm limitations, restricted current density, hindered high-pressure operation | Low-temperature electrochemical water splitting with high efficiency even at 100 °C, low-cost nickel alloy electrocatalysts, insights from magnetic properties, shaping the design of highly efficient electrocatalysts | 17 and 23 |
Solid oxide electrolysis cells (SOECS) | Leveraging steam at elevated temperatures (750–1000 °C) | Hybrid-SOEC with a current per unit area of 3.16 A/cm2 at 750 °C and 1.3 V | Temperature-dependent efficiency, cation removal challenges | The hybrid-SOEC with BaZr0.1Ce0.7Y0.1Yb0.1O3−δ electrolyte, no performance decline over 60 hours of uninterrupted operation | 24 and 25 |
Proton exchange membrane electrolysis (PEM) | Solid polymer electrolyte (e.g., Nafion), effective proton conductivity at around 80 °C | 50–70% | Gas crossover reduction, compact system design | Introduction of solid polymer electrolytes, a significant departure from traditional methods | 26 |
Photo-electrolysis | Harnessing sunlight to separate water into hydrogen and oxygen, typically at 20 °C to 30 °C | Photo-current density of 4.30 mA/cm2, stability demonstrated over 70 hours in seawater splitting | Challenges in yield and efficiency, economic viability | A durable photo-electrode layout featuring an ultra-stable PEC cell with a BiVO4 photo-anode and a MoO3 barrier layer, ensuring stable and efficient hydrogen production from seawater | 27 |
Thermochemical water splitting | Converting water into hydrogen and oxygen via high-temperature endothermic (500 °C to 1800 °C) and low-temperature exothermic processes | Potential for sustainable, large-scale hydrogen generation | Energy and exergy efficiency variations, design challenges | Favourable assessment of sulfur-iodine and hybrid sulfur cycles, achieving approximately 77% efficiency | 28 |
Eutectic molten salt electrolysis | Voltage range: 1.5–2 V | 59.30% (molten chloride), 87.70% (molten hydroxide), 99% (molten carbonate) | Variable product rates, CO2 conversion varies with molten electrolyte type | Longer-chain hydrocarbons are exclusively produced within molten carbonates at 1.5 V. Promising for sustainable hydrocarbon fuel production, providing a foundation for fundamental investigations. Adequate heating values of produced fuels for subsequent applications | 29 |
Temperature range: 225–475 °C | |||||
Electrolyte: titanium cathode, graphite anode |
Additionally, the NiS2–MoS2 hetero-nanorods catalyst, leveraging high valence state Ni and Mo synergism, exhibits a current density of 103.41 mA/cm2 at 1.54 V for urea-assisted water electrolysis, reducing the cell potential by 224 mV compared to general water electrolysis.33 Another promising catalyst, the 15% Ni/ZrO2 synthesized via the wetness impregnation method, shows high conversion rates of methane (62.9%) and carbon dioxide (64.9%) in the dry methane reforming (DRM) process at 750 °C with a gas hourly space velocity (GHSV) of 72000 mL h−1 gcat−1, indicating its potential for efficient hydrogen production with enhanced stability.34 The NiSe2/MoSe2 heterostructured catalyst demonstrated optimized interfacial electron redistribution and enhanced urea adsorption energies,35 achieving a current density of 10 mA/cm2 at a potential of 1.33 V for urea oxidation, and requiring a cell voltage of only 1.47 V for the urea–water electrolyzer to drive 10 mA/cm2, highlighting significant energy savings.
In line with these advancements, the conversion of formaldehyde (HCHO) using a direct formaldehyde fuel cell achieved remarkable results, with a selectivity greater than 99% and a faradaic efficiency of 200%. Similarly, methanol (CH3OH) co-electrolysis with water demonstrated a promising reaction potential of approximately 1.3 V vs. RHE, producing hydrogen and formate with a faradaic efficiency exceeding 95%. The co-electrolysis of urea (CO(NH2)2) and water also showed significant potential, resulting in the production of hydrogen and carbon monoxide with a reaction potential of around 1.3 V vs. RHE. Moreover, the use of nickel-based catalysts, such as Ni(OH)2, significantly enhanced the electro-oxidation of methanol, achieving a current density increase of 8.2 times when modified with platinum (Pt).36 These results collectively underscore the potential for efficient and sustainable hydrogen and value-added chemical production using small organic molecules, while maintaining high selectivity and energy efficiency.
