Recent advances in transition metal phosphide nanomaterials: synthesis and applications in hydrogen evolution reaction.

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Recent advances in transition metal phosphide nanomaterials: synthesis and applications in hydrogen evolution reaction Yanmei Shia and Bin Zhang*ab The urgent need of clean and renewable energy drives the exploration of effective strategies to produce molecular hydrogen. With the assistance of highly active non-noble metal electrocatalysts, electrolysis of water is becoming a promising candidate to generate pure hydrogen with low cost and high efficiency. Very recently, transition metal phosphides (TMPs) have been proven to be high performance catalysts with high activity, high stability, and nearly B100% Faradic efficiency in not only strong acidic solutions, but also in strong alkaline and neutral media for electrochemical hydrogen evolution. In this tutorial review, an overview of recent development of TMP nanomaterials as catalysts for hydrogen generation with high activity and stability is presented. The effects of phosphorus (P) on HER activity, and their synthetic methods of TMPs are briefly discussed. Then we will demonstrate the specific strategies to further improve the catalytic efficiency and stability of TMPs by structural engineering.

Received 28th May 2015

Making use of TMPs as cocatalysts and catalysts in photochemical and photoelectrochemical water

DOI: 10.1039/c5cs00434a

splitting is also discussed. Finally, some key challenges and issues which should not be ignored during the rapid development of TMPs are pointed out. These strategies and challenges of TMPs are instructive

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for designing other high-performance non-noble metal catalysts.

Key learning points (1) (2) (3) (4) (5)

A short, comprehensive overview of the recent advances on TMPs. Basic principles of electrochemical HER and evaluation approaches of HER electrocatalysts. A summary of synthetic methods of TMPs and structural engineering strategies to improve their HER performance. A short review on TMPs’ application in photochemical and photoelectrochemical hydrogen evolution. New trends and challenges of TMPs as HER electrocatalysts, which are instructive for designing other non-noble metal electrocatalysts.

1. Introduction The increasing global energy demand and accompanying climate changes as well as environmental issues are driving scientists to search for sustainable and environmentally friendly alternative sources of energy to replace exhaustible fossil fuels.1 Molecular hydrogen (H2), with the highest gravimetric energy density compared with other fuels and only non-polluted water as the combustion product, is proposed as the potential candidate for the future energy supply.2 In addition, H2 is also applied

a

Department of Chemistry, School of Science, Tianjin University, and Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China b Tianjin Key Laboratory of Molecular Optoelectronic Science, Tianjin University, Tianjin 300072, China. E-mail: [email protected]

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in the synthesis of ammonia for fertilizer and petroleum refining, making H2 a critical chemical feedstock in the modern industry. At present, the steam reforming process from fossil resources uses both H2 and CO2 as the primary source for large-scale production of H2. However, this production route not only aggravates the consumption of fossil fuels, but also contributes to the global CO2 emission.3 Therefore, seeking a clean, renewable and efficient strategy for H2 production is urgently needed. The urgent need of H2 production stimulates intensive research interests on the electrolysis of water (H2O (l) - H2 (g) + 1/2O2 (g), DG0 = +237.2 kJ mol1, DE0 = 1.23 V vs. normal hydrogen electrode (NHE)). With a number of advantages such as using water as starting material, no emission of greenhouse gases and other polluted gases, high hydrogen production efficiency and highpurity product, electrolysis of water is considered to be a clean and efficient method to produce H2. The electrolytic process of

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water constitutes two half-reactions, hydrogen evolution reaction (HER, 2H+ (aq) + 2e - H2 (g)), and oxygen evolution reaction (OER, 2H2O (l) - 4e + 4H+ (aq) + O2 (g)). Assuming an ideal Faradic efficiency of 100%, the amount of hydrogen generated is twice than the generated amount of oxygen, and both are proportional to the total electrical charge flow in the solution. Under acidic conditions, HER undergoes a multistep reaction process, which is suggested as two different mechanisms with three possible reactions.4 The dominant reaction mechanism in the HER process can be theoretically indicated by a Tafel slope, which can be obtained from experimental data. The first step is the discharge process, also known as the Volmer reaction. In this step, an electron is transferred to the surface of the cathode to capture a proton in the electrolyte, forming an intermediate state of an adsorbed hydrogen atom on the active site of the catalytic surface: H+ (aq) + e - Hads (Step 1, Volmer reaction) b1;V ¼

2:3RT aF

(1)

where b is the Tafel slope, a is the symmetry coefficient with a value of 0.5, F is the Faraday constant, R is the ideal gas constant, and T is the absolute temperature. The following step has two different ways to generate the final product of H2. When the Hads coverage is low, the adsorbed hydrogen atom prefers to couple with a new electron and another proton in the electrolyte to evolve H2. This electrochemical desorption step is named Heyrovsky reaction: Hads + H+ (aq) + e - H2 (g) (Step 2, Heyrovsky reaction) b2;H ¼

2:3RT ð1 þ aÞF

(2)

However, at a relatively high Hads coverage, the recombination between the adjacent adsorbed hydrogen atoms is dominant, which is called as Tafel reaction, also known as the chemical desorption step:

Yanmei Shi received her BS degree in applied chemistry from Tianjin University in 2013. She is currently working on her PhD degree at Tianjin University under the supervision of Professor Bin Zhang. Her research focuses on the development of non-noble metal electrocatalysts for hydrogen and oxygen evolution reactions.

Yanmei Shi

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Hads + Hads - H2 (g) (Step 2 0 , Tafel reaction) b20 ;T ¼

2:3RT 2F

(3)

At 25 1C, Tafel slopes of the above three reactions are calculated to be b1,V = 118 mV dec1, b2,H = 39 mV dec1, and b2 0 ,T = 29 mV dec1, respectively. The Tafel slope is the intrinsic property of an electrocatalyst. In practice, it is obtained from the slope of the linear part of Tafel plots, whose equation is shown as follows. Z = b log( j/j0)

