Recent developments in noble metal–based hybrid electrocatalysts for overall water splitting

Abstract

The quest for sustainable energy sources has accelerated the exploration of water splitting as a method of clean hydrogen production. Among the various electrocatalysts designed to drive water splitting, noble metal–based electrocatalysts have recently emerged as promising candidates. This review highlights the recent developments in noble metal–based electrocatalysts for overall water splitting. These electrocatalysts integrate the exceptional catalytic properties of noble metals, such as platinum, iridium, and ruthenium, with diverse materials, including transition metals, carbon substrates, and metal oxides, to enhance their efficiency, stability, and cost-effectiveness. This review discusses recent developments on noble metals such as platinum, palladium, rhodium, ruthenium, iridium, and noble metal–based hybrid materials as bifunctional electrocatalysts for overall water splitting. In addition, the existing obstacles and prospects for bifunctional water-splitting electrocatalysts are also focused.

Graphical abstract

Introduction

The ever-increasing use of fossil fuels generates a series of catastrophes, including air pollution, energy shortages, and climate change, enabling swift progress in renewable, environmentally friendly alternative energy sources [1,2,3,4,5]. In the context of pollution issues, finding clean, cost-effective, and renewable energy resources seems to be a critical objective for achieving long-term growth [6,7,8]. In such a case, hydrogen remains a viable alternative to traditional fossil fuels since it is a sustainable, more efficient, and flexible energy source. H₂ is an outstanding energy transporter and a strong applicant for prospective low-carbon energy systems because of its very high gravitational energy density and combustion by-product that remains environment-friendly, especially with water [9,10,11]. Furthermore, hydrogen is the most plentiful element in the cosmos and is mostly stored in the world’s vast water resources. As a result, water decomposition serves as a better approach for hydrogen production due to its sustainability and environmental friendliness [12, 13]. The water-splitting process can be accomplished using electricity or light. Electrocatalytic water splitting is a more practical technology than photocatalytic water splitting because it has a better conversion efficiency and higher hydrogen generation purity [14,15,16,17]. In general, water splitting demands significantly greater voltage than the theoretical voltage (1.23 V ) due to the slow kinetics of the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) of water electrolysis [18, 19]. As a result, developing high-efficiency catalysts for decreasing the overpotential associated with the cathodic HER and anodic OER is critical for accelerating H₂ and O₂ generation [20,21,22]. Despite significant efforts toward hydrogen production, noble metal and its alloy continue to be the efficient and effective electrocatalyst. To date, noble metal–based catalysts and their derivatives have proven to be effective electrocatalysts for HER, whereas OER electrocatalysts rely on iridium and ruthenium at the industrial level [23,24,25,26]. However, noble metals’ high price and scarcity tend to restrict their application in industries. As an outcome, scientists have devised several solutions to these issues, including shrinking noble metals to single atoms anchored on a porous conductive carbon-based matrix by hybridizing and alloying them with less expensive transition metals. Additionally, interface engineering is also used to solve their high price and instability [27, 28].

In the past few decades, many advanced noble metal–based electrocatalysts have been successfully developed for boosting water splitting, and some reviews regarding the electrocatalytic performance studies of noble metal catalysts have been published. However, the advances in noble metals and their derivatives as electrocatalysts for accelerating water splitting have not been systematically summarized. Considering the significant potential of water electrolysis and the great merits of noble metals and their derivatives as electrocatalysts, a comprehensive review should be made. Herein, we summarized the recent progress toward noble metals and their derivatives in the field of electrocatalytic water splitting with a particular emphasis on the merits of nanostructures. Moreover, some representative noble metal–based electrocatalysts and many effective strategies for optimizing their electrocatalytic performance have also been manifested. Lastly, the challenging issues and future directions of noble metal–based electrocatalysts in electrochemical water splitting are also outlined.

Electrochemistry of overall water-splitting reaction

The overall water splitting is a very straightforward chemical reaction: 2H₂O → 2H₂ + O₂. It primarily consists of two half-reactions that practically co-occur: the cathodic hydrogen evolution reaction and the anodic oxygen evolution reaction, which necessitates the four-electron transfer. However, the overall water-splitting reaction varies depending on the pH of the solution. Along with pH, the electrolyte involved also plays a major role. According to the Nernst equation, acidic environments encourage HER, while OER proceeds more easily in alkaline environments. Figure 1 shows the schematic representation of a conventional water electrolyzer [29].

In alkaline medium:
At anode:	4OH− → 2H2O + O2 + 4e−	E° = −0.40 V
At cathode:	4H2O + 2e− → 2H2 + 4OH−	E° = −0.83 V
In acidic medium:
At anode:	2H2O → O2 + 4H+ +4e−	E° = −1.23 V
At cathode:	4H+ + e− → H2	E° = 0 V

Fig. 1

Schematic representation of conventional water electrolyzer [29] reproduced with permission from ref. 27, copyright 2020, International Journal of Hydrogen Energy

Hydrogen evolution reaction (HER)

The HER process is a two-electron reaction that starts with the adsorption of water molecules at catalytic active sites (Volmer step) and ends with the successive desorption of water molecules from the surface of the electrocatalyst through either electrochemical desorption (Heyrovsky step) or chemical desorption (Tafel recombination). Therefore, the generic response path could be represented by either the Volmer-Heyrovsky or Volmer-Tafel path.

Mechanism of HER catalysis in alkaline medium

Step I:

Volmer reaction: H₂O + e⁻ → OHv⁻ + H_ads

Step II:

Heyrovsky step: H₂O + H_ads + e⁻ → OHv⁻ + H₂

Step III:

Tafel reaction: 2H_ads → H₂

Mechanism of HER catalysis in acidic medium

Step I:

Volmer reaction: H⁺ + e⁻ → H_ads

Step II:

Heyrovsky step: H_ads + H⁺ + e⁻ → H₂

Step III:

Tafel reaction: H_ads + H_ads → H₂

Oxygen evolution reaction (OER)

As the OER process comprises complex routes with multistage four-electron transfer, particularly with slower kinetics, the OER process is significantly more difficult than the HER.

Mechanism of OER catalysis in acidic medium

The total oxidation process of water is

$$2{\mathrm{H}}_2{\mathrm{O}}_{(l)}\to {\mathrm{O}}_2+4{\mathrm{H}}^{+}+4{e}^{-};{\varDelta \mathrm{G}}_0=4.92\mathrm{eV}$$

The underlying mechanisms for a typical catalytic surface are as follows [30]:

$$\mathrm{M}+{\mathrm{H}}_2{\mathrm{O}}_{(l)}\to \mathrm{M}-\mathrm{OH}+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}$$

$$\mathrm{M}-\mathrm{O}\mathrm{H}+{\mathrm{O}\mathrm{H}}^{-}\to \mathrm{M}-\mathrm{O}+{\mathrm{H}}_2{\mathrm{O}}_{(l)}+{\mathrm{e}}^{-}$$

$$\mathrm{M}-\mathrm{O}\to 2\mathrm{M}+{\mathrm{O}}_{2\left(\mathrm{g}\right)}$$

$$\mathrm{M}-\mathrm{O}+{\mathrm{H}}_2{\mathrm{O}}_{(l)}\to \mathrm{M}-\mathrm{O}\mathrm{OH}+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}$$

$$\mathrm{M}-\mathrm{OOH}+{\mathrm{H}}_2{\mathrm{O}}_{(l)}\to \mathrm{M}+{\mathrm{O}}_{2\left(\mathrm{g}\right)}+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}$$

Mechanism of OER catalysis in alkaline medium

The overall reaction is as follows [29,30,31]:

$$4{{\mathrm{O}\mathrm{H}}^{-}}_{\left(\mathrm{aq}\right)}\to 2{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{aq}\right)}+4{\mathrm{e}}^{-}+{\mathrm{O}}_{2\ \left(\mathrm{g}\right)}$$

The OER follows the multi-step process below [31]:

$$\mathrm{M}+{\mathrm{OH}}^{-}\to \mathrm{M}\hbox{--} \mathrm{OH}$$

$$\mathrm{M}-\mathrm{O}\mathrm{H}+{\mathrm{OH}}^{-}\to \mathrm{M}-\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$$

$$2\mathrm{M}\hbox{--} \mathrm{O}\to 2\mathrm{M}+{\mathrm{O}}_2\left(\mathrm{g}\right)$$

$$\mathrm{M}\hbox{--} \mathrm{O}+{\mathrm{OH}}^{-}\to \mathrm{M}\hbox{--} \mathrm{O}\mathrm{OH}+{\mathrm{e}}^{-}$$

$$\mathrm{M}-\mathrm{OOH}+{\mathrm{O}\mathrm{H}}^{-}\to \mathrm{M}+{\mathrm{O}}_2\left(\mathrm{g}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$$

Parameters for the evaluation of electrocatalytic performance

Though several factors must be considered to assess an electrocatalyst for water splitting, they can be grouped under three main categories: activity, stability, and efficiency. Furthermore, the activity is influenced by several variables, including overpotential, Tafel slope, and exchange current density, which can be deduced from polarization curves. The stability is described by the overpotential or current change over time. On the other hand, the efficiency comprises the Faradaic efficiency and the turnover frequency (TOF).