Further, FeCoMoS@NG catalysts, used in zinc–air batteries and water splitting, demonstrate excellent stability and low cell voltages, requiring only 1.58 V to achieve a current density of 10 mA/cm2 and maintaining stability over 100 hours of operation. Rh/RhOx catalysts achieve remarkable turnover frequencies of 2.19 s−1 at 1.53 V and 20.30 s−1 at 1.63 V, reflecting a significant increase in activity compared to standard Rh catalysts and exhibiting exceptional stability over 20 hours at a current density of 50 mA/cm2.39 Additionally, a recent study developed a highly efficient water electrolysis system powered by a cellulose sponge-based hydrovoltaic power generator (CHPG) optimized for long-term stability and high performance. A single CHPG achieved an open-circuit voltage (Voc) of approximately 0.47 V and a short-circuit current (Isc) of approximately 477 μA under relative humidity of 45–50% at 25 °C.40 By connecting six CHPG modules in parallel, the system produced a Voc of 2.09 V and an Isc of 3.11 mA, sufficient to drive water electrolysis for hydrogen production. The HER was facilitated by a cobalt phosphide/nickel foam (PCO/NF) electrode, which showed a significantly lower overpotential and a Tafel slope of 85 mV/dec compared to other electrodes. For OER, the Co5O4/NiFe-LDH/NF electrode demonstrated an overpotential of 1.5 V to achieve 1 mA and a Tafel slope of 74 mV/dec. These optimized electrodes exhibited excellent long-term stability, maintaining performance over 24 hours at a current density of 100 mA/cm2. The integrated system achieved a hydrogen production rate of 81.0 μmol/h, highlighting its potential for sustainable and efficient hydrogen generation.40
Building on recent advancements, several studies have demonstrated significant improvements in hydrogen yield and production efficiency through innovative catalyst designs. For instance, Chai et al.41 reported a 71% increase in hydrogen yield using Fe nanoparticles and a 35.2% increase with NiO nanoparticles in wastewater treatments. Furthermore, they observed a 623% enhancement in hydrogenase activity with 20 mg/L Ec-NiO-NP synthesized from Eichhornia crassipes, significantly boosting hydrogen production. Similarly, Liu et al.42 showed that dual-doped Co3N electrodes achieved high current densities at low overpotentials, retaining 92.3% of their current density after 10 hours of operation. Additionally, Qian et al.43 developed Ni3N–Co3N heterointerfaces on Ni foam, achieving an overpotential of 43 mV for HER at 10 mA/cm2 and an ultralow working potential of −88 mV for HzOR, maintaining 25 mA/cm2 for 40 hours without significant decay. Moreover, Zhu et al.44 demonstrated that Ruc/NiFe-LDH catalysts achieved an industrial-scale current density of 1 A/cm2 at 0.43 V, resulting in an energy saving of 3.94 kW h m3 of H2, while maintaining performance over 120 hours at 5 A/cm2. Zhu et al.45 showcased Ni3N/Co3N nanowires, achieving a 94.6% faradaic efficiency for formate production at 1.35 V, producing 11 mmol/cm2/h of formate and 21.4 mmol/cm2/h of hydrogen, and maintaining stability for over 200 hours at 1 A/cm2. Collectively, these insights underscore the potential of advanced catalysts to significantly enhance hydrogen production efficiency. In situ and ex situ characterizations, along with advanced techniques like Density Functional Theory (DFT), provide a deeper understanding of catalyst evolution and reaction mechanisms, essential for optimizing performance and ensuring long-term stability. Comprehensive testing and analysis are crucial for addressing overpotential, efficiency, and durability in water electrolysis systems.
For instance, at a temperature of 427 °C, the conductivity of NaOH is twice that of NaNO3 at the equivalent temperature. This heightened conductivity facilitates faster reaction kinetics and minimizes energy loss resulting from electrode over-potential. Consequently, it proves advantageous for high-temperature fuel cell systems and water-splitting procedures.47
Consequently, there is a general improvement in system efficiency. Moreover, this technology eliminates the necessity for valuable catalytic metals by employing base metals to generate hydrogen gas, with the molten hydroxide acting as a catalyst instead.48 The increased operational temperature of molten salt reduces the decomposition potential of water, thereby enhancing efficiency even further. By effectively isolating the electrolysis system, sustained energy savings can be consistently attained for prolonged usage.49 Moreover, molten salt, without the need for extra heating, sustains the required temperature through the passage of current during electrolysis, provided that the system is properly insulated.50 The process can also leverage waste heat from other systems. However, the primary drawback limiting its industrial application is the requirement for a working temperature of less than 300 °C.51 According to the Carnot cycle for thermodynamic efficiency, this leads to heat wastage, preventing widespread industrial utilization.
H2O → H2(g) + ½O2(g) | (1) |
ΔG = ΔH − QE = ΔH − TΔS | (2) |
![]() | (3) |
![]() | (4) |
![]() | ||
Fig. 1 (A) Traditional electrocatalytic water splitting in alkaline environments. (B) (a) X-ray powder diffraction (PXRD) patterns of α-Ni(OH)2 obtained from the nonwoven fabric (NF) before (above) and after (below) oxygen evolution reaction (OER). (b) Transmission electron microscopy (TEM), (c) atomic force microscopy (AFM), and (d) high-resolution TEM images of α-Ni(OH)2. (C) (a) TEM and (b) HRTEM images of α-Ni(OH)2 after OER. High-resolution X-ray photoelectron spectroscopy (XPS) spectra for (c) Ni 2p and (d) O 1s of α-Ni(OH)2 before (above) and after (below) OER. The original data is represented by the black curves in panels (c) and (d). (D) Scanning electron microscopy (SEM) images at different magnifications of platinum nanoparticles (Pt NPs) electrodeposited on self-supporting electrodes manufactured at various electrolyte levels (EL): (a and b) 0.05 V, (c and d) −0.2 V, (e and f) −0.35 V, and (g and h) −0.5 V. Edited with permission from ref. 52–54. Copyright ACS ©2018, ©2021 and Elsevier ©2022. |
In a related study, Zhang et al.65 aimed to creat self-supporting electrodes (SSEs) for use in direct methanol fuel cells (DMFC) and direct alcohol fuel cells (DAFC) by utilizing a simple square-wave potential (SWP) method to electrodeposit platinum nanoparticles (Pt NPs) onto carbon paper.65 The study targeted an enhancement in the utilization rate and catalytic durability of Pt in these fuel cells. Specifically, Fig. 1(D) showcased SEM images of the electrodeposited Pt NPs on SSEs prepared at different lower potentials, revealing morphological changes in Pt NPs with varying potentials (E). For example, At E = 0.05 V, smooth sphere-like Pt NPs were observed. However, as the potential negatively shifted to −0.2 V, the PtNPs surface became rough, presenting a cauliflower-like morphology.65 At E = −0.35 V, observations revealed platinum nanoparticles (Pt NPs) resembling coral formations, with surface protrusions. Finally, at E = −0.5 V, thorn-like Pt NPs were formed.65 These outcomes suggest that managing the potential had a notable effect on the quantity, distribution, and structure of the electrodeposited Pt NPs on carbon paper, potentially influencing the catalytic activity and endurance of the Pt catalysts in fuel cells.
