Electron Transfer, Proton Transfer, and Molecule Transfer: Chemical Engineering and beyond

1. Introduction

Nowadays, we are standing at a historic inflection point driven by two major revolutions. One is the energy revolution driven by “carbon peak and carbon neutrality” goals and the rise of renewable energy. The other is the information revolution driven by booming artificial intelligence (AI). These two revolutions are promoting chemical processes from the era of matter-centeredness to an era of matter–energy coupling, and further toward an era of matter-energy- information coupling. In this context, chemical engineering, by integrating multidiscipline knowledge, is entering into “new-quality chemical engineering”. Compared with “green chemical engineering”, “new-quality chemical engineering” has three distinct features: green energy, advanced materials, and AI. For instance, the green energy replaces conventional energies; advanced materials, like metal–organic frameworks (MOFs), covalent organic frameworks (COFs), quantum dots, single atoms, and microdroplets, play more important role; AI accelerates the transition from green chemical engineering to “new-quality chemical engineering”. Moreover, the feedstocks shift from fossil resources to renewable carbon resources; the processes move in the direction of intelligence and sustainability; and the products become more diverse and highly value-added. All bring about significant and even disruptive changes in a variety of fields beyond chemical engineering, such as photo/electrochemical energy conversion, hydrogen energy, emerging energy storage, green biomanufacturing, microfluidic chemical engineering, and single-atom catalysis.

For “new-quality chemical engineering”, the research focus is shifting from macroscopic to microscopic scale, from continuum description to continuum-discrete coexistence, and from conceptual description to precise quantitative description. More importantly, microscopic particles such as electrons, protons, molecules (ions), photons, and waves are universally involved in the reaction processes of “new-quality chemical engineering”. Among these, the representative particles are electrons, protons, and molecules, where the electron, the smallest negatively charged particle, is associated with energy, the proton, the smallest positively charged particle, associates with both matter and energy, and molecules (charged (ions) or neutral) are substrates, intermediates, or products associated with matter. These three types of particles maintain their own independence while exhibiting strong coupling relationships in “new-quality chemical engineering”, thereby governing the efficiency of matter–energy coupling.

Since the 1950s, momentum transfer, heat transfer, and mass transferthe “Classic Three Transfers”have been recognized as the cornerstone of chemical engineering. Meanwhile, “Classic Three Transfers” theory has some inherent restrictions in describing the microscopic particle behaviors in “new-quality chemical engineering” processes. Fortunately, the development of advanced materials has made it possible to manipulate the electron transfer, proton transfer, and molecule transfer at atomic/molecular scale. These materials not only serve as platforms for decoupling the electron transfer, proton transfer, and molecule transfer, but also provide physical and chemical microenvironments where transfer phenomena can be synergistically regulated. Also, thanks to advanced characterization techniques with high spatiotemporal resolution and multiple dimensions as well as powerful computational simulation capabilities, the understanding of electron transfer, proton transfer, and molecule transfer is greatly facilitated. The three transferselectron transfer, proton transfer, and molecule transferas representatives among the myriads of microscopic particle transfers and as the basis of matter–energy coupling, are defined as the “New Three Transfers”. Revealing the intrinsic laws of “New Three Transfers” not only extends and complements the “Classic Three Transfers” theory at the microscopic scale, but also captures the transfer behaviors of various microscopic particles within matter–energy coupling. (Figure ). Research on the “New Three Transfers” is of great significance for the development of “new-quality chemical engineering” and undoubtedly radiate to many other disciplines.

1.

Relationship network between the “New Three Transfers” and matter transfer/energy conversion.

As green energy is increasingly used in chemical industrial processes, the matter–energy coupling at the microscopic scale has attracted increased attention and deserves in-depth investigation. With research going into the transfer behaviors of electrons, protons, and molecules, as well as inspiration from natural phenomena such as photosynthesis, this Viewpoint presents the theoretical framework of “New Three Transfers”. It integrates electron, proton, and molecule transfer, highlighting that the synergistic regulation of these three transfers is the core of achieving high efficiency in “new-quality chemical engineering”.

2. Features of “New Three Transfers”

2.1. Understanding of the “New Three Transfers”

2.1.1. Electron Transfer

Upon excitation by energy or matter interaction, electrons become free carriers and subsequently migrate through media driven by electric fields or electrochemical potential gradients. Their transfer velocity depends on carrier mobility and field intensity. Electrons are stored as chemical or electrical energy and utilized in processes such as water electrolysis and fuel cells. The scenarios of electron transfer (Figure ) can be classified as follows: (1) single-electron or multielectron transfer, based on the number of electrons involved; (2) short-distance transfer mediated by orbital overlap or quantum tunneling or long-distance transfer facilitated by conductive pathways such as molecular wires, based on the transfer distance; (3) bulk or interfacial transfer, where the efficiency is dominated by the material conductivity or interfacial potential; and (4) independent or coupled transfer, such as proton-coupled electron transfer (PCET), based on the coupling form. Currently, electron lifetime is commonly used to reflect the electron transfer dynamics, which can be measured by techniques such as ultrafast freeze-quenching, time-resolved X-ray absorption spectroscopy, ultrafast spectroscopy, and attosecond photoelectron interferometry. Accordingly, electron transfer events from milliseconds to attoseconds can be detected, including intercluster electron transfer in biomacromolecules, intermolecular photoelectron transfer, intramolecular electron motion, as well as the dynamics of valence, conduction, and core electrons during chemical bond breaking.