Electrocatalytic activity

Overpotential

Overpotential (η) is a crucial criterion for assessing electrocatalyst performance. At standard conditions of 25 °C and 1 atm, the potential required for water splitting is 1.23 V [32]. Nonetheless, this potential (1.23 V) is more than the stated value. This unanticipated reaction is so ubiquitous in electrochemistry that is known as overpotential. As a result, the applied potential must be raised for an electrolytic reaction to occur.

The overpotential is given by the formula:

$$\eta ={\mathrm{E}}_{\mathrm{Applied}}-{\mathrm{E}}_0-\mathrm{iR}$$

where iR is the ohmic correction, E₀ is the theoretical equilibrium value, and E_applied is applied potential. In short, overpotential (η) is largely utilized to overpower the intrinsic activation barriers existing on the anode (η_a) and cathode (η_c) and a variety of additional resistances (η_other), such as solution and contact resistances [32]. For example, N-doped carbon-wrapped Ir-doped Ni nanoparticle composites supported on Ni foam exhibit low overpotentials of 32 mV at 10 mA cm⁻² for HER, while it exhibits an overpotential of 329 mV at 50 mA cm⁻² for OER [33]. Similarly, SrIrO developed by L. Zhang et al. displays an extremely low overpotential of 245 mV for OER and 18.2 mV for HER at 10 mA cm⁻² [34]. In summary, a good catalyst should attain a greater current density while having a smaller overpotential [35].

Tafel slope and exchange current density

The Tafel slope (b) and the exchange current density (j₀) utilized to assess the electrocatalyst activity are calculated using the Tafel equation [30, 36, 37]:

$$\eta =a+ blog\left(|j|\right)$$

$$\left(\mathrm{a}=2.303\mathrm{RT}\ \log \left({\mathrm{j}}_0\right)/\alpha \mathrm{nF},\mathrm{b}=2.303\mathrm{RT}/\upalpha \mathrm{nF}\right)$$

where η is the overpotential which is the variation in potential of the electrode and standard potentials (η = E − E₀), b is the Tafel slope value, j is the measured current density, j₀ is the exchange current density, T is the temperature, R is the gas constant, n is the number of transferred electrons in the electrochemical redox reaction, F is the Faraday constant (96,485 C mol⁻¹), and α is the charge transfer coefficient. In addition, the exchange current density (j₀) is the overpotential value when the current density is zero, indicating the electrochemical process rate [30, 37]. For example, S. Zhang et al. reported the synthesis of nickel (Ni)-iridium (Ir) alloy nanoparticles/graphene hybrids for overall water splitting. As a result, the Tafel slope of Ni_0.93Ir_0.07/rGO (64.3 mV dec⁻¹) is found to be smaller than that of Ir/rGO (118.7 mV dec⁻¹) and Ir/C (74.3 mV dec⁻¹) [38]. Therefore, the electrocatalyst with a lower Tafel slope (b) and higher exchange current density (j₀) seem to be the desirable characteristics for water splitting.

Electrocatalytic efficiency

Faradaic efficiency

Faradic efficiency is an important measure that reflects the efficiency of electron usage during the electrochemical process. It is defined as the ratio of the experimentally discovered amount of H₂ or O₂ to the theoretically estimated amount of H₂ or O₂. Because of the rare side reaction, water splitting has a high Faradic efficiency, which benefits practical applications for energy usage efficiency [32]. For example, NiFe-LDH nanosheets developed by H. Sun et al. exhibited a faradaic efficiency of ~ 100% demonstrating that all the electrons are involved in catalytic reactions [39].

Turnover frequency (TOF)

The term TOF describes the amount of reactant a catalyst can transform into the desired product per catalytic site in a given amount of time [40]. The TOF is calculated by the formula:

$$\mathrm{TOF}=\mathrm{JA}/\alpha \mathrm{Fn}$$

where j represents current density and A represents the working electrode’s surface area. α indicate the electron number (electrons/mol) of the target product. F is Faraday’s constant (96,485.3 C mol⁻¹), and the number n denotes the number of moles of active materials determined by calculating electrochemically active surface area (ECSA). Furthermore, not all atoms are catalytically active and equally accessible. As a result, obtaining an exact TOF value is challenging.

Bifunctional electrocatalysts

Electrochemical water splitting is recognized as a viable method for producing hydrogen, which can be used to replace fossil fuels and store energy sustainably. Developing highly efficient and low-cost electrocatalysts is vital to the large-scale implementation of hydrogen energy devices. Bifunctional electrocatalysts that work for both HER and OER are favorable for the overall water-splitting systems, promoting the system’s performance and simplifying the system design. In recent years, noble metal–based compounds have been extensively explored and employed as bifunctional electrocatalysts for overall water splitting, such as platinum, palladium, ruthenium, iridium, inter-noble metals, and hybrids.

Noble metal–based electrocatalysts for water splitting

Initially, non-precious metals and non-metal-based electrocatalysts have garnered significant interest in the field of electrochemistry due to their potential that reduce costs and alleviate the reliance on scarce and expensive precious metals like platinum and palladium. However, they also come with some drawbacks, which can limit their widespread adoption in various electrochemical applications. Here are some of the disadvantages associated with non-precious metals and non-metal-based electrocatalysts such as lower catalytic activity and limited selectivity and stability. Non-precious metal electrocatalysts often exhibit lower catalytic activity than their precious metal counterparts, which means they may require higher overpotentials (applied voltage exceeding the thermodynamic minimum) to drive electrochemical reactions effectively, which can result in lower energy efficiency. In addition, they are susceptible to degradation over time, especially under harsh electrochemical conditions, reducing their long-term stability and performance.

The best and most practical materials in terms of performance for electrocatalytic water splitting are the noble metal–based compounds such as platinum (Pt), palladium (Pd), rhodium (Rh), ruthenium (Ru), and iridium (Ir), collectively called the “platinum-group metal” (PGM). To date, Pt/C is set as a benchmark for comparing the performance of upcoming electrocatalysts and is commercially used worldwide in essential places. Nevertheless, its limited storage capacity, availability, and high price hinder the commercialization of noble metal–based catalysts. To meet this problem, sensible catalyst designs with minimal metal loading and high metal utilization are required.