In the process of water electrolysis, hydrogen ions are drawn towards the cathode, while hydroxide ions are directed towards the anode. Meanwhile, gas receivers gather hydrogen and oxygen generated at the cathode and anode, respectively, with a membrane separating them. T notably, the combined potential of the reactions is −1.23 V, indicating the theoretical cell voltage for the procedure. However, various barriers must be overcome, including electrode surface boundary layers, electrode and electrolyte phases, separator, electrical resistance, and activation energies, analyzed in thermodynamic, kinetic, and transport process principles contexts.64 Specifically, resistance in a water electrolysis system includes external electrical circuit resistance (R1), anode overpotential (Ranode), resistance due to oxygen bubbles (Rbubble of O2), electrolyte resistance (Rions), membrane resistance (Rmembrane), resistance due to hydrogen bubbles (Rbubble of H2), cathode overpotential (Rcathode), and electrical resistance at the cathode . Therefore, the total resistance is expressed in eqn (5).66
![]() | (5) |
![]() | (6) |
![]() | (7) |
![]() | ||
Fig. 2 (A) Illustration demonstrating various electrolyte concepts in distilled water and in combination with aqueous KOH, including (a) porous diaphragms, (b) anion-exchange membranes (AEM), and (c) ion-solvating membranes. (B) Visual representation outlining the sequential steps of high-temperature water electrolysis. (C) Representation of cell potential for hydrogen gas production through the water electrolysis process as a function of temperature. (D) Schematic depicting the electrical double layer near a negatively charged electrode surface. (E) Depiction of the effect of potential change on Gibbs energy: the overall relationship between energy change and reaction state. (F) Enlarged view of the highlighted section in (E). Reproduced with permission from ref. 69–73. Copyright ACS ©2023 and 2019, Elsevier ©2022, 2015, ACS ©2020 and Springer ©2023. |
Connecting this insight to the broader context of the section discussing equilibrium potential and thermodynamics, it's evident that these electrolyte concepts play a pivotal role in influencing overpotentials, ion conductivity, and overall efficiency in water electrolysis.69 The interaction among these ideas and the thermodynamic principles deliberated upon provides insights into the complex factors influencing the functionality of alkaline electrolysis cells. The equilibrium potential is derived by deducting the cathode electrode's equilibrium potential from that of the anode electrode, as outlined in eqn (8).74 The equilibrium cell potential correlates with the Gibbs free energy alteration of the entire cell reaction, as delineated in eqn (9).74 In the case of a spontaneous cell reaction, the driving force possesses a negative value of ΔGcell, articulated as eqn (10).74
![]() | (8) |
![]() | (9) |
ΔGcell = ∑ΔGproduct − ∑ΔGreactant = ΔHcell − TΔScell | (10) |
During high-temperature water electrolysis, several reaction steps occur as highlighted in Fig. 2(B). Initially, H2O adheres to the perovskite surface or the three-phase boundary (TPB), leading to the formation of an OH− ion by relinquishing a hydrogen atom. Subsequently, the OH− ion sheds another hydrogen atom, yielding O2− ions, while the liberated hydrogen atoms merge to generate hydrogen gas.78 The generated O2− ions move towards the YSZ phase and traverse the YSZ electrolyte's interior towards the anode side, where they unite to generate oxygen gas. These steps are influenced by factors such as electrode material properties, TPB nature, and microstructure. It's important to highlight that the conversion of water into hydrogen and oxygen is not thermodynamically favourable, necessitating an external supply of electrical energy to initiate the cell reaction.79 The cell potential must exceed the applied potential, and reactions can vary in speed, with some requiring overpotential for the necessary current density.80 The kinetics discussed earlier, reveal an increase in overpotential with rising current density. Additional energy introduces a potential drop influenced by electrolyte properties, electrode shape, and cell design. As expressed in eqn (11), the cell potential consistently ranges between 1.8–2.0 V, with a current density value between 0.001–0.003 A/cm2 for an industrial water electrolysis system.80
![]() | (11) |
The efficiency of electrolysis can be assessed using different parameters. One crucial indicator is voltage efficiency, which signifies the proportion of the actual voltage employed for water splitting compared to the total voltage applied to the cell. This relationship is articulated in eqn (12).85 Thermal efficiency, as delineated in eqn (13) is grounded in the energy variations of the water electrolysis reaction.85 It covers the complete thermal equilibrium of the process. Another approach to assessing efficiency involves evaluating the rate of hydrogen gas generation compared to the total electrical energy input to the electrolysis system, as depicted in eqn (14).85
![]() | (12) |
![]() | (13) |
![]() | (14) |
i = icathodic − ianodic = FAK°(C0(0,t)e−αf(E−E°) − CR(0,t)e(1−α)f(E−E°)) | (15) |
The Butler–Volmer equation describes the connection between current density, surface potential, and the composition of the electrolyte near the electrode surface. Expressed as a one-electron reaction, it involves parameters such as A (electrode surface area), K° (typical rate constant), α (transfer coefficient ranging from 0 to 1 for a single-electron reaction), and f (the ratio). The variables “t” and “0” within the brackets represent the particular time and distance from the electrode, respectively.91
