Rare‐Earth Doping Transitional Metal Phosphide for Efficient Hydrogen Evolution in Natural Seawater

Electrolysis of inexhaustible seawater offers a promising way for harvesting practically infinite hydrogen energy without worsening freshwater shortage. Complicated ionic environment of saline seawater, however, places a great burden on catalyst performance for hydrogen evolution. Herein, tailoring the electronic structure of transitional metal phosphide by rare‐earth doping for effectively propelling hydrogen evolution in a wide pH range and natural seawater is reported. The rare‐earth doping leads to not only a nearly zero Gibbs free energy of H* adsorption and lower work function but also faster OH* desorption for rapid release of the active sites, thereby accelerating the hydrogen evolution reaction (HER) kinetics under nonacidic conditions. On this basis, highly conductive and hydrophilic MXene is introduced to further boost the adsorption of hydrogen carriers and charge transfer across the catalyst. Together, they allow the activity, kinetics, and durability of the electrocatalyst for HER to be improved overall. The obtained electrocatalyst shows superior activity to commercial Pt/C in terms of specific surface area and active mass and turnover frequency, as well as 400 times longer lifetime than Pt/C with high Faradaic efficiency in natural seawater. This study presents new insight into the development of viable electrocatalysts for harvesting hydrogen energy from abundant‐reserve seawater.

phosphides (TMPs) have been considered as a promising system of HER electrocatalyst due to their electronic structure resembling to Pt-group metals, good conductivity, and high chemical stability. [21][22][23] Various strategies, including downsizing to the nanoscale, surface modification, phase hybridization, defect engineering, and heteroatom doping, have been adopted to optimize the HER activity of TMP electrocatalysts. [24][25][26][27][28] However, there are only a handful of TMP-based electrocatalysts for short-term HER in seawater, such as Ni 5 P 4 , [29] Mo 2 C-MoP embedded N, P co-doped carbon nanofibers, [30] featherlike NiCoP/nickel foam, [31] NiCoN/Ni x P/NiCoN, [32] and CoMoP@C composite. [10] A lack of high-performance seawater-efficient electrocatalysts, especially under neutral condition, remains a significant obstacle to the harvesting of hydrogen energy from direct seawater electrolysis. [33] Heteroatom doping offers an efficient way to preciously tailor the intrinsic properties of the catalyst by distorting the crystalline lattice and redistributing the local electronic structure. [34] With larger atomic radius but lower electron negativity than most of transition metals (e.g., Co, Mo, Ni, and Fe), rare-earth (Ln) elements may act as effective dopants to change the structural and electronic structure of TMP significantly for optimizing HER activity, which, however, has been rarely explored. [35] In this work, we report to design an efficient electrocatalyst for natural seawater electrolysis by rare-earth doping of TMPs. As a demonstration of proof-of-concept, La-doping is applied to alter the d-band center of MoP coupling with highly conductive and hydrophilic MXene. This effect not only balances the H* adsorption-desorption but also promotes the OH À desorption, thereby reducing the occupation of catalytic sites by these species for accelerating the HER ( Figure 1). Meanwhile, an overall enhancement in charge access and water adsorption is achieved by coupling highly conductive MXene with hydrophilic properties to propel HER under nonacidic conditions. As a result, the obtained catalyst can outperform the commercial Pt/C catalyst for HER in a wide pH range of 7.2-14 and natural seawater in terms of the activity normalized to a specific surface area or the active mass. It also exhibits superior durability for 400 h with high Faradaic efficiency of 95% for HER in natural seawater, highlighting the high robustness in complex ionic environments.