Platinum (Pt)-based materials

Platinum (Pt) and Pt-based alloys are currently regarded as state-of-the-art for HER catalytic materials, owing to the beneficial development of Pt–H_ads at the active sites, which allows for adequate water molecule adsorption. However, the high cost, limited elemental sources, and inferior durability of Pt-based catalysts have severely hampered their wide usage. Thus, using a bifunctional electrocatalyst simplifies the water-splitting process, reduces costs, and enhances the overall electrocatalytic efficiency. However, poor OER performance of platinum has hampered the development of perfect water-splitting systems. Though Pt serves as a benchmark for HER catalyst, the catalytic activity of Pt is hampered by water dissociation due to an insufficient oxyphilic surface to divide the O–H bond of water molecules. Among various metal hydroxides, Ni (OH)₂ has been found to promote dissociation of water and hydrogen generation. A tremendous synergic effect exists when Ni (OH)₂ is combined with Pt. Capitalizing on this fact, H. Liu et al. prepared Pt-decorated Ni (OH)₂/CeO₂ as a hybrid electrocatalyst. The Pt NPs with an average size of 3.1 nm are homogeneously deposited on the Ni (OH)₂/CeO₂ (abbreviated as NC) nanosheets, which could increase the exposure area of active sites and improve the utilization of Pt. Additionally, the mosaic-structured NC nanosheets not only effectively catalyze the OER but also enhance the HER activity of Pt due to electronic modulation and promoted dissociation of water. The as-prepared PNC hybrid possessed an overpotential of 76 mV for HER and 186 mV for OER at 100 mA cm⁻². The PNC hybrid demanded a cell potential of 1.45 V to attain 10 mA cm⁻² in 1 M KOH. The calculated turnover frequency (TOF) of the PNC hybrid was found to be 3.488 s⁻¹, remarkably performing better than Pt/C (0.455 s⁻¹) and Pt/G (0.254 s⁻¹). In addition, the PNC hybrid was found to perform well at high current density with fast formation and liberation of bubbles without any catalyst degradation, which seems suitable for industrial utilization. In an extended stability test, a constant potential of 2.06 V was maintained for 85 h with minor degradation at the current density of 1000 mA cm⁻². This higher performance was attributed to the large scattering of platinum NPs plus the electronic interaction between CeO₂, Ni (OH)₂, and Pt. In contrast, the stability enhancement was attributed to the strong deposit/graphite adhesion, hydrophilicity of the electrode, and aided release of gas bubbles [41]. Thus, alloying Pt with 3d transition metals serves as a prominent strategy for boosting the catalytic activity by enhancing the electronic and geometric structures of Pt. Furthermore, platinum electrocatalysts, including Pt–M alloys (M = transition metals, transitional metal chalcogenides/carbides/phosphides), have also been extensively investigated for efficient catalysts [42,43,44,45]. Therefore, Pt-based electrocatalysts are produced by lowering the amount of Pt while retaining electrochemical activity and stability. It is also known that nickel oxides or hydroxides catalyze the breakage of water molecules, producing H intermediates that adsorb on neighboring Pt to form H₂ [46,47,48,49,50]. As a result, Ni-based electrocatalysts serve as favorable electrocatalysts for OER and Pt for HER. Hence, it is advantageous to fabricate PtNi alloy nanoparticles as bifunctional catalysts for overall water splitting. Recently, X. Chen et al. electrodeposited a trace amount of Pt on NiFeO_xH_y film with a copious amount of Ni³⁺ structure to improve the performance of water electrolysis. The Pt particles with a size of several nanometers are dispersed uniformly in NiFeO_xH_y film. The short-brush NiFeO_xH_y film undergoes easier surface transformation than nanosheets to form an active NiFe(OOH)_x phase during OER activation. During OER activation, the NiFeO_xH_y facilitates surface modification, forming NiFe(OOH)_x phase, as it reached 10 mA cm⁻² with an overpotential of 204 mV, performing better than typical NiFeO_xH_y nanosheets. Furthermore, the Pt-modified materials possessed high Pt mass-specific activity and excellent HER catalytic activity with a cell potential of 1. 54 V, whereas commercial RuO₂ and Pt/C on NF required more than 1.65 V. For HER, the hybrid Pt/NiFeO_xH_y/NF catalyst resulted in an overpotential (η) of 48 mV, which remains lower than the NiFeO_xH_y/NF catalyst. Furthermore, a Tafel slope of 42 mV decade⁻¹ is observed for this catalyst. The low Tafel slope indicates that the Heyrovsky step, rather than the Volmer step, determines the electrocatalytic HER kinetics on hybrid Pt/NiFeO_xH_y/NF, demonstrating that the hybrid catalyst successfully stimulates water splitting in alkaline conditions. In the case of OER, this catalyst required an overpotential of 204 mV (@10 mA cm⁻²), and their corresponding Tafel slopes were as small as 28 mV dec⁻¹, as shown in Fig. 2. Hence, this electrocatalyst performs better than the recently reported Ni-based compounds [46].

Fig. 2

a Schematic illustration of the synthesis process of the NiFeO_xH_y/NF and hybrid Pt/NiFeO_xH_y/NF electrodes, b SEM images of NiFeO_xH_y/NF-1 prepared on the Ni foam, c Tafel plots of the Ni foam and NiFeO_xH_y/NF samples, and d Tafel plots of Ni foam, NiFeO_xH_y/NF, Pt/NiFeO_xH_y/NF, and Pt/C/NF 49 reproduced with permission from ref. 50, copyright 2022, Applied Surface Science

Similarly, through a simple and scalable corrosive-coordinate method, Chen et al. constructed a 2D-3D nanostructured hybrid with trace Pt (Pt-NiFe PBA) via a corrosion-coordination method. Herein, a 2D-3D nanostructure composed of metal hydroxides and Prussian blue analogous (PBA) was in situ decorated onto the NiFe foam (Pt-NiFe PBA) through a facile and scalable corrosive-coordinate approach. This specifically designed morphology favors the provision of abundant active sites, optimizes the reaction pathway, and accelerates mass transport during the electrocatalytic process. The as-synthesized catalyst required the overpotentials of 210 mV for OER and 29 mV for HER in an alkaline medium. Meanwhile, a favorable reaction kinetics was also evident with the Tafel slope, for HER value of 58.3 mV dec⁻¹, which was lesser in comparison to NiFe-LDH (148.9 mVdec⁻¹), NiFe PBA (124.3 mVdec^ͨ−1), and even Pt/C (67 mVdec⁻¹), and the corresponding OER value was 43.5 mVdec⁻¹. They suggested that the high performance may be attributed to a variety of factors, including coupling effects, specific nanostructure, binder-free feature, and hydrophilicity/hydrophobicity features, which resulted in favorable mass/charge transport, abundant exposed active sites, and improved reaction kinetics. Furthermore, to propel forward from the traditional water splitting, they have set out a distinct experiment of approaching seawater electrolysis, which would be a suitable alternative. The electrocatalytic performance on 1 M KOH in seawater was excellent as it required the smallest overpotential of 21 mV to attain 10 mA cm⁻² and 47 mV to 50 mA cm⁻² [51]. Recently, layered double hydroxide has been focused as an electrocatalyst for water splitting. Several studies on catalytic water splitting with LDH-based materials as catalysts have been published, showing a fast growth trend [51,52,53,54,55]. This exposure to LDH was attributed to their abundance, lower price, and resource-richness compared to the activity and durability of traditional noble metals. Similarly, X. Zhang et al. anchored Pt clusters on NiMo hydroxide nanosheet via the chlorine etching method and found that this catalyst showed excessively high stability for 160 h. The SEM image indicated the formation of Pt clusters with an average size of 1.6 nm. These Pt clusters were found to be uniformly anchored on amorphous NiMo-OH/NF nanosheets. Regarding catalytic activity, the catalyst exhibited only a small overpotential of 34 mV (η₁₀) and 190 mV (η₁₀) for HER and OER, respectively. The high TOF number, mass activity, and ECSA proved their better HER performance. This excellent HER activity was caused by the collaborative electron transfer between platinum, nickel, and molybdenum. In terms of kinetics, the Tafel slope of Pt-NiMo-OH was 30.5 mV dec⁻¹, which was lesser than bare nickel foam (182.23 mVdec⁻¹) and Pt/C/NF (55.1 mVdec⁻¹). This better OER performance was attributed to the formation of high valence Mo and Ni, which was instrumental in the formation of OER active materials [56]. Y. Feng et al. also worked on platinum-nickel-based hydroxide. They synthesized vanadium-doped Pt clusters on NiFe-LDH (Pt-NiFeV) for overall water splitting. The continuous electrochemical CV cycling dissolutes Pt atoms from Pt net to get deposited onto the nucleation sites of the working electrode: defective NiFe-LDHs-Ar. Since the Fe and Ni atoms around the defect sites in NiFe-LDHs-Ar were active, the electrochemical cycling at negative potentials reduces the part of Ni and Fe ions. The Pt atoms diffuse from the counter electrode preferred to couple with the in situ reduced FeNi atoms to form PtFeNi alloys. Their research emphasized on the importance of doping vanadium in Pt clusters onto NiFe-LDH. The TEM analysis revealed that the self-reduced Pt nanoparticles on the NiFe-LDH nanosheets (Pt/NiFe) are apparently larger than those in Pt/NiFeV, which indicates that V doping could prevent Pt overgrowth or agglomerate into large nanoparticles. This catalyst possessed a very high catalytic activity with an overpotential of 19 mV at 10 mA cm⁻² and Tafel slope of 33 mV dec⁻¹ for HER and 254 mV at 50 mA cm⁻² and Tafel slope of 48 mV dec⁻¹ for OER. This outstanding performance was also supported by DFT simulation, which showed that V doping modified the d-band center of Ni active sites, weakening the adsorption of *OH intermediates and facilitating the desorption of oxygen molecules at the anode. As a result of V doping, more electrons were transferred to neighboring Fe atoms, leading to an elevated reducing ability and optimized free energy of hydrogen adsorption and proton recombination kinetics at the cathode [57]. Combining an electrocatalyst with a support material results in significant advantages such as coalescence during electrochemical testing, which provides vital connections between nanoparticles and electrolytes, embeds the nanoparticles against detachment, and provides long-term stability against disintegration. Carbon nanofiber mats (CFMs) have recently gained much attention among the numerous carbon-based support materials because of their excellent mechanical stability, chemical resilience, and high electrical conductivity for charge transfer. D. Yin et al. proposed a Pt-WO_3−x@rGO hybrid composed of Pt, WO_x, and graphene. In this work, a new structure-defined cis-platin-modified Dawson-type POM complex of K₆[P₂W₁₇O₆₁{cis-Pt (NH₃)₂}₂] 32H₂O (P₂W₁₇Pt₂) has been successfully fabricated by reaction of cis-platin and mono-lacunary Dawson-type POM. Then, through photochemical reduction, the in situ generated P₂W₁₇Pt₂@rGO hybrid can be achieved by anchoring POM on the reduced graphene (rGO) substrate. In the subsequent calcination process, the P₂W₁₇Pt₂@rGO hybrid forms a new ternary hybrid of Pt-WO_3−x@rGO, which exhibits excellent electrocatalytic water-splitting performance due to the combination of surface small Pt⁰ nanoparticles on the WO_3−x substrate, which shows the better HER and OER activities. This catalyst demanded a cell potential of 1.55 V to attain 10 mA cm⁻² in alkalinity. The spherical Pt⁰ nanoparticles were widely dispersed on the WO₃ surface. The element analysis revealed a very low content of Pt, with a Pt ratio of 1.58%. However, this catalyst resulted in better performance for HER, with an overpotential of about 13 mV at 10 mA cm⁻², which was lower than 20% Pt/C catalysts (29 mV). Meanwhile, the prepared hybrid resulted in a trivial Tafel slope value (35 mV dec⁻¹) than the standard benchmark Pt/C (45 mVdec⁻¹). In the case of OER, the overpotential was 174 mV at 10 mA cm⁻² in an alkaline medium. In addition, the low Tafel slope (48 mVdec⁻¹) confirmed the highest OER activity. In the alkaline condition, it exhibited a mass activity of 27 A mg⁻¹ at an overpotential of 30 mV [58]. In addition, the performances of platinum-based electrocatalysts for water splitting are summarized in Table 1.

