Perspective of an external magnetic field-assisted catalytic process for green H₂ generation and CO₂ conversion

Abstract

H₂ generation via water splitting and CO₂ conversion to value-added chemicals are two key reactions that have immense importance for deep decarbonization. Being energy-intensive processes, water splitting and CO₂ conversion are often carried out in the presence of catalysts. Electrocatalysis, photocatalysis and thermocatalysis are three major catalytic conversion pathways for such conversions. To boost the energy efficiency of the catalytic conversions, the role of an external magnetic field (as an external physical force) has been explained in detail in this review. Fundamentals of water splitting and CO₂ conversion, the underlying mechanism in the presence of a magnetic field, and the role of different types of magnetic fields and their effect on the chemical conversion and energy efficiency of the mentioned processes have been elaborated in this article. In conclusion, the future scope to utilize the present magnetic field-based green process at a large scale has been discussed elaborately.

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Sudeshna Das Chakraborty

Dr Sudeshna Das Chakraborty is currently a researcher at the R&D division of Tata Steel Ltd, India, focusing on various catalytic & non-catalytic pathways for industrial-scale CO2 reutilization & green H2 production as a sustainable solution for the CO2 intensive steel sector. She previously served as a postdoctoral fellow at CSIR-National Metallurgical Laboratory in India, where she worked on hydrogen generation via electrocatalytic and photocatalytic methods using heterostructured 2D catalysts. She has earned her PhD in 2019 from the Saha Institute of Nuclear Physics, HBNI, India. Her doctoral research explored the effects of magnetic fields on excited-state molecular dynamics on nanosurfaces and involved the synthesis of various 0D, 1D, 2D, and 3D nanocatalysts. To date, she has authored 13 research papers, contributed one book chapter, and filed a patent related to magnetic field effects on chemical reactions for eco-friendly fuel production.

Samik Nag

Dr Samik Nag is the Chief of the Iron Making Research Group of R&D, Tata Steel Ltd, Jamshedpur, India. With over 23 years of experience at the organization, he brings deep expertise in metallurgy and chemical engineering and has authored many research papers. Dr Nag holds a BTech from NIT, Durgapur, in mechanical engineering. He has earned his MTech (mechanical engineering) & PhD (chemical engineering) from IIT Bombay, India. He is actively involved in several decarbonization initiatives at Tata Steel, contributing to the company’s sustainable transformation efforts by deploying specific technologies at the existing steel manufacturing plants.

Trilochan Mishra

Dr T. Mishra has completed his PhD at Utkal University, India, and postdoctoral studies at Friedrich–Alexander University, Germany. He is currently working as a Chief Scientist and Professor of CSIR-National Metallurgical Laboratory, a premier research organization in India. Throughout his career, he has been mostly involved in translational research in the areas of nanomaterials, functional coatings, catalysis, and surface chemistry. He made significant contributions in the innovative synthesis of various size and shape-controlled functional nanomaterials, such as TiO2 nanotubes, porous catalytic materials, and 2D materials, including metal chalcogenides and MXenes for green energy applications. He has published many papers in high impact journals, book chapters and patents (both Indian & US). In addition, he has been serving as an editorial board member of reputed journals. He has developed eight technologies/processes, out of which four are already commercialized by industries, and one is implemented in the strategic sector. In addition to various awards, Dr Mishra is an elected fellow of the Indian Chemical Society (FICS) and the Royal Society of Chemistry (FRSC).

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1. Introduction

The rampant use of fossil fuels (coal and liquid hydrocarbons) is leading the whole world towards a sustainability crisis. The global average temperature increase of the Earth's surface is more than ever before in the last fifty years, primarily due to excessive greenhouse gas emissions, with CO₂ accounting for the largest share. Considering the criticality of the situation globally, governments and industries are now seeking sustainable solutions through green fuel production, with specific targets to be achieved over the next ten years. Green H₂ is becoming an alternative carbon-neutral fuel to address this critical problem. In addition, alternative ways for the recycling of CO₂ from heavily emissive industries like iron & steel (8% of the global CO₂ emission) to value-added fuels like syngas, CH₄, and CH₃OH are being researched for a holistic solution.¹ Research on all possible fronts is being done to meet the upcoming demand for sustainable fuels. Being energy-intensive processes, both green H₂ generation and CO₂ conversion are cost-intensive and involve a high energy requirement to break the thermodynamically stable H₂O or CO₂ molecule. Catalytic pathways are best suited to decrease the energy demand of the mentioned processes. Various chemical routes like electrochemical, photochemical, photo-electrochemical, and thermochemical are under trial to produce green H₂ and for CO₂ conversion. However, most of the catalysts are not effective enough, so there is a need to explore stable noble metal-free sustainable catalysts for upscaling the mentioned chemical conversions to higher technology readiness levels (TRLs) to meet the demand of the industrial sectors. Low-cost transition metal-based catalysts are important for large-scale use. To enhance catalytic performance, several common approaches are employed in catalyst development, such as defect engineering,² interfacial strain induction,³ z-scheme catalysis,⁴ doping⁵ and increasing the number of active sites.^6,7 However, creating complexity in the catalyst structure increases the cost. Hence, the utilisation of external physical forces, including gravity,⁸ ultrasound⁹ and electric fields¹⁰ in enhancing reaction kinetics, is increasingly gaining interest. These physical forces can be applied with minimal complexity in the system. Recent studies indicate that magnetic fields, as an external physical force, significantly affect chemical reactions that involve charged intermediates. The role of the external magnetic field in excited state electron transfer processes,¹¹ hydrogen atom transfers,¹² reactions in ionic liquids,^13,14 hydrogen adsorption and desorption kinetics¹⁵ and hole hopping¹⁶ draws attention. The role of the external magnetic field in water splitting and CO₂ conversion (both processes go through various charged intermediates and spin interconversions) is a very cutting-edge research area.

Magnetic field influence is also extended to other sustainable reactions, like organic decomposition¹⁷ and toxic metal reduction.¹⁸ It can influence the crystal structure, morphology, and energy gap of a nanosized molecule, as reported elsewhere.^19,20 An external static magnetic field can improve molecular alignment and help enhance the electrical conductivity of organic molecular nanowires.²¹ It can influence electronic transitions, such as by applying an external magnetic field, the electronic emission (fluorescence) can be delayed.²² Moreover, it can split the electronic energy level (Zeeman splitting), altering the energy gap between the ground and the excited state. For a photoexcited solute molecule, the singlet–triplet conversion efficiency of a donor–acceptor radical ion pair can be improved to a great extent in the presence of a tiny (<1T) magnetic field, as previously reported by Das Chakraborty et al.¹¹ Interestingly, coupling the effect of the magnetic field with conventional (electro/photo/thermal) catalysis by developing a suitable magneto-catalyst in the presence of external magnetic fields is a promising and novel strategy. In addition, the process can be considered a green approach as it does not demand extra energy or cost to improve the catalytic performance, particularly when a static magnetic field is used. Several recent studies are in progress to explore the role of an external magnetic field in the catalytic processes of H₂ generation and CO₂ conversion.

The present review summarises (schematically presented in Fig. 1) the effect of an external magnetic field to enhance the catalytic reactions for efficient water splitting to produce green H₂. In addition, the effect of the magnetic field on the CO₂ reduction reaction (CO₂RR) to produce value-added products like syngas, CO, CH₄, CH₃OH, and HCOOH is discussed. The possible underlying mechanisms and challenges of magnetocatalysis are discussed in the subsequent sections. The simplicity of the magnetic field effect to reduce the overall energy efficiency of H₂ generation via water splitting and CO₂ conversion makes it attractive from an industrial perspective. However, further studies are still required for optimising the magneto-catalysis process. In the present review, a comprehensive analysis of reported data has been carried out in the work on the magnetic field effect on electrocatalytic/photocatalytic and thermo-catalytic conversions of water and CO₂ through multiple driving forces—mass transfer, spin selectivity, electron–hole separation, and magnetothermal effects (as illustrated in Fig. 1). The objective of the present review is to produce insight for further improvement and development of research in this area.

