Johnson Matthey Technol. Rev., 2021, 65, (2), 207
Critical Review of Platinum Group Metal-Free Materials for Water Electrolysis: Transition from the Laboratory to the Market
Earth-abundant borides and phosphides as catalysts for sustainable hydrogen production
- Alexey Serov§
- Pajarito Powder, LLC, Albuquerque, New Mexico 87109, USA
- Kirill Kovnir‡
- Department of Chemistry, Iowa State University, Ames, Iowa 50011, USA; Ames Laboratory, US Department of Energy, Ames, Iowa 50011, USA
- Michael Shatruk¶
- Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306, USA; National High Magnetic Field Laboratory, Tallahassee, Florida 32310, USA
- Yury V. Kolen’ko*
- International Iberian Nanotechnology Laboratory, Braga 4715-330, Portugal
To combat the global problem of carbon dioxide emissions, hydrogen is the desired energy vector for the transition to environmentally benign fuel cell power. Water electrolysis (WE) is the major technology for sustainable hydrogen production. Despite the use of renewable solar and wind power as sources of electricity, one of the main barriers for the widespread implementation of WE is the scarcity and high cost of platinum group metals (pgms) that are used to catalyse the cathodic hydrogen evolution reaction (HER) and the anodic oxygen evolution reaction (OER). Hence, the critical pgm-based catalysts must be replaced with more sustainable alternatives for WE technologies to become commercially viable. This critical review describes the state-of-the-art pgm-free materials used in the WE application, with a major focus on phosphides and borides. Several emerging classes of HER and OER catalysts are reviewed and detailed structure–property correlations are comprehensively summarised. The influence of the crystallographic and electronic structures, morphology and bulk and surface chemistry of the catalysts on the activity towards OER and HER is discussed.
1. Sustainable Hydrogen Generation by Water Electrolysis
Hydrogen is the first element of the Periodic Table and the most abundant element in the Universe. Currently, we are witnessing the emergence of hydrogen gas as an increasingly powerful energy vector for storing and delivering electricity (1). Firstly, besides having very low physical density, hydrogen has the highest gravimetric energy density of any known non-nuclear fuel (for example three times higher than gasoline and 150 times higher than a state-of-the-art lithium-ion battery), which sets the stage for hydrogen as an efficient energy-storage solution (2). Secondly, hydrogen is an environmentally benign fuel, since only energy and water are the end products of the reaction between hydrogen and oxygen, giving access to zero-emission electricity production when used in fuel cells. These benefits have made hydrogen a priority area within the “climate and resource frontrunners” of the European Green Deal.
Currently, most hydrogen (ca. 95%) is produced through energy-demanding steam reforming reaction of water with natural fossil fuels. Unfortunately, this translates into undesired generation of greenhouse gases. As an alternative, WE (Equation (i)) can be employed for hydrogen production, providing a completely carbon-neutral solution when working on renewable electricity from wind, solar, wave, tide, biomass or geothermal, thus eliminating carbon dioxide emissions.
Importantly, the resultant high-purity hydrogen can be stored in a compressed or liquefied form or in chemical compounds (metal hydrides and liquid organic hydrogen carriers), and then delivered as needed for fuelling small- and large-scale applications (3).
Although the era of WE began more than two centuries ago, its worldwide implementation is still limited to hydrogen production up to the megawatt range using liquid alkaline electrolysis (4). WE is a kinetically controlled process characterised by slow charge transfer and insufficient chemical reaction rates. A large overpotential, defined as the difference between the required and thermodynamic value of the WE voltage (E0 = 1.23 V vs. reference hydrogen electrode (RHE)), needs to be applied to drive this reaction. Therefore, catalysts, primarily based on pgms, are used to accelerate WE by reducing the value of the applied overpotential to conduct the cathodic HER and the anodic OER, which are the two half reactions of the WE (5).
The best-performing catalysts for WE are platinum for HER and iridium oxide/ruthenium oxide for OER, featuring topmost activity and long-term stability in both acidic and alkaline electrolytes. Unfortunately, these pgms are scarce and expensive, having the status of critical raw materials in the European Union (EU). Iridium and ruthenium are scarce even when compared to platinum because both are byproducts of platinum mining. Hence, the use of pgms has been recognised as one of the major bottlenecks for hydrogen production by WE on terawatt scale, and, therefore, the development of pgm-free catalysts is an economically sound strategy towards technological and industrial expansion of WE.
2. Membrane-Based Water Electrolysis
On the electrolyser side, the commercially available technologies for hydrogen generation are based on: (a) alkaline electrolysers (AELs); (b) solid oxide electrolysers (SOELs); and (c) (semi)solid polymer electrolysers (SPELs) (4, 6–9). The AEL technology has been commercially available for >50 years. It is characterised by long lifetime and low cost. Recent developments of AEL deal with the employment of high temperatures and pressures, targeting improvement of efficiency (7). The AEL electrodes are typically composed of abundant nickel with a catalytic coating. Shortcomings of the AEL are the use of corrosive liquid electrolytes (for example potassium hydroxide), low purity of the resultant hydrogen due to high cross-permeation through cathode and anode separator, and long start-up times not suitable for dynamic operation. The currently developing SOEL (also known as steam electrolysis) omits the need for corrosive liquid electrolyte, but operates at temperatures above 700°C, making the whole process energy demanding. Another issue is the instability and degradation of the electrode materials at the high operating temperatures. WE technology based on SPEL is well-suited to intermittent supply applications at scale (dynamic operation), offering high current densities compared to AEL, high-purity hydrogen at pressures up to 30 bar and efficiency of ca. 70%. Nevertheless, a number of obstacles, such as high capital costs, prevent the widespread use of SPEL (9).
Advantageously, SPELs operate at low temperatures (50–80°C), and the thin humidified polymer membranes feature high conductivity and low resistance, resulting in minimal current losses and low cross-permeation of hydrogen. There are three SPEL technologies: (a) proton-exchange membrane (PEM); (b) anion-exchange membrane (AEM); and (c) bipolar membrane (BPM).