Table 1 Summary of platinum-based electrocatalysts for water splitting

Palladium (Pd)-based materials

Palladium (Pd) nanocatalyst has recently been used as a replacement for platinum (Pt) nanocatalysts due to its increased electrocatalytic activity in alkaline conditions and availability. Palladium’s electronic structure and catalytic activity can be manipulated by altering the local structure by alloying Pd with foreign metals or supporting Pd on metal oxides. Recently, S. Lu et al. identified a hitherto undiscovered two-dimensional palladium carbide using the structure swarm intelligence technique. The proposed monolayer, known as α-PdC, consisted of completely dispersed Pd atoms and a stiff carbon backbone with sufficient stability and strong electrical properties as suggested by theoretical calculation. As a result, it exhibited an excellent catalytic performance of about − 0.01 V (HER overpotential) and a low activation energy barrier (0.16 eV) [73]. In another study, Ipadeola et al. successfully anchored Pd nanocatalysts on chemically etched Ni metal-organic framework-derived carbon using microwave-assisted solvothermal synthesis. The resultant Pd/Ni/MOFDC reveals that the nanocatalysts’ surfaces have flake-like morphology and exhibit better kinetics toward the HER and OER than the acid-treated nickel counterpart. This catalyst showed moderately improved kinetics and mass transport because of increased exchange current density (j_o,s = 0.1079 mA cm⁻²), lower activation energy (21 kJ mol⁻¹), and Tafel slope (b_c = 173.0 mV dec⁻¹) values. The high j values of the Pd/Ni/MOFDC toward alkaline HER may be traced to the strong synergy between Ni/Ni(OH)₂ interface and nanostructured Pd, as the Ni/Ni(OH)₂ can facilitate continuous H₂O cleavage with immediate desorption of Hads from the active sites of Pd, as well as prompt recombination of Hads making the Ni/Ni(OH)₂ interface beneficial for alkaline HER [74, 75]. H. Zhang et al. designed an N-doped PdCoNi composite (Pd-e-NiCo-PBA-C) via pyrolysis of a Prussian blue analogue. Furthermore, the composition was manipulated by an ion exchange technique, and its catalytic performance was explored. As a result, the morphology and structure of the Pd-e-NiCo-PBA-C change from a nanocube to a hollow nanocage and finally to tapered nanosheets. Impressively, encapsulation of CoPd₂ nanoparticles onto N-doped graphite carbon matrix successfully prevented the prevailing corrosion and resulted in the catalytic activity of more than 50 h of stability for total water disintegration, which was better than other twin-function electrocatalysts that have been recently reported. Meanwhile, when tested with LSV, it showed a minor shift after 5000 cycles, demonstrating excellent stability performance. The prepared electrocatalyst was tested in both alkaline and acidic conditions. As a result, the obtained electrode resulted in an overpotential of 47 mV at acidic conditions with a corresponding Tafel slope of 55 mV dec⁻¹. Furthermore, at the alkaline (pH = 14) condition, the overpotential of 147 mV and the corresponding Tafel slope of 67 mV dec⁻¹ were achieved. For OER, this catalyst delivered the 10 mA cm⁻² at overpotentials of 309 mV, which produced results comparable to those of commercial IrO₂ (343 mV) [76]. The electrochemical analysis revealed that the conductive graphite carbon matrix possessed various advantages such as electrolyte diffusion property, high specific surface area, and electronic configuration adjustment using nitrogen and nickel co-doping, accompanied by the distinctive dodecagon nanosheet morphology, which contributed to the exceptional HER performance. The significant OER activity, especially in acidic solutions, was attributed to the carbon conducting support with metallic composite, which provides a comparatively hydrophobic environment for electrocatalysts in acidic media. Karuppasamy et al. prepared defect-enriched palladium nanocrystals encapsulated on a half-hollow nanotube-structured N-MoO₂-Mo₂C (HHNT) heterointerface. The icosahedral Pd NCs finely encapsulated the N-MoO₂-Mo₂C surface with the average size of the icosahedral Pd NCs of approximately 48.5 nm.The obtained N-MoO₂-Mo₂C HHNT resulted in a significant boost in overall water solubilization activity. Furthermore, on assembly of the electrolyzer cell, this composite displayed a cell voltage of 1.56 V at a current density of 10 mA cm⁻². For HER, it demonstrated an overpotential of 65 mV at 10 mA cm⁻². The synergistic effect of Pd NCs and MoO₂-Mo₂C heterointerface was responsible for the remarkable electrocatalytic activity. Additionally, the Tafel slope was used to investigate the catalytic reaction kinetics and processes of HER electrocatalysts. On investigation, the Pd/N-MoO₂-Mo₂C resulted in a lower Tafel value of 46.1 mV dec⁻¹ compared with Pt/C of 48.6 mV dec⁻¹. On the other hand, with the availability of active sites and unique structure, the Pd/N-MoO₂-Mo₂C electrocatalyst resulted in improved electrocatalytic performance with reduced activation energy. For OER, this as-prepared catalyst delivered a small overpotential (η) of 180 mV, and these results were considerably smaller than Pd icosahedral (η = 420 mV) and Pt/C (η = 440 mV). To further examine the interfacial reactions and kinetics of electrode, EIS was conducted, of which the lower R_ct value was responsible for the outstanding performances. The R_ct value of Pd/N-MoO₂-Mo₂C was 6.18 Ω, indicating more rapid electrode kinetics and increased catalytic activity. Here, the molar ratio of the produced O₂ and H₂ was determined quantitatively by gas chromatography and compared with the amount that had been theoretically calculated, where it was discovered to be 1.97:1 near to 100% Faradic efficiency [77]. W. Zhang et al. reported a method to hasten the hydrogen and oxygen evolution kinetics of NiFe-LDH by combining it with Pd-based material to form Pd/NiFeO_x nanosheets. This catalyst was tested under various conditions and found that the catalyst showed excessively high stability for 50 h. In case 1 M KOH solution, the as-synthesized Pd/NiFeO_x nanosheets exhibited outstanding HER activity as it needed an overpotential of 76 mV and an overpotential of 180 mV in 0.5 M H₂SO₄ solution. Moreover, the catalyst attained a 10 mA cm⁻² with overpotentials of 46 mV for HER and 169 mV for OER. Using 1 M phosphate-buffered saline (PBS) as an electrolyte, the catalyst exhibited a small overpotential of 75 mV (η₁₀) for HER and 310 mV (η₁₀) for OER. Because of the exceptional bifunctional activities of Pd/NiFeO_x throughout a broad pH range, an electrolyzer was built using Pd/NiFeO_x nanosheets as both anode and cathode to investigate the total water-splitting performance. The Pd/NiFeO_x‖Pd/NiFeO_x cell demanded a voltage of only 1.57 V to reach 20 mA cm⁻² in 1 M KOH solution with nearly 100% faradic efficiency [78]. Furthermore, the HER and OER performance of palladium-based electrocatalysts for water splitting are given in Table 2.