The Butler–Volmer equation, derived through the transition-state theory, utilizes curvilinear coordinates along reaction pathways (Fig. 2(E)), where potential energy varies as a function of independent coordinates within the system. An increase in potential (ΔE) results in a reduction in the electron's relative energy by F(E − E°) (as shown in Fig. 2(F)), subsequently influencing the Gibbs free energy of hydrogen ions and hydrogen in the reaction as highlighted in Fig. 2(F).92 This behaviour does not impose any restrictions on mass transfer. The Butler–Volmer equation (eqn (16)) can be simplified as follows.92 Where i0 represents the current density of exchange, which is defined as the current related to the reversible process of water cleavage. The overpotential at each electrode is derived from this simplified equation.92 Neglecting one term in the total resistance equation is permissible in the absence of mass transfer influence and at large over-potentials (>118 mV at 25 °C), i.e., when e−αfη > e−(1−α)fη. The relation between i and η(E − E°) is represented by the Tafel eqn (17).92
i = i0(e−αfη − e−(1−α)fη) | (16) |
η = a + b![]() ![]() | (17) |
Composition of electrolyte | Temperature (°C) | Atmosphere | Reference electrode | Working electrode | Crucible material | Anode | Cathode | Applied voltage (V) | Reference |
---|---|---|---|---|---|---|---|---|---|
NaOH | 550 | Ar | Ni rod | Pt wire, Ni wire, and Ni pallet | Ni | — | — | — | 94 |
NaOH–KOH | 200 | Ar | Pt wire | Pt, Au, Fe rod | Platinum | — | — | — | 95 |
NaOH–KOH (50![]() ![]() |
347 | N2 | Cu plated Platinum sheet | Pt | Vitreous carbon crucible | — | — | — | 96 |
LiOH–NaOH, LiOH–KOH, NaOH–KOH | 250–300 | — | Ag/AgCl | Pt, Pd, Ni, Ag, Al | Nickel crucible | — | — | — | 97 |
KOH | 110–160 | — | Fe | Fe, FeSi, and FeC | PTFE | — | — | — | 98 |
KOH | 35–400 | Steam | — | — | Ni 400 | Ni, Ni400, Co plated Ni | Ni 400 | 99 | |
KOH, NaOH, Ba(OH)2, LiOH | 200–700 | Ar | — | — | Alumina | Ni, Pt and lithiated Ni | Ni | 1.1–2.3 | 100 |
NaOH | 25 | Air | SCE | Cu rod | — | — | — | — | 101 |
NaOH (8.0 M) | 90 | Hg/HgO/1 M NaOH | Co–Mo alloy | — | — | — | — | 102 | |
KOH (10 wt%) | 20, 40, 60 | Air | — | — | Vinyl chloride | Ni–Cr–Fe alloy | Ni–Cr–Fe alloy | 6 | 103 |
NaOH–NaHS (50![]() ![]() |
80 | Air | Hg/HgO (1 M NaOH) | — | Glass | Graphite, Ni, Ni–Cr alloy | Ni, graphite | — | 104 |
NaOH | 320–400 | Argon | —— | — | — | Ni | Ni | 105 | |
Aqueous, KOH–NaOH, LiOH–NaOH | 110–140, 350, 300–400 | Argon | — | — | — | Activated Ni | Activated Ni | 1.55,1.3, 1.45 | 106 |
KOH (1.0 M) | 25 | Air | Ag/AgCl | C-felt supported Ni, Co, Ni–Co | — | — | — | — | 107 |
NaOH (0.1–5.0 M) | 25 | Ag/AgCl | Ni–Mo–Fe coated stainless steel | — | — | — | — | 108 | |
NaOH–KOH (57![]() ![]() |
200–220 | Ar or NH3 | Ag wire | Pt plate | Ni-400 (commercial Monel® alloy) | — | — | — | 109 |
NaOH | 530 | Ar | —— | — | Ni-covered by alumina | Ni | FeO | 1.7 | 110 |
KOH (50 wt%) | 80, 150, 208, 264 | — | (DHE) | Ni | —— | — | — | 111 | |
NaOH–KOH (51![]() ![]() |
200–450 | NH3 | — | Ni tube | Ni-400 (commercial Monel® alloy) | —— | — | — | 112 |
NaOH–KOH (50![]() ![]() |
200 | N2 and steam | — | — | — | Planar Ni | Mesh Ni monel | 1.2 | 113 |
NaOH | 550 | O2 + air | Ag wire | Ni alloy | Alumina | — | — | — | 114 |
NaOH–KOH (54![]() ![]() |
400 | O2 + air | Ag wire | Ni alloy | Alumina | —— | — | — | 114 |
NaOH (8.0 M) | 70 | Air | Hg/HgO | Ni | Plastic | — | — | — | 115 |
In 1976, Miles et al.116 demonstrated a temperature-dependent influence on electrode kinetics, influencing the oxygen evolution reaction to a greater extent compared to the hydrogen evolution. Subsequently, Divisek et al.117 expanded this inquiry to molten NaOH, achieving an efficiency of approximately 38–39% in water electrolysis at elevated temperatures. Furthermore, in a further study, Divisek et al.118 broadened their scope to the electrolytic splitting of water in aqueous KOH and molten hydroxides (NaOH, LiOH–NaOH) at different temperature ranges. This research achieved a notable current efficiency of 90% in a NaOH melt at 350 °C, revealing a unique side reaction of peroxide production, which was notably diminished within a LiOH–NaOH melt. Moreover, Anani et al.119 demonstrated the potential for hydrogen production by electrolyzing sulfur hydrogen at 80 °C in an equimolar aqueous solution of NaOH. A novel approach was taken by Abouatallah et al.,120 investigating the addition of soluble V2O5 to an 8 M KOH aqueous solution at 70 °C to reactivate a nickel cathode during hydrogen evolution. While the V2O5 additive proved effective, the electro-catalytic activity of vanadium-modified nickel did not surpass that of a fresh nickel electrode.
Additionally, Miles et al.121 explored various eutectic molten hydroxides (NaOH–KOH, LiOH–KOH, LiOH–NaOH) with different working electrodes. Proposing Ag/AgCl as a reference electrode due to its swifter reaction kinetics, the study emphasized the applicability of molten hydroxides in thermal battery applications, underscoring the need to minimize H2O and O2 for the effective use of lithium or sodium anodes. Nagai et al.122 investigated the effect of bubble formation between electrodes on water electrolysis efficiency. Using a 10 wt% KOH aqueous solution and Ni–Cr–Mo alloy electrodes, they found that adjusting the space between electrodes influenced the void fraction, impacting electrolysis efficiency. Similarly, Zabinski et al.123 investigated augmenting hydrogen evolution by incorporating carbon into the Co–Mo alloy cathode. They conducted experiments in an 8 M NaOH solution at a temperature of 90 °C. Despite increased hydrogen evolution activity, preventing molybdenum dissolution proved elusive. Híveš et al.124 pioneered the electrochemical production of ferrate(VI) in eutectic molten hydroxide NaOH–KOH at 200 °C. Ferrate(VI) formation was detected at the inert electrode in contact with ferrate(III) ions and an iron electrode.