(4)

where Z is the overpotential, j is the current density, and j0 is the exchange current density. If the experimental Tafel slope of an electrocatalyst is 29 mV dec1, it is suggested that the Heyrovsky reaction (electrochemical desorption step) is the rate determining step, and the HER catalyzed by this electrocatalyst proceeds via the Tafel–Heyrovsky mechanism. An electrolysis experiment can nearly never be conducted under the theoretical decomposition potential. The absolute value of the difference between the experimental potential and the thermodynamically determined reduction potential is overpotential. The overpotential, which is caused by polarization of the electrode, always demands much energy for electrolysis. A key assessment of an excellent electrocatalyst is low overpotential in electrolysis. The most well-known efficient electrocatalysts for HER are Pt and Pt-group metals with nearly zero overpotential. However, these noble metal catalysts are unfortunately limited by their scarcity and accompanying expensive price. Thus the development of non-noble metal electrocatalysts, such as Ni-based alloys, metal chalcogenides, carbides, borides and nitrides, as well as newly developed non-metal catalysts is urgently needed.5 Currently, transition metal phosphides (TMPs) are acting as typical representatives of the burgeoning non-noble metal electrocatalysts. The discovery of TMPs can be dated back to the 18th century. Unexpectedly, no notable use of TMPs is found for

Bin Zhang received his PhD degree from the University of Science and Technology of China in 2007. He carried out postdoctoral research in the University of Pennsylvania (2007.7–2008.7) and worked as an Alexander von Humboldt fellow in Max Planck Institute of Colloids and Interfaces (2008.8– 2009.7). Currently, he is a professor in the chemistry department at Tianjin University and Collaborative Innovation Center of Bin Zhang Chemical Science and Engineering (Tianjin). He mainly focuses on the development of porous and ultrathin nanomaterials for catalytic applications, such as electrocatalysis, photocatalysis, and photoelectrocatalysis.


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nearly two hundred years.6 Until the 1960s, TMPs are gradually applied to metallurgy, hydrodesulfurization (HDS), hydrodenitrogenation (HDN), hydroprocessing (HPC), pesticides, photocatalytic degradation, lithium ion batteries, etc. In the early time, most of the syntheses of metal phosphides undergo high temperature and/or high pressure with flammable elemental phosphorus (P) as the phosphorus source, making the experiments difficult and dangerous to be conducted, which greatly hindered the developments and applications of TMPs for many years. The early research studies demonstrated that the amorphous transition metal–phosphorus ‘‘alloy’’ film electrodes, which were prepared by electrodeposition at room temperature, exhibited high activity towards HER. And people at that time speculated that the high HER activity of these electrodes originated from the adjusted electronic structure of metals by hydrogen adsorption during electrochemical preparation more than the catalysts themselves. In 2005, Liu and Rodriguez found that the Ni2P(001) behaved somewhat like the [NiFe] hydrogenase based on density functional theory (DFT) calculations, predicting Ni2P to be a highly active HER catalyst (Fig. 1).7 Then the first experimental report on nanoscale TMPs for high performance electrochemical hydrogen evolution is raised by our group in 2013.8 In this study, nanoporous FeP nanosheets were synthesized by an anionexchange pathway, which performed high electrocatalytic activity toward HER with low overpotential and a small Tafel slope. Nearly at the same time, Schaak, Lewis and coworkers reported Ni2P hollow nanoparticles as an active HER electrocatalyst (Fig. 2).9 This work was inspired by the similar mechanism between HDS and HER, both of which underwent the reversibly associating and dissociating hydrogen atom in the process. So Ni2P with extremely high HDS conversion of nearly 100% was chosen as the research model. Moreover, the as-prepared Ni2P

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Fig. 2 (A–C) Crystal structure of Ni2P: (A) four unit cells stacked on top of one another, with a single unit cell outlined, (B) top-down view of the Ni2P(001) surface, and (C) a two-dimensional slice of Ni2P, showing the (001) surface on the top. (D) HRTEM image of a representative Ni2P nanoparticle, highlighting the exposed Ni2P(001) planes. (E) Polarization data for three individual Ni2P electrodes in 0.5 M H2SO4, along with glassy carbon, Ti foil and Pt for comparison. Reproduced with permission from ref. 9, Copyright 2013 American Chemical Society.

nanoparticles possessed a high density of exposed (001) facets, confirming the previous theoretical prediction authentically. In 2014, Sun and coworkers grew different TMP nanostructure arrays directly on three-dimensional (3D) substrates through gas–solid reaction without surfactants, which strongly enhanced the activity and stability of TMPs towards HER.11–13 These pathbreaking reports usher in the golden era of TMPs as highly efficient HER electrocatalysts. To date, the current counts of published papers on TMPs for HER are more than 30 every year and still increasing. In this review, we try to provide an overall understanding of TMP nanomaterials as catalysts and cocatalysts with high activity and stability for hydrogen generation from electrochemical, photochemical and photoelectrochemical water splitting. Some main evaluation approaches of HER electrocatalysts and frequentlyused synthetic strategies to fabricate nanostructured TMPs for HER will be summarized first. Then the specific structural modification methods for TMPs to further optimize the HER performance along with recent advances in the hydrogen evolution process are described. Taking advantages of TMPs as cocatalysts and catalysts for photochemical and photoelectrochemical water splitting are highlighted as well. At last, we point out some scientific challenges that existed in the rapid development of TMPs for hydrogen evolution and provide outlooks for the future research.

2. Evaluation approaches of HER electrocatalysts 2.1.

Fig. 1 (A) Schematic representation of the reaction mechanism suggested by Liu and Rodriguez. (B) Structural excerpt of the proposed active site on Ni2P[001] and (C) the structurally similar site on Ni5P4[001]. Reproduced with permission from ref. 10, Copyright 2015 Royal Society of Chemistry.


Overpotential

The standard electrode potential of HER is zero under standard conditions. The absolute value of difference between zero and the onset potential to initiate HER using an electrocatalyst is the corresponding overpotential. A high performance electrocatalyst with lower overpotential requires less energy to achieve the same current density. Plotting current density vs. overpotential can obtain the polarization curves. Based on the origins of polarization on the electrode, the overpotential can be mainly divided into activation overpotential and concentration overpotential. The former can be greatly lowered by using suitable electrocatalysts. The concentration difference of

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the involved ions between the bulk solution and the electrode surface, which is caused by the slow diffusion rates of ions, is the reason for concentration overpotential. However, due to the presence of a diffusion layer, the concentration overpotential can only be partially reduced by stirring, which may in turn disturb the electrode reaction. Another important overpotential is resistance overpotential, also known as junction overpotential, which occurs at surfaces and interfaces of the measurement system. The resistance across surfaces and interfaces will cause extra voltage drop, making the measured overpotential of the electrode larger than the true value. A useful method to eliminate this kind of overpotential is IR compensation to get accurate overpotential of electrocatalysts. In a three electrode measurement system, resistance between the tip of the Luggin capillary and the surface of the working electrode is the main source of the resistance overpotential. This resistance is the R in the IR compensation. Many electrochemical workstations can measure the value of R directly. Or the value can be directly read out from leftmost intersection between the Nyquist curve and X-axis at high frequencies. The IR compensation is expressed as the following relationships. Ecorrection = I R

(5)

Ecorrected = Euncorrected Ecorrection = Euncorrected I R (6) where E is the potential and I is the current flowing through the system. According to the formula, the correction affects slightly on the onset overpotential because the current is quite small at that point. However, with the current increasing, both the Ecorrection and the shift of the polarization curves will become larger and larger. 2.2.

