Cobalt on Tungsten Titanium Carbide MXene: A Electrocatalyst for Industrial-Scale High Current Density Alkaline Hydrogen Evolution Reaction

doi:10.21203/rs.3.rs-4427524/v1

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Cobalt on Tungsten Titanium Carbide MXene: A Electrocatalyst for Industrial-Scale High Current Density Alkaline Hydrogen Evolution Reaction

https://doi.org/10.21203/rs.3.rs-4427524/v1

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Hydrogen (H₂) is considered the most promising clean energy source to replace conventional fossil fuels. Despite the exceptional catalytic activity of Pt-group metals, the high cost and relatively low durability of these noble metal-based catalysts remain key bottlenecks for sustainable hydrogen production at the industrial scale. In this report, we synthesized a new type of MXene, tungsten titanium carbide (W₂TiC₂). By loading cobalt (Co) onto the surface of W₂TiC₂, we developed a highly effective and stable catalyst for alkaline hydrogen evolution reaction (HER). The optimized catalyst exhibited a small overpotential of 62.58 mV at 10 mA/cm² and a low Tafel slope of 44.34 mV/dec. At high current density of 100 mA/cm² and 1000 mA/cm², the low overpotentials of 191.18 mV and 407.72 mV were achieved, significantly outperforming the commercial Pt/C electrode. Under both current ranges, our catalyst exhibited excellent stability of 500 h at 10 mA/cm² and for 100 h at 1000 mA/cm² without any obvious degradation. In flow cell tests, by pairing with Ni foam, our catalyst also required much lower cell voltage than commercial Ni foam||Pt/C and maintained ~ 100% H₂ faradaic efficiency over the 15h continuous tests from 100 to 400 mA/cm². X-ray absorption near edge structure (XANES) demonstrated a temperature induced redistribution of Co on MXenes from single sites and large nanoparticles (NPs) to uniform small NPs. Density functional theory (DFT) calculations, combined with X-ray photoelectron spectroscopy (XPS) and extended X-ray absorption fine structure spectroscopy (EXAFS), revealed that the HER activity was sensitive to the coordination environment of Co on W₂TiC₂, and the Co-W interface is the likely HER active site . This work sheds light on structure design and utilization of metal-support interaction (MSI) in MXene-supported metal catalysts for the efficient electrochemical water splitting at industrial scale.

Physical sciences/Energy science and technology/Renewable energy

Physical sciences/Chemistry/Catalysis

MXenes

Non-noble metal catalyst

Metal support interaction

Alkane hydrogen evolution

High current density

Hydrogen (H₂) has long been considered the most promising candidate to replace fossil fuels due to its high energy density and carbon-neutral nature ^1–3. Electrochemical water splitting, when synergistically combined with electricity derived from renewable resources, holds great potential in achieving zero-carbon emission, which is considered a crucial method for sustainable production of “green” hydrogen ^{4, 5}. In this context, several nations have defined strategies to support hydrogen production through regulations, fundings, and tax programs ⁶. According to the projections made by the International Energy Agency (IEA), water electrolyzers with a capacity of 720 GW are needed by the year 2030 to follow the path towards the net zero greenhouse gas emissions by the year 2050, with a projected required-capacity of 3670 GW in the year 2050. While achieving these projections is an ambitious goal for the current petroleum-based energy sector, developing an efficient, durable, and affordable electrode materials for “green” hydrogen production can help maintain the globally averaged temperature rise below 1.5 $\text{℃}$ by the year 2050 ⁷.

Despite the exceptional catalytic activity of Pt-group metals for the hydrogen evolution reaction (HER) under acidic conditions, the high cost, concern over corrosion in acidic condition, and relatively lower durability of these noble metal-based catalysts remain the key bottlenecks for sustainable hydrogen production at the commercial scale ^8–10. Alkaline water electrolysis (AWE) is the most technologically mature way of continuous water electrolysis at large scale ^{11, 12}, in which the HER takes place at the cathode, and the oxygen evolution reaction (OER) takes place at the anode. However, its widespread practical application is still limited by the economic burden posed by substantial energy losses owing to the high overpotential of the half-reactions, and scarcity of noble metal catalysts ^13–15. Furthermore, under industrial operating conditions, the commonly used HER catalyst, e.g., Pt/C also experiences durability decline and significant activity drop in alkaline media compared with its performance in acidic electrolysis ^16–20. Therefore, the rational design of stable, low-cost catalysts that maintain high activity in alkaline electrolytes at high current densities is essential for development of a commercial process for HER ^21–23.

In this regard, Co-based electrocatalysts have risen to prominence benefiting from their cost-effectiveness, reduced environmental impact, good catalytic activity, and structural diversity ²⁴. When combined with other materials such as carbon-based materials or transition metals, Co-based catalysts can exhibit synergistic effects that enhance not only their HER activity but also stability ^{25, 26}. Extensive research has been dedicated to developing cobalt related composite structures with transition metal oxides, sulfides, phosphides, carbides, and hydroxides ^27–37; however, few studies have demonstrated good performance with comprehensive coverage across a wide range of current densities (e.g., from 10 mA/cm² to 1 A/cm²) while also achieving long-term stability under such conditions, such as exceeding 100 hours.