Fig. 1 The overall scheme representing the external magnetic field effect on water splitting and CO₂ conversion for various catalytic pathways (including electro-catalytic, photo-catalytic, photo-electrocatalytic, and thermo-catalytic processes) alongside the roles of mass transfer, spin selectivity, electron–hole separation, and the magneto-thermal effect as underlying driving forces.

2. Fundamentals of electrochemical photochemical & thermochemical reactions

The basic mechanisms of water splitting & CO₂ conversion via electrochemical, photochemical, and thermochemical pathways are explained in this section.

2.1. Electrochemical conversion

Electrochemical conversion is an attractive way to achieve water splitting or the conversion of CO₂ molecules to valuable chemicals. This process requires electrical energy, and when supplied by renewable sources, it becomes eco-friendly. The mechanism of water splitting & CO₂ conversion at different pH values and energy requirements is given in Table 1.

Table 1 Electrochemical reaction & energy requirements of water splitting & CO₂ conversion for different (electro/photo/thermal) catalytic pathways^23–25

Reaction	Electrocatalysis	Photocatalysis	Thermo-catalysis
Water splitting	H2O → H2 + 1/2O2, ΔE^0 = 1.23 V vs. RHE, ΔG^0 = 237.2 kJ mol^−1	Band gap required: ≥ 1.23 eV	O–H: 459 kJ mol^−1
	Mechanism:
	In an acidic medium:
	Cathode: 4H^+ + 4e^− → 2H2, E° = 0.0 V
	Anode: 2H2O → O2 + 4H^+ + 4e^−, E° = +1.23 V
	In a basic medium:
	Cathode: 4H2O + 4e− → 2H2 + 4OH−, E° = −0.828 V
	Anode: 4OH^− → O2 + 2H2O + 4e−, E° = +0.401 V
	In a neutral medium:
	Cathode: 4H2O + 4e^− → 2H2 + 4OH^−, E° = +0.413 V
	Anode: 2H2O → O2 + 4H^+ + 4e^−, E° = +0.817 V
CO2 conversion	CO2 reduction reactions:	CO2/CO = −0.53 eV	C=O 745 kJ mol^−1,
	CO2 + 2H^+ + 2e− → CO + H2O, E° = −0.52 V,	CO2/HCHO = −0.48 eV	C–C (336 kJ mol^−1),
	CO2 + 6H^+ + 6e− →CH3OH + H2O, E° = −0.43 V	CO2/CH3OH = −0.38 eV	C–O (327 kJ mol^−1),
	2CO2 + 6H^+ + 6e− → C2H5OH + 12OH^−, E° = −0.33 V	CO2/C2H5OH = −0.24 eV	C–H bond (411 kJ mol^−1)
	H2O oxidation reactions:
	2H2O → O2 + 4H^+ + 4e−, E° = +0.81 V

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A device that utilizes electrical energy to drive the above-mentioned reactions is called an electrolyser. Different electrolysers are used for electrochemical water splitting or CO₂ conversion, namely a proton exchange membrane electrolyser (PEM), anion exchange membrane electrolyser (AEM), alkaline electrolyser and solid oxide electrolyser. The first three are used for low-temperature electrolysis, and the last one is used for high-temperature electrolysis. As shown in Fig. 2a–d, different electrolyser systems illustrate electrochemical approaches for water splitting and CO₂ conversion. The elementary details of the kinetics and thermodynamics of the aforementioned reactions, without the influence of a magnetic field, have been elaborated elsewhere.²³ Here, the HER and OER represent the hydrogen evolution reaction and oxygen evolution reaction.

Fig. 2 (a–d) Different types of electrolyser systems for electrochemical conversion; (e and f) electron–hole separation in single and heterojunction photocatalytic systems; (g) photo-electrochemical water splitting; (h and i) thermal heating and the activation energy requirement; (j) different thermal pathways for CO₂ conversion.

2.2. Photochemical conversion

Photochemical reactions are mainly driven by photonic energy. Generally, they are carried out on a photoactive semiconducting catalyst. Upon photon absorption in semiconductors, electrons and holes are generated; the electron facilitates the reduction reaction, while the hole induces the oxidation reaction. The hole remains in the valence band (VB), and the electron transits to the conduction band (CB). Generally, in a single semiconducting catalyst, recombination of the electron–hole pair takes place easily and reduces productivity.

Hence, to reduce the recombination, heterojunction catalysts, Z scheme catalysts, and p–n junction catalysts are under development as described in detail elsewhere.²⁴Fig. 2e–j depicts photocatalytic and thermocatalytic mechanisms under varying activation conditions. When a combination of photon energy and electrical energy is used to drive a chemical reaction, it is called photo-electrochemical water splitting, as shown in Fig. 2g. Here, mostly light energy is converted to electrical energy to initiate an electrochemical reaction.

The minimum band gap required for a photocatalyst to carry out the water-splitting reaction & CO₂ conversion successfully is presented in Table 1. The elementary details of photocatalytic chemical reactions without a magnetic field have been described elsewhere.²⁶

2.3. Thermochemical reactions

These reactions are driven by heat, where heat is used to cross the activation energy barrier (Fig. 2h–j). The schematic of the reaction is presented in Table 1. Previous studies show that the splitting of water by heat requires very high energy;²⁷ however, CO₂ conversion with thermal pathways is under extensive research. A few details of CO₂ thermal cracking are presented in Table 1. The fundamental details of the thermal CO₂ conversion pathways in the absence of a magnetic field have been detailed elsewhere.²⁵

3. Why should a magnetic field influence the reaction kinetics?

In this section, the fundamental role of the external magnetic field in the electrochemical, photochemical, and thermochemical pathways has been described in detail, as elaborated below.

3.1. Influence on electrochemical conversions

The progress of an electrochemical reaction depends mainly on two factors: (a) the mass transport and (b) the reaction progress (as shown in Fig. 3a and b). Reaction kinetics depend on the reaction mechanism, activation energy, and electron transfer feasibility, whereas mass transport is the back-and-forth movement, migration, or diffusion of ionic species in the electrolyte on the electrode surface. By applying an external magnetic field, the overall reaction rate can be altered by modulating the reaction kinetics and mass transport, as explained in the following section.

Fig. 3 (a) Schematic presentation of an electrocatalytic reaction at the interface of the electrolyte/catalyst (b) illustration of the polarisation curve, current vs. potential with respect to the reference hydrogen electrode (RHE) associated with the electrochemical reduction reaction (reproduced from P. Vensaus et al.).²⁸

3.1.1. Mass transport effect. During electrolysis, the measured current density depends on the potential applied. Primarily, the potential requirement depends on the reaction kinetics. By improving the kinetics, the potential requirement to get a particular current density can be lowered. However, after a certain kinetic enhancement, further reduction in potential becomes impossible, as at this stage, the reaction proceeds much faster than the availability of reactants at the electrode surface. At that point, mass transport becomes the rate-limiting factor. Hence, stirring the reaction mixture in the beaker leads to an increase in the reaction rate at this stage. However, stirring is not possible for an electrolyzer system and here comes the role of a magnetic field. An external magnetic field, as a physical force, increases the mass transport of the ions to the electrode surface and helps to enhance electrochemical performance.