Table 2 Tabulation of HER and OER performance of palladium-based electrocatalysts

Rhodium-based materials

Initially, Duan et al. reported a new rhodium phosphide (Rh₂P) nanocube with excellent HER and OER performance in an acidic solution [85]. Furthermore, Q. Qin et al. reported rhodium phosphide which exhibited excellent trifunctional activities for oxygen reduction, oxygen evolution, and hydrogen evolution reactions. Similarly, F. Yang et al. also found that the rhodium-based nanoparticles showed superior results in terms of HER. Likewise, Y. Zhao et al. prepared Rh nanosheets (Rh NSs) as bifunctional electrocatalysts for water electrolysis [86]. Thus, due to various advantages such as corrosion resistance, outstanding durability, anti-toxic nature, and strong electrochemical performance, metal rhodium (Rh) should be given special consideration as an electrocatalyst across various pH values. Recently, layered double hydroxide has received much attention as a catalytic material due to its good electrocatalytic stability. Among the hydroxides, nickel and iron hydroxides exhibit better catalytic activity for OER but poor HER. Therefore, H. Sun et al. prepared Rh-doped NiFe-LDH by the hydrothermal method, exhibiting nanosheet morphology. It demonstrates an overpotential of 24 mV and a Tafel slope of 27 mV dec⁻¹ in an alkaline medium. The addition of Rh atoms to NiFe-LDH accelerates the dissolution of water molecules, improving HER performance. The improved kinetics were tested with EIS measurements. The low R_ct value demonstrated the involvement of a quick faradaic process and advantageous charge transfer kinetics. The intrinsic catalytic activity was examined with the help of electrochemical active surface areas. The ECSA of NiFeRh-LDH was normalized to be 1336.3 cm⁻² (high), which could be due to the nanosheet nature of NiFeRh-LDH, as well as the vast number of oxygen vacancies, and availability of a large number of active sites. An overpotential of 204 mV was needed for oxygen evolution catalytic performance, which was much lower than NiFeVLDH, RuO₂, and IrO₂. The electrocatalytic performance of the as-prepared electrocatalyst is shown in Fig. 3. Furthermore, this catalyst was also excellent in urea electro-oxidation reaction (UOR), which was proved when the urine-mediated electrolysis cell required a potential of just 1.3 V for delivering a 10 mA cm⁻² of current density. Additionally, the DFT calculation showed that the shift in the d-band center and the oxygen vacancies were responsible for the increased activity [39].

Fig. 3

Electrocatalytic HER properties. a Polarization curves of catalysts for HER (with iR compensation). b The derived overpotentials at 10, 50, and 100 mA cm⁻² for the as-prepared catalysts. c Nyquist plots of the as-prepared catalysts. Electrocatalytic OER properties. d Polarization curves of catalysts for OER (with iR compensation). e The bar chart comparing overpotentials at 10, 200, and 400 mA cm⁻². f Nyquist plots of the catalysts86. reproduced with permission from ref. 86, copyright 2021, Applied Catalysis B: Environmental

Cao et al. constructed a bifunctional catalyst for all pH values. Their strategy was to alloy noble metal with non-precious metal, reducing the high price and enhancing the electrochemical performance. Due to their higher electrocatalytic activity, they employed Cu-based 1D nanocrystals, especially nanotubes. In addition, the experiments and DFT calculations reveal that the formation mechanism of RhCu NTs is due to the etching of nanowires from both ends. The developed electrocatalyst resulted in various advantages such as increased interior and exterior active sites and preventing material aggregation. As a result, they synthesized defect-rich RhCu NTs via the wet chemical method. Specifically, the RhCu NT sample showed excellent HER activity with η₁₀ of 8, 12, and 57 mV for 1 M KOH, 0.5 M H₂SO₄, and 0.1 M PBS conditions, respectively. As a result of the bigger exchange current density (j₀) and lower Tafel slope, RhCu NTs had higher activity and faster HER kinetics than Pt/C at all pH values. Furthermore, to investigate their extraordinary HER performance, DFT calculations were performed, which revealed that the HER amplification in an alkaline medium was due to the transition in electronic structure caused by higher Rh content [87]. Rare earth metal phosphides are commonly used in industry as catalysts for oxygen removal (HDO), nitrogen removal (HDN), sulfur removal (HDS), and other processes. This has prompted researchers to look into their usage in electrocatalysis. Among them, rhodium phosphide exhibits different crystalline phases and therefore shows an impressive electrocatalytic performance. X. Wu et al. in their experiment synthesized rhodium phosphide nanoparticles implanted in a nitrogen/phosphorus-doped carbon matrix with a very low Rh loading of 39.2 μg cm⁻². The resultant bifunctional electrocatalyst (Rh₂P-N/P-CC) outperformed well against Pt/C. In terms of stability for HER, after a time of 40 h only, a mere surge of 13 mV was observed at 100 mA cm⁻². This catalyst displayed significantly enhanced HER activity with an overpotential of 5 mV at 20 mA cm⁻². The Rh₂P-N/P-CC had a Tafel slope of 47 mV dec⁻¹, similar to Pt/C (49 mV dec⁻¹), showing a kinetically accelerated HER process. Similarly, the required overpotentials for OER was 274 mV to achieve a current density of 20 mA cm⁻². The sensible integration of its unique structural and compositional characteristics resulted in its exceptional electrocatalytic performance. Thus, Rh2P nanoparticles proved to be efficient electrocatalysts with minimal loading of Rh, with their uniform distribution and partial attachment of the doped atoms significantly improving stability at high current density electrolysis [88]. Due to their huge specific surface area, rich coordinated atom content, and high density of edges and flaws, two-dimensional is the most alluring electrocatalyst for accelerating water electrolysis. The intrinsic properties of graphene-based materials, such as graphene as an electroactive component, reduced graphene, graphene quantum dots, doped graphene, and functionalized graphene supports, have attracted a lot of interest in the catalysis field, and have shown particular promise in water electrolytic application. Narwade et al. developed a hybrid electrocatalyst of rhodium nanoparticles decorated onto graphene oxide with elevated exposure to active Rh nanosphere (NS) sites. For HER, the faster kinetics can be proved by the smallest Tafel slope of 10 mV dec⁻¹, Rh−GO catalyst showed majestic HER activity, as it possessed a minute overpotential of 2 mV at 10 mA cm⁻². To further probe the activity of this, catalyst electrochemical impedance spectroscopy (EIS) was used. The R_ct values of 6.6 Ω indicated the substantial enhancement in electron transport during HERs in acidic media. In OER studies, the catalyst reached 10 mA cm⁻² at an overpotential of 0.23 V and provided a Tafel slope of 27 mV dec⁻¹. These outcomes supported hydroxyl regeneration poisoned by Rh, resulting in excellent performance and reduced hydrogen production cost via electrolysis. This outstanding performance was attributed to the decoration of Rh NSs onto GO, which increased viability toward interfacial electron transfer [89]. In addition, the HER and OER performances of rhodium-based electrocatalysts are summarized in Table 3.