PEM electrolysers are the present state-of-the-art SPEL technology. Relatively compact PEMWE systems are available from several small, medium and large companies, such as ITM Power, NEL, Giner Labs, Enapter and Siemens. Despite the fact that PEMWE has high technology readiness level (TRL), it is still mainly used for demonstration projects, largely subsidised by local or national governmental funding organisations. The main reasons for the limited commercial adoption are high capital expenditures (CAPEX) and operational expenditures (OPEX), making the produced hydrogen economically non-competitive compared to the traditional steam-reforming process.
More specifically, due to the extremely acidic protonic matrix of the ionomer and membrane (for example NafionTM, Aquivion®, Celtec®), the technology relies on scarce platinum/carbon and iridium oxide/ruthenium oxide catalysts, which exhibit high intrinsic activity and durability (10). Notably, only these materials can withstand the acidic PEMWE operation conditions over a 20 year electrolyser lifetime. Additionally, highly acidic and oxidative operation conditions dictate that the porous transport layers (PTLs) have to be made from corrosion-resistant but rather expensive titanium, which, combined with pgm catalysts, represents ca. two thirds of the cost of the PEMWE stack (several individual PEM cells stacked together to achieve higher hydrogen production).
The alternative AEMWE approach is rapidly growing from TRL2–3 to the level of TRL6–7 (11–13). AEM combines the advantages of AEL and SPEL, eliminating the need of corrosive liquid electrolyte and leveraging efficient membrane-based WE (14). Until recently, the main bottleneck in AEMWE was the lack of highly hydroxide conductive, stable and durable AEMs as compared to PEMs (15, 16). Fortunately, several promising AEMs have recently emerged on the market (17), such as those from Tokuyama, Fumatech, Ionomr Innovations, W7energy®, Ecolectro, Dioxide MaterialsTM and AGC. The non-aggressive AEM environment at pH ≈ 13 and the possibility to produce AEMs without the use of fluorine (regulated in several countries) makes AEM production more environmentally friendly and safer for workers and nearby inhabitants (11).
Importantly, AEMWE may occur efficiently on pgm-free catalysts based on inexpensive metals (such as iron, cobalt or nickel) that can also withstand prolonged operation in alkaline membrane environment. Therefore, the development of an electrolyser based on emerging AEMs and pgm-free catalysts should enable a significant reduction in both CAPEX and OPEX of electrolysis (18). Interesting work on AEMWE within the EU Horizon 2020 ANIONE project additionally illustrates these points very well. Notably, a further CAPEX reduction can be achieved by replacing the expensive titanium PTLs by simple stainless steel, which is stable under AEMWE operation. Nevertheless, the detailed technoeconomic analysis of AEMWE with pgm-free catalysts is complicated, and the projected cost of the as-produced hydrogen strongly depends on the selected model and input parameters (19, 20).
A third, significantly less explored, avenue for SPEL is the BPM (21). Here, a PEM and an AEM are connected in series, thus allowing the use of non-iridium or ruthenium catalysts on the anode side. Notably, BPM provides a thermodynamic advantage due to the pH gradient across the membrane, which makes it possible to conduct the electrolysis under applied potential below the standard potential of WE, thus requiring less current input. BPM technology is still in its infancy, and more research is required to improve its design, performance, stability and cost.
Our research is focused on the development of pgm-free catalysts for alkaline HER and OER that could be further translated into real-life application of AEMWE. Various classes of pgm-free materials, such as alloys, (oxy)hydroxides, borides, carbides, nitrides, phosphides, chalcogenides, oxides, spinels, perovskites, metal-organic frameworks (MOFs) and metal-free carbon-based compounds have been investigated as catalysts for alkaline WE. In this review, we will highlight several titled prospective pgm-free catalysts that have been studied in our laboratories.
3. Alkaline Water Electrolysis
In the simple scheme of a AEMWE cell (Figure 1), the OER and HER catalyst layers are directly coated on the respective sides of the AEM membrane, thus forming a membrane electrode assembly (MEA). Alternatively, the catalysts can be incorporated into the respective gas-diffusion layer (GDL), thus forming a gas-diffusion electrode (GDE). Using porous GDLs is preferential compared to planar ones, since the porosity accelerates the mass transfer of the reactant or product over the catalyst layer. The main role of the AEM is to ensure hydroxide transfer from the cathodic side of the cell to the anodic side. Normally, AEM electrolysers use a water feed with the addition of HCO3–/CO32– or dilute potassium hydroxide electrolytes to achieve better performance.
As shown in Figure 2(a), HER starts with electrochemical hydrogen adsorption according to Equation (v) (Volmer reaction), where H* designates a hydrogen atom chemically adsorbed on an active site of the electrode surface (M).
The initial adsorption can be followed by either electrochemical desorption according to Equation (vi), (Heyrovsky reaction), or chemical desorption in Equation (vii) (Tafel reaction). The exact mechanism and rate-determining step of the HER over certain catalysts can be established by analysis of the respective Tafel slope data (22).
The proposed alkaline OER (Figure 2(b)) commences with the electrochemical adsorption of hydroxide anion on the electrode active site (Equation (viii)), followed by the electrochemical formation of O- and OOH-bound intermediates (Equations (ix) and (x)). Finally, O2 molecule is generated as a result of the last single-electron charge-transfer step according to Equation (xi).
Notably, a scaling relation (23–26) between binding energies of the reaction intermediates results in a minimum theoretical excess potential, the so-called ‘overpotential wall’ of 0.37 V (27). This means that one would need to apply a minimum potential of 1.23 V + 0.37 V (i.e., 1.6 V vs. RHE) to conduct the OER reaction, which has found experimental confirmation (28). Breaking the OER scaling relation and thus reducing or eliminating the ‘overpotential wall’ still remains a challenge (29).
Table I summarises parameters typically used to present laboratory half-cell data for the evaluation of the catalyst performance and comparison between different catalysts with similar mass loading per geometric area. A number of these parameters will be used in the subsequent discussion. Readers interested in best practice for investigating novel HER and OER catalysts are referred to (30), while (31) provides useful guidance on analysing and presenting the catalytic data. Moreover, some critical aspects of electrochemical data evaluation are reported elsewhere (22, 32–34).