3.1.2. Lorentz force effect. An external magnetic field gives a stirring effect or a rotatory motion by inducing the Lorentz force on the moving ionic species in the electrolyte that facilitates mass transport. The effect of the Lorentz force is shown schematically in Fig. 4. The Lorentz force can be expressed by using the following eqn (1):

F_L = q(E + v × B) (1)

where F_L is the Lorentz force, B is the magnetic field, E is the electric field, q is the charge, and v is the velocity of the charges. The Lorentz force is assumed to be perpendicular to the magnetic field and the ion's velocity direction, which induces a rotational movement of the ions. This consequently disturbs the anionic and cationic movement from their usual linear path toward the electrode in the absence of an applied magnetic field. This phenomenon is illustrated in Fig. 4a and b. In this context, the ionic current for the oxidation and reduction reaction exhibits opposing polarity, thereby influencing the Lorentz force in a reverse direction that leads to the formation of a whirlpool/swirl in effect. Work by Shigenori Mitsushima et al.²⁹ has shown that chaotic convection flow increased the shear stress acting on the electrode surface, and shear stress drastically reduced the mass transfer resistance due to bubbles, as shown in the following figure (Fig. 4c). The convection in the electrolyte effectively promotes the mass transfer via the upward pumping effect (Fig. 4c). This convection effect is called the magnetohydrodynamic effect (MHD). The Lorentz force is F_L = j × B (j is the local current density and B is the external magnetic field strength) (Fig. 4d). When B and j are perpendicular, the Lorentz force is the largest, and when B and j are parallel, the Lorentz force is zero.

Fig. 4 (a and b) Lorentz force effect on OH⁻ ion movement (reproduced from P. Vensaus et al.),²⁸ (c) Lorentz force on product gas bubbles produced on the electrode's surface and (d) direction of Lorentz force when B and j act perpendicular to each other.

From a fluid-dynamic perspective, the Navier–Stokes equations with an MHD term is shown in eqn (2),^30,31

ρ(∂v/∂t + v·∇v) = −∇p + µ∇²v + j × B (2)

where current density (j) interacts with B to induce chaotic convection, decreasing bubble adhesion and mass transfer resistance. ρ is the mass density of the fluid, ∂v/∂t: local acceleration—how the velocity changes with time at a fixed point and v·∇v: convective acceleration. Together, this term represents the total acceleration of a fluid element, scaled by its mass density—essentially Newton's second law (F = ma) for a fluid. −∇p = gradient of pressure, µ∇²v = viscous forces and j × B: Lorentz force per unit volume.

Along with the MDH effect under the main magnetic field, small eddy currents are created due to the induced magnetization near the edges of the electrodes, resulting in secondary micro-MHD eddy currents.³²

3.1.3. Maxwell stress effect. Electrochemical reactions mainly take place at the electrode–electrolyte interface. Hence, the influence of the magnetic field on the electrode/electrolyte interface should be carefully studied. There are two layers at the electrode–electrolyte interface: the layer near the electrode with a linear change of potential distribution is called the Stern layer (0.5–10 nm) or the Helmholtz layer. The Stern layer is divided into two parts, inner Helmholtz planes (IHP) and outer Helmholtz planes (OHP). After the OHP, there is a diffusion layer (1−100 µm) where the potential change occurs nonlinearly and a different layer is associated with different resistance (as shown in Fig. 5a and b). The nonlinearity is caused by the thermal motion of the ions. Natural or forced convection in the solution can alter the diffusion layer potential, whereas the change in surface tension & additional charge distribution can affect the double layer.^33,34 Dunne and Coey have shown that under the influence of a magnetic field, the curvature and wettability of the paramagnetic ionic droplet change (Fig. 5c), which shifts the location of the OHP and, in turn, affects the electrochemical double layer and the double layer capacitance.³⁵

Fig. 5 (a) Formation of the Stern layer/Helmholtz layer & diffusion layer near the electrode–electrolyte interface, (b) resistance in different layers, and (c) size deformation of the solvated ions under a magnetic field (MF).

As reported by Dunne et al., due to the Maxwell stress effect under a 0.5 T magnetic field, the OHP gets contracted by 0.25 nm, with a subsequent decrease in double-layer capacitance and the charge transfer resistance³⁵ which consequently boosts the electrochemical reaction.

The three effects described so far are for enhancing the mass transport and thereby increasing the efficiency of the electrochemical water splitting. The role of the magnetic field in enhancing the kinetics of the electrochemical reactions has been discussed in the subsequent sections.

3.1.4. Spin selectivity effect. The half-reactions of water splitting (mainly the OER) or CO₂RR (mainly the ORR) usually involve a combination of intermediate free radical pairs or radical ion pairs with singlet (↑↓) or triplet (↑↑) spins (as shown in Fig. 6a).

Fig. 6 (a) Zeeman effect and spin–orbit coupling causing the singlet–triplet interconversion, reducing the recombination; (b) representation of how spin interconversion is helping product formation for the CO₂RR (reproduced with permission from the cited ref. 36).

An external magnetic field can cause the interconversion of two spin states of the intermediates, which determines the final product. Magnetic field-induced spin selectivity can promote the reaction efficiency due to the constrained or unconstrained spin state flipping.³⁷ Magnetic field-induced spin polarisation increases the transfer efficiency of the electronic spin by reducing the kinetic energy barrier.³⁸ The external magnetic field induces spin–orbit coupling, and the Zeeman effect helps in singlet-to-triplet interconversion. The Zeeman effect takes place when the orbital angular momentum of an electron interacts with an external magnetic field.

The radical ion-pair produced during the electrochemical reaction experiences Zeeman splitting, described by using eqn 3:

ΔE = gµ_BBm_s, (3)

where g is the Landé g-factor, µ_B is the Bohr magneton, B is the magnetic field strength and m_s is the magnetic moment of the charged particles.³⁹

This splitting can:

• Alter the spin alignment of radical pairs

• Promote singlet–triplet interconversion

• Reduce spin-forbidden recombination⁴⁰

• Enhance reaction selectivity and the reaction rate

As shown in Fig. 6b, the magnetic field helps in spin flipping from the singlet state to the triplet via the spin–orbit coupling state in the CO₂RR, which reduces recombination (spin-forbidden transition) and promotes product formation.³⁶

3.2. Influence on photocatalytic conversions

When photons fall on a photoactive material, an electron–hole pair is generated (Fig. 7a), and the photogenerated electron and hole participate in the reaction. The efficiency of a photochemical conversion mainly depends on (1) the recombination rate of the electron–hole pair, (2) the photostability of the catalyst and (3) the band gap of the semiconducting material. The magnetic field can influence the electron–hole recombination; hence, by applying an external magnetic field, the rate of a photocatalytic reaction can be improved.

Fig. 7 (a) Formation of an electron–hole pair upon light irradiation on a semiconducting photocatalyst, (b) direction of Lorentz force on the electron and hole, and (c) Jablonski diagram showing the singlet–triplet transition under a magnetic field.

3.2.1. Lorentz force. As described earlier, in the presence of an external magnetic field, the direction of the Lorentz force on positively and negatively charged particles is opposite to each other. As electrons and holes contain opposite charges, they experience the Lorentz force in opposite directions under a magnetic field (Fig. 7b). So, the electron–hole recombination gets delayed, which in turn helps in improving the photocatalytic efficiency.

Under a magnetic field, electrons and holes, being oppositely charged, experience Lorentz forces in opposite directions (as shown in Fig. 7b). This spatial separation reduces their overlap, delaying recombination and increasing the probability of charge transfer to surface reaction sites. The result is a prolonged excitonic lifetime and improved quantum efficiency.

3.2.2. Spin selectivity effect. The spin states of photoexcited charged particles (electrons and holes) experience the same force by the external magnetic field and undergo Zeeman splitting as discussed in detail in the previous section. This splitting arises due to the interaction between the magnetic field and the magnetic moment of the photo-generated electron and hole. During photochemical excitation, the electron moves from the ground singlet state to the excited singlet state. Then the excited singlet state electron takes part in the reaction to give the product, or the excited electron emits energy and reverts to the ground state. The singlet–singlet recombination is a nanosecond process, but if the electron from the excited singlet state can go to the excited triplet state through intersystem crossing (as shown in Fig. 7c), the recombination becomes slow, as the singlet–triplet recombination is a spin-forbidden process. The spin polarisation can be done by applying an external magnetic field, as reported earlier.⁴¹ An external magnetic field helps in spin flipping by increasing the spin–orbit coupling¹¹ and helps in delaying electron–hole recombination. Under a magnetic field, the excited state lifetime increases, as shown by Chen et al. in a semiconducting electron–hole transition in the MnCsPbBr₃ system.⁴¹ The delayed recombination increases the photocatalytic performance.⁴²

Due to the spin selectivity effect on the electrochemical and photocatalytic reactions, radical intermediates often form as spin-correlated pairs. Their recombination pathways are dictated by quantum spin dynamics, described by the radical pair Hamiltonian (eqn (4))^40,43

H = H_Z + H_HF + H_ex (4)

where H_Z = gµ_BB·S (Zeeman interaction), H_HF (hyperfine coupling), and H_ex (exchange interaction). The Zeeman effect induces (ΔE = g µ_BBm_s) singlet–triplet interconversion. This modifies spin state populations, increasing the lifetime of reactive triplet intermediates and decreasing spin-forbidden recombination.