Table 3 Summary of HER and OER performance of rhodium-based electrocatalysts

Iridium-based materials

Combining Ir with non-noble metals is not only an effective way to reduce Ir consumption, but it also has the potential to change the electronic structure and adsorption energy, resulting in increased intrinsic activity for water splitting. R. Fan et al. doped Ir with NiCo-layered double hydroxide via the solvothermal method. The FESEM image of the Ir-NiCo LDH exhibits homogeneously distributed bright dots (~ 0.2 nm), confirming the presence of single Ir atoms dispersed in the NiCo LDH nanosheets. This as-synthesized Ir-NiCo LDH on NF demonstrated an exceptional long-term stability of over 200 h, which makes it the best among Pt/C||Ir/C couple and other recently reported bifunctional catalysts. This can be ascribed to the following two factors: (1) Ir-NiCo LDH’s lattice, Ir atom existed as independently scattered single atoms. Single-atomic Ir in Ir-NiCo LDH was remarkably stable due to the strong electrical connection between the iridium atom and the NiCo LDH, which played a vital role in increasing catalyst stability under working conditions. (2) Significant crystal defects like OVs were found due to spontaneous galvanic displacement between NiCo LDH and Ir precursor. The as-prepared catalyst displayed an exceptional catalytic activity for its application. To achieve 10 mA cm⁻², overpotentials of 21 mV (for HER) and 192 mV (for OER) were required. Moreover, their resultant Tafel slopes were only 35 mV dec⁻¹ and 41.2 mV dec⁻¹. The Ir-doped NiCo LDH electrocatalyst achieved an extraordinary performance with a modest cell voltage of 1.45 V in an alkaline medium due to synergistic effects in the HER and OER process, along with the ECSA and enhanced electrical conductivity. Additionally, EIS studies were employed to understand better the kinetics related to catalytic activity. With the help of the Nyquist plot, it was found that the Ir-NiCo LDH exhibited only minor charge transfer resistance R_ct of 0.65 Ω. Thus, the complete water-splitting efficiency of the Ir-NiCo LDH coupling was assessed. Furthermore, in the case of the water electrolysis carried out for 90 min, the Faradaic efficiency was approximately 100% [96]. To improve the catalytic performance, interface control has recently received much attention. The creation of defect active sites results in improved electrocatalytic activity, which might be greatly enhanced by homointerface. Homointerfaces are believed to cause defects due to lattice distortion, which is critical for enhancing electrocatalytic activity. Investigating this distinctive morphology of Ir-based materials is crucial for enhancing electrocatalytic water-splitting performance. Gao et al. developed homointerface IrCo_x nanorings (NRs) with bracelet-like architecture using a simple, controlled, surfactant-free method. The nanoring-structured IrCo_x was synthesized by combining reduction and an etching process. With the help of EDX, it was confirmed that the bracelet-like IrCo_x morphology was formed with ample homostructure interfaces. The LSV shows that, when compared to the C-IrCo_0.14, the IrCo_0.14 nanowire had an enhanced HER performance, with an overpotential of 16 mV and a corresponding Tafel slope of 28.8 mV dec⁻¹, showing improvement in HER activity and reaction kinetics. Likewise, for OER, the anodic reaction reached an overpotential of 278 mV at 10 mA cm⁻² [97]. In addition, Fig. 4 indicates the (a) schematic of the solar water-splitting system in 0.1 M HClO₄ and (b) photos of an electrolyzer device powered by a Si cell in 0.1 M HClO₄.

Fig. 4

a Schematic of solar water-splitting system in 0.1 M HClO₄. b Photos of full water-splitting device powered by a commercial Si cell in 0.1 M HClO₄ [97], reproduced with permission from ref. 97, copyright 2020, Electrochimica Acta

Similarly, J. Chen et al. synthesized an iridium-iron alloy with a volcano-like morphology, providing a high specific surface area and abundant active sites to enhance catalytic activity. This distinctive volcano-like morphology FeIr alloy was grown hydrothermally with 0.546 wt% Ir loading. This material presented exceptional catalytic activity and stability for HER, OER, and overall water splitting. The bifunctional electrocatalyst possessed an overpotential of only 220.0 and 25.6 mV for OER and HER, respectively. Even at an elevated current density of 1000 mA cm⁻², the overpotentials are only 327.0 mV (HER) and 410.0 mV (OER) with exceptional stability and negligible declines after being maintained for 124 h. As soon as the HER long-term stability test was complete, the SEM pictures and HRXPS spectra were analyzed. Its outstanding stability is evidenced by the fact that the structural and elemental states were maintained. Thus, water splitting required 1.51 and 1.75 V to arrive at 10 and 500 mA cm⁻². Furthermore, it was operated at 500 mA cm⁻² for 100 h in 30 wt% KOH solution at 60 °C, which seems useful for widespread applications. Thus, the excellent performance was predominantly due to (i) Fe and Ir alloying that improved the inherent catalytic activity, (ii) a distinctive volcano-like structure that provided adequate active sites, and (iii) a self-supporting structure that maintained superior stability [98]. The synthesis process of the as-prepared FeIr alloy is shown in Fig. 5.

Fig. 5

Schematic representation of synthesis illustration of volcano-like FeIr/NF [98] reproduced with permission from ref 98, copyright 2021, Chemical Engineering Journal

Recently, multi-metal oxides formed by the combination of iridium and metal serve as a promising strategy to address insufficient activity and stability of Ir as an electrocatalyst [99,100,101]. Many Ir-based 3D perovskites (AIrO₃), where A represents an alkaline-earth/alkaline/ metal ion, have been investigated recently. L. Zhang et al. constructed strontium iridate perovskites, SrIrO (laminar Sr₂IrO₄ modified with 6H phase SrIrO₃) [34]. The in-plane corner-shared IrO₆ octahedral framework’s structural stability after cation leaching was improved, improving its stability during catalysis, as shown in Fig. 6. This is because of the layered nature of the material, which makes it easier for the cations (interlaminar) to be extracted without changing that structure’s fundamental makeup. The deformed monoclinic hexagonal structure of the 3D 6H SrIrO₃ has been theoretically and experimentally shown to be more active for the OER.

Fig. 6

a Crystal structures of quasi-2D Sr₂IrO₄ and 3D SrIrO₃. b SrIrO-1100-based two-electrode electrolyzer driven by a battery of 1.5 V. c LSV curves for OER and HER in 0.5 M H₂SO₄. d The overall water-splitting LSV curves for SrIrO-1100 and commercial IrO2‖Pt/C in 0.5 M H₂SO₄ [34] reproduced with permission from ref 102 copyright 2021, Chemical Engineering Journal

Taking advantage of the benefits, the as-prepared SrIrO catalyst exhibited exceptional catalytic activity, with an overpotential of 18.2 mV at 500 mA cm⁻². A layer of IrO_x with IrO₆ octahedral structure domains was generated on the surface during the testing. As a result, it is inferred that SrIrO’s extraordinary HER activity was predominantly due to the as-fabricated IrO6 octahedra-dominated IrO_x layer after Sr leaching. As a result, higher HER activity during continuous HER tests was due to metallic Ir’s occurrence on the catalyst surface. As a result, SrIrO-1100 resulted in a better Tafel slope value of about 30.6 mV dec⁻¹, which was near to the benchmark Pt/C (27.5 mV dec⁻¹), showing that it had quick response kinetics that proceeds via the Volmer-Tafel reaction mechanism. In a full cell analysis for total water splitting, SrIrO endowed a small cell potential of 1.50 V at 10 mA cm⁻², significantly superior to most electrocatalysts in an acidic environment [102, 103]. Recent studies have shown that combining iridium with 3d transition metals can manipulate the electronic structure of Ir by improving the intrinsic activity and reducing the usage of Ir. Meanwhile, doping a heteroatom within carbon materials plays a significant role in stability by modifying electronic structures and interacting metal with support. As a result of this foresight, Z. Zhang et al. achieved a record-low overpotential of 4.7 mV@10 mA cm⁻² with Ir/Fe@NCNT (Ir content = 4 wt%) composite for cathodic reaction. The catalyst was synthesized by pyrolyzing ferric chloride, melamine, and iridium trichloride to obtain Ir(20)/Fe@NCNT-900. High standards were maintained well in terms of the kinetic behavior of the as-prepared catalyst. A conclusion was reached that both Ir(20)/Fe@NCNT-900 and Pt/C have approximately closer Tafel values. Another cardinal parameter for evaluating kinetics was the R_ct value. The R_ct value of 0.93 Ω was obtained from the Nyquist plot, indicating its rapid charge transfer during the HER. Meanwhile, the turnover frequency (TOF) was estimated to be 6.65 s⁻¹, superior to the reported Ir-based catalysts. This indicates that Ir(20)/Fe@NCNT-900 has a high intrinsic activity for OER. As a result, this outstanding performance was aroused from the synergistic effect of NCNT, Fe, and Ir species. Thus, the NCNTs serve as conductive support and armor, shielding the metals from direct contact with the electrolyte. In the case of Fe, the Ir–Fe interaction helped optimize the iridium’s electronic structure, making OER and HER thermodynamically easier. The Ir–Fe interaction and the wrapping of NCNT limited the aggregation and dissolution of atomically distributed Ir. Thus, the two forms (atomically distributed Ir in the Fe nanoparticle lattice and Ir nanoclusters on NCNT) generated enhanced active sites for HER and OER, including the Ir⁰ and Ir⁴ species. Furthermore, the HER and OER performances of iridium-based electrocatalysts are summarized in Table 4.