4. From Precatalyst to Catalyst During Alkaline OER
OER, as a kinetically slow 4e– transfer reaction, governs the overall efficiency of WE. Since the largest overpotential stems from OER, catalyst development for this half reaction will offer the largest efficiency gains (26).
Interestingly, initial reports employing transition metal borides, carbides, nitrides, phosphides and chalcogenides (in general, ‘Xides’) as catalysts for alkaline OER putatively attributed the observed catalytic activity to the pristine materials. However, a careful consideration of following reasons indicated that all the aforementioned Xides should undergo oxidation during the OER into the respective oxides or (oxy)hydroxides (35):
the alkaline OER is conducted under very oxidative conditions at pH ≥ 13, with the applied potential of ≈1.6 V vs. RHE;
the potential- and pH-dependent phase diagrams (Pourbaix diagrams) indicate existence of several oxo-containing phases for the corresponding metals under the conditions of alkaline OER;
the enthalpy of the formation of Xides is more positive than that of the respective transition metal oxides or (oxy)hydroxides.
Detailed characterisation studies, including diffraction, spectroscopy and microscopy analyses of the catalysts before and after alkaline OER, provided strong support for in situ oxidation. Accordingly, all transition metal Xides could be considered as precatalysts or ‘active’ supports for alkaline OER. The degree of oxidation could be different and largely controlled by the chemical nature of the Xide precatalysts. Moreover, the size and microstructure of the catalysts play an important role. If the catalyst nanoparticles (NPs) are ultrasmall or the active catalyst surface layer is very thin, the compounds will rapidly undergo full oxidation. In contrast, if the catalyst NPs or films are reasonably large or thick, then partial oxidation of the surface occurs, forming distinct core@shell NPs and nanoheterostructured films, preserving the bulk Xide structure underneath. In other words, in the case of poor catalysts, the oxidation of the bulk compound occurs forming non-conductive oxides or hydroxides, while for the good catalysts the oxide or hydroxide layer passivates the catalyst surface, preserving the bulk structure of the original compound.
The surface oxidation yielding the real catalyst is an unavoidable but crucial phenomenon to obtain highly active and stable OER catalysts. In the vast majority of studies, the resultant in situ formed catalysts show remarkably higher OER performance than their respective metal oxide or hydroxide counterparts. Full understanding of this performance enhancement is still lacking. Generally, there are several factors that should be pointed out: (a) the development of high surface area due to the formation of amorphous-like (oxy)hydroxide surface; (b) the high electrical conductivity of the non-oxide Xide precatalyst underneath the real catalyst; and (c) synergetic catalyst−precatalyst electronic interactions within the as-formed nanoheterointerface (35–37). These factors give rise to a larger number of catalytically active sites and faster charge transfer kinetics over the anode/electrolyte interface, thus beneficially boosting the OER performance.
Currently, there is a limited knowledge of the mechanism of surface transformation during OER because mainly ex situ studies are performed before and after OER. In situ surface-sensitive studies (such as spectroscopy, diffraction, imaging, electrochemical imaging, cyclic voltammetry, nanoimpacts and combination thereof), despite being extremely challenging, are crucial for the rational development of the OER precatalysts (38–42). Such knowledge will be essential to achieve control over the formation of the real catalyst, thus guiding future efforts towards active, stable and economic catalysts for alkaline OER (Figure 3).
To summarise, the structure of the bulk phase or core of NPs of a prominent OER catalyst is preserved during the reaction. The surface is, undoubtedly, oxidised due to the extreme oxidation potential applied and the presence of multiple active species, such as hydroxide radicals. The formed oxide layer passivates the surface, thus preventing further oxidation of the bulk of the precatalyst. The same oxide layer is an active catalyst. Thus, the formation of the oxide layer should not be avoided but rather judiciously directed to enhance stability and activity of the catalysts. A combination of computational and detailed in situ studies is a promising strategy in this research direction.
5. Current Industrially-Relevant pgm-Free Catalysts for Alkaline HER/OER
Thorough and comprehensive studies conducted by research groups around the globe have narrowed down the range of industrially-relevant pgm-free HER/OER catalysts for AEMWE to: (a) nickel, nickel alloys and intermetallic compounds as the most active and stable for the HER side of the MEA (43–51); and (b) base metal oxides, (oxy)hydroxides, spinels and perovskites for the OER side of the MEA (52–58).
In general, the catalysts for alkaline HER are similar to those used in AEM fuel cells (AEMFC) (59). Carbon-supported nickel, nickel-molybdenum, nickel-copper and other nickel-based catalysts have been synthesised and investigated (43–51). As a lighter analogue of pgms (especially being chemically similar to palladium), nickel plays the main role in HER, while the addition of other elements increases the cathode durability. Nickel-molybdenum catalysts have been intensively studied for both hydrogen oxidation reaction and HER under realistic AEMFC and AEMWE conditions, respectively (45, 48, 50). The nickel-molybdenum alloys enriched with nickel up to ≈87 at% were found to be not only active in alkaline HER, but also less prone to poisoning by functional groups of ionomers, which is a well-known shortcoming of pgm catalysts in AEMFC and AEMWE (45, 48). The properties of carbon supports, such as the surface area, level of graphitisation, bulk and surface chemical composition and hydrophobic or hydrophilic properties, significantly affect the AEMWE performance and should be carefully tuned as a function of the type, composition and weight loading of the HER catalysts (60).
Considering the OER catalysts, nickel (oxy)hydroxides or oxides with iron impurities in alkaline electrolyte, as well as nickel-iron (oxy)hydroxides, have become benchmark catalysts for AEMWE (61, 62). Although the exact nature of the active catalytic sites in such systems is not well understood (63), the nickel-iron-based materials for alkaline OER deserve a more careful study in the future, especially with respect to their low durability issues (64, 65).