2.

Main scenarios of the “New Three Transfers”.

2.1.2. Proton Transfer

Protons are released via acid dissociation or electrochemical reactions and transferred within media driven by their electrochemical potential gradient. The proton transfer velocity depends on proton mobility and the conduction mechanism. Protons can be stored by bonding in compounds, by potential gradients, or by adsorption in porous materials. These protons could be used in energy conversion, catalysis, and pH regulation. The scenarios of proton transfer are relatively complex (Figure ). In water, protons move quickly via a relay mechanism along hydrogen bonds, known as the Grotthuss mechanism. In contrast, the vehicle mechanism involves slower diffusion of proton-carrying water clusters. At solid–liquid interfaces, confinement effect alters proton dynamics. Fixed ionic groups can attract protons and increase local proton concentration, thereby reorganizing local hydration structures and stabilizing stronger (shorter) hydrogen bonds. Moreover, proton transfer is central to energy conversion in nature; in mitochondria, electron transport pumps protons to create a gradient, which then drives ATP synthesis. Currently, proton conductivity (σ, S cm^–1) is commonly used to reflect the proton transfer, primarily measured by electrochemical impedance spectroscopy. The emerging of proton-conducting materials such as MOFs, COFs, and HOFs has made proton conductivity increase by 10⁶-fold, approaching 10⁰ S cm^–1. Proton transfer kinetics and timescales are obtainable through NMR and transient absorption spectroscopy.

2.1.3. Molecule Transfer

Molecule transfer occurs in various media through diffusion or convection, driven by chemical potential, concentration, pressure gradients, etc. The velocity for molecule transfer is determined by diffusion coefficients and flow velocity. Molecules can be retained or stored in a region through chemical bonding, surface adsorption, dissolution into the medium, or biological uptake/integration. These molecules are utilized in energy conversion, biological systems, industrial manufacturing, and environmental processes. The scenarios of molecule transfer can be classified by driving force and spatial scale (Figure ). In terms of driving force, active diffusion is characterized by the movement of molecules against chemical potential gradient, which requires energy input. In contrast, passive diffusion proceeds downhill, driven solely by thermal energy and chemical potential gradient. In terms of spatial scale, bulk diffusion occurs where the characteristic length far exceeds molecular size and mean free path, with negligible boundary effects. Conversely, confined diffusion occurs in confined space where boundary interactions dominate. Currently, the diffusion coefficient D (cm²/s) is commonly used to reflect molecular transfer (diffusion) behavior, primarily measured by techniques such as gas diffusion coefficient test apparatus, zero-length chromatography, frequency response method, and rapid time-resolved infrared spectroscopy. The diffusion coefficient of molecules in gas, liquid, and solid phases, as well as at material interfaces and within pores, can generally vary over a range of approximately 10^–12 to 10^–1 cm²/s.

2.2. Theories Evolution of “New Three Transfers”

The “New Three Transfers” have been studied for a long time. For electron transfer, key developments include Faraday’s law, Marcus theory of electron transfer reactions, and studies of electron transfer in biological systems and molecular chains. Research has moved from short-lived local events to long-range continuous transfer. For proton transfer, key developments include the Grotthuss mechanism, Brønsted–Lowry theory, the vehicle mechanism, and the role of hydrogen-bond networks. For molecule transfer, since the introduction of Fick’s law in 1855, the understanding has matured significantly through theories describing gas diffusion dynamics, diffusion in multicomponent systems (Maxwell–Stefan equations), and diffusion in liquids/fluids (Stokes–Einstein model).

2.3. A Preliminary Universal Model for the “New Three Transfers”

Even though the mechanisms differ, based on nonequilibrium thermodynamics, the transfer rates of electrons, protons, and molecules can be described by a unified equation:

$$v_i=-{\mu }_i·{\mathrm{\nabla }F}_i$$ 1

where v _i is the average transfer velocity, ∇F _i is the gradient of the driving force, and μ _i is the corresponding diffusion coefficient or mobility.