Tafel slope and exchange current density

By replotting the polarization curves (current density vs. potential) into Tafel plots (overpotential vs. log|current density|), the Tafel slope can be determined by fitting the linear regions of Tafel plots to the Tafel equation (eqn (4)). This is the most common way to get the value of the Tafel slope. The smaller value of the Tafel slope means that increasing the same current density required smaller overpotential, implying a faster charge transfer kinetic. Another way to acquire the Tafel slope is proposed by Hu’s group, which began to gain the acceptance of researchers in recent years.14 They obtained the Tafel slopes from the impedance data by calculating the slope of the linear fitting plots of log Rct vs. overpotential. The electrochemical impedance spectroscopy (EIS) data of HER reaction catalyzed by the electrocatalyst can be fitted with an equivalent electric circuit, in which Rct is the charge transfer resistance in the equivalent circuit. Tafel slopes obtained through this method can purely reflect the charge transfer kinetic of the electrode reaction. By contrast, Tafel slopes acquired from polarization curves may include the contribution from catalyst resistance, which may be caused by high catalyst loading or poor electrical conductivity of the electrocatalysts (Fig. 3).

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Fig. 3 (A) The equivalent circuit for the impedance data using a transmission line model. Rc represents the contact resistance of the electrode with the film. Rm represents a non-negligible electronic resistance of MoSx. (B) Plots of the extracted parameters for Rct, Rm and Cct for the MoS3-modified electrode. Reproduced with permission from ref. 14, Copyright 2013 Royal Society of Chemistry.

The intersection of the extrapolated linear part of Tafel plots and the X-axis is the exchange current density. At equilibrium, the anodic current density is equal to the cathodic current density. This value of current density in both directions is the exchange current density. Exchange current density is the intrinsic property of the electrode reaction, which depends only on catalyst materials, electrolyte and temperature. It reflects the ability of electron transfer and the difficulty of an electrode reaction. The internal reason of overpotential is exchange current density. Electrode reaction with larger exchange current density needs less driving force (smaller current density) to conduct the reaction. For instance, the best well-known HER electrocatalyst: Pt, has a large exchange current density of about 1 103 A cm2 in 0.5 M H2SO4. However, when mercury is employed as an HER electrocatalyst, the exchange current density can be as small as 5 1013 A cm2 in 0.5 M H2SO4, showing an extremely poor HER activity. 2.3.

Hydrogen bonding energy

Generally, a good HER electrocatalyst should have a neither too weak nor too strong free energy of hydrogen adsorption.15 The weak adsorption results in difficulty in the combination between the proton and electrocatalyst. In contrast, the strongly adsorbed Hads will be desorbed difficultly from the catalytic surface. As a result, the active sites on the catalytic surface are always occupied, hence poisoning the catalyst. The standard hydrogen electrode potential is defined as zero, which means that the Gibbs free energy of hydrogen bonding of a good HER electrocatalyst should be close to zero as well. The Gibbs free energy of hydrogen adsorption is always acquired through DFT calculations. Plotting the exchange current density vs. Gibbs free energy of hydrogen adsorption can obtain the Sabatier Volcano. The electrocatalyst with a plot closer to the summit of the volcano exhibits better HER activity.16 2.4.

Stability

Stability is another important evaluation method for HER electrocatalysts. There are two techniques for the stability measurements: cyclic voltammetry (CV) and galvanostatic or potentiostatic electrolysis. In CV, potential cycles are repeated in the region including the onset HER potential. After hundreds of CV cycles, the less the overpotential for the same current


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density increases, the better the stability of the electrocatalyst is. Reading intuitively from voltammograms, the polarization curve of a steady electrocatalyst after CV cycles should change little compared with the initial one. Galvanostatic (or potentiostatic) electrolysis is the time dependence of the potential (or current density) at constant current density (or overpotential) of an electrocatalyst. A current density of 10 mA cm2 is often used in this kind of electrolysis, because this value is the most frequently used standard in HER electrocatalysis and solar fuel synthesis. The duration can last from several to dozens of hours, and longer duration means better stability. 2.5.

Faradic efficiency

In HER, the Faradic efficiency is the efficiency by which the electrons provided by external circuit are transferred to drive the HER. Faradic losses may occur when heat or byproducts of the electrode reaction are generated. To measure Faradic efficiency, both the theoretical and practical amounts of H2 production are needed. The theoretical hydrogen production can be calculated from galvanostatic or potentiostatic electrolysis by integration. At the same time, the practical hydrogen production can be measured by gas chromatography (GC) or a water–gas displacing method. The ratio between the practical and theoretical hydrogen production is the Faradic efficiency.

3. Effects of P on HER activity TMPs can be viewed as doping P atoms into crystal lattices of transition metal. Until now, only six different transition metals (Fe, Co, Ni, Cu, Mo and W) are found to form TMPs that can be used for electrochemical hydrogen evolution. Other TMPs like titanium phosphides, zinc phosphides and cadmium phosphides, which hydrolyze easily in water or acid, are not capable of HER. 3.1.