Furthermore, a general direction in the development of catalysts for alkaline OER is to improve the electronic conductivity of already established active oxide catalysts, since the conductivity of the oxides is typically very low. Such an improvement is critical to reducing the applied overpotential during real AEMWE operation, where loading of OER catalysts can often be as high as 4–12 mg cm–2 (53–55). Unfortunately, the oxidative operation conditions during OER at high potential of ≈1.9–2 V forbid the utilisation of traditional highly conductive carbon supports, which would simply oxidise and degrade during prolonged AEMWE. At the same time, using carbon as a supporting material for transition metal oxide catalysts is a common laboratory practice in short half-cell experiments for the initial assessment of intrinsic catalytic activity of the materials and performance losses due to insufficient electronic conductivity (52). For instance, highly graphitic carbon nanotubes or high surface area graphenes are widely used during the screening and selection of promising alkaline OER materials (12, 52). An alternative strategy is based on employing non-stoichiometric mixed oxides, perovskites, delafossites or spinels (58). Although these materials possess high conductivity (> 0.1 Ω–1 m–1) (58), specific synthetic methods should be developed to produce such conductive oxides on a large scale.
Notably, the commercial metal oxide products made on the multi-tonne scale are mainly silica, zeolites and titania. The protocols used to manufacture these materials are well-matured and based on the sol-gel, precipitation, hydrothermal, flame or spray pyrolysis. Nevertheless, these approaches have a limited ability to control physicochemical properties required for OER catalysts, including phase purity (required for high electronic conductivity), the surface area (required for higher density of active sites) and the primary particle size (required for the uniform distribution in the electrode structure).
For the past eight years Pajarito Powder redesigned and modified their proprietary manufacturing platform VariPoreTM for upscaling mixed oxide catalysts production. The schematic of the method is shown on Figure 4. The main idea of this method is to design material by a bottom-up approach with the ability to control all required properties of the final catalyst at every stage of the process. For example, selecting particle and pore forming agents (P&PAs) with different morphology will afford a final catalyst with an inverted structure of the respective P&PAs. The chemical nature of the precursors will allow selection of manufacturing conditions to obtain materials under low energy demanding regime, substantially decreasing the cost of final materials.
The method was successfully used for the preparation of phase-pure CuCo2O4 spinel catalyst for OER (Figure 5(a)). The P&PAs used in this design derived from high surface area spherical silica particles, which prevented NPs from agglomeration. The phase purity of the spinel was achieved by thermal treatment at just 550°C, allowing to preserve the unique spherical shape of agglomerates (Figure 5(b)). The latter is important for manufacturing dense electrodes for OER, allowing maximal catalyst utilisation on the GDEs. At the moment, this method is established as a robust, flexible and modular manufacturing platform for making different classes of HER/OER catalysts and practiced for commercial production at the kilogram scale per batch (54).
Among the large variety of pgm-free catalysts that are currently extensively explored, the interest of our groups have been focused on transition metal phosphides and borides, which belong to the larger family of non-oxide catalysts referred to as Xides. In the next two sections, we focus specifically on these two groups of Xides, highlighting some of our results along with important findings from other research groups. For a more general overview of pgm-free non-oxide electrocatalysts, the readers are referred to several recent reviews relevant to this topic (36, 66, 67).
6. Emerging Transition Metal Phosphide Catalysts
6.1 Crystal and Electronic Structure
Transition metal phosphides (TMPs) are considered promising alternatives to pgm catalysts for WE. Phosphides of earth-abundant transition metals iron, cobalt, nickel and molybdenum are mainly studied with stoichiometries ranging from M2P to MP2. The crystal structures of TMPs are quite different from the structures of the corresponding metals. In the crystal structures of phosphorus-rich MP and MP2, each M atom is surrounded by six or seven P atoms forming P6 octahedra (iron, cobalt, nickel), P6 trigonal prisms (MoP), or P7 monocapped trigonal prisms (MoP2) centred by M atoms. The coordination of M atoms in metal-rich M2P phosphides is composed of four or five P atoms forming tetrahedra or square pyramids around the M atoms (Figure 6).
In all TMPs, strong covalent bonds between P and M atoms are formed due to substantial overlap of M–3d / M–4s and P–3p orbitals, resulting in significant changes in the electronic structure and chemical properties as compared to elemental metals. These changes lead to the higher chemical stability of TMPs under HER/OER conditions compared to elemental iron, cobalt, nickel and molybdenum metals. The majority of the studied phosphides have metallic properties with non-zero density of states (DOS) at the Fermi level (EF). Iron, cobalt and nickel phosphides with identical composition are either isostructural or closely structurally related, which allows fine tuning of the Fermi level position, as illustrated in Figure 7 for M2P compounds. FeP, CoP and MoP also exhibit metallic properties. In the case of nickel phosphides, NiP is a high-pressure phase while at ambient conditions Ni5P4 is a stable metallic phosphide with excellent catalytic properties. Further increasing the P content in TMPs results in semimetallic properties for MoP2 and narrow-bandgap semiconducting properties for MP2 with M = Fe, Co and Ni. Again, nickel is a special element due to the existence of a cubic metallic polymorph NiP2. This compound was originally assumed to be a high-pressure phase but recently it has been shown to be a metastable polymorph that can be synthesised at ambient pressures (68).
6.2 Hydrogen Evolution Reaction Catalysis
Currently, several hundred research papers describing TMP catalysts for HER and OER are published annually (37, 69, 70). Despite the large number of studies, there are two issues in this field: (a) the reported data are very scattered in terms of performance, and (b) very few attempts have been reported that go beyond half-cell measurements in the research laboratory by making an actual electrolyser using TMPs. The former issue is illustrated in Figure 8, which shows the data published for the HER performance of CoP catalysts in acidic media. Although in all cases the material is reported to be single phase CoP, mainly in the form of NPs, the performance is quite scattered. While certain variability in the data is due to different setups, testing protocols, and sample preparation routines used in various laboratories, the reported CoP NPs have different shapes, crystal facets exposed, surface termination by ligands and surface area, thus making the analysis of the reported data non-trivial.