Table 4 Tabulation of HER and OER performance of iridium-based electrocatalysts

Ruthenium-based materials

Reier et al. studied the activity and stability of the oxides of Ru, Ir, and Pt nanoparticles along with their bulk counterpart. The electrocatalyst activity of these nanoparticles decreased in the order of oxidized Ru > oxidized Ir > oxidized Pt. The Pt nanoparticles experienced extra deactivation compared to the bulk catalyst, making them an inferior candidate [122]. Ruthenium (Ru) plays a significant role in electrocatalysis because of its lower cost and similar HER activity of Pt. Cen et al. synthesized a Ru-doped NiCoP nanowire catalyst for high-performance overall water splitting. The unique 3D interconnected nanowires ensure the uniform distribution of Ru coupled with NiCoP and endow the Ru-NiCoP/NF with the large ECSA, fast electron transport, and favorable reaction kinetic attributes. In addition to high intrinsic activity and a large number of accessible active sites, the Ru-NiCoP/NF catalyst also has improved electron transfer and expedited reaction kinetics in catalytic processes. As a result of this, the electrocatalyst provided better catalytic activity as it needed only 32.3 mV at 10 mA cm⁻² and 233.8 mV at 50 mA cm⁻² for HER and OER, respectively, with higher activity than the earlier reported electrocatalysts. The corresponding Tafel slope was analyzed to provide insight into the HER kinetics. This catalyst has a very low Tafel slope compared to other control samples, such as bare Ni foam. Furthermore, the enhanced HER activity was reassured by the higher value of C_dl obtained from ECSA, which was due to higher exposed active sites. In an aqueous electrolyte, the OER produces a current density of 50 mA cm⁻²; a small overpotential of 233.8 mV was required. According to the EIS Nyquist plots, the R_ct value of this catalyst was less, which implied that it could transfer charge more rapidly for OER. A full cell evaluation was carried out as part of the three-electrode studies. Furthermore, the electrolyzer cell was tested for 140 h at a current density of 100 mA cm⁻², demonstrating that the prepared catalyst possessed exceptional long-term stability for water-splitting applications. Moreover, the Faraday efficiency was investigated by comparing the theoretical and actual gas volumes at a constant current density of 25 mA cm⁻². For H₂ and O₂ evolution, the Ru-NiCoP/NF catalyst has nearly 100% [123]. Likewise, doping noble metal with transition metal dichalcogenides is the best technique. Layered 2H-molybdenum sulfide (MoS₂) has attracted attention due to its unique structural and electronic properties. Additionally, 1T-MoS₂ has an edge-rich structure, which produces a high ratio of exposed surface atoms and an ultrahigh specific surface area, which increases the HER catalytic activity. This is reflected in a recent pioneering study by Maiti et al., where they fabricated a Ru-doped CuO/MoS₂ (MSCR) hybrid. The electrochemical measurement revealed a distinct activity in the case of cathodic investigation with an overpotential of 198 mV. Furthermore, the as-prepared catalyst exhibited a Tafel slope of 113 mV dec⁻¹. In the case of anodic examination, the MSCR hybrid catalyst exhibited a superior overpotential (201 mV at 10 mA cm⁻²) and Tafel slope (229 mV dec⁻¹). Additionally, an increase in the active sites of hybrid was due to the combination of copper oxide and ruthenium which helped in improving MSCR hybrid catalytic activity. Furthermore, with the help of cyclic voltammetry (CV) and electrochemical active surface area (ECSA), the various performance indexes such as C_dl and R_f were found. From this, they conclude that the astonishingly high double-layer capacitance, large electrochemical active surface area, and significant roughness factor of the MSCR hybrid in the alkaline medium could be attributable to its robust OER performance [124]. Precious metal aerogels have recently gained much attention due to their higher electrical conductivity, porous nature, and several hundred active sites for electrochemical applications. Taking advantage of this fact, S. Yan et al. demonstrated the development of a three-dimensional ruthenium (Ru) aerogel with a unique porous structure in alkaline and acidic conditions, resulting in better electrocatalytic water-splitting performance compared to Pt/C. As a result, ruthenium aerogel delivered the overpotential of 36 mV, which was better than Ru, Ru-60, and 20% Pt/C catalyst. In addition, in an alkaline environment, the HER behavior followed the Volmer-Tafel mechanism. As a result, the thermodynamic energy barrier to the HER lowers, leading to increased catalytic activity. Moreover, the Ru-30 required a cell potential of 1.48 V to achieve 10 mA cm⁻² and stay stable [125]. Meanwhile, B. Yang et al. reported RuCoP nanoparticles in nitrogen-doped carbon, which presents rhombus faces and straight edges and is labeled as RuCoP@CN. This catalyst was engineered by in situ carbonization with the template ZIF-67. Transmission electron microscopy was utilized to comprehend the influence of structure on catalytic performance; Ru atom addition caused an increase in the number of lattice fringes of RuCoP. Thus, RuCoP@CN electrocatalyst affords an overpotential of 24 mV. Furthermore, to investigate the intrinsic catalytic activity, ECSA was employed. The C_dl value of RuCoP@CN was found to be 133 mF cm⁻². Furthermore, the exchange current density (j₀) was calculated by an extrapolation method from the Tafel slope. Overwhelmingly, RuCoP@CN exhibited more activities than the others, benefiting from the existence of P and Ru atoms. Furthermore, the RuCoP@CN could retain 100 h of stability during the overall water done by chronopotentiometry. According to DFT simulations, Ru and P doping could optimize the free energy and promote electron enrichment for HER and OER [126]. Cen et al. synthesized a hybrid electrocatalyst comprising Ru-MoO_3−x NF. Additionally, comparative studies were conducted to demonstrate that urea is crucial for the creation of unique structures, morphologies, and metal reduction. As a result, the as-prepared hybrid resulted in the overpotential of 12 mV at 10 mA cm⁻², 49 mV at 100 mA cm⁻², and high endurance of at least 17 h in alkaline conditions. In addition, the MoO_3−x with oxygen deficiency provided larger active sites and improved intrinsic activity, facilitated electron transport, and successfully prevented the loss of Ru species during the catalytic process. Therefore, the as-prepared electrode exhibited better HER performance. Furthermore, on assembly of electrolyzer cells using Ru-MoO_3−x NF and Fe₃O₄-NiFe-LDH NF, the cell voltage of 1.46 V was acquired and demonstrated high stability without obvious activity loss after continuous operation of 180 h [127]. Pei et al. synthesized Ru loaded on nickel foam (Ru/NF) by substitution reaction, which exhibited an outstanding bifunctional catalytic performance with reasonable overpotential and better reaction kinetics, superior to commercial Pt/C. This catalyst labeled Ru/NF was prepared by wet chemistry strategy with only 0.3 wt% of Ru loading, endowed with an HER overpotential of 10 mV and Tafel slope of 34 mV dec⁻¹. Due to their better catalytic activity, the complete water-splitting system was tested directly with Ru/NF-2 as the anode and cathode. It offered an applied cell voltage of 1.56 V to accomplish 10 mA cm⁻² without visibly declining performance. The superior performance can be accredited to the addition of Ru, which causes the d-band of Ru/NF to move to the Fermi level, modulating the surface adsorption properties of the reaction intermediate (H*). Likewise, adding Ru increased the adsorption of H₂O onto the Ru/NF surface, resulting in a quick Volmer step. These findings show that the Ru/NF has incredible potential for HER. In the case of OER, the formation of O–O bonds are favored by electron transport between Ru and Ni substrates. Adding NiO_x or other oxides to the catalyst’s surface boosted its OER performance, which proved an effective method. Furthermore, Ni²⁺ on the surface of Ru/NF increased the water splitting, while Ni³⁺ on the surface stimulated the production of oxides and hydroxides (NiOOH), providing more OER active sites [128]. J. Zhang et al. produced an extraordinary result of an ultralow overpotential of 2.04 mV at 10 mA cm⁻² for hydrogen evolution reaction in alkaline media. They constructed RuCoFe nanosheet (RuCoFe NS) via a rapid heating strategy. The electrochemical testing showed that RuCoFe NSs required only 180 mV of overpotential to reach 10 mA cm⁻². This impressive performance was again proved when the Tafel slope value (69.18 mV dec⁻¹) was revealed. Stability is another important attribute to consider when evaluating the performance of OER electrocatalysts. Even more unexpectedly, RuCoFe NS OER activity has not changed after 5000 cycles. As mentioned earlier, they achieved 2.04 mV@10 mA cm⁻² in terms of the cathodic process of water splitting. Furthermore, it also exhibited outstanding kinetics and durability as it possessed a Tafel slope of 33.07 mV dec⁻¹ and sustained 5000 cycles of ADT test using cyclic voltammetry and the typical chronoamperometry. The two-electrode device has good catalytic performance as it reached a current density of 10 mA cm⁻² with just 1.56 V [129]. Furthermore, the HER and OER performances of ruthenium-based electrocatalysts are summarized in Table 5.