In this work, we report a non-noble metal based HER catalyst by dispersing Co on a novel MXene (W₂TiC₂). MXenes are a family of two-dimensional early transition metal carbides, nitrides, and carbonitrides ³⁸. As catalyst support, MXenes can significantly enhance charge transfer and increase surface area with exposed active sites owing to their metallic electrical conductivity and two-dimensional large surface ^{25, 39–41}. Together with the high hydrophilicity and good chemical stability under reactive conditions, MXenes have been widely applied in electrocatalysis as part of composite structures. Our group, together with many others, have discovered MXenes can modulate the geometric and electronic properties of metal catalyst through various metal-support interactions (MSI) ^42–49. During our studies, the catalytic performance of our Co/W₂TiC₂ catalyst were not only evaluated at low current densities relevant to direct solar driven water splitting (10 mA/cm²), but also at industrial electrochemical AWE level (1000 mA/cm²) ^{50, 51}. In 1 M KOH, the catalyst showed a low overpotential of only 62.58 mV at 10 mA/cm² with a Tafel slope of 44.34 mV/dec and exhibited excellent stability for 500 h under such current density. At industry-level conditions, the catalyst maintains the incredible performance, exhibiting overpotential of 407.72 mV and a high stability of at least 100 h at 1000 mA/cm². Our density functional theory (DFT) calculations, in conjunction with X-ray absorption near edge structure (XANES) studies, identified interfacial Co-W sites as the likely active site. Our results showed a strong correlation between the d-band center of interfacial W atoms and hydrogen adsorption strength, indicating a strong structurally modulated metal-support interaction at Co-W interfaces.

W ₂ TiC ₂ MXene and Co/W₂TiC₂ catalysts. The synthesis procedure of W₂TiC₂ MXene is shown in Fig. 1. The bulk W₂TiAlC₂ MAX precursor was first prepared by spark plasma sintering of commercial powders. Two dimensional W₂TiC₂ was then obtained by selective etching of the aluminum layers with hydrofluoric acid (HF). Tetramethylammonium hydroxide (TMAOH) was used followed by sonication to exfoliate the MXene to thinner layers. The X-ray diffraction (XRD) pattern of W₂TiAlC₂ showed the characteristic peak of (004) plane at 2θ = 19° (Fig. 1b), and the peak positions exhibited good agreement with the pattern simulated in VESTA ⁵² by importing the lattice parameters derived by the 2θ values of the (110), (101), and (004) peaks of the experimental results. XRD refinement was conducted by GSAS ⁵³ software and the alignments of main peaks including (006), (103), (104), (008) and (106) were achieved, confirming the expected W₂TiAlC₂ composition and structure. The (002) diffraction peak showed up in a lower angle (2θ = 6.8°) after the HF etching, indicating a larger c lattice parameter of W₂TiC₂ due to the removal of aluminum layers. After exfoliation, the (002) plane peak shifted further to 2θ = 5.85°, consistent with the expansion along the [001] direction. The experimental XRD also aligned with VESTA simulated XRD for W₂TiC₂ (Supplementary Fig. 1d), showing the c lattice parameter increasing from to 1.873 nm to 3.009 nm. Scanning electron microscopy (SEM) image of W₂TiC₂ MXene showed a morphology of stacked nanosheets (Fig. 1d). More magnified observation through transmission electron microscopy (TEM) revealed that the nanosheets were composed of few layers of MXenes, as shown in the side view (Fig. 1e) by the contrast of light and shaded regions changing with number of MXene flakes. Lateral structures of alternating atom layers of various contrast were also observed from the sidelined pieces, further validating the M₃C₂ structure of W₂TiC₂.

Co was then loaded on the W₂TiC₂ MXenes by incipient wetness impregnation of cobalt chloride precursor, followed by reduction under H₂/N₂ atmosphere at different temperatures, denoted as Co/W₂TiC₂-n (n is applied temperature in degree Celsius, n = 500, 600, 700). The X-ray diffractograms of the Co/W₂TiC₂-500, 600 and 700 catalysts (Fig. 2a) exhibited broad small Co peaks at around 2θ = 44° region, suggesting the existence of metallic Co. The broadening of the peaks were resulted from small nanoparticle sizes, which was confirmed later by the Extended X-ray absorption fine structure spectroscopy (EXAFS) fitting results. The decrease in the intensity of the MXene peaks and slight shift to lower 2θ of the (002) peak with increasing temperature suggested a slight decrease in the inter-flake spacing after the introduction of Co. The SEM image in Fig. 2b showed that the thin layer morphology of the MXene was maintained after the Co loading with no large Co grain being observed, indicating the uniform distribution of Co on the MXene surface. The TEM image of Co/W₂TiC₂-500 showed the MXene layers, with a consistent d spacing measured as 0.24 nm for the (01-10) lattice planes. The selected area electron diffraction (SAED) pattern further confirmed the basal plane hexagonal symmetry structure (Figs. 2c, d). Owing to the high extra-nuclear electron density of tungsten element, Co imaging was hindered