The Kovnir and Kolen’ko groups have been exploring TMPs as an alternative to pgms in WE since 2014, partially through EU Horizon 2020 CritCat project, with significant progress in understanding the limitations and potential of new pgm-free catalysts (69). A Ni–P material was synthesised by gas-transport phosphorisation of commercial nickel foam (71). The resultant self-supported foam cathode was highly active toward acidic or alkaline HER in terms of overpotentials, exchange current densities and Tafel slopes (71). To reduce the mass loading of the Ni–P catalyst in the foam cathode (60 mg cm–2), a thin-film Fe0.2Ni0.8P2 supported on carbon paper was prepared through combination of sputtering and gas-transport phosphorisation. The material demonstrated excellent acid or alkaline HER performance with only 1 mg cm–2 catalyst mass density (72).
Notably, surface characterisation of TMPs during reaction is quite limited. In the vast majority of cases, the surfaces are characterised before and after the reaction. X-ray photoelectron spectroscopy (XPS) and related spectroscopic techniques often reveal significant presence of oxides on the surface of the used catalysts. This observation has led to a common suggestion that the surface of the TMP catalysts is restructured or oxidised during the HER. An open question remains whether this assumption is valid and how the bulk crystal structure of the phosphide impacts its catalytic properties. A recent study of two polymorphs of NiP2 shows that the bulk structure has a significant impact on the catalytic properties of the corresponding phosphide (68). One possible explanation is that the monoclinic polymorph of NiP2 is a semiconductor, while the cubic polymorph has metallic properties, which might be sufficient to explain the observed difference in reactivity, without the need to invoke the differences in their bulk crystal structures.
A recent study on single crystals of FeP and monoclinic NiP2 has shown that different facets of the same crystal exhibit different catalytic activities (73). Moreover, the activity was demonstrated to correlate with the computed surface H adsorption energy on the P atoms of the corresponding facet. This finding clearly highlights that both the underlying bulk structure and the specific surface termination play an important role in the performance of HER catalysts. Formation of surface oxides may be explained by oxidation of P–H bonds upon exposure of used catalysts to ambient conditions.
In contrast to hundreds of papers reporting fascinating properties of various TMPs, very few TMPs were tested beyond simple HER half-cell measurements in solution. To the best of our knowledge, there are only a few reports, one for each metal phosphide, NiP2, CoP, FeP and MoP, of assembling a complete PEMWE single cell using TMPs as cathodes and iridium or iridium/ruthenium oxides as anodes (74). An interesting example of achieving a current density of 0.88 A cm–2 at applied potential of 2 V with CoP NPs as HER catalyst in a commercial-scale 86 cm2 PEMWE was reported by King and coworkers (75). Further, it has been shown that for cubic NiP2 only a 13% increase in potential was required in gas-phase PEMWE operation to achieve a current density of 50 mA cm–2 as compared to reference platinum electrode (1.86 V for cubic NiP2 vs. 1.64 V for platinum) (68). Very recently, a cathode of highly crystalline FeP NPs supported on commercial conductive carbon was used to achieve a current density of 200 mA cm–2 at 2.06 V cell compared to 1.71 V with reference platinum cathode, corresponding to a difference of only 0.07 W cm–2 in the power input (74). Separate experiments showed up to 100 h of cathode operation in PEMWE, as well as stable switch-on and shut-down cycle dynamic operation during 36 h. Importantly, these NiP2 and FeP catalysts show PEMWE HER performance on par with the best reported platinum-free materials (75–77).
6.3 Alkaline Oxygen Evolution Reaction Catalysis
While for HER Pt has not been beaten, TMPs excel at alkaline OER, outperforming simple reference iridium and ruthenium oxide catalysts with rutile structure. Table II provides several examples of TMPs studied as OER electrocatalysts, summarising their performance characteristics and the conditions used for the catalyst synthesis. The surface chemistry of TMPs under OER conditions is more complex than that for HER. As discussed in Section 4, under OER conditions the top surface layers of phosphides undergo oxidation while the bulk material remains intact. The active catalyst is thus an oxygen-containing phase, which can be any combination of transition metal oxide, hydroxide and phosphate. In the best-performing OER TMP catalysts, this oxidised layer is relatively thin and coherently interfaced to the underlying metal phosphide bulk. The formed heterostructure is crucial for high performance because simple catalysts composed of transition metal oxides or hydroxides show much lower activity. The role of TMPs is to serve as precatalyst to template thin in situ oxidised active surface layer and efficiently supply current to this layer through the bulk conductivity of the TMPs (37).
|Compound||η10, mV||b, mV dec–1||Synthesisa||Ref.|
|Cubic NiP2||340||56||NiCl2 + P, 40 h at 773 K; H2O; ball-milling||(68)|
|Monoclinic NiP2||410||61||Ni + P, 76 h at 973 K; ball-milling||(68)|
|Cubic Fe0.2Ni0.8P2||140||49||Fe0.2Ni0.8/C paper + P, 1 h at 773 K, 12 h at 523 K||(72)|
|Ni2P||310||48||Ni/C paper + P, 6 h at 773 K||(78)|
|Mg-modified Ni2P||280||71||Ni/Mg/C paper + P, 6 h at 773 K||(78)|
|Ni5P4–Ni2P||250||54||Ni foam + P, 2 h at 873 K||(79)|
|Al-modified Ni5P4–Ni2P||180||27||Ni foam/Al + P, 2 h at 873 K||(79)|
At the same time, the importance of the bulk structure of TMPs for OER catalysis has been demonstrated by a comparative study of two polymorphs of NiP2 (68). Cubic NiP2 is a metallic conductor, while monoclinic NiP2 is a semiconductor. During the OER reaction both polymorphs are expected to oxidise to a similar surface phase because of identical composition. However, the OER performance of the polymorphs was different (Table II), emphasising that the bulk structure of the TMP OER catalysts is an important factor determining the catalytic activity.