Crucial roles of P

It has been proven that P atoms in the TMPs play crucial roles for HER by DFT calculations.7,15 P atoms with more electronegativity can draw electrons from metal atoms. The negatively charged P can act as base to trap positively charged proton during electrochemical HER. So for the same metal phosphides, increasing the atomic percentage of P may effectively improve the corresponding HER activity. For instance, monodispersed nickel phosphide nanocrystals with different phases were synthesized by simply changing the raw ratio. The result showed that among Ni2P, Ni5P4 and Ni12P5, Ni5P4 with the highest P content (44 at% P) displayed highest catalytic activity (Fig. 4A and B).17 Morphologically equivalent Co2P and CoP hollow nanoparticles synthesized by adjusting reaction temperature could be made direct comparisons as well. Similarly, the results demonstrated that CoP nanoparticles showed significantly lower overpotential than Co2P to produce the same current density (Fig. 4C and D).18 The same conclusion also came from comparison between MoP and Mo3P even in the bulk form.15 All the mentioned experimental results indicate


Fig. 4 (A) Possible formation mechanism of the as-synthesized nickel phosphide nanocrystals with different phases and morphologies. (B) CV curves of Ni12P5, Ni2P and Ni5P4 nanocrystals before (solid) and after (short dash) long-term 500 cycles. Reproduced with permission from ref. 17, Copyright 2015 Royal Society of Chemistry. (C) Crystal structures of (top) Co2Si-type Co2P and (bottom) MnP-type CoP. Unit cells are shown as dashed black lines. Both CoP and Co2P are the derivatives of the NiAs structure type, so that both of them have similar Co–P bond lengths. (D) Polarization data in 0.5 M H2SO4 for Co2P/Ti and CoP/Ti electrodes before (solid lines) and after (dashed lines) 500 CV cycles, along with a Pt mesh control. Insets are the TEM images of (top) Co2P and (bottom) CoP nanoparticles. Reproduced with permission from ref. 18, Copyright 2015 American Chemical Society.

that TMPs with higher P content always perform better HER activity. 3.2.

Conductivity

With continuously doping P into metals, electrically conductive metals gradually become semi-conductive or even insulated metal phosphides. The reason is that P atoms with more electronegativity will greatly restrict the electron delocalization in metal, resulting in weakened conductivity.6 However, with appropriate difference in electronegativity and atomic ratio of metal and P, TMPs are able to exhibit a metallic character or even superconductivity, especially for the metal-rich phosphides. Mar and co-workers’ work indicated that the di- and tri-metal phosphides showed a similar electronic structure with the corresponding pure metals,19 confirming excellent conductivity of these phosphides. Unexceptionable conductivity of some TMPs is advantageous for the electrocatalytic process. 3.3.

Gibbs free energy of H adsorption

As mentioned above, a good HER electrocatalyst should have moderate free energy of hydrogen adsorption as well. As early as 2005, DFT calculations done by Liu et al. indicated that the P atoms on the surface of a metal phosphide could not only have a small negative charge to trap protons as the base, but also provide high activity for the dissociation of H2, thus preventing the system from deactivation caused by high coverage of strongly bonding hydrogen atoms as pure nickel does.7 Recently, Wang’s group demonstrated a more detailed discussion

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4. Synthetic strategies


4.1.

Fig. 5 Top view of H adsorbed P terminated (001)-MoP surfaces and associated calculated Gibbs free energy of H adsorption. Only the first two atomic layers are shown. Reproduced with permission from ref. 15, Copyright 2014 Royal Society of Chemistry.

on how P affected the hydrogen absorption of a metal phosphide. As shown in Fig. 5, Gibbs free energy (DG0H) of the P terminated surface on (001)-MoP was slightly negative at first. With H coverage increasing from 1/4 monolayer (ML, one hydrogen adsorbed on 2 2 slab) to full coverage, DG0H gradually became positive, which meant that P atoms on the P terminated surface of (001)MoP could bond hydrogen at low coverage whilst desorbed H at high coverage, just as a ‘‘hydrogen deliverer’’. This behaviour of the P terminated surface in molybdenum phosphides quite resembles the S-edges in MoS2, which played a key role in creating large numbers of active edges for HER.20 This feature makes metal phosphides meet the criteria of good HER electrocatalysts further. 3.4.

Corrosion resistance

In addition, corrosion resistance of TMPs in acidic media is directly correlated with P content. Kucernak et al. proposed that when alloyed with P, metal dissolution is thermodynamically less favoured.21 At the same time, the much less soluble phosphate formed on the surface by oxidation can effectively protect the TMPs from dissolution. Experimental data show the same results as well. As shown in Fig. 4B, it is clearly presented that the increased overpotential to reach the same current density of Ni5P4 with highest P contents is the least among the three nickel phosphides after 500 CV cycles, performing the best HER stability. The same conclusion can be drawn from cobalt phosphides, too. In Fig. 4D, CoP with the higher P content performs not only higher HER activity, but also the better stability. Both the cases indicate that the increased P content of the phosphides can effectively improve the corrosion resistance during the electrolysis in acid media. In general, there are many different phases with different P content naturally for the same metal phosphides, such as CoP and Co2P, WP and WP2, FeP and Fe2P, and Ni2P, Ni5P4 and Ni12P5. As discussed above, TMPs with higher P content can possess more active sites to trap protons and improving the corrosion resistance to benefit HER. However, with the P content increasing, P atoms will greatly restrict the electron delocalization of the metal atoms, and thus impede the conductivity, therefore resulting in a weakened HER activity. So a balance is needed. Therefore, it is reasonable to speculate that without severely hurting the conductivity of TMPs, higher P content always leads to higher HER activity and better stability.

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Solution-phase reaction

There are two main methods to synthesize TMP nanostructures applied for HER. One is solution-phase synthesis with using tri-n-octylphosphine (TOP) as a phosphorus source. TOP is a versatile and attractive phosphorus source. At elevated temperature (ca. 300 1C), the C–P covalent bond can be broken, and then different precursors, including metal (both bulk metal and metal nanoparticles), metal acetylacetonates, metal carbonyl compounds and metal oxides, can be phosphorized. Since TOP has a strong coordination effect, the wise use of TOP can efficiently facilitate the reaction and lead to the fabrication of some unusual structures. For example, our group has successfully converted the solid nanosheet precursors into porous FeP nanosheets by taking advantage of TOP as a phosphorus source and ligand to extract S from Fe18S25–TETAH (TETAH = protonated triethylenetetramine) nanosheets for the formation of TOP–S (Fig. 6).8 Nevertheless, the insolubility of TOP in water and its high decomposition temperature restricts the system to be only conducted in high boiling-point organic solvent (e.g. 1-octadecylene, octyl ether and squalane), leading the reaction to be highly flammable and corrosive. So this kind of experiment should be carried out under severely oxygen free conditions with skilled workers. In addition, the oxide of TOP (tri-n-octylphosphine oxide (TOPO)) and other organic phosphines (like tri-phenylphosphine (TPP)) have a similar effect, and can be mixed with TOP as common phosphorus sources and capping molecules (Table 1). 4.2.