Overall, the interplay of Zeeman, hyperfine, and exchange interactions under an external magnetic field governs spin-selective reaction pathways, facilitating singlet–triplet interconversion, prolonging reactive triplet lifetimes, and thereby enhancing both selectivity and efficiency in electrochemical and photocatalytic processes through spin-controlled kinetics.

3.3. Influence on thermocatalytic conversions

In the case of thermocatalytic conversion, magnetic hyperthermia plays a key role in enhancing the catalytic activity. However, enhanced mass transfer or electron density enhancement also plays a role.

3.3.1. Magnetic hyperthermia (localized heating). The effect of a magnetic field on the thermocatalytic reactions is mainly due to the magneto-thermal heating and can be explained by using the Arrhenius rate equation, as explained in the previous section. It is the role of magnetic local heating or magnetic hyperthermia in catalysis. According to the Arrhenius equation, k = Ae^−E_a/RT, with the increase in temperature, the rate of the reaction increases. Heating the whole solution consumes more heat, leading to energy loss and corrosion, whereas magnetic hyperthermia is energy efficient and works via local heating processes without increasing bulk solution temperature, reducing energy losses.

For magnetic heating, the Arrhenius rate equation (eqn (5)):

k = A exp(−E_a/RT_local), where T_local = T_bulk + ΔT_MF (5)

ΔT_MF is the temperature increase due to magnetic field-induced heating.⁴⁴ Even small ΔT_MF (20–50 °C) significantly boost reaction rates, especially for endothermic pathways such as reverse water–gas shift (RWGS).⁴⁵

3.3.2. Thermodynamic coupling and Gibbs free energy. Magnetic fields influence the activation free energy barrier.⁴⁶ The effective Gibbs free energy under an applied field becomes (eqn (6)):

ΔG^‡(B) = ΔG^‡(0) − ΔE_Zeeman − ΔQ_MHD (6)

ΔG^‡(B) is the activation free energy under magnetic field B, ΔG^‡(0) is the activation free energy in the absence of a magnetic field, ΔE_Zeeman is the energy change due to Zeeman splitting of spin states, and ΔQ_MHD is the contribution from magnetohydrodynamic (MHD) effects, such as spin polarisation, Lorentz force, or magnetic field-induced charge separation.

This modifies the Arrhenius rate constant (eqn (7)):

k(B) = A exp(−ΔG^‡(B)/RT) (7)

K(B) is the rate constant under magnetic field B, A is the pre-exponential factor (frequency of collisions between reactant molecules), ΔG^‡(B) is the activation free energy under a magnetic field, R is the universal gas constant, and T is the temperature in Kelvin.⁴⁷

Thus, the application of an external magnetic field effectively lowers the activation free energy barrier, thereby enhancing the Arrhenius rate constant and accelerating the overall reaction kinetics through combined Zeeman and magnetohydrodynamic (MHD) contributions.

3.3.3. Enhanced mass transfer in thermocatalytic reactions under an external magnetic field. In thermocatalytic reactions, magnetic fields markedly enhance reactant diffusion and mixing dynamics within the catalytic system. In thermocatalytic systems, the application of an external magnetic field induces magnetohydrodynamic (MHD) effects through the Lorentz force acting on moving charged species, enhancing micro-convection and fluid circulation within the reactor. These effects thin the concentration boundary layer around catalyst surfaces and promote efficient transport of reactive species.⁴⁸ Within porous and zeolitic catalysts, magnetic fields further modify molecular trajectories and induce magnetophoretic migration, enabling deeper penetration of reactants such as CO₂ and H₂ into active sites. The resulting improvement in mass transfer increases effective diffusivity (D_eff), enhances reactant–active site collision frequency, and elevates turnover rates and catalytic efficiency.⁴⁹

In thermocatalytic CO₂ conversion systems, such enhancement ensures uniform heat and reactant distribution, minimizes local mass-transfer limitations, and stabilizes reactive intermediates—collectively improving conversion efficiency and product selectivity under magnetic influence.⁵⁰ This phenomenon has also been reported in magnetically stabilized bed reactors, where the applied field reduces channelling and external film resistance, confirming the magnetic-field-induced intensification of mass and heat transfer in gas–solid thermocatalytic processes.⁵¹

3.3.4. Eddy currents and electron density enrichment. Under an external magnetic field, electromagnetic induction in metallic catalysts generates eddy currents that circulate near the surface due to the skin effect, enriching electron density at active sites. These localized surface currents facilitate faster electron transfer to adsorbed molecules, effectively lowering the activation energy of surface reactions and enhancing catalytic kinetics.^52,53 In thermocatalytic CO₂ conversion systems such as the Reverse Water–Gas Shift (RWGS) and Sabatier reactions, this electron enrichment accelerates rate-determining steps like CO₂ activation and hydrogenation, leading to higher activity and selectivity.⁵⁴

4. Role of different types of magnetic fields in the catalytic pathways

Irrespective of the nature of the magnetic field, the magnetic field influences the kinetics of water splitting and CO₂ conversion reactions, though the underlying mechanism may change depending on the type of magnetic field. Four different types of magnetic fields and their influence on the catalytic conversion of H₂O and CO₂ have been discussed in the following sections.

4.1. Alternating magnetic field

When an alternating magnetic field is applied to a magnetically active catalytic material, the catalyst gets heated by local heating or magnetic hyperthermia is created by the alternating magnetic field (AMF) (Fig. 8a). As discussed earlier, as per the Arrhenius equation of the rate constant, with every 10 °C rise in temperature, the reaction rate increases by a factor of two. With the increase in the rate constant, the overpotential requirement of the reaction decreases. As observed by Chatenet et al., magnetic nano-iron carbide coated with catalytically active metallic nickel was prepared for both the anode and cathode.⁵⁵ In the presence of a high-frequency alternating magnetic field of 48 mT, the overpotential at 20 mA cm⁻² decreases by 200 mV for the OER and 100 mV for the HER. In the presence of an alternating magnetic field, the FeC core of the nanoparticle undergoes magnetic hyperthermia, and the Ni shell favours the electrolysis of water.⁵⁶ The effect of alternating magnetic fields on the thermocatalytic and photocatalytic reactions has been explained in the subsequent sections.

Fig. 8 Representation of electrochemical processes under (a) alternating, (b) pre-magnetized,⁵⁶ (c) variable,⁵⁸ and (d) static magnetic fields (reprinted from the cited references).