Interestingly, the aforementioned Fe0.2Ni0.8P2 catalyses the OER in alkaline media as well, showing reasonably high activity (Table II). More importantly, Fe0.2Ni0.8P2 serves as precatalyst for the in situ generation of the active catalyst during water oxidation. Since the Fe0.2Ni0.8P2 catalyst film was quite thin, it underwent complete oxidation during OER into amorphous-like iron-containing Ni(OH)2, which remained active and stable for at least 60 h of alkaline OER. Based on the concept of in situ formation of real catalysts, the activity and stability of this system was further improved by interfacing the Ni2P NP precatalyst with magnesium oxyhydroxide (78). In contrast to the complete oxidation of Fe0.2Ni0.8P2 observed in the previous study, it was found that Ni2P NPs form distinct core@shell structures during alkaline OER (Figure 9), and the resultant catalyst shows η10 of only 280 mV (Table II), while being stable over eight days.
A self-supported aluminium-doped Ni–P foam cathode has been recently developed as a highly active material for HER. The substitution of aluminium for nickel atoms in the crystal structure of nickel phosphide favourably modifies the electronic structure of the resultant cathode (80). The knowledge that aluminium dissolves in sodium or potassium hydroxides (similar to selective etching of aluminium from aluminium-nickel alloys to form Raney nickel catalyst) and the catalyst is formed in situ from the Ni–P precatalyst has led to implementation of a foam anode with good physical mixing of aluminium and Ni–P at the foam surface (79), in contrast to the chemical doping above. Sacrificial leaching of the aluminium phase coupled to the oxidation of the Ni–P precatalyst produced a high-surface area real catalyst exhibiting an impressive Tafel slope of 27 mV dec–1 and offering anodic current densities of 10 mA cm–1, 100 mA cm–1 and 300 mA cm–1 at overpotentials of merely 180 mV, 247 mV and 312 mV, respectively (Table II). In addition, the anode demonstrated an excellent stability during galvanostatic electrolysis, providing steady j = 10 mA cm–2 at very low η ≈ 185 mV for over eight days. This is one of the best-performing pgm-free anodes reported for alkaline OER (79), marking that aluminium scaffolding plus in situ precatalyst oxidation has unprecedented potential towards alkaline OER anodes (81).
7. Emerging Transition Metal Boride Catalysts
7.1 Crystal and Electronic Structure
Transition metal borides (TMBs) represent another appealing group of Xides for WE. Similar to phosphides, borides of various stoichiometries are known for the majority of transition metals, including the earth-abundant iron, cobalt and nickel. A distinct characteristic of TMBs is the relatively small size and electron-deficient nature of boron atoms, which result in dense crystal packing and extensive contacts between metal atoms (82), even in the structures with a relatively high boron content (for example MB). Therefore, in contrast to TMPs, a more appropriate way to describe TMB structures with more than 50 at% of M is to consider boron-centred polyhedra of metal atoms.
Representative TMB structures are provided by iron borides: Fe3B, Fe2B, FeB and FeB2. As one traverses this series, the Fe–B distances remain consistent within the range from 1.95 Å to 2.35 Å, while each B atom becomes surrounded by fewer Fe atoms and the B–B bonding becomes more extensive (Figure 10). The structure of Fe3B is assembled of tricapped trigonal prisms, Fe9B, that fill up the volume by edge-sharing. The nearest B–B distance is 3.24 Å, suggesting the lack of any bonding between B atoms. In Fe2B, each B atom is enclosed in a bicapped trigonal prism of Fe atoms that can be also described as a flattened square antiprism. The antiprisms share basal faces that are perpendicular to the c axis of the tetragonal lattice, resulting in short B–B distances of 2.13 Å along that direction. Thus, linear chains of B atoms are observed along the c axis. The crystal packing is completed by the antiprisms sharing edges in the ab plane. Even shorter B–B distances, 1.79 Å, are found along zigzag chains observed in the FeB structure, where each B atom is surrounded by only seven Fe atoms that form a monocapped trigonal prism. Finally, a substantial change is observed in the structure of FeB2, where Fe atoms are sandwiched between honeycomb layers of B atoms, which can be considered isoelectronic to graphene if the oxidation state of +2 is assigned to Fe while the coordination environment of Fe is reminiscent of the ferrocene molecule. The B–B distance in the layer is 1.76 Å, only slightly shorter than the distance observed in the zigzag chains of B atoms in FeB.
Other TMBs exhibit structures similar to those of iron borides. It should be also mentioned that Fe3B, Fe2B and FeB2 are representatives of the structure types of Fe3C (cementite), Al2Cu and AlB2, respectively, while FeB is the parent of its own structure type.
Due to the smaller size of B atoms, extensive M–M contacts permeate the crystal structures of metal-rich TMBs (Mx By with x/y ≥ 1). As a result, the electronic structures of these materials feature non-zero DOS at the EF, resulting in metallic behaviour. High electrical conductivity can be beneficial when TMBs are used as catalysts or precatalysts for electrochemical reactions that afford rapid transport of electrons between the external circuit and the catalyst-electrolyte interface. Moreover, the substantial hybridisation of 3d orbitals in borides of first transition row metals leads to appearance of pronounced DOS peaks in the electronic structure. When the EF is tuned to cross these peaks, the resulting electronic instability is resolved by spontaneous spin polarisation that manifests itself as magnetic ordering in the macroscopic response of the material (83). For example, metallic Ni3B can be tuned into ferromagnetic behaviour by partial substitution of cobalt for nickel, which lowers the electron count in the system, changing the position of the EF with respect to the DOS features (Figure 11). The predictability of such changes is facilitated by the isostructural nature of 3d metal borides with specific compositions. A strong magnetic response may be beneficial for improving the efficiency of WE in the presence of an applied magnetic field, as has been shown in several recent reports (84–87).