Gas–solid reaction

The other method is a gas–solid reaction way. PH3 is efficient and active in phosphorization. However, it is extremely toxic and lethal at a few ppm. So substitutes which can in situ

Fig. 6 (A) SEM image and (B) TEM image of porous FeP nanosheets. (C) Scheme illustrating the synthesis of the porous FeP nanosheets through the anion exchange reaction of the inorganic–organic hybrid Fe18S25– TETAH nanosheets with P ions. Reproduced with permission from ref. 8, Copyright 2013 Royal Society of Chemistry.


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Selected summary of transition metal phosphides synthesized by liquid-phase reaction

Catalyst


28

Co2P FeP8 Ni12P529 Ni2P, Ni5P4 & Ni12P517 Ni2P27 Ni2P9

Precursor

P source

T/1C

t/h

Solvent, additive and atmosphere

Resulting morphology

Co nanoparticles Fe18S25–TETAH Ni(Ac)24H2O Ni(acac)2 Oxidized Ni foam Ni(acac)2

TOP TOP TPP TOP TOP TOP

320 320 300 320 320 320

1 5 0.5 2 2 2

1-Octadecene, oleylamine, Ar 1-Octadecene, Ar Oleylamine, N2 Oleylamine, 1-octadecene, Ar flow 1-Octadecene, Ar 1-Octadecene, oleylamine, Ar

Nanoparticles Porous nanosheets Nanoparticles Nanoparticles Nanosheets Hollow nanoparticles

generate PH3 like hypophosphites (NH4H2PO2 and NaH2PO2) are widely utilized. Hypophosphites decompose to release PH3 when heated over 250 1C, which can further react directly with a variety of metal oxides, hydroxides, metal–organic frameworks (MOFs), and some other compounds to form TMPs. The reaction equation is shown as follows: 2NaH2PO2 = PH3 m + Na2HPO4

(7)

This gas–solid reaction strategy is surfactant-free and very applicable to retain the dimension and morphology of the precursors. And it should be noted that post-treatment of tail gas is extremely necessary to absorb excessive PH3. Reducing metal orthophosphates directly by H2 at higher temperatures (over 650 1C) is another kind of gas–solid reaction to fabricate TMPs, mainly used for molybdenum and tungsten phosphides.22 Briefly, a mixture of stoichiometric poly tungstate or molybdate, orthophosphate and water is evaporated under a water bath and calcined at B500 1C in sequence. Then the calcined mixtures are milled and calcined again in a flow of H2 mixed with inert gas at B650 1C to get the final product of phosphides. TMPs acquired through this way are always macrosized particles with large grain size and irregular morphology (Table 2). 4.3.

Other synthetic methods

There are also many other methods to prepare TMPs. Cathodic electrodeposition is employed to synthesize cobalt phosphide film on a copper substrate at room temperature.23 White and red phosphorus can react with metal salts under hydrothermal conditions at relatively low temperature (ca. 180 1C) to form TMPs with abundant nanostructures, especially hollow and branched nanomaterials.24 Pyrolysis of P-containing porous ionic polymers can easily yield MoP and FeP nanomaterials

Table 2

Catalyst

wrapped with porous carbon.25 Diversified synthetic strategies make it feasible to construct various TMPs with abundant structures.

5. Applications of TMPs in electrochemical HER 5.1.

Strategies for enhancing electrochemical performance

Although the intrinsic structures of TMPs meet the criteria of outstanding electrocatalysts, there still exist many methods that could further improve their HER performance by structural modification. 5.1.1. Fine surface design. With the electrolysis going on, gas bubbles will gradually generate from the surface of the electrode. This phenomenon should be positive. However, plenty of bubbles adsorbed on the catalytic surface would impede the contact between electrocatalysts and the electrolyte, which would greatly decrease the electrochemical active area, hence lowering the activity of the electrocatalysts. To avoid such a negative effect, it is of great significance to develop electrocatalysts with hydrophilic and aerophobic surfaces from which bubbles are detached easily. In this regard, an MoS2 nanosheet array-like film with a ‘‘superaerophobic’’ surface is reported. The ‘‘superaerophobic’’ MoS2 film can greatly promote the HER electrocatalytic performance, which even surpasses Pt/C at high current density.26 Inspired by this report, our group synthesized vertical Ni2P nanosheets grown on Ni foam as high performance HER electrocatalysts. The vertical Ni2P nanosheets have been proven to be aerophobic by providing a rapid renewal of small gas bubbles and a fixed working area, thus increasing the efficiency of HER.27 Therefore, the design of the electrode with an aerophobic surface is very important for HER and other gas evolution reactions. 5.1.2. Coupled with carbon materials. Electrocatalytic efficiency is also affected by the electrical conductivity and

Selected summary of transition metal phosphides synthesized by gas–solid reaction

Precursor

P source

T/1C

t/h

Atmosphere

Resulting morphology

CoP Cu3P30 FeP11 Mo3P15

Co(OH)F nanowires Cu(OH)2 nanowires Fe2O3 nanowires (NH4)6Mo7O244H2O

NaH2PO2 NaH2PO2 NaH2PO2 (NH4)2HPO4

(NH4)6Mo7O244H2O MoO3 Ni-BTCa WO3 nanorods

(NH4)2HPO4 Red phosphorus NaH2PO2 Red phosphorus

1 1 2 2 2 2 3 or 5 1 1

Static Ar Ar Ar H2 Ar H2|N2(5%|95%) flow Vacuum-sealed

Nanowire arrays Nanowire arrays Nanowire arrays Bulk

MoP22 MoP231 Ni12P532 WP12

300 250 300 650 800 650 850 325 800

13

a

Static Ar

Film Capsules Nanoparticles Nanorods arrays

BTC: 1,3,5-trimesic acid.