4.2. Pre-magnetised field

A magnetically active material can retain the induced magnetisation (Fig. 8b) for some time due to the coercivity effect; after switching off the magnetic field, its effect on the electrochemical processes is retained. Pre-magnetised carbon florets decorated with Co₃O₄, Co, and Ni–Co ferro−paramagnetic nanoparticles can improve the H₂ generation by 2.5 times with ∼85% decrease in charge transfer resistance, and the kinetics of the process is increased by 650%.⁵⁷

4.3. Variable magnetic field

A magneto-electrochemical setup for a variable magnetic field (0–371 mT) has been developed (Fig. 8c) by Berlinguette et al., where the magnetic flux density was measured as a function of distance using a Hall probe.⁵⁸ In this study, magnetic flux density was calculated. There is an increase in current density of ∼4.7% at a magnetic field of 371 mT.⁵⁸

4.4. Static magnetic field

By applying a continuous magnetic field (Fig. 8d) on the anode and cathode material, electrochemical performances can be improved. As reported by José Ramón Galán-Mascarós et al., a Ni-foam electrode decorated with mixed metal oxide NiZnFe₄O_x shows an increment in current density above 100% in the presence of a magnetic field of ≤450 mT for water electrolysis in an alkaline medium.⁵⁹

5. Influence of a magnetic field on the electrochemical reactions

5.1. Water splitting

Both the half–cell reactions of water splitting, the HER and OER, can be influenced by applying an external magnetic field. The role of an external magnetic field in the water splitting process has been explained in this section. Four-electron transfer and metal−oxygen intermediate (M−O) processes lead to sluggish kinetics and large overpotential for the OER, consequently restricting overall water-splitting processes for hydrogen production. Generally, precious Ir or Ru-based catalysts are used to have excellent OER performance. For the reduction of OER overpotential and to accelerate the reaction kinetics, the use of transition metal (Fe, Ni, and Co) based catalysts with an external magnetic field can be a reasonable alternative to precious noble metal catalysts. The effects of magnetic field strength and direction on electrocatalysis have been investigated in detail by Yiyi Li et al., showing a practical way to improve the performance of catalysts efficiently with Co₃O₄/NF for the OER. Under a 125 mT external magnetic field, perpendicular to the electric field, an overpotential decrease of 252 mV at a current density of 20 mA cm⁻² and a lowering of Tafel slope up to 26.7 mV dec⁻¹ are observed due to the magnetohydrodynamic effect near the electrode.⁶⁰ Using noble-metal-free metal–organic framework (MOF) surfaces, Liu et al. addressed how orbital interactions between catalysts and intermediates and spin-related charge transfer can improve oxygen evolution reaction (OER) kinetics.⁶¹ This contribution helps to create effective, reasonably priced catalysts for the oxidation of water. The researchers manipulated spin electron configurations in Co₀.₈Mn₀.₂ metal–organic frameworks (MOFs) through an inexpensive magnetic stimulation technique.

This method uses a thermally differentiated superlattice, in which spin flips can occur at specific active locations through localized magnetic heating with periodic spatial distribution. This demonstrates a spin-dependent reaction pathway where the spin-rearranged Co_0.8Mn_0.2 MOF displays a mass activity of 3514.7 A g per metal with an overpotential of ∼0.27 V, which is 21.1 times that of the pristine MOF. These findings help to design a spin electrocatalyst with improved reaction kinetics.⁶¹

5.2. CO₂ conversion

Conversion of CO₂ to value-added products is a very recent and important research area, considering the sustainable goal. At present, the reaction suffers from low conversion efficiency, selectivity of product formation and overall energy efficiency. The magnetic field effect is also observed in the case of the electrochemical CO₂ reduction reaction. Taking formic acid as the product, the singlet radical pair configuration ([CO₂⁻˙↑⋯H˙↓]¹) is the favourable spin state for the CO₂ conversion, whereas the triplet radical pair ([CO₂⁻˙↑⋯H˙↑]³) is of unfavourable spin for the CO₂ reduction reaction. The applied magnetic field can promote the spin evolution of spin-related radicals from the triplet state to the singlet state, to promote the CO₂RR. It has been observed that the reduction current, Faraday efficiency, and formic acid yield increased simultaneously under the 0.9 T magnetic field. The role of the magnetic field was considered to transform the triplet [CO₂⁻˙↑⋯H˙↑]³ into the singlet [CO₂⁻˙↑⋯H˙↓]¹_, increasing the yield of formic acid.⁷²

A comparative electrochemical efficiency of different electrocatalysts with or without an applied magnetic field with respect to electrochemical water splitting & CO₂ conversion is presented in Table 2.

Table 2 Effect of the applied magnetic field on electrochemical water splitting & CO₂ conversion efficiency

Sr no.	Catalytic material	Processes	Magnetic field intensity	Reaction parameters without a MF	Reaction parameters with a MF	% improvement	Ref.
1	NiZnFe4Ox	HER	≤450 mT	Current density: 0.7 A cm^−2	Current density: >1 A cm^−2	40%	59
2	Co3O4, Co, and Ni–Co	HER	100 mT	Current density: 1.2 mA cm^−2, overpotential @ 10 mA cm^−2: 320 mV	Current density: 7.8 mA cm^−2, overpotential @ 10 mA cm^−2: 210 mV	650% increase in HER kinetics	57
3	FeC–Ni core–shell nanoparticles	OER	Alternating magnetic field	Current density: 1.65 V @ 20 mA cm^−2	Current density: 1.45 V @ 20 mA cm^−2	∼200 mV drop – (∼12%) of required potential	62
4	Co/Pt superlattice	HER	Magneto-optical effect	—	7% increase in current density	7%	63
5	Platinum with an area of 50 mm × 50 mm	HER	220 mT	13V@220 mA cm^−2	11.3 V@220 mA cm^−2	Conductivity increases 13.3%	64
6	CoOx|FTO anode	OER	371 ± 1 mT	—	4.7% increase in current density	4.7%	58
7	Co3O4/NF	OER	125 mT	η = 280 mV @ 20 mA cm^−2	η = 252 mV	10%	60
8	Co0.8Mn0.2 MOF	OER	Alternating MF	Mass activity: 166 A g^−1	Mass activity: 3515 A g^−1	2011%	61
9	Ni(OH)2, NiO, and Ni	OER	0–1.4 T	η = 320 mV @ 10 mA cm^−2	η = 300 mV	6.25%	65
10	CoFe2O4, Co3O4, and IrO2	OER	1 T	Tafel slope: 120 mV dec^−1	Tafel slope: 90 mV dec^−1	∼25% kinetic improvement	66
11	Metallic cobalt (Co) nanodots (∼10 nm) embedded within macroporous carbon nanofibers	ORR/OER	350 mT	Overpotential: 370 mV@ 10 mA cm^−2	Overpotential: 355 mV@ 10 mA cm^−2	4.05% improvement in OER performance	67
12	S-doped SnO2 nanoparticles	CO2RR	900 mT	1.4 V w.r.t RHE	1.1 V w.r.t RHE	Formic acid yield: increased by 99.9%	68
13	NiFeDAT	CO2RR	1.48 T	515 mA cm^−2 at 3V	565 mA cm^−2	9.7% improvement	69
14	Cu–ZnO/ZrO2 catalyst	CO2RR	20.8 mT	CO2 conversion: 12.5%	CO2 conversion 25.0%	50% enhancement in CO2 conversion	70
15	Iron-loaded bimodal mesoporous silica (Fe/MCM-41)	CO2RR	27.7 mT	CO2 conversion (%): 3.8	CO2 conversion (%): 6.8	79% enhancement in CO2 conversion	71

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5.3. Magnetic field effect on an electrolyser

Currently, electrochemical water splitting is projected to be the most feasible green hydrogen generation technique for large-scale production. Therefore, large-scale electrolysers are tested and commercially evaluated for their cost calculation and future practical utility. The role of the external magnetic field in the electrolyser system has been studied to explore its effect on large-scale H₂ generation. One such experiment has been done on a proton exchange membrane (PEM) electrolyser under an external magnetic field by Kaya et al. in 2019. The overall process involves little capital cost.⁷³ At lower water flow rates, around 33 to 56% cell performance improvement is achieved under a 0.5 T magnetic field. To explore magneto-electrocatalysis at a large scale for green H₂ generation, big projects like spin-polarised Catalysts for Energy-Efficient AEM Water Electrolysis by the EU Horizon have been ongoing from 2021 to 2025. The magnetic field can influence the CO₂ to CO conversion efficiency for the large-scale conversion of CO₂ to CO in the CO₂ electrolyser, too. As observed in a gas diffusion electrode-based flow electrolyser system, the power saving can be enhanced from 7% to 64%. The overall cell energy could be reduced by reducing the anode overpotential, using a bimetallic NiFe catalyst at the anode and carrying out the experiment under an external magnetic field. Paul J. A. Kenis et al. achieved CO₂ to CO conversion with an energy efficiency of 45%.