7.2 Hydrogen Evolution Reaction Catalysis
Similar to TMPs, the substantial hybridisation between the metal d orbitals and boron p orbitals results in strong covalent M–B bonding in TMBs. As a result, the free energy of H atom absorption is lowered in comparison to pure metal catalysts, and such TMBs as RuB2 (88), MoB2 (89), FeB2 (90) and VB2 (91) exhibit high HER activity in both acids and bases. In fact, RuB2 shows activity similar to that of platinum metal under acidic conditions and outperforms platinum under alkali conditions (92). Besides the higher catalytic activity, RuB2 was also shown to be more acid- and base-resistant as compared to borides with higher ruthenium content. Lower activities in HER have been observed for other metal-rich TMBs, for example MoB (93), Mo2B (94) and Fe2B (95). Yet again, similar to TMPs, the majority of studies on the catalytic properties of TMBs have been limited to the half-cell HER measurements (Table III). A notable exception is the study of the overall water splitting in an alkaline electrolyser utilising FeB2, which achieved the current density of 10 mA cm–2 at 1.57 V (90), comparable to the state-of-the art systems.
|Compound||pH||η10, mV||b, mV dec–1||Synthesisa||Ref.|
|RuB2||0||16||30||K2RuCl5 + MgB2, 3–10 h at 973–1223 K; 0.5 M H2SO4; H2O/ethanol||(88)|
|RuB2||14||25||28||K2RuCl5 + MgB2, 3–10 h at 973–1223 K; 0.5 M H2SO4; H2O/ethanol||(88)|
|FeB2||14||61||88||FeCl2 + LiBH4 in THF, 2 h at reflux; centrifugation; H2O; 2 h at 873 K||(90)|
|MoB2||0||149||76||Mo + B, 15 min at 2073 K and 5.2 GPa||(89)|
|VB2||0||192||68||VCl3 + B + Sn, 8 h at 1073 K; 10% HCl; H2O/ethanol||(91)|
|AlMoB||0||301||–||Al(excess) + Mo + B, 10 h at 1673 K; 3 M HCl; H2O||(96)|
|Mo2B||0||>400||128||Mo + B, arc-melting; manual grinding||(94)|
An interesting approach to modifying the catalyst structure was suggested by Schaak and coworkers, who studied the HER catalysed by AlMoB (96). The structure of this material is derived by insertion of layers of Al atoms into the FeB-type structure discussed above (Figure 10). This spatial separation of the [MoB] layers is expected to enhance their catalytic activity. The HER at pH = 0 proceeded with a rather high overpotential, η10 = 400 mV. To increase the access to the catalytic sites, the authors soaked the material in sodium hydroxide, which led to partial removal of aluminium. After treatment with acid, to decrease the concentration of OH-terminated sites, the overpotential η10 recorded at pH = 0 decreased to 301 mV. These findings suggest that the spatial separation of the catalytically active blocks by insertion of other structural fragments might be a viable strategy toward enhancing the catalytic activity of Xides, in general.
7.3 Alkaline Oxygen Evolution Reaction Catalysis
Recently, TMBs have been shown to exhibit excellent electrocatalytic activity toward OER under alkaline conditions. As emphasised in Section 4, the nature of OER leads to complex chemical processes that result in inevitable surface oxidation of the catalyst. Thus, works that attribute the OER activity to intrinsic behaviour of TMBs should be taken with a grain of salt. Careful studies of these reactions confirm the formation of a shell of catalytically active oxides and hydroxide NPs around the core TMB structure, which, therefore, should be classified as a precatalyst. The role of the precatalyst is similar to that already discussed for the case of TMPs: it provides the structural support to the in situ formed oxidised surface shell and also facilitates electron transport between the catalytically active surface and the external circuit.
In contrast to HER, where MB2 borides perform better, in the case of OER (Table IV) higher catalytic activities have been shown by the systems that include metal-rich borides, such as M3B and M2B (M = Fe, Co, Ni). Solid solutions of these borides have been also explored to optimise the catalytic performance. While the surface of such catalyst, quite obviously, undergoes substantial oxidation, as demonstrated by XPS and TEM measurements (97, 98), it is worth noting that they exhibit better performance than corresponding metals (102) or dispersed metal oxide NPs (98). These observations strongly support the hypothesis that the TMB core underlying the catalytically active shell enhances the overall stability and activity of such catalyst or precatalyst nanoheterostructure.
|Compound||η10, mV||b, mV dec–1||Synthesisa||Ref.|
|AlFe2B2||240||42||Al + Fe + B, arc-melting; ball-milling||(98)|
|FeB||270||49||Fe + B, arc-melting; ball-milling||(98)|
|Co2B||287||51||Co + B, ball-milling, 10 h||(99)|
|FeB2||296||52||FeCl2 + LiBH4 in THF, 2 h at reflux; centrifugation; H2O; 2 h at 873 K||(90)|
|Ni2B||296||58||NiCl2 + NaBH4 + NaOH in H2O/ethylenediamine, electrodeposition on Cu foil; 3 h at 473–573 K||(100)|
|Ni3B||302||52||Ni(OAc)2 + NaBH4 + NaOH in H2O, 10 min; centrifugation; H2O/ethanol; 0.5–2.5 h at 623 K||(101)|
|Co3B||≈370||n/a||CoBr2 + LiBH4 in THF, 2 h; filtration; 2 h at 773 K||(97)|
The layered structure of ternary boride, AlFe2B2 (Figure 12), is similar to that of AlMoB, which was shown to exhibit an increased catalytic activity toward HER after the aluminium layers had been partially etched with sodium hydroxide solution. While AlFe2B2 dissolves rather quickly in acids (103), it is very stable in alkali solutions. Therefore, the Shatruk and Kolen’ko groups explored this material as a potential precatalyst for alkaline OER (98). The initial electrocatalytic cycles revealed a slight decrease in the η10 value, which stabilised at 240 mV after about 20 CV cycles and remained remarkably constant for 10 days. Examination of the catalyst after this activation period revealed that aluminium had been partially etched from the structure and a stable shell of catalytically active Fe3O4 NPs had been formed around the AlFe2B2 core particle. Importantly, much poorer performance was observed when Fe3O4 NPs were used by themselves, without the underlying AlFe2B2 precatalyst.