Simultaneously, Jayalakshmi et al.125 explored the electrochemical catalytic activity of a stainless steel substrate coated with a composite film of Ni–Mo–Fe in alkali solutions, showcasing improved hydrogen evolution activity. The potential for ferrate(VI) production was further explored by Híveš et al.,126 using a NaOH–KOH eutectic hydroxide at temperatures between 170–200 °C with a stationary iron electrode. The formation of ferrate(VI) at the anodic oxidation of the iron electrode was confirmed, achieving a current efficiency of Fe(VI) formation of up to 72%. In a different domain, Ganley et al.127 pioneered a direct ammonia fuel cell using eutectic molten hydroxides (NaOH–KOH) at temperatures from 200 to 450 °C, achieving a maximum power density of 40 mW/cm2 at 450 °C. On a separate note, Cox and Fray128 effectively converted iron(III) oxide to iron in molten sodium hydroxide at 530 °C, accomplishing a current efficiency of around 90%. In addition, Ganley129 extended the investigation, examining the impacts of elevated temperature and pressure on alkaline electrolysis, employing concentrated KOH at 400 °C across different pressure conditions. The research utilized nickel Monel as a cathode and evaluated different anode materials. It determined that the most optimal cell performance was achieved when employing a cobalt-plated nickel anode at 400 °C and under a steam partial pressure of 8.7 MPa.
Continuing the journey to enhance hydrogen evolution, Döner et al.130 employed a 1.0 M KOH water solution at ambient temperature. They supported cathodes made from various materials (Ni, Co, and Ni–Co alloy) with C-felt coating, resulting in significant increases in current densities, particularly with the C-felt/Ni–Co combination recording the highest current density. Extending the versatility of molten hydroxide, Guo et al.131 demonstrated a direct carbon fuel cell with enhanced performance up to 400 °C by employing the eutectic mixture as an electrolyte. Moreover, Hrnčiariková et al.132 contributed to this area by examining how the composition of the anode affects the electrochemical production of ferrate(VI) using molten KOH. They analyzed three distinct anode materials in molten KOH: pure iron (Fe), white cast iron (FeC), and silicon-rich steel (FeSi) electrodes. The research unveiled the concurrent existence of ferrate(VI) formation alongside significant oxygen evolution. Recent advancements include Yang et al.'s platinum electrode, achieving a maximum power of approximately 16 mW/cm2.133 Licht et al.100,134 in their studies demonstrated ammonia production feasibility while observing a decrease in hydrogen gas evolution efficiency at high temperatures. The hydrogen gas evolution efficiency of the mixed hydroxide electrolyte (NaOH–KOH; 50–50 mol%) dropped from 96% to 13.4% with increasing temperature from 200 to 600 °C, as illustrated in Fig. 3(A). Ultimately, the evolving narrative illustrates the interconnected nature of these studies, providing insights into the intricate world of molten hydroxide electrochemistry.
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Fig. 3 (A) The efficiency of hydrogen production as a function of temperature and the selected hydroxide electrolyte under atmospheric pressure was assessed at 1 A between two flat nickel electrodes. (B) A cyclic voltammogram of a nickel wire electrode in the NaOH melt at 550 °C was conducted at a scan rate of 100 mV/s. The reference electrode was a nickel wire, with the potential limited between −0.4 V and 1.3 V. (C) A cyclic voltammogram of a platinum wire electrode in the NaOH melt at 550 °C was conducted at a scan rate of 100 mV/s. The reference electrode was a nickel wire, with the potential limited between −0.4 V and 1.3 V. (D) Cyclic voltammograms of a platinum electrode in NaOH–KOH at 200 °C were performed at a scan rate of 20 mV/s, using argon gas or ammonia as the atmosphere, and with Ag reference electrode. (E) (a) Cyclic voltammograms of a Pt plate electrode under Ar or NH3 at 200 °C (sweep rate: 20 mV/s); (b) an enlarged view of the profiles between −0.75 and −0.35 V from (a). Edited with permission from ref. 100, 109 and 135. Copyright IOPscience ©2016, 2015 and Elsevier ©2014. |
NiO + 2OH− → NiO2 + H2O + 2e− | (18) |
Litch et al.138 explore alternative molten electrolytes for water splitting to generate hydrogen fuel. Their study demonstrates that the electrolysis potential needed for water splitting at 400 °C in molten NaOH is remarkably low, measuring below 1.4 V (Fig. 3(B)). Under these circumstances, elevated current densities, potentially reaching A/cm2, are readily attainable. Anticipations suggest that utilizing reticulated or textured electrodes with enhanced microscopic or nanoscopic surface areas may further decrease electrolysis overpotentials. Anodes composed of Ni, Pt, monel, and Ir, paired with Ni cathodes, demonstrate notable stability, maintaining effective current densities of 1 A/cm2 under conditions where both anode and cathode limitations are present.138 Notably, at low current density, the electrolysis potential notably drops to less than 1 V in 700 °C LiOH, even yielding some hydrogen gas. In a parallel study, Ge et al.57 delved into the oxidation of nickel-to-nickel oxide in a one-step reaction within molten NaOH, albeit at a higher temperature of 550 °C. This transformative process was succeeded by a secondary reaction involving hydrogen evolution. Fig. 3(C) vividly depicts the cyclic voltammogram of a nickel wire working electrode in molten NaOH at 550 °C, revealing clear redox peaks. The oxidation peak O3 indicates the oxidation of the Ni wire in NaOH, leading to the creation of a thin oxide film on the Ni surface. Concurrently, peak C3 captures the reduction of the oxide C3. Noteworthy is the intensified current observed at the cathode in C2 and at the anode in O2, corresponding to the evolution of hydrogen and oxygen gas, respectively, as articulated in eqn (19) and (20).
2H2O + 2e− → H2(g) + 2OH− | (19) |
2OH− → ½O2(g) + H2O + 2e− | (20) |
Transitioning to another facet of research, Sayed et al.139 conducted an extensive investigation on an independent nickel-layered double hydroxide characterized by a hierarchical nanosheet architecture. This unique structure, synthesized efficiently on nickel foam via hydrothermal treatment and later transformed into NiO at 500 °C, demonstrated impressive activity in methanol oxidation.139 The synthesized electrodes displayed an onset potential of 0.35 V in a 1 M KOH solution, with the nanosheet structure enhancing charge and mass transfers, resulting in superior overall activity. Notably, these electrodes demonstrated prolonged stability during extended oxidation activity, maintaining a discharge current at 0.5 V for more than 1 hour without any decline in performance represents a noteworthy improvement compared to bare nickel foam.139 These findings emphasize the effectiveness of nickel-layered double hydroxide and nickel oxide electrodes as anodes in alkaline direct methanol fuel cells, suggesting substantial promise for energy conversion systems.