Results and Discussion
Ti 3 C 2 T x MXene is first made by selectively etching Al in the ceramic Ti 3 AlC 2 followed by ultrasonic exfoliation ( Figure S1, Supporting Information). Afterward, a uniform thin layer of Mo-based polyoxometalate (POM) and rare-earth metal salts residing in Cu-based MOF is coated on MXene surface with abundant negatively charged groups. The nanospaceconfinement effect of molecular cages in Cu-based MOF restricts the uniform distribution of Mo-based POM and rare-earth metal species as molecular/ionic clusters. It enables the formation of rare-earth metal-doped MoP nanocrystallites on P-doped carbon matrix surrounding the MXene (denoted as Ln x Mo 1Àx P/PC/ MXene) after subsequent phosphorization and acidic etching of Cu 3 P ( Figure S2, Supporting Information). [36,37] Taking La-doped catalyst as a representative, the catalyst well inherent the sheet-like shape of MXene (Figure 2a (Figure 2f ). [38] The Mo 6þ signals disappeared after ionic sputtering for 20 s, implying that it is originated from the oxidation of MoP on particle surface in the atmosphere instead of the bulk phase. The P 2p spectrum can be resolved to P 2p 3/2 /2p 1/2 (129.7/131.8 eV), P-C (133.5 eV), and P-O (134.6 eV) ( Figure S3b, Supporting Information). [39] The La 3d spectrum features a 3d 5/2 /3d 3/2 doublet at 837.  Figure S3c, Supporting Information). [40] The presence of 2D MXene with largely extended surface area endows the The intrinsic HER activity of La x Mo 1Àx P/PC/MXene with various La/Mo ratios is first evaluated in 1.0 M KOH. All of them exhibit superior activity to La-free counterparts (denoted as MoP/PC/MXene), showing greatly reduced overpotential to achieve a current density of 10 and 100 mA cm À2 (η j ¼ 10 and η j ¼ 100 ). Among them, the La 0.17 Mo 0.83 P/PC/MXene yields the lowest η j ¼ 10 (65 mV) and η j ¼ 100 (135 mV), lowering by a half than that of MoP/PC/MXene (η j ¼ 10 ¼ 136 mV and η j ¼ 100 ¼ 273 mV), respectively (Figure 3a, S5, Supporting Information). The HER overall all the La x Mo 1Àx P/PC/MXene catalysts undergo an accelerated Volmer-Heyrovsky route with smaller Tafel slopes (42.1 À 51.2 mV dec À1 ) relative to MoP/PC/MXene (55.8 mV dec À1 ) ( Figure 3b). [41] The activity of La 0.17 Mo 0.83 P/PC/MXene can also exceed 20% Pt/C at the current densities above 40 mA cm À2 (Figure 3c) and is superior to most reported Mo-based catalysts for alkaline HER (Table S1 The MoP-free La/PC/MXene exhibits poor HER activity with large η j ¼ 10 (479 mV) and Tafel slopes (129.8 mV dec À1 ), which is close to that of PC/MXene. It suggests the MoP instead of La species or MXene acts as the active phase for catalyzing HER. Despite the poor HER activity, the presence of MXene contributes greatly to high activity of La x Mo 1Àx P/PC/MXene catalysts in extending a large and conductive catalytic interface. As a result, the activity La 0.17 Mo 0.83 P/PC/MXene can be enhanced by 2.8 folds in contrast to MXene-free La 0.16 Mo 0.84 P/PC with a similar Mo:La ratio (η j ¼ 10 ¼ 185 mV and η j ¼ 100 ¼ 260 mV).
The HER activity is normalized to the specific surface area, and the active mass is evaluated to further assess the activity of the catalyst for practical use. When normalized to specific surface area, the La 0.17 Mo 0.83 P/PC/MXene catalyst still exhibits superior activity to 20% Pt/C, MoP/PC/MXene and La 0.16 Mo 0.84 P/PC at high potential ( Figure 3e). Remarkably, the mass activity of La 0.17 Mo 0.83 P/PC/MXene outperforms the 20% Pt/C at the overpotential beyond 137 mV. It can even deliver a much higher mass activity of 3.39 A mg cat À1 than that of 20% Pt/C (2.44 A mg cat. À1 ) at an overpotential of 200 mV ( Figure S8, Supporting Information). The turnover frequency (TOF) reflecting the number of H 2 molecules evolved per second at each active site is another important indicator to evaluate the activity of electrocatalyst. [42,43] The TOF of La 0.17 Mo 0.83 P/PC/MXene (2.45 s À1 ) at an overpotential of 100 mV is 3.3 times higher than The above results suggest the significant effect of La doping on promoting intrinsic HER activity of MoP. As well known, the rare-earth metals usually show high similarity in electron structure (4f 0%14 5d 0%1 6s 2 , e.g., La(5d 1 6s 2 ) Ce(4f 1 5d 1 6s 2 ) Pr(4f 3 6s 2 ) Nd(4f 4 6s 2 )), chemical properties, and electron negativity (e.g., 1.1, 1.12, 1.13, 1.14 for La, Ce, Pr, and Nd, respectively). The