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dispersity of the catalyst. An electrocatalyst with poor electrical conductivity results in a voltage drop across the electrode, producing an extra overpotential to lower the apparent activity of the catalysts and consume more energy. On the other hand, excellent dispersity of electrocatalysts allows full use of active sites on catalysts to participate in electrode reaction to improve the electrocatalytic efficiency. Compounding TMPs with carbon materials like carbon nanotubes (CNTs), graphite, graphene, and porous carbon can not only increase the dispersity of the active components due to their huge surface area, but also improve the conductivity of the hybrid catalysts.16 Not only so, the coupled carbon materials can also modulate the electron density and the distribution of electronic potential in the hybrid materials, thus enhancing their HER activity and stability.33 Taking CoP as an example, a nanohybrid which consists of CNTs decorated with CoP nanocrystals exhibits greatly improved HER activity than CoP nanocrystals only.34 In a word, compositing catalysts with carbon materials can make full use of the excellent conductivity and favourable dispersity of carbon materials, and benefit for improving the electrocatalytic activity. 5.1.3. Doping with other elements. Doping TMPs with other elements is another way to improve their catalytic activity. Jaramillo and coworkers successfully doped S into MoP to form molybdenum phosphosulfide (MoP|S) on the surface of MoP by a post-sulfidation process.22 At the same catalyst loading, reaching a hydrogen generating current density of 10 mA cm2 required the overpotentials of 86 and 117 mV for MoP|S and MoP, respectively, indicating that MoP with a phosphosulfide surface substantially improved the activity over that of pure MoP (Fig. 7). The reason might be that S and P can tune each other’s electronic properties to produce an active catalyst phase. Jin’s group investigated the structure and HER activity of a sequence of pyrite-phase nickel phosphoselenide nanomaterials through thermal conversion of Ni(OH)2 nanoflakes. By simply adjusting the raw ratios of phosphorus and selenium, they acquired four different nanomaterials, respectively, i.e. NiP2, Se-doped NiP2, P-doped NiSe2 and NiSe2. The results showed that Se-doped NiP2 performed the highest HER activity, providing another example of improving the HER catalytic activity by doping.35 Cation doping is also accessible. Tungsten-doped nickel phosphide microspheres were proved to be high activity HER catalysts. It might be conjectured that by doping tungsten into NixP, more lattice defects would form, and many electrons would be offered to P atoms for better activity.36 To date, exact reasons for the high activity of the doped materials are not entirely clear yet, inspiring researchers to elucidate their origins by means of further characterization and DFT calculations. 5.2.

New trends in electrochemical hydrogen generation

Trends in electrochemical HER undergoing rapid development are briefly discussed in this section. Such trends are not only suitable for TMPs, but also instructive for designing many other non-noble metal electrocatalysts. 5.2.1. From coating nanocrystal dispersion on planar electrodes to 3D self-supported electrodes. In recent years,

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Fig. 7 (A) XRD patterns of MoP and MoP|S supported on Ti foil with a reference diffractogram of the MoP crystal. (B–D) Corresponding MoP crystal structure. The purple rhombic prism displays the unit cell (molybdenum blue, phosphorus magenta). (E) Linear sweep voltammograms of MoP and MoP|S in 0.5 M H2SO4. The mass loading of MoP is 1 mg cm2. The solid and dotted blue lines represent MoP|S with a loading of approximately 1 and 3 mg cm2, respectively. Reproduced with permission from ref. 22, Copyright 2014 Wiley-VCH.

3D self-supported electrodes are becoming more and more popular in electrochemical research studies. Compared with traditional electrodes coated on two-dimensional (2D) substrates by spin-coating, dip-coating, and sputtering, 3D self-supported electrodes can always exhibit much better performance than the planar ones.37 Common 3D current collectors like carbon cloth (CC), Cu foam and Ni foam possess excellent conductivity, robust skeleton and large surface area, and thus effectively increasing contact area between catalyst and electrolyte. To combine electrocatalysts and substrates, polymer binders are always necessary. However, the polymer binders will increase the series resistance, block active sites and inhibit diffusion, resulting in a weakened catalytic activity. If the electrocatalysts grow directly on the 3D structures to form 3D self-supported electrodes, polymer binders are free of use to avoid the above disadvantages. For example, Sun and coworkers have successfully developed a series of well-designed TMPs (FeP,11 WP,12 CoP13) with different structures on CC as HER electrocatalysts with extremely enhanced HER activity. Generally, the use of 3D self-supported electrodes can greatly increase the current density by nearly 5–6 times at the same overpotential. 5.2.2. Combining experiment and theory. With the rapid development of computer technology, theoretical calculation is becoming more and more powerful to predict promising catalysts, ascertain reactive sites and get insight into the reaction mechanism. Extraordinary progress in DFT has created infinite possibilities in the computer-aided design of catalysts with most favourable activity.38 As mentioned above, Liu and Rodriguez predicted that Ni2P(001) had comparable HER activity with the [NiFe] hydrogenase by DFT calculations, which was proven by Schaak and coworkers later.7,9 However, Hu et al. recently found that if the HER completely occurred on Ni2P(001), the theoretical overpotential should be at least as high as 0.31 V, which was inconsistent with latest reports, implying the existence of other widely available active sites. To verify this conjecture, they synthesized uniform hexagonal Ni2P nanowires without exposing (0001) facets as calculated models, and found that either (1% 1% 20) or (112% 1) facets are the active sites as well (Fig. 8).39 Even so, various active sites or facets of Ni2P are still to be studied. Wang’s group also


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Fig. 8 (A) TEM image of the Ni2P nanowires. (B) Polarization curve of Ni2P nanowires in 1 M H2SO4 at 25 1C at a scan rate of 5 mV s1. (C) Free energy diagram assuming a Tafel mechanism. Calculated barriers for the Tafel slope are shown by the peak of the splines for the configurations, which are most stable at 0 V vs. RHE. The insets show transition states of the two facets with the lowest Tafel barriers. Ni atoms are shown in green, P in yellow and H in white. Reproduced with permission from ref. 39, Copyright 2015 Royal Society of Chemistry.

employed DFT calculations to elucidate the ‘‘hydrogen deliverer’’ behaviour of the P terminated surface on (001)-MoP as mentioned.15 Not only that, when Mo3P is calculated in the same way, the corresponding hydrogen binding energy was positive even with the initial two hydrogen adsorbed, substantially explaining the origin of poor HER activity of Mo3P. 5.2.3. From acid to all pH. The water splitting reaction can be considered as a combination of two half reactions: HER and OER. In consideration of thermodynamic convenience and potential in proton exchange membrane fuel cells, most HER catalysts are studied and functioned well in acidic media (Table 3). However, almost all the best OER catalysts work well only in neutral or basic media.40 In order to achieve overall water splitting, it is of great importance to find HER catalysts that work well in a wide pH range so that they can have the Table 3

Selected summary of the HER performance of some TMPs in 0.5 M H2SO4

Catalyst

Substrate

Mass loading (mg cm2)

CoP23 CoP13 Cu3P30 FeP8 FeP25 FeP11 MoP15 MoP|S22 MoP25 Ni12P532 Ni12P517 Ni12P529 Ni2P9 Ni2P32 Ni2P27 Ni2P17 Ni5P417 NixWP36 WP12