8. Conclusions and Outlook
Electrocatalysis is a rapidly evolving research area, and there are plenty of pgm-free catalysts reported in the literature, including nickel alloys, oxides, TMPs and TMBs, as detailed here. Recent findings have shown that, while HER most likely takes place on the surface of the pristine Xide catalyst, the in situ formation of oxide or oxohydroxide NPs as the true catalyst is clearly observed during OER as a result of surface oxidation of the Xides. Even though pgm-free nickel alloys, TMPs and TMBs do not outperform platinum, their promising HER activity can be of practical importance due to significantly lower costs. In contrast, many iron-, cobalt- and nickel-based catalysts outperform the standard rutile-type iridium/ruthenium oxides in OER activity, but little is known about the durability of these pgm-free materials. This is mainly due to the fact that the vast majority of the literature reports the results of laboratory half-cell measurements and the durability is only tested for a few weeks at most.
With respect to the synthesis, it is difficult to formulate the exact key requirements (for example chemical composition; promoters; crystallographic, electronic and surface structures; physical properties; morphology; fine microstructure; specific surface area; particle size distribution and supporting material) that should be fulfilled for the HER/OER catalysts to be economic, active, stable and durable for AEMWE. This challenge is mainly explained by the ever increasing complexity of the reported catalysts, their dynamic reconstruction during reactions and large scattering of the resultant half-cell testing data. The way to solve this shortcoming is to foster the international community working on WE to follow well-defined characterisation and testing protocols. Recent efforts in this direction (30, 31) provide hope that the aforementioned requirements will be established in the near-term, also involving the help of data mining and computation. It would be also interesting to see how Xides will compete and, more importantly, participate in the rapidly developing area of mass-efficient atomically dispersed catalysts for HER/OER (104, 105).
Considering the discovery of new, more efficient catalysts, solid-state chemistry offers hundreds of ternary and multinary phosphides, borides and other Xides, in addition to the relatively limited range of binary materials that have been studied thus far as potential HER/OER electrocatalysts. Such materials have unique crystal and electronic structures and may exhibit quite different catalytic properties as compared to their binary counterparts. Tuning the transition-metal d-orbital filling, local metal and non-metal (boron, carbon, nitrogen, silicon, phosphorus, sulfur, selenium, tellurium) coordination environments, and the density of states at the Fermi level is important to optimising the HER/OER catalytic performance. The large structural and compositional variety is a challenge as the synthesis and property characterisation protocols are tedious. Application of computational and machine learning approaches can substantially accelerate identification of the most promising candidates. Coupling the computational screening, which accounts for surface dynamics, together with experimental research should result in emergent catalysts with improved performance.
Assembling even a single AEMWE cell is a tedious process that requires expertise not commonly found in academic research laboratories. For instance, to the best of our knowledge, no full AEM electrolyser has been tested with promising TMP/TMB cathodes and anodes. Nevertheless, real AEMWE testing is quite important to demonstrate the applied potential of Xides as catalysts. Here, the early involvement of industry will be highly beneficial, allowing the real future prospects of the reported pgm-free catalytic systems to be understood.
Another issue is the lack of benchmarked AEMWE components (membrane, ionomer, GDL and PTL) and standard HER/OER catalysts accepted by the electrolyser community. This state of matters makes comparison of performances not only difficult, but in many cases meaningless. Substantial input from academia, national laboratories and industry into standardisation of materials and test protocols is needed. This work has been initiated in the international US–EU collaboration under the HydroGEN consortium and has already resulted in the harmonisation of testing protocols for PEMWE, which will be followed by expansion to the promising AEMWE field. Establishing such protocols will allow the electrolyser community to work at the device level to engineer AEMWE electrolysers with high performance and durability (16, 106).
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density of states
high-angle annular dark-field scanning transmission electron microscopy
hydrogen evolution reaction
membrane electrode assembly
oxygen evolution reaction
particle and pore forming agent
platinum group metal
porous transport layer
reference hydrogen electrode
solid oxide electrolyser
solid polymer electrolyser
transition metal boride
transition metal phosphide
technology readiness level
transition metal borides, carbides, nitrides, phosphides and chalcogenides
X-ray photoelectron spectroscopy
We thank all our collaborators and team members for fruitful discussions and support. Alexey Serov acknowledges the US Department of Energy (DOE) Office of Energy Efficiency & Renewable Energy (EERE) (DE-EE0008419, Active and Durable pgm-free Cathodic Electrocatalysts for Fuel Cell Application) and the US DOE EERE (DE-EE0008833, High-Performance AEM LTE with Advanced Membranes, Ionomers and pgm-Free Electrodes). Kirill Kovnir acknowledges support by the National Science Foundation under Grant No. 1955456. Michael Shatruk acknowledges support by the Petroleum Research Fund of the American Chemical Society under Grant No. 59251-ND10. Yury Kolen’ko acknowledges the EU’s Horizon 2020 research and innovation programme (CritCat Project, grant agreement No. 686053), and Portuguese National Funding Agency for Science, Research and Technology (CritMag Project, PTDC/NAN-MAT/28745/2017).
Alexey Serov is a Chief Scientist in Pajarito Powder, a US-based company manufacturing electrocatalysts for fuel cells and electrolysers. He has more than 10 years of industrial experience in the design, synthesis and manufacturing of new energy-related materials.
Kirill Kovnir is an Associate Professor of Chemistry at Iowa State University, USA, and Faculty Scientist at US DOE Ames Laboratory. He enjoyed solid-state chemistry research starting from freshman year at Lomonosov Moscow State University, Russia. Kirill’s research interests are in the broad field of solid-state and materials chemistry with focus on the development of unconventional synthetic routes towards novel inorganic solids; prediction, synthesis and structural characterisation of complex pnictides, tetrelides and borides; and development of novel thermoelectric, magnetic, catalytic and non-linear optical materials.
Michael Shatruk is interested in applications of intermetallic materials to problems of catalysis and magnetism. His fascination with the structures and properties of intermetallics nucleated during graduate studies at Lomonosov Moscow State University, Russia, and a postdoctoral stint at Cornell University, USA. He continues to explore and share this passion as professor of inorganic and materials chemistry at Florida State University, USA.
Yury V. Kolen’ko enjoys synthesis, investigation and application of unconventional nanomaterials. Particular emphasis is placed on catalysis, clean energy technology and advanced coatings. He is happy to collaborate on these topics with his university friends Alexey, Kirill and Michael.