versatility of rare-earth doping on enhancing HER performance of MoP is explored by using Ce, Pr, and Nd as the dopants. Typically, the Ce 0.14 Mo 0.86 P/PC/MXene, Pr 0.19 Mo 0.81 P/PC/ MXene, and Nd 0.16 Mo 0.84 P/PC/MXene were synthesized in a similar way of making La 0.17 Mo 0.83 P/PC/MXene catalyst with careful tuning of Ln/Mo ratio ( Figure S11, Supporting Information). All these catalysts show greatly enhanced activity and kinetics for HER in 1.0 M KOH, as characterized by much lower η j ¼ 10 (70 À 80 mV) and Tafel slopes (48.6 À 51.5 mV dec À1 ) relative to Ln-free  MoP/PC/MXene (136 mV and 55.8 mV dec À1 ) ( Figure S12, Supporting Information). This observation suggests that doping of rare-earth metals could be a general approach to boost the HER activity of TMP electrocatalysts. In a wide range of nonacidic pH, the La 0.17 Mo 0.83 P/PC/ MXene also exhibits excellent electrocatalytic performance for HER. It requires smaller η j ¼ 10 of 276, 192, 107 mV than those of 20% Pt/C (η j ¼ 10 ¼ 315, 235, 138 mV) under pH 7.2, 9.3, and 11.2, respectively (Figure 3g and S13, Supporting Information). Tafel slopes under different pH indicate that the HER over this catalyst undergoes a similar kinetic mechanism over a wide nonacidic pH range (Figure 3h). As compared to 20% Pt, the La 0.17 Mo 0.83 P/PC/MXene always enable faster kinetics regardless of the pH change, manifesting as smaller Tafel slopes (48.9, 71.7 and 51.2) relative to Pt/C (57.4, 87.9 and 87.5) under pH 7.2, 9.3, and 11.2, respectively.
Long-term durability of La 0.17 Mo 0.83 P/PC/MXene is also evaluated for HER in 1.0 M KOH. The negligible shift of polarization curves is observed after 5,000 sweeps relative to the initial state. The chronoamperometry tests show that this catalyst can keep working stable for over 400 h (Figure 3i). Post-mortem analysis reveals that the original structure of La 0.17 Mo 0.83 P/PC/MXene is well maintained in terms of the structure, crystalline phase, and the covalent state of Mo after long-term use, exhibiting high robustness of the catalyst ( Figure S14, Supporting Information). Under pH 7.2, 9.3 and 11.2, this catalyst shows a negligible shift of polarization curves after 3,000 sweeps relative to the initial state and stable chronoamperometry performance for over 50 h in contrast to fast failure of 20% Pt/C in a short time ( Figure S15, Supporting Information).
The La 0.17 Mo 0.83 P/PC/MXene electrocatalyst retains good activity for HER in natural seawater (Bohai Sea, China) with poor ionic conductivity and saline composition. It delivers the η j ¼ 10 of 158 mV and η j ¼ 100 of 365 mV, much lower than that of 20% Pt/C (η j ¼ 10 ¼ 263 mV) (Figure 4a). The fast decrease of the HER activity after 10 cycles is due to the deposition of Mg 2þ and Ca 2þ from the natural seawater on the active sites of the catalysts. In natural seawater, the Pt/C encountered fast failure at high potential when the current density exceeds 100 mA cm À2 . The polarization curves of La 0.17 Mo 0.83 P/PC/MXene can maintain stable with small shift after 2,000 sweeps in seawater at various current densities from 10 to 500 mA cm À2 . Whereas the 20% Pt/C is rapidly deactivated after only 10 sweeps (Figure 4a  www.advancedsciencenews.com www.small-structures.com well retained with negligible change in terms of crystalline phase and chemical composition, showing high stability against corrosive saline seawater ( Figure S17, Supporting Information). As a contrast, the 20% Pt/C suffers apparent Pt leaching after shortterm use ( Figure S18, Supporting Information), leading to poor stability in seawater. The Faradaic efficiency of the La 0.17 Mo 0.83 P/ PC/MXene is as high as 95% in natural seawater (Figure 4c). It also outperforms most of the reported electrocatalysts in both activity and durability for HER in seawater, determining the high promise in hydrogen production from seawater ( Figure 4d and Table S2, Supporting Information). Density functional theory (DFT) calculations are utilized to analyze the effect of La doping on the electronic structure of MoP for boosting HER activity in neutral/alkaline conditions. The slab models of (001) surface of MoP, La 0.08 Mo 0.92 P, La 0.17 Mo 0.83 P, and La 0.28 Mo 0.72 P are considered for this purpose ( Figure S19, Supporting Information). The La has much lower electron negativity (1.1) than that of Mo (2.16) and P (2.19), inducing