Copper disk Carbon cloth Cu foam GCEa GCE Carbon cloth GCE Ti foil GCE GCE GCE Ti foil Ti foil GCE Ni foam GCE GCE Ni foam Carbon cloth

0.92 15.2 0.28 0.24 1.5 0.86 3 0.24 0.35 1.99 3 1 0.35 3.5 1.99 1.99 1.5 2

a

chance to cooperate with OER catalysts. The first attempt to use the TMPs as HER catalysts in alkaline solution is done by Schaak et al. However, their Ni2P hollow nanoparticles decomposed to metal nickel quickly in 1.0 M KOH with the HER activity declining rapidly.9 The reason is not clear yet. Later reports have shown that under both strong acidic and strong alkaline conditions, not only Ni2P, but also other TMPs like CoP, MoP, Ni5P4, and WP2 can offer excellent catalytic performance and maintain the high performance for dozens of hours (Table 4). This pH-tolerable ability makes TMP one of the most attractive electrocatalysts for overall water splitting. 5.2.4. From unifunctional HER to bifunctional overall water splitting. Another idea for cost-effective overall water splitting is to design earth-abundant bifunctional electrocatalysts for both HER and OER. This concept can be accomplished by either tactfully integrating high active HER and OER electrocatalysts, or developing electrocatalysts which can be highly active and stable for both HER and OER under the same pH conditions. In particular, TMPs can be partially oxidized under oxidizing potentials, and these oxidized intermediates (oxides and phosphates) are always highly capable of OER. In this way, the highly active HER catalysts (TMPs) and highly active OER catalysts (the oxidized intermediates) are integrated. Shalom’s group explored the OER performance of Ni5P4 in alkaline media, which was even superior to metal Ni. They attributed the OER activity of Ni5P4 to the fast electrochemical formation of highly OER active NiOOH.41 Yoo et al. found that operating CoP nanoparticles upon anodic potential under alkaline conditions would cause unique metamorphosis, leading to efficient but irreversible performance of CoP towards OER.42 Based on XPS, XANES (X-ray absorption near edgy spectroscopy) and EXAFS (X-ray absorption fine structure), they expected that the local structure of the post-OER sample was composed mainly of bis-oxo/hydroxo-bridged Co ions in discrete molecular units arranged into a phosphate-enriched amorphous

GCE: glassy carbon electrode.

b

Overpotential (mV) Zonset 38 62 100 20 50

Z10b

Z20

85 67 143

100

52 58 64 51

380 80 75 80 62 34 50 50

208 107

78

141 130

172 137 118 110 130

Tafel slope (mV dec1)

Exchange current density (A cm2)

50 51Z=5–60 67 67 49 45 54 50 45 270 75 63.0 46 62 68 49 42 39 69Z=100–200

2 104 2.88 104 1.8 104 5.0 104 3.4 105 5.7 104 4.5 105 2.857 105 3.3 105 7.1 105 4.592 105 5.702 105 4.4 105 2.9 104

Z10: overpotential of the electrocatalyst at the current density of 10 mA cm2.


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Table 4

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Summary of the HER performance of TMPs in alkaline media (pH = 14)

Catalyst

Substrate

Mass loading (mg cm2)

CoP13 FeP11 MoP15 Ni2P27 Ni5P410 NixWP36 WP12

Carbon cloth Carbon cloth GCE Ni foam Ti foil Ni foam Carbon cloth

0.92 1.5 0.86 3.5 177 1.5 2

network (Fig. 9). Besides the good OER activity of cobalt oxide, the assistance of the surface phosphate group and the synergetic effect between cobalt oxides and phosphates were suggested to make a contribution to the high OER activity. It needs to be emphasized that irreversible changes occur on TMPs after OER experiments, and the post-OER TMPs are not capable of HER anymore. It is also allowed to directly combine TMPs with OER active materials during the synthetic process. By reacting porous anodized Co oxide films with phosphorus vapour, Tour’s group directly fabricated porous Co phosphide/phosphate thin films, which performed superior activity for both HER and OER with onset overpotentials of 35 mV and 220 mV for HER and OER, respectively.43 The development of bifunctional electrocatalysts for both HER and OER is still an ongoing challenge. It deserves more efforts to design and fabricate new and efficient bifunctional electrocatalysts.

Fig. 9 (A) XANES profiles, (B) P 2p XPS spectra, and (C) EXAFS spectra (Fourier transforms of k-space oscillations) of CoP/C before and after potential cycling. Insets are possible molecular structures representing the local environment of the present catalyst. Nonbridging oxygen ligands (phosphate, hydroxide, and water) are marked in green. (D) HER polarization curves of original CoP/C, 20 wt% Pt/C, and post-OER state of CoP/C obtained at H2 saturated 0.1 M KOH with a scan rate of 2 mV s1 (catalyst loadings: 0.05 mg cm2 for Co mass and 0.016 mg cm2 for Pt mass). All given data are IR compensated (R B 30 ohm). Reproduced with permission from ref. 42, Copyright 2015 American Chemical Society.

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Overpotential (mV) Zonset

Z10

115 86

209 218

Z20

Tafel slope (mV dec1) 129 146 48 50 98

41 49 160 150

102Z=120–250

6. Applications in photocatalytic and photoelectrocatalytic hydrogen evolution 6.1. As cocatalysts in photocatalytic and photoelectrocatalytic hydrogen evolution In photocatalytic water splitting progress, semiconductors always have large surface overpotentials for water oxidation or reduction, which need to be compensated with extra photovoltage or the application of external bias. Electrocatalysts with high HER or OER activity have been used as cocatalysts to lower the overpotential on the surface of semiconductors.44 The high activity of TMPs towards HER makes them potential cocatalysts for photocatalytic water splitting. Besides, TMPs are in favour of charge separation at the cocatalyst/semiconductor interface which is advantageous for the high photocatalytic efficiency, especially when a ‘‘Z-scheme’’ heterogeneous structure is formed. Schaak, Lewis and co-workers have successfully synthesized FeP nanoparticles with a uniform, hollow morphology.45 When immobilized on TiO2, FeP nanoparticles could act as cocatalysts to enhance the photocatalytic generation of hydrogen under UV irradiation in neutral aqueous solution. Fu’s group has also found that both Ni2P and Co2P can be used as efficient cocatalysts for hydrogen evolution in a system with CdS nanorods as photosensitizers (Fig. 10A).28 TMPs can be used as cocatalysts in photoelectrochemical hydrogen evolution as well. Huang and coworkers demonstrated

Fig. 10 (A) Comparison of photo-generating hydrogen with different components after 10 h irradiation. LED light: l Z 420 nm, 30 3 W. Reproduced with permission from ref. 28, Copyright 2015 Royal Society of Chemistry. (B) A time course of H2 production from aqueous solution containing 50 mg of MoP2 catalyst: (i) Unmodified MoP2 and (ii) ball-milling and 2.0 wt% Pt-deposited MoP2 photocatalyst. The light source is a 300 W Xe lamp with a cut-off filter (l 4 400 nm). Reproduced with permission from ref. 31, Copyright 2015 Royal Society of Chemistry.