6. Influence of a magnetic field on the photochemical reactions

Photochemical reactions are a greener route for chemical transformation. However, they are associated with low quantum yield. Application of an external magnetic field can promote the photochemical reaction kinetics, and this has been established by many previous studies.^{11,12,74–79}

6.1. Water splitting

Tsang et al. showed a dramatic improvement in the overall photocatalytic performance for an Au-supported Fe₃O₄/N–TiO₂ superparamagnetic photocatalyst under a magnetic field. In the presence of an external weak magnetic field of 180 mT, a strong local magnetic field is induced, which promotes the quantum efficiency up to 88.7% at 437 nm at 270 °C. The theoretical calculation and lifetime study established that the increase in activity was mainly due to the Lorentz force and spin-polarisation which induced a prolonged excitonic lifetime. By varying the local magnetic field, a hydrogen efficiency of 11.9 ± 0.5% and energy efficiency of 1.16 ± 0.05% were achieved under solar light for overall water splitting under AM 1.5 G simulated solar illumination⁸⁰

6.2. CO₂ conversion

Along with the magnetic field effect on photocatalytic water splitting performance, the photocatalytic CO₂ reduction pathway can also be altered by applying an external magnetic field. Liu et al. showed the importance of an external alternating magnetic field for promoting photocatalytic CO₂ reduction. Cu/Cu₂O/Ni(OH)₂/NF exhibited alternating magnetic field-enhanced photocatalytic conversion of CO₂ to CH₄ with 96.1% selectivity. The yield under a magnetic field is 11 and 6 times higher than that for photocatalysis.⁸¹

6.3. Large photochemical reactor

Being a green and simple process, photocatalytic reactions are being explored for large-scale H₂ generation via water splitting. The following figure shows that research has already been initiated with panel reactors (Fig. 9) to upscale the photocatalytic water splitting for large-scale applications as explained in detail elsewhere.⁸²

Fig. 9 Photocatalytic water splitting and CO₂ conversion to methane in a large panel reactor setup. Reprinted with permission from Yamada et al.⁸³

CO₂ reforming for large-scale photocatalytic conversion is under exploration. Large-scale photocatalytic reactions have so far been performed without the influence of a magnetic field,⁸³ and this presents a promising opportunity to investigate the effects of an external magnetic field on large photocatalytic reactors.

7. Influence of a magnetic field on the photo-electrochemical reactions

Photo-electrochemical reactions are carried out with photonic energy, followed by an electrical potential. Initially, the photoactive material/semiconducting catalyst absorbs photonic energy and electron holes are generated. The electrical potential helps in the separation of the electron and hole and migration of the charge carrier to the electrode–electrolyte interface. Spin-manipulated photo-electrochemical water splitting using Janus MoSSe/GaN heterostructures under an external low magnetic field has been observed by Kumar et al.⁸⁴ 2D-MXene (Ti₃C₂T_x/MoSSe/GaN) manifests ∼1.37 times photocurrent enhancement and ∼1.50-fold enhancement in product (H₂/O₂) formation under a low applied magnetic field (0.4 T).⁸⁴ A superior photoelectrochemical response has been observed in NiO and Co₃O₄ nanocatalysts under a magnetic field and the highest photocurrent density of 0.12 and 0.55 mA cm⁻² have been achieved under the magnetic field.⁸⁶ Three times enhancement in photocurrent has been achieved due to the multi-field coupling-assisted PEC water splitting system.⁸⁷ 55.40% and 43.22% enhancement was observed in photocurrent density in FeCoSe₂.⁸⁸ Two-dimensional polar MoSSe intensifies the possibility of (Fig. 10) spin-dependent photo-excited charge transfer for efficient catalysis under an external magnetic field. Due to the synergic effect, the optimized Mo₂CT_x/MoSSe/SiNW photocathode shows a 52% increase in the photocurrent under a 0.4 T magnetic field at zero bias.⁸⁹

Fig. 10 (a) Schematic of extended carrier lifetime via spin polarization in Mn–CsPbBr₃ under a magnetic field, (b) spin-polarized density of states (DOS) of Mn–CsPbBr₃; the inset shows real-space charge distribution and (c) the normalized photoinduced transient reflectivity changes (ΔR/R) under a magnetic field. (Reprinted from Chang Chien et al.)⁸⁵

Doping magnetic elements like Mn into CsPbBr₃ can enhance spin-polarized electron generation (Fig. 10a), improving photocatalytic CO₂ reduction. Lin et al. showed that Mn-doped CsPbBr₃ exhibited asymmetric spin-polarized DOS (Fig. 10b) and significantly higher (∼6 times) CO and CH₄ yields than undoped CsPbBr₃, due to longer carrier lifetimes (Fig. 10c) and reduced recombination. Under a 300 mT magnetic field, Mn–CsPbBr₃ exhibited superior photocatalytic CO₂ reduction performance. This was due to Zeeman-enhanced spin polarisation, which increased spin-polarised charge carriers and suppressed electron–hole recombination.⁸⁵ The effect of the external magnetic field on photocatalytic and photo-electrocatalytic water splitting and CO₂ conversion efficiency is presented in Table 3. It is observed that the H₂ evolution rate is the maximum on the Au-supported Fe₃O₄/Ns-TiO₂ catalyst. Of course, the applied magnetic field intensity is also the maximum in this case compared to other reported results. Due to the availability of only a few reported results, it is difficult to conclude the importance of magnetic field intensity and the catalyst parameters. So, it is required to investigate a large number of catalysts under an applied magnetic field with varying intensities towards water splitting and CO₂ conversion, so as to have a deeper insight into the process mechanism and advantages.

Table 3 Effect of a magnetic field on the water splitting & CO₂ conversion efficiency of different photocatalysts & photo-electrocatalysts

Sr no.	Catalytic material	Processes	Magnetic field intensity	Without a MF	With a MF	% Improvement	Ref.
1	Au-supported Fe3O4/Ns-TiO2	Photocatalytic overall water splitting (POWS)	180 mT	H2 evolution rate: 7600 µmol g^−1 h^−1	H2 evolution rate: 21 230 µmol g^−1 h^−1	180%	80
2	CoFe2O4–BiFeO3 core–shell nanoparticles	H2 evolution	22.3 mT, 1.19 kHz AC magnetic field	H2 evolution: 7.5 (µM g^−1)	H2 evolution: 17.5 (µM g^−1)	133%	93
3	α-Fe2O3/rGO	O2 evolution	1 T	Photocurrent density: 6.2 µA cm^−2	Photocurrent density: 12.5 µA cm^−2	102%	94
4	NiO/TiO2	CO2 to CH4	Alternating magnetic field (∼5 mT at a frequency of 60 Hz)	CO2 to CH4: 13.2 µmol g^−1 h^−1	CO2 to CH4: 41.3 µmol g^−1 h^−1	213%	95
5	TiO2 nanotubes	Conversion of CO2 to C2H5OH under visible light	1000 Gs	CO2 to C2H5OH conversion: 6.63%	CO2 to C2H5OH conversion: 66.71%	Conversion efficiency: 910%	96
				Ethanol yield: ∼0.28 µmol g^−1 h^−1	Ethanol yield: 6.16 µmol g^−1 h^−1	Yield: 2100%

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8. Thermal CO₂ conversion under a magnetic field

In this section, the role of an external magnetic field in the thermal pathways of conversion of CO₂ to value-added chemicals has been described. There are two major thermal pathways, like the hydrogenation reaction (Sabatier reaction and reverse water gas shift reaction) & reforming reaction (like dry reforming) for CO₂ conversion (as presented in Table 4). The role of magnetic heating in classical heating has also been discussed (Fig. 11a and b).