Table 5 Summary of HER and OER performance of ruthenium-based electrocatalysts

Inter-noble materials

It is well known that RuO₂ and IrO₂ are promising catalysts for acidic OER. However, RuO₂ has the disadvantage of having low stability in acidic conditions at high potential. To improve the stability of acidic OER, Shan et al. reported that strained Ru core and IrO_x shell heterostructures with significant charge redistribution can act as stable catalysts for acidic OER [149]. To further investigate the stability issue, J. Wang et al. reported the single-site Pt-doped RuO₂ hollow nanospheres (SS Pt-RuO2 HNSs) with interstitial C, which serves as highly active and stable electrocatalysts for overall water splitting. They have addressed the durability issue, where this as-prepared catalyst exhibited better stability for more than 100 h at 100 mA cm⁻². Additionally, the catalytic efficacy of this catalyst during OER was outstanding, showing minor overpotentials of 228 mV to achieve 10 mA cm⁻² and 282 mV for reaching 100 mA cm⁻². Concerning HER performance, at current densities of 10, 50, and 100 mA cm⁻², the overpotentials were 26, 47, and 67 mV, which matches the Pt/C catalyst. The experiments show that interstitial C can lengthen the M–O bonds (where M = Ru and Pt) and the added single-site Pt has a considerable impact on the electrical interaction of RuO₂. According to theoretical calculations, the strong synergy promotes OER performance by lowering energy hurdles and increasing the dissociation energy of *O species [150]. Core/shell morphology has recently been widely investigated in many energy storage applications. Electrocatalytic studies were conducted on these structures, and the nature of the two underlying mechanisms such as ligand effect and strain effect was not elucidated clearly in prior research. Therefore, M. Li et al. prepared precise PdCu/Ir nanocrystals with core/shell morphology. This study specifically tuned heteroepitaxial coating and monolayer (ML) Ir shells onto the PdCu nanocrystals to get the exclusive strained PdCu/Ir core/shell nanocrystals. In addition, it exhibited an OER overpotential of 283 mV corresponding to 10 mA cm⁻² and high mass activity of 1.83 in acidic media, significantly superior to their non-strained alloy counterparts. DFT calculations further supported the predominance of compressive strain in improving activity by showing how it affects the adsorption strength toward oxygenated intermediates. The result was a very effective and stable electrocatalytic water-splitting electrolyzer with a cell voltage of 1.583 Vat 10 mA cm⁻² for HER and a 20 mV toward HER overpotential reported by the strained PdCu/Ir core/shell nanocrystals [150]. High entropy alloys (HEAs) have gained attention as they can perform unique water-splitting catalytic processes. Even though there are several synthesis methods, the HEA nanoparticle produced has a common issue of uneven composition. As a result, Y. Lu et al. proposed an easy and quick approach to synthesize FeCoNiCuPtIr HEA nanocatalysts via pulsed laser irradiation, as depicted in Fig. 7. Pulsed radiation, on the other hand, prevented the Ostwald ripening phenomena by effectively controlling the size of HEA nanoparticles and accelerating reactions with its intrinsic hollow active sites (i.e., OER and HER), which showed an overpotential of 255 mV and a modest Tafel slope (61.7 mV dec⁻¹) in alkaline conditions for OER and an exceptionally small overpotential 21 mV and a small Tafel slope (54.5 mV dec⁻¹) for HER. Meanwhile, DFT studies revealed that adding Ir atoms reduces the coupling strength of adsorbate and alloy surface by donating valence electrons to surrounding hollow sites [150]. The most effective catalysts in PEM water electrolyzers are Ir- and Ru-based electrocatalysts, the peak of the “volcano figure.” Ru-based electrocatalysts, on the other hand, are prone to disintegration when exposed to high-acid electrolytes. N. Liu et al. synthesized bifunctional electrocatalysts with elevated performance and low precious metal content. By a two-step dealloying procedure, they developed multimetallic nanoporous nanowires (NiIrRuAl) with various compositions. The enhanced water-splitting performance was evident in its overpotential, as NiIrRuAl-1/3 NPNWs in OER needed an overpotential of just 237 mV at 10 mA cm⁻². In contrast, the 0.1 M HClO₄ solution for HER required a little overpotential of 14 mV. Importantly, a minor cell potential of 1.464 V was sufficient to operate the bifunctional catalyst to generate 10 mA cm⁻² and showed better stability. Specifically, the enhanced electrocatalytic performance of NiIrRuAl NPNWs was attributed to (i) electrocatalyst alloying with transition metal, (ii) the combined action of ruthenium and iridium, and (iii) the highly ordered nanostructure [151]. In addition, the HER and OER performances of inter-noble metal–based electrocatalysts are given in Table 6.

Fig. 7

a–c Schematic illustration showing the fabrication process of NiIrRuAl NPNWs [151] reproduced with permission from ref.154, copyright 2020, Journal of Materials Chemistry A

Table 6 Summary of HER and OER performances of inter-noble metal–based electrocatalysts

Summary and future outlook

In electrochemical water splitting, noble metal–based hybrid materials exhibit outstanding electrocatalytic efficacy because of their inherent high activity, mechanical stability, and more readily accessible surface-active sites. This paper systematically summarizes recent advances in developing electrocatalysts for water splitting based on precious metals. Structure engineering tactics such as adding heteroatoms, defect and strain engineering, single-atom manipulation, and morphology manipulation have all been shown to improve electrocatalytic performance. Overpotentials as low as 2 mV and their rational cause are explained in this article. It has been thoroughly researched that increased surface-active site exposure, synergetic effect, oxygen vacancy, coupling effect, distinct electronic interaction, and string effects are essential for improved electrocatalytic performance in water electrolysis. Despite these significant efforts to improve the composition and morphology of noble metal hybrids, there is still much work to be done before they can be utilized on a wide scale for electrolytic cells, and some outstanding difficulties with noble metal–based hybrids should not be neglected. Given noble metals’ high cost and low natural abundance, limiting their consumption is eagerly anticipated. Due to their high price and limited availability of noble metals, minimizing their consumption is eagerly anticipated. Because of their high atom usage and excellent electrocatalytic activity, single-atom catalysts are gaining popularity in electrocatalysis. However, due to the high surface energy, increasing the loading of single atoms will certainly result in significant accumulation and limit the catalytic activity. More efficient ways to increase noble metal usage are desperately needed in the near future. In addition, the synthesis of alloys based on noble metals is substantially more complicated and complex, and several bimetallic structures transform in the reaction. As a result, determining the atomic positioning of these alloys is crucial. In addition, diffusion and segregation of metals can occur under reaction conditions due to interface engineering efforts, particularly during long-standing durability tests. Therefore, additional theoretical and experimental research is required in noble metal–based materials in water-splitting applications.