Additionally, Al-Shara et al.140 made a valuable contribution to the field by developing a novel reference electrode, Ni/Ni(OH)2, specifically designed for electrolysis in eutectic molten hydroxides. This electrode was manufactured by employing a eutectic molten hydroxide (NaOH–KOH; 49–51 mol%) at a temperature of 300 °C.140 Cyclic voltammetry tests were conducted to evaluate its performance, revealing exceptional stability and reusability. In a comparative analysis with platinum and silver quasi-reference electrodes, the Ni/Ni(OH)2 electrode emerged as a promising candidate, demonstrating suitability, stability, reproducibility, and reusability as a reference electrode in a molten hydroxide electrolyte. In summary, these crucial findings underscore the potential of the Ni/Ni(OH)2 electrode as a stable and efficient reference electrode for electrolysis in eutectic molten hydroxides.140
Enter Ge et al.,135 pioneers in this symphony. Cyclic voltammetry, the maestro's baton, gracefully wielded on a platinum electrode basking in molten NaOH at 550 °C. A material chosen for its chemical steadfastness, platinum graced the stage with cyclic voltammograms. The cathodic reverie, C1, whispers the reduction of a platinum-clad oxide film; O1, anodic in nature, resounds oxidation. C2's crescendo echoes the hydrogen evolution reaction, while O2, a sonnet, harmonizes with the oxygen evolution reaction. The saga extends as Ge et al.135 venture into the realms of platinum wire, a virtuoso in three molten hydroxides NaOH–KOH at 280 °C, LiOH–NaOH, and LiOH–KOH at 270 °C. In NaOH–KOH's embrace, a cathodic crescendo at −0.32 V, a ballet of superoxide ions (O−2) reduction, unravelled. Cyclic voltammetry gracefully revealed the nuances of a platinum (Pt) electrode in a molten hydroxide electrolyte, a narrative skillfully crafted by Yang et al..109 Reduction and oxidation peaks pirouetted elegantly at distinctive potentials, with reduction currents exhibiting their dance below −0.55 V. The stage was then seized by a commanding oxidation peak between −0.55 V and 0.1 V, rising dramatically above 0.17 V, where the evolution of oxygen took center stage.109
In the presence of ammonia (NH3), an ethereal onset potential of approximately −0.67 V marked the beginning of anodic currents, with a crescendo leading to a maximum of around −0.2 V. However, the drama unfolded swiftly above 0.15 V as oxygen evolution claimed the spotlight. The forward scan from −0.2 to 0.15 V witnessed a diminishing oxidation current, a subtle interplay involving Pt oxidation and the reduction of the active surface for ammonia oxidation. In a seamless transition, Yang et al.109 continued their electrochemical tale, this time exploring the ammonia-driven drama on a platinum stage immersed in molten NaOH–KOH at 200 °C. Fig. 3(E) gracefully presented cyclic voltammograms, a visual symphony offering choices of argon and ammonia, while Ag stood as the stoic ref. 109. The dashed line served as a blank canvas, encapsulating platinum's narrative in the hydroxide embrace, while the solid line painted the enchanting transformation of ammonia to N2, all harmonized by the eutectic molten hydroxides. The platinum electrode, a versatile protagonist, showcased its prowess in every experiment, contributing a lyrical stanza to the grand saga of electrochemistry.
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Fig. 4 (A) Depictions of (a) mullite tube reference electrode (RE), (b) MgO tube RE, and (c) Mg working electrode (WE) following a 31 days testing period. (B) Cyclical voltammograms recorded for a Pt plate electrode under Ar or NH3 at 200 °C with a sweep rate of 20 mV/s. (C) Illustration of the synthesis process of Nb-doped Nb-NiFe-LDH via molten salt. (D) Electrochemical evaluations conducted in 1.0 M KOH, comprising (a) linear sweep voltammetry (LSV) curves for Nb-NiFe-LDH, NiFe-LDH, Nb-Ni-LDH, Ni-LDH, NF, and RuO2 on NF, accompanied by (b) corresponding Tafel plots; (c) comparative assessment of overpotential at 10 mA/cm2 and Tafel slopes compared to other catalysts; (d) calculation of the double-layer capacitance (Cdl) value from cyclic voltammetry (CV) curves at 1.30 V vs. reversible hydrogen electrode (RHE); (e) polarization curves before and after 5000 CV scans; and (f) chronopotentiometric testing at 100 mA/cm2. (E) Comparison of cyclic voltammograms for Ni, Pt, Ag, Mo, and stainless steel working electrodes in molten hydroxide at 300 °C with a scan rate of 100 mV/s, utilizing a 0.5 mm Ni/Ni(OH)2 reference electrode and a 5 mm stainless steel counter electrode with an immersion depth of 14 mm, under Ar gas at a flow rate of 40 cm3/min. Edited with permission from ref. 143–145. Copyright IOPscience ©2023, Elsevier ©2013, ©2022, and ©2020. |
In a separate study by Choi et al.,146 the exploration of CV in molten hydroxides, particularly with Mg metals, takes center stage. The cyclic voltammetry (CV) technique allows for the qualitative and quantitative determination of various species in the electrolyte, surpassing open circuit potentiometry. The introduction of hydroxide ions and dissolved magnesium metal to the MgCl2–KCl–NaCl salt mixture induces effects on redox potential and electrochemical behaviour. While presenting a solubility measurement of approximately 0.0205 wt% at 500 °C (Fig. 4(B)), the study acknowledges a high associated error, necessitating further understanding in future studies.143 This study significantly enhances the understanding of the electrochemical behaviour of Mg metals in molten hydroxides. Turning attention to Zhou et al.'s147 investigations, they delve into the synthesis process of Nb-doped Nb-NiFe-LDH via molten salt (Fig. 4(C)) and examine the effect of high-valence Nb doping in NiFe hydroxides on improving the oxygen evolution reaction (OER) in water splitting. The findings depicted in Fig. 4(D) underscore the remarkable electrochemical efficacy of Nb-NiFe-LDH, as demonstrated by a notable decrease in overpotential at 50 mA/cm2 from 280 mV (for NiFe-LDH) to 242 mV with Nb doping, signifying considerable advantages for the oxygen evolution reaction. This observation finds further reinforcement in the overpotential values recorded for Ni-LDH (428 mV) and Nb-Ni-LDH (342 mV). Tafel slopes reveal that Nb-NiFe-LDH (31.3 mV/dec) exhibits enhanced electron transport compared to NiFe-LDH (47.5 mV/dec), NbNi-LDH (42.8 mV/dec), and Ni-LDH (38.8 mV/dec), thus accelerating OER kinetics. Electrochemical Impedance Spectroscopy (EIS) at 1.37 V vs. RHE illustrates the commendable electrode conductivity and electron transfer rate of Nb-NiFe-LDH, as indicated by the smallest fitted semicircle diameter.147