2.3.1. Equation for Electron Transfer

For electron transfer, ∇F _i corresponds to the electrochemical potential gradient. Based on the carrier mobility formula, the electron transfer velocity can be derived as eq :

$$v_i={-\mu }_i\mathrm{\nabla }\varphi$$ 2

where μ _i is the electron mobility. Theoretically, for metals, μ = qτ/m _e , where q is the magnitude of the electron charge, τ is the average free time (the time between two scattering events for electrons), and m _e is the electron mass. For semiconductors, m _e is corrected to the effective electron mass m*. The mobility can be measured through the Hall effect, or the field-effect transistor (FET) method.

2.3.2. Equation for Proton Transfer

For electric field-dependent proton transfer such as in proton exchange membranes, according to the Nernst–Planck equation, neglecting diffusion, the value of transfer velocity can be approximated by eq :

$$v_i=\frac{N_{{\mathrm{H}}^{+}}}{c_{{\mathrm{H}}^{+}}}\approx -{\mu }_i·\mathrm{\nabla }\varphi$$ 3

The apparent mobility μ can be derived from conductivity σ of the membrane via electrochemical impedance spectroscopy. Furthermore, the potential gradient ∇ϕ is calculated from ∇ϕ = V/L, where V is the voltage either directly applied across the membrane or measured with a high-impedance voltmeter, and L is the membrane thickness.

2.3.3. Equation for Molecule Transfer

For molecule transfer, ∇F _i is the chemical potential gradient. Under isothermal condition, it simplifies to RT∇ ln a _i (where a _i is the activity of component i). The diffusion coefficient D _i (which determines the mobility μ _i ) depends strongly on the medium and molecule properties. The velocity can be described by the Maxwell–Stefan equations (eqs and ).

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$$-\mathrm{\nabla }{\mu }_i+F_i=RT\sum _{j=1,j\ne i}^n\frac{x_j}{D_{ij}}({\upsilon }_i-{\upsilon }_j)$$

$${\upsilon }_i=-D_{ij}\frac{\mathrm{\nabla }{x}_i}{x_i}$$ 5

where ∇μ _i is the chemical potential gradient of i; R is the ideal gas constant; T is the thermodynamic temperature of the mixture; F _i is the external force acting per mole of component i; x _j is the mole fraction of component j; n is the number of components in the mixture; D _ij is the binary diffusion coefficient between components i and j; v _i or j is the diffusion velocity of component i or j. The diffusion coefficient of molecule transfer can be obtained experimentally or computationally. The chemical potential gradient can be determined indirectly by measuring concentration gradients or activity coefficients.

More importantly, machine learning, particularly physics-informed neural networks (PINNs), provides a feasible pathway to correlate characterization data, calibrate model parameters, and accelerate predictions across scales. High-throughput calculations or experiments generate datasets of transfer parameters. Neural networks can be trained to directly predict k _ET, k _PT, D for new materials. PINNs enforce both data fitting and physical laws. The physical laws are expressed by the “New Three Transfers” framework.

3. “New Three Transfers” in Natural and Non-Natural Processes

3.1. “New Three Transfers” in Natural Process (Photosynthesis)

In natural processes, cellular systems leverage their precisely designed complex structures to coordinate the “New Three Transfers”. This coordination enables the system to perform energy transduction processes (e.g., photosynthesis and respiration) with excellent quantum efficiency, while also supporting energy-intensive biosynthetic reactions such as nitrogen fixation (Figure ):

• (1)The precisely aligned electron transfer chains on the thylakoid membrane ensure efficient transfer of photogenerated electrons to the terminal acceptor for NADPH synthesis.
• (2)During electron transfer, proton electrochemical gradients are generated across the thylakoid membrane. This gradient not only drives ATP synthase to produce ATP, but also facilitates the movement of other ions, such as Mg²⁺ and Cl^–, which help balance charge and stabilize the membrane potential.
• (3)With the assistance of transfer proteins on the cytoplasmic, chloroplast, and thylakoid membranes, CO₂ (HCO₃ ^-) is transferred to the chloroplast stroma and finally participates in enzymatic CO₂ fixation.

3.

Scheme of “New Three Transfers” in photosynthesis.