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that Ni12P5 nanoparticles had efficient and stable catalytic activity for HER. Furthermore, introducing Ni12P5 nanoparticles to silicon nanowires displayed efficient hydrogen generation in the voltage range of 0.1 V to 0.4 V, and the solar power conversion efficiency of the resulting composite was even larger than that of silicon nanowires decorated with Pt particles under the illumination of a 350 W Xe lamp, whose light intensity was adjusted to 100 mW cm2.29 These wonderful studies make TMPs competent earth-abundant cocatalysts for sustainable photocatalytic and photoelectrochemical hydrogen production. 6.2.

As photocatalysts in hydrogen evolution

It is also worth noting that since the P content can adjust the band gap of metal phosphides by limiting the electron delocalization, choosing metal phosphides with appropriate P content may have possibilities to achieve photocatalytic hydrogen evolution by phosphides themselves. Wu et al. demonstrated that with the assistance of Pt and triethanolamine as a cocatalyst and electron donor, respectively, semimetallic MoP2 could perform actively in water reduction to generate hydrogen under visible light (Fig. 10B).31 This attempt may motivate researchers to find and design appropriate TMPs as novel photocatalysts for effective water splitting under visible light.

7. Summary and outlook In conclusion, we have summarized the application development of TMPs in electrochemical, photochemical and photoelectrochemical hydrogen evolution. Recently, TMPs have been proven to be one of the most highly active, stable and costeffective HER electrocatalysts in a wide pH range. It is proven that P content in TMPs is crucial to the HER performance. Without severely hurting the conductivity, TMPs with higher P content always lead to higher HER activity and better stability. We next discuss the universal evaluation methods of good electrochemical HER, synthetic methods of TMPs for HER, strategies for enhanced HER activity of TMPs including combining with carbon materials, doping and surface designing, as well as methods to optimize catalytic performance which are not only suitable for TMPs, but also instructive for designing other non-noble metal catalysts. At last, we present recent advances of TMPs in photochemistry and photoelectrochemistry. TMPs used as HER electrocatalysts are undergoing rapid development. However, the booming development always causes insufficient understanding. Many researchers focus on synthesizing new phosphides with novel morphology and characterizing the electrochemical performance in very detail. Much attention should be paid to mechanistic and fundamental insights of TMPs in a HER process. Some of the points are discussed as follows. And they are expected to be not only suitable for TMPs, but also instructive to other electrocatalysts and other electrochemical processes. (I) Surface reconstruction. As mentioned above, when using CoP for electrochemical OER, it is the oxidized intermediates rather than the original CoP which are active for OER.


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Besides oxidation, previous research studies have shown that the electrochemical process may cause reduction, amorphization and enhanced hydrogen adsorption to the electrocatalysts. It is of great significance to understand the reconstruction of electrocatalysts during electrolysis because it is the restructured surface rather than the bulk which defines the steady-state performance.46 In particular, the adsorption of generated hydrogen may affect the interatomic force on the surface of the catalyst, resulting in a change in the surface state, which may have an influence on the following hydrogen evolution performance of the catalysts. Seldom researchers focus on the reconstruction of TMPs during the HER process. Therefore much effort should be devoted to this field. (II) Facet effect. For heterocatalysis like HER, the surface of the catalyst is extremely important for the catalysis, while the bulk of the catalyst, besides acting as a current collector, contributes little to HER.47 Therefore, to explore and control the arrangement of atoms on the surface of the catalysts, the exposed facets are of great significance for HER. However, most of the TMPs under research are amorphous or polycrystalline until now. The synthesis of TMPs with specific exposed facets is still a great challenge. So it is highly desirable to synthesize facet-controllable TMP nanocrystals and further study their facet effect on HER. (III) In situ spectroscopic surface analysis of electrode reaction. Probing into the molecular-level interfacial structure of catalysts during electrochemical hydrogen evolution is another noteworthy issue. Spectroscopic measurements are general ways to in situ investigate the interfacial structure during the electrochemical process. However, in the HER process, huge Faradic currents and plenty of hydrogen bubbles make the measurements difficult. By employing a confocal microprobe Raman system, Tian’s group has successfully observed the stable and reproducible Raman spectra of hydrogen bound to a platinum electrode under severe HER conditions, with surmounting difficulties of the weak Raman effect of Pt and complexity of the near surface electrolyte.48 The shell-isolated nanoparticle-enhanced Raman spectroscopic technique,49 also recently developed by Tian, Ren and coworkers, may open a new avenue to observe interface signals of non-noble metal HER electrocatalysts including TMPs. The Yang group investigated the structure of amorphous cobalt sulfide operating under HER conditions through an operando spectroscopic approach.50 With the information from Raman spectroscopy, XANES, and EXAFS, they proposed a possible atomic model for the catalyst under HER conditions, which differs from the as-prepared one, and further speculates the highly HER active sites based on the model. This excellent work may inspire the interest of getting insight into the interfacial structure of TMPs in electrochemical hydrogen evolution by using in situ Raman and X-ray absorption measurements. (IV) Taking full advantage of DFT calculations. DFT calculations are becoming more and more powerful to design and predict the performance of new catalysts. For TMPs, how the crystal structure, chemical constitution and electronic state of TMPs affect the HER performance, why doping with other

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elements can improve the activity, and how the interaction between the coupled materials and TMPs takes place, it is difficult to answer all these questions without the assistance of theoretical calculations. These fundamental understandings will help us get deep insight into the mechanism on how the TMPs work and remain active and stable during the electrolysis. Enough basic knowledge will guide us to design and optimize TMP catalysts with high performance more effectively.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No. 21422104 and 21373149).

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