the enrichment of charge density around Mo and P atoms by La doping into the lattice of MoP (Figure 5a). This phenomenon is characterized by continuous decrease of the valent state of Mo with increasing La doping in high-resolution Mo 3 d XPS spectra of La x Mo 1Àx P (Figure 5b). The P 2p XPS peak also tends to shift to lower binding energy with La doping due to the bonding with La atoms ( Figure S20, Supporting Information). [44] Apparently, the La is an efficient electron donator to enrich the   (Figure 5c). [45] As a result, the ΔG H* can be gradually reduced to nearly zero (À0.16 eV) by La doping in contrast to a very negative value for MoP (À0.73 eV) (Figure 5d). A nearly zero ΔG H* allows neither a too strong nor too weak H* binding, which is desired to accelerate H* conversion to H 2 without limiting H 2 release, thereby enhancing HER activity. [46] The energetics diagram of neutral/alkaline HER is further modeled by three elementary steps, namely, water dissociation to H* and OH* intermediates, OH* desorption from catalyst surface, and then H* desorption to generate H 2 . [47] It reveals the weaker H* adsorption but faster OH* desorption with a lower energy barrier from La x Mo 1Àx P relative to MoP (Figure 5e). These features are important to release more active sites from the coverage of H 2 and OH À intermediates, thereby accelerating the HER kinetics. Ultraviolet photoelectron spectrometer (UPS) analysis reveals that the La doping can reduce the work function of MoP, which promises fast electron transfer and also help to boost the HER ( Figure S21, Supporting Information). The MXene also plays a significant role in promoting HER due to high conductivity and unique 2D nanostructure with large exposed surface. [48][49][50][51] Its presence enables a great increase in electrochemical active surface area of the catalyst, indicated by higher electrochemical double-layer capacitance (C dl ) of La 0.17 Mo 0.83 P/PC/MXene (15.2 mF cm À2 ) relative to La 0.16 Mo 0.84 P/PC (7.08 mF cm À2 ) (Figure 6a and S22, Supporting Information). Meanwhile, the presence of abundant oxygen-containing groups on the surface endows the MXene with superb hydrophilic properties. Smaller water contact angle (52°) of the La 0.17 Mo 0.83 P/PC/MXene verifies the better hydrophilicity to MXene-free catalyst (74°) ( Figure S23, Supporting Information), which is desired to enhance electrode wetting and access of active sites to water, the hydrogen carrier under neutral/alkaline conditions. This character improves the capability of water adsorption on La 0.17 Mo 0.83 P/PC/MXene (1.93 g g cat. À1 ) by 1.6 times with a faster rate than that of  high conductivity (260 S cm À1 ), [54] the MXene also contributes to faster charge transport across the electrocatalyst for reducing the potential polarization. In the presence of MXene, the electrical conductivity of La 0.17 Mo 0.83 P/PC/MXene (%196 S cm À1 ) can be enhanced by nearly 8 times relative to MXene-free catalyst (25 S cm À1 ). Accordingly, this improvement dramatically reduces the charge-transfer impedance of La 0.17 Mo 0.83 P/PC/ MXene (R ct ¼ 2.1 Ω) by 27 folds with respect to MXene-free catalyst (R ct ¼ 56 Ω) (Figure 6d and S24, Supporting Information).
Working together with rare-earth doping, the HER performance of MoP can be boosted by overall enhancement in intrinsic catalytic properties and accessibility of charge and hydrogen carrier to the catalyst.

Conclusion
In conclusion, a seawater-efficient electrocatalyst was developed by anchoring rare-earth doped molybdenum phosphide nanocrystallites on MXene. Doping of rare earth significantly improves the HER activity of MoP by balancing the H* binding energy and weakening the adsorption of OH* for reducing the undesirable coverage of active sites by H 2 and OH À . MXene with high conductivity and hydrophilicity is further induced to improve the electrical conductivity of the electrocatalyst and the adsorption of water molecules on the catalytic interface. A synergy of rare-earth doping and MXene greatly boosts the HER activity and kinetics beyond the 20% Pt/C in a wide pH range. Such catalysts, which display excellent Faradaic efficiency, outstanding activity, and stability to 20% Pt/C, can be employed to efficiently harvest hydrogen from natural seawater thanks to their strong structure and high activity.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.