Table 4 Different thermochemical CO₂ conversion pathways

Sr. no.	Thermal processes for CO2 conversion	Reaction
1	Reverse water–gas shift (RWGS) reaction (hydrogenation)	CO2 + H2 ⇌ CO + H2O, ΔH° = 41 kJ mol^−1
2	Sabatier reaction (hydrogenation)	CO + 4H2 ⇌ CH4 + H2O, ΔH° = −165 kJ mol^−1
3	Dry reforming	CH4 + CO2 ⇌ 2CO + 2H2, ΔH° = 247.3 kJ mol^−1

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Fig. 11 (a and b) Comparison between conventional heating & induction heating.^92,101,102

8.1. Hydrogenation reactions (RWGS & Sabatier reaction)

A magnetic field can influence the hydrogenation reaction of CO₂. Chareonpanich et al. established the role of an external magnetic field in catalytic CO hydrogenation.⁹⁰ The reaction was carried out in a conventional thermal reactor with a bimetallic 5Fe–5Co/ZSM-5 molecular sieve catalyst. Under a magnetic field of −20.8 and −25.1 mT, the CO conversion efficiency is increased by 1.9 times, and the selectivity of CH₄ formation is increased by 1.3 times. The magnetic field helps in mass transfer inside the zeolite cavities and promotes diffusivity & reactivity.⁹⁰ Another study by Chaudret et al. compared the activity of CO₂ conversion to CH₄ with the Sabatier process on a supported nickel catalyst (5 wt% Ni supported on titania) under classical heating conditions and magnetically activated catalysis by using iron wool as a heating agent.⁹¹ The results show that under an applied magnetic field, the CO₂ conversion reached up to ∼85% at ∼335 °C, whereas by only thermal heating, even at ∼380 °C, a conversion efficiency of only ∼78% can be achieved. The improvement in catalytic activity at a low temperature in the presence of a magnetic field is mostly due to the generation of local hotspots or catalytically active sites and better heat distribution under induction heating. The induction heating was carried out for up to 45 hours to test the long-term endurance of the process, which could help in understanding the potential of induction heating for pilot-scale reactor development.⁹¹ Further study is on the role of an external magnetic field in another CO₂ hydrogeneration process, that is, the reverse water gas shift reaction (RWGS). A study by Bruno Chaudret et al. has shown that electromagnetic induction heating can boost the selective hydrogenation of CO₂ to CO on Fe fibre-based catalysts. With a 3% Ru/Fe catalyst, the CO₂ conversion temperature decreases by 100 °C under an applied magnetic field. The enhancement in catalytic activity is attributed to the electromagnetic field-induced eddy current.

This results in the enrichment of electron density on the catalytic surface and goes via HCOOH-intermediate formation. The HCOOH intermediate-based pathway has a lower energy barrier than the redox pathway of traditional heating.⁹² Overall, these encouraging results demand more systematic investigation to draw a meaningful conclusion for future application.

8.2. Dry reforming

Dry reforming is the process of forming syngas from CO₂. As reported by Sangregorio et al., the activation of the catalytic dry reforming process is enhanced by induction heating.⁹⁷ The reaction is carried out by using a radiofrequency alternating magnetic field. The alternating magnetic field is applied to a magnetic catalyst (Ni₆₀Co₄₀ alloy) to induce heat to carry out the reaction. The Ni₆₀Co₄₀ alloy also acts as a catalyst for the dry reforming reaction. Hence, in this case, the heat is directly transferred by the catalyst to the reactant gases, which decreases the heat loss by heat dissipation.

Moreover, high heating rates have been achieved by magnetic induction (200 °C min⁻¹), which helps to reach the required temperature (800–1000 °C) fast and decreases the chance of carbon deposition due to methane cracking (takes place at ∼600–700 °C).⁹⁷ Other reports also show the very optimistic effect of the magnetic field on the steam reforming of methane and dry reforming with different magnetic and non-magnetic catalysts.^101–103 The activities of various thermocatalysts on CO₂ conversion under a magnetic field are presented in Table 5.

Table 5 Activities of the thermocatalysts on CO₂ conversion under a magnetic field

Sr. no.	Catalytic material	Process	Magnetic field intensity	Without a magnetic field	With a magnetic field	% Improvement	Ref.
1	5Fe–5Co/ZSM-5 molecular sieve	CO hydrogenation	20.8 to 25.1 mT	CO conversion (%): 35	CO conversion (%): 66	Conversion: 87 (%)	90
				CH4 selectivity (%): 60	CH4 selectivity (%): 78	Selectivity: 30 (%)
2	Cu–ZnO/ZrO2	CO2 hydrogenation	27.7 mT	CO2 conversion (%): 10.5	CO2 conversion (%): 28.0	Conversion: 167%	70
3	Fe–Cu bimetallic	CO2 hydrogenation to hydrocarbon	28.91–58.59 mT	CO2 to (C2 + C3) hydrocarbons: 0.015 mol%	CO2 to (C2 + C3) hydrocarbons: 0.53 mol%	Conversion: 3400%	98
4	Core-shell ferrite nanoparticles (CoFe2O4 and CoFe2O4–Fe3O4)	CO2 to CH4	55 mT	CO2 conversion: 71%	CO2 conversion: 100%	CO2 conversion: 43%	99
				CH4 selectivity: 65%	CH4 selectivity: 100%	CH4 selectivity: 54%
5	CoFe2O4 and CoFe2O4–Fe3O4 nanoparticles	CH4 synthesis by CO2 hydrogenation	0.45 T	CO2 decomposition ratio (α): ∼12%	CO2 decomposition ratio (α): ∼20%	CO2 decomposition: 67%	100
				CH4 selectivity (β): ∼30%	CH4 selectivity (β): ∼60%	CH4 selectivity: 50%
6	3% Ru/Fe	CO2 into CO reverse water–gas shift (RWGS)	—	Temperature for ∼40% CO2 conversion: ∼500 °C	Temperature for ∼40% CO2 conversion: ∼400 °C	Reduction in temperature requirement: 20%	92
7	5 wt% Ni supported on titania	Sabatier process, CO2 to CH4 conversion	25 mT	CO2 conversion: ∼78% at 380 °C	CO2 conversion: ∼85% at 335 °C	CO2 conversion: 9% at lower temperature	91

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9. A comparative analysis between magnetic and non-magnetic systems across electro-, photo-, and thermo-catalytic pathways

As summarized in Table 6, both magnetic and non-magnetic catalysts exhibit performance gains under external magnetic fields, but the underlying mechanisms differ. Non-magnetic systems primarily benefit from mass transport enhancements (via magnetohydrodynamic convection) and minor Lorentz effects. In contrast, magnetic catalysts experience synergistic advantages: in addition to transport and Lorentz effects, they exhibit Maxwell stress-driven bubble release, spin-selective kinetics, and magnetic hyperthermia under AC fields, which together produce significantly larger activity gains. These distinctions underscore why magnetic catalysts often outperform non-magnetic analogs under identical magnetic field conditions.^104–106 As mentioned in the previous section.