Calculations of double-layer capacitance (Cdl) from cyclic voltammetry curves highlight Nb-NiFe-LDH's larger Cdl value (5.2 mF/cm2) compared to NiFe-LDH (4.5 mF/cm2), Nb-Ni-LDH (3.0 mF/cm2), and Ni-LDH (1.8 mF/cm2), indicating a higher exposure of effective active sites through Nb integration.147 These comprehensive findings significantly advance our understanding of electrochemical processes, providing valuable insights for optimizing cyclic voltammetry experiments in both chloride and hydroxide systems. Comparing various working electrodes (Ni, Pt, Ag, Mo, and St. st) in eutectic molten hydroxide at 300 °C using a potential scan rate of 100 mV/s, each electrode displayed a unique reduction potential value.145 Platinum had the most positive reduction potential value, followed by nickel and then stainless steel (Fig. 4(E)). Silver and molybdenum electrodes demonstrated lower reduction potential values. The obtained reduction potential values are consistent with previous literature findings, highlighting the uniqueness of each working electrode. The values, ranging from approximately −0.47 V for platinum (Pt) to −0.56 V for molybdenum (Mo), underscore the diverse electrochemical characteristics of these materials.145 In conclusion, this collective exploration sheds light on crucial aspects of electrode stability, material selection, and electrochemical behaviour in molten hydroxides, providing valuable insights for optimizing future cyclic voltammetry experiments.
The versatility of molten salt electrorefining is evident in the recovery of metals such as tungsten, cobalt, manganese, zirconium, lead, uranium, silicon, and ruthenium.151 Tailored techniques like selective electrodeposition and electrorefining for specific metals such as Te, Cu, U, and actinides enhance efficiency and reduce radioactive sludge formation, contributing to overall process sustainability. Additionally, Fig. 5(A) illustrates a schematic diagram delineating the recycling process based on regenerative redox targeting for used LiFePO4 material, facilitating comprehension of the steps and compartments involved in recycling LiFePO4 material using the regenerative redox targeting method.151 Shen et al.156 explore the diverse applications of hydrogen generated in molten salts, showcasing its potential for large-scale production of advanced carbon nanostructures. The molten salt-assisted electrolysis strategy emerges as an innovative pathway for reducing CO2 and H2O into syngas and hydrocarbon fuels. Operating in molten salts as a reaction solvent, this process benefits from high thermal stability and outstanding CO2 dissolving capacity, further amplified by the integration of renewable solar energy.156 This green and efficient electrolysis process converts CO2 into valuable carbon nanostructures, leveraging rapid ion migration and diffusion, high thermal stability, and superior CO2 dissolving capacity offered by molten salts. Moreover, the method shows promise for large-scale production of advanced carbon nanostructures, capitalizing on the hydrogen generated in molten salts. Notably, the glassy carbon retains its initial characteristics post-electrolysis.156 At 650 °C, thick carbon sheets assemble from quasi-spheres, while at 750 °C, fiber-like nanostructures of hollow multi-walled CNTs measuring 20 nm in diameter and having a wall thickness ranging from 5 to 10 nm appear.156
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Fig. 5 (A) Illustrated depiction outlining the regenerative redox targeting-based recycling process for spent LiFePO4 material. (B) Schematic representation of hydrogen production through molten salt and its subsequent application in the direct reduction of suspended Fe2O3 powders within the melt. (C) Diagrammatic representation of electrochemical CO2 fixation in molten salts utilizing a liquid Zn cathode. (a) The alumina tube sealed at one end, (b) configuration of the liquid Zn cathode, and (c) formation of Zn@C spheres with a core–shell structure within a carbon flake. (D) Overview emphasizing key aspects of the direct sustainability of metallic materials. (E) Illustration showcasing various scales concerning kinetic and thermodynamic simulations and experiments, focusing on the solid-state direct reduction of iron oxides using hydrogen. Edited with permission from ref. 152–155. Copyright ACS ©2020, Elsevier ©2023, ©2019 and ACS ©2023. |
Moreover, carbon materials originating from chitosan via a single-step carbonization process using LiCl–ZnCl2 molten salt exhibit a substantial specific surface area of 2025 m2/g and a significant nitrogen content of 5.1 wt%. These materials demonstrate noteworthy CO2 uptake under various temperatures and pressures.157 The conversion of PET into nitrogen-doped porous carbon using melamine and ZnCl2/NaCl eutectic salts at 550 °C yields a considerable specific surface area of 1173 m2/g and abundant nitrogen dopants. Additionally, porous biochar derived from wood waste with eutectic molten salts shows enhanced CO2 sorption, with the most efficient biochar displaying the highest CO2 uptake and selectivity.157 Furthermore, N-doped microporous carbons synthesized from melamine and sucrose using molten LiCl/KCl salts show significant CO2 uptake under specific conditions. Fig. 5(B) elucidates the specific steps and conditions integral to the borate-enhanced molten salt electrolysis process, directing the mass production of sophisticated carbon nanostructures of high worth using hydrogen produced within molten salts.152 Additionally, Fig. 5(C) illustrates a schematic representation of the process where hydrogen produced in molten salt is employed for the direct reduction of suspended Fe2O3 powders in the melt.155 This sustainable method involves the electrochemical decomposition of water in molten salt, generating hydrogen gas that reduces Fe2O3 particles to metallic iron.