3.2. “New Three Transfers” in Typical Non-Natural Processes

We choose CO₂ electroreduction as a showcase to illustrate the synergistic regulation of the “New Three Transfers”. During CO₂ electroreduction, the electrons transfer from the external circuit to the cathodic active sites, the protons originate from the electrolyte and diffuse to the catalyst surface, and the CO₂ molecules diffuse from the gas phase through the gas diffusion layer into the electrolyte and then to the active sites. These three species react at the active site to produce high-value products. The transfer pathway of CO₂ molecules from the gas phase to the active sites is rather long, and the transfer efficiency is restricted by the low solubility and small diffusion coefficient of CO₂ in the electrolyte. The efficiency of electron transfer from the external circuit to the active sites is constrained by factors such as electrode conductivity, contact resistance between catalyst and support, and interfacial electronic state matching. The efficiency of proton transfer from the proton donors in the bulk electrolyte to the cathodic active sites is restricted by the proton diffusion gradient, local pH value, buffering capacity, and competition from the hydrogen evolution reaction. Recently, strategies have switched to focus on the synergistic regulation of electrons, protons, and molecules in CO₂ electroreduction. For example, Chen et al. dispersed cobalt phthalocyanine (CoPc) atoms on a hollow carbon (HC) carrier to form the CoPc@HC, and they constructed a CoPc@HC/sCu tandem electrode by adding a CoPc@HC catalyst on top of the sputtered Cu (sCu) layer to reduce CO₂ to a C₂₊ compounds under acidic conditions. The gas diffusion layer and the high specific surface area of CoPc@HC respectively promoted the transfer of CO₂ and CO to the active site. The high dispersion of CoPc and the tight connection between the catalytic layers promoted the electron transfer. Regulating the pH of the electrolyte promoted the proton transfer. At 800 mA cm^–2 and 25 °C, the optimized series electrode achieved a Faradaic efficiency of 61% for C₂H₄ and 82% for C₂₊. Beyond CO₂ electroreduction, the synergistic regulation of “New Three Transfers” has proven effective in other areas, such as thermal/photo/electrocatalysis, electrochemical gas compression, energy conversion and storage, biomass and plastics degradation, biocatalysis and synthetic biotechnology, and biophoto-electro-devices (Figure ).

4.

Scheme of “New Three Transfers” in diverse applications.

4. Conclusions and Implications

With the progress of “new-quality chemical engineering”, transfer theory needs to move from the macroscopic level to the microscopic level. This Viewpoint establishes and clarifies the “New Three Transfers” theoretical framework, manifesting its fundamental role in connecting matter and energy flows, creating a universal dynamic model, and revealing shared principles across different fields. Synergistic regulation of the “New Three Transfers” will pave the way to breaking current efficiency bottlenecks. Particularly, the rational design of advanced materials remains the primary base for achieving independent and synergistic regulation of the “New Three Transfers”.

Looking forward, constructing next-generation chemical reaction systems based on the “New Three Transfers” faces several major challenges:

• (1)Future research should transcend the classical paradigm of macroscopic statistical averaging and delve into the quantum and atomic-scale origins of the “New Three Transfers”. We need to develop real-time, in situ, multidimensional operando characterization techniques with ultrahigh spatiotemporal resolution (e.g., attosecond-angstrom level), and leverage AI-powered analysis of massive dynamic data, aiming to visualize electron pathways, proton hopping routes, and molecule transfer trajectories.
• (2)The core objective of the “New Three Transfers” should be shifted to the synergy of “process intensification” and “materials creation” for efficient coupling of matter, energy and information. In advanced material manufacturing involving photoelectric, piezoelectric, thermoelectric, and mechanochemical effects, the synergy between energy (light, electricity, force, heat) and matter (molecules, ions) is fundamentally a manifestation of the “New Three Transfers”. Importantly, information carrierssuch as chemical concentration gradients, molecular conformational changes, electrical potential distributions, and optical signalsare intrinsically linked to the transfer behaviors of electrons, protons, and molecules. The rates, pathways, and fluctuations of these transfers carry abundant information that can be captured by real-time sensors and used for feedback control. In the future, the “New Three Transfers” should be more actively employed to guide the design of next-generation materials with specific electronic structures, precise proton channels, and stimuli-responsive molecular recognition sites. Meanwhile, by embedding sensing units and information feedback modules, we can create “information–matter–energy” integrated material systems that perceive, respond to, and optimize their own states.
• (3)As the boundaries between physical and chemical processes become increasingly blurred under extreme conditions such as nanoconfinement and quantum confinement, the “New Three Transfers” affords a universal perspective for understanding complex reaction systems. Looking ahead, we should strive to construct intelligent chemical engineering systems based on the synergistic regulation of matter, energy, and information. By deciphering the intrinsic quantitative relationships between information carriers (e.g., molecular conformation, chemical gradients, interfacial potential) and the matter/energy/information flows of the “New Three Transfers”, we can ultimately realize a new paradigm of self-adaptive, self-optimizing “Intelligent Chemical Engineering”. This will be further empowered by digital twins, multimodal data fusion, and real-time decision-making algorithms, imparting the ultimate solution for precise, efficient, low-carbon, and sustainable chemical processes.

We present this framework of the “New Three Transfers” not as a destination, but as a starting point to stimulate further thinking and practice. We earnestly hope that this Viewpoint may serve as an initiator, sparking insightful discussions, constructive critiques, and collaborative exploration that will promote our understanding of complex phenomena and underlying logic in chemical engineering and beyond.

The authors declare no competing financial interest.