Table 6 The comparative table of the magnetic field effect for magnetic and non-magnetic catalysts with respect to different mechanisms

Mechanism	Magnetic systems (ferro/ferrimagnetic catalysts and magnetic composites)	Non-magnetic systems (noble metals, TiO2, BiVO4, g-C3N4, and carbon-based)
Mass transport (MHD)	j × B convection thins the diffusion layer and accelerates bubble detachment; reported current gains up to 30–40% in NiFe- and Co-based oxides^107	Beneficial but weaker (∼5–20% improvement), primarily from electrolyte convection; e.g., TiO2 photocatalysis improved ∼24% under 0.28 T^108
Lorentz force	Alters charge-carrier trajectories and improves electron extraction in porous magnetic electrodes^109	Present but subtle; generally indistinguishable from MHD effects^110
Maxwell stress	∇B couples with magnetic domains and paramagnetic O2, strongly enhancing bubble detachment and reducing blockage during the OER^111	Weaker, limited to O2 paramagnetism; modest bubble-release enhancement^112
Spin selectivity (Zeeman)	Spin-polarized bands in ferromagnetic semiconductors reduce OER onset potentials, lower Tafel slopes, and improve CO2RR selectivity^113	Minimal spin leverage; improvements in activity dominated by transport; exceptions exist in chiral or spin-filtering oxides^114
Magnetic hyperthermia (induction)	Under AC B-fields, ferro/ferrimagnetic nanoparticles act as local heat sources, reducing bulk temperature requirements for CO2 methanation by 50–100 °C while maintaining high conversion^115	Not applicable; no magnetic losses → no induction response
Thermodynamic coupling & Gibbs free energy (ΔG and ΔG^‡)	External B does not change reaction equilibrium ΔG° for non-magnetic reactions; observed enhancements arise from kinetic effects (lower ΔG‡) via spin-selective pathways and localized T rise from induction (magnetic induction heating, MIH), which effectively increases rate constants k ∝ e^−ΔG^‡/RTlocal and can shift apparent selectivity (pathway-dependent). In magnetic materials, a magnetic work term (−M·B) can slightly bias energetic landscapes, but the contribution is typically small vs. thermal/spin effects^116	Magnetic field does not alter ΔG°; improvements are apparent/kinetic (transport, modest ΔG‡ via charge-separation in PEC or photochemical systems). No MIH; thus no field-driven thermally localized lowering of effective barriers^117

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9.1. Electrocatalysis

Both magnetic and non-magnetic systems benefit from the Lorentz force and magnetohydrodynamic (MHD) effects, which enhance mass transport and bubble detachment. However, intrinsic magnetic catalysts (e.g., Fe, Co, and Ni-based oxides) show additional improvements due to magnetic ordering and spin-polarisation effects, leading to higher current densities and reduced overpotentials compared to non-magnetic analogues.

9.2. Photocatalysis

Non-magnetic semiconductors (TiO₂, BiVO₄, and g-C₃N₄) primarily benefit from delayed electron–hole recombination under a magnetic field via Lorentz force-induced charge separation. In contrast, magnetic photocatalysts (Fe₃O₄/TiO₂ composites and Mn-doped perovskites) show not only this benefit but also spin-selectivity and Zeeman splitting effects, resulting in significantly prolonged exciton lifetimes and enhanced hydrogen or CO₂ reduction yields.

9.3. Thermocatalysis

Non-magnetic catalysts experience negligible direct field effects, relying only on bulk heating. Magnetic catalysts, however, enable magnetic hyperthermia/induction heating, providing localized nanoscale hotspots that lower the apparent activation energy and improve CO₂ hydrogenation and methanation efficiency. For instance, Ru/Fe and Ni–Co alloys under induction heating achieved higher conversion rates at 50–100 °C lower temperatures than comparable non-magnetic catalysts under conventional heating.

A comparative summary of how magnetic and non-magnetic catalysts respond to external magnetic fields across electro-, photo-, and thermocatalytic processes is presented in Table 6. The analysis highlights the universal benefits (e.g., magnetohydrodynamic convection and Lorentz force effects) as well as the unique enhancements enabled only in magnetic systems (e.g., spin-selectivity and magnetic hyperthermia).

It can be concluded that intrinsic catalysts synergistically improve the efficiency due to several mechanistically favourable conditions under an applied magnetic field. Therefore, to utilise the advantage of an external magnetic field, one has to look into an appropriate catalyst with an intrinsic magnetic property.

10. Conclusion and future direction

Magnetic field-assisted kinetic improvement of different catalytic processes for two major reactions, water splitting and CO₂ conversion to achieve carbon neutrality, has been discussed in this review. A comprehensive discussion has been attempted to cover the role of an external magnetic field in lowering the energy input (electrical/photonic/thermal) required for various chemical processes, including photocatalysis, the HER, the OER, the ORR, and the CO₂RR. The magneto-catalysis approach proved to be beneficial in improving reaction efficiency, activity, and selectivity, in comparison to conventional catalysis for all the above reactions. However, to date, this remains restricted to laboratory-scale research and requires a proper understanding of the process, development of the desired catalyst and additional effort to scale it up through an appropriate engineering approach. A magnetic field can directly influence the electron–hole recombination during the photocatalytic reaction, resulting in delayed recombination and an improved rate of reaction. With an appropriate catalyst, it is possible to increase the photocatalytic hydrogen generation and CO₂ conversion to 280% and 213% respectively, under an applied magnetic field. In this review, mainly the roles of the Lorentz force effect, the MDH effect, the spin selectivity effect, and Arrhenius heating have been explained, though another inner lying mechanism may also be involved, which needs further exploration or detailed understanding. Until now, substantial progress has been made in exploring the mechanism and its effect. To derive the maximum benefit of the applied magnetic field, appropriate and stable catalyst development is important. Hence, there is scope to explore different electrocatalysts with inherited magnetic properties like ferromagnetism, paramagnetism, ferrimagnetism, antiferromagnetism, and superparamagnetism. External magnetic field strength and applied direction sufficiently influence the electro–catalysis reaction due to the direct effect on electron mobility. With the application of a magnetic field, electrocatalytic hydrogen generation efficiency can be increased to 650% while CO₂ conversion can be increased to 180%. It is well concluded from the available studies that the static magnetic field gives better results and is easier to scale up in comparison to the variable magnetic field. The effect of intrinsic and non-intrinsic magnetic catalysts under external fields on the catalytic efficiency is a subject of interest. Therefore, it is imperative to choose an intrinsic magnetic catalyst so as to derive the advantage of an external magnetic field for all types of discussed catalytic reactions. Future progress and practical application of this process depend on the proper catalysts development and device fabrication for large-scale H₂ production or CO₂ conversion under an external magnetic field. Therefore, further research needs to be accelerated to bring the cost-effective & feasible technology to an industrial scale. The present discussion regarding the magnetic field effect on decreasing the energy input can help to build the future for large-scale application of the applied magnetic field as a green approach. Some research with magnetic field-assisted PEM & alkaline electrolyzers has already been initiated & preliminary results show positive outcomes with increased efficiency. In this regard, catalyst design and composition are of utmost importance with proper magnetic field application. Systematic optimisation of several magnetic–field parameters is crucial for establishing a quantitative correlation between the magnetic field and the behaviour to provide theoretical guidance is important and needs attention to improve the activity. Another aspect that requires control and investigation before scaling up the process is the thermal effect associated with magneto-catalysis. Electrode and catalyst stability under a strong magnetic field is another area of concern and needs proper evaluation. Still, there is a need to have an advanced characterization technique to study the in situ reaction intermediates and the exact interfacial behaviour under a magnetic field to improve each stage of the technology in the future. Along with the mentioned reactions, many other useful reactions, like the nitrogen reduction reaction to produce NH₃, energy storage systems and electrodeposition, are useful reactions that need to be explored. Some recent studies on the effect of chirality on magnetic field-enhanced electrocatalysis are being conducted, which may open up many exciting opportunities.^114,118 Currently, the industrial application of magneto-photocatalytic technology faces some challenges, primarily due to slow kinetics. Improvements in these areas with breakthrough technologies are essential for better industrial viability. This comprehensive review has given an insight that magneto-catalysis has potential in reducing the operational costs (OPEX) and reducing the electrical energy consumption for H₂ generation & CO₂ conversion when implemented on a large scale. Reduction in grid power consumption will also lead to a reduction in CO₂ emissions, which in turn helps the CO₂-intensive industries like steel and cement sectors, which are looking for sustainable solutions through green fuel production.

Conflicts of interest

There is no conflict to declare.

Data availability

No primary research results, software or code have been included and no new data were generated or analyzed as part of this review.

Acknowledgements

The authors would like to express their sincere gratitude to Prof Em. Seshadri Seetharaman (Royal Institute of Technology, Stockholm, Sweden) and Mr Siddhartha Misra (Process Research, R&D Tata Steel), for their valuable suggestions & comments in improving the manuscript. The authors are also grateful to the management of Tata Steel Ltd & director of CSIR-NML for granting permission to publish this work.

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