Furthermore, the remarkable faradaic efficiency of approximately 94.5% in the one-pot generation of syngas through CO2 electrolysis at a low temperature of 450 °C is achieved by utilizing renewable solar energy for electricity generation and concentrated solar light for eutectic melting.152 The research also highlights achievements in the electrolysis of CO2 to produce a range of hydrocarbon fuels utilizing eutectic molten salts. Raabe et al.153 further explore the potential of electrochemical hydrogen generation in high-temperature molten salts as a versatile method for producing metals and alloys. The electrolysis procedure entails conducting an electrical current through a molten salt electrolyte containing metal ions, resulting in the deposition of metals and alloys on the cathode. During the electrochemical fabrication of graphene in molten LiCl, the procedure produced interconnected graphene nanosheets/nanoscrolls. The reduced graphene exhibited a specific surface area of 565 m2/g and a specific capacitance of 255 F/g. Even after 5000 charge/discharge cycles in 6 M KOH, it retained 95% of its capacitance.158 Additionally, the electrochemical etching process using molten salt to produce carbide-derived carbon (CDC) demonstrated encouraging outcomes. For example, employing a molten salt (CaCl2) electrochemical etching method on silicon carbide led to the creation of porous nanospheres, exhibiting a specific capacitance of 176 F/g at 1 A/g in a 6 M KOH electrolyte.159 Notably, this electrochemical method excels in producing exceptionally pure metals, utilizing sustainable electrical energy sources like wind or hydropower directly.160 This makes it an attractive alternative to conventional reduction methods reliant on fossil reductants. Moreover, integrating renewable electrical energy in electrolytic cells contributes to higher overall efficiency.
Challenges accompany this technology as the high operating temperatures required for electrolysis can induce aggressive interactions with electrodes and insulation materials. The development of carbon-free electrode materials becomes crucial to minimize CO2 emissions.161 Additionally, the variability in sustainable electrical power supply, particularly from sources such as solar or wind energy, underscores the need to design electrolysis cells capable of adapting to variable power supply and interruptions. Fraction of Renewable Energy System values exhibit variation across different scenarios and months, with a fluctuation range of 30–35%. The fluctuations mainly stem from seasonal shifts in renewable resources and changes in demand.162Fig. 5(D) provides a comprehensive summary of the crucial aspects of direct sustainability in metallic materials production, including various ores, such as low-grade and high-grade, used for synthesizing metals from mineral raw materials, as well as the utilization of scrap and waste as alternative feedstock sources.153 Additionally, Fig. 5(E) illustrates the intricate multi-scale nature of simulations and experiments related to kinetics and thermodynamics involved in the hydrogen-driven solid-state reduction of iron oxides, providing a visual representation of the process's complexity.153 In conclusion, electrochemical hydrogen generation in high-temperature molten salts offers a promising approach to the sustainable production of metals and alloys. This method utilizes renewable electrical energy sources and has different conversion efficiencies depending on the energy source used. For example, solar PV + electrolysis has a conversion efficiency of 10.5%, while wind + electrolysis, hydroelectric + electrolysis, and tidal + electrolysis has a conversion efficiency of 70%.163 These technologies are at different stages of development, as indicated by their technology maturity level (TML) ratings. Ongoing research and development efforts are essential to address challenges related.163
Scaling up from laboratory to industrial-scale operations also poses difficulties, particularly in maintaining efficiency and managing costs. Laboratory setups achieving high efficiencies often do not translate directly to industrial applications, with efficiency drops of around 5–10%.167 Furthermore, the safe handling of molten salts at high temperatures is critical to prevent accidents, as any spillage or leak can cause severe burns and equipment damage. Managing these materials in large-scale operations introduces additional safety and regulatory concerns. Despite these challenges, the prospects for molten hydroxide electrolysis are compelling. The high operating temperatures significantly enhance reaction kinetics, reducing overvoltages and improving overall efficiency. Studies have reported a reduction in cell voltage by 3.4 to 4 mV/K between 100 °C and 200 °C, substantially higher than the thermodynamic reduction of 0.8 mV/K for liquid water in the same range. Additionally, the conductivity of the electrolyte, particularly KOH solutions, increases with temperature, further boosting efficiency. Ongoing research into more efficient and durable catalysts is critical for advancing molten hydroxide electrolysis. For instance, NiFeOxHy nanosheets have demonstrated impressive performance characteristics, with a low overpotential of 216 mV in freshwater electrolytes and 232 mV in saline solutions.168 These nanosheets show exceptional stability, maintaining functionality for 1000 hours at a current density of 0.1 A/cm2 in freshwater electrolytes and for 550 hours at 1 A/cm2 in saline solutions. The Tafel slope of these nanosheets is as low as 51 mV/dec, indicating efficient kinetics and a rapid increase in current density with increasing overpotential. Their robust nanosheet structure provides abundant electroactive sites, reduces charge transfer resistance (Rct of 1.54 Ω), and enhances mechanical stability, allowing for efficient mass and charge transfer.168
The development of new materials that can better withstand harsh conditions could reduce maintenance costs and improve system longevity. Ni-based alloys, for example, show promise due to their durability in molten hydroxide environments. Integrating advanced materials and nanostructures in these catalysts could significantly enhance the lifespan and efficiency of molten hydroxide electrolysis systems, reducing the need for frequent catalyst replacement and lowering overall operational costs. Furthermore, integrating renewable energy sources, such as solar or wind power, with molten hydroxide electrolysis could significantly lower operational costs, potentially reducing the overall cost of hydrogen production by 15–20%.169 Process optimization techniques, such as advanced thermal management and automated control systems, can improve efficiency by 10–12%, making operations more cost-effective. For instance, an ET-PEMEC system using concentrated solar power and thermal energy storage showed improved overall efficiency and operational flexibility. As technology advances and production scales up, the cost of hydrogen production through molten hydroxide electrolysis could become competitive with traditional methods. With technological advancements and increased production scales, the cost of hydrogen production could potentially lower to below $2 per kilogram of hydrogen, making it a viable option for large-scale hydrogen production.169 For example, a PtM system utilizing a LSGM-based tubular SOEC stack achieved a CO2-to-CH4 conversion ratio of 98.7% and a PtG efficiency of 94.5%, demonstrating the potential for economic feasibility and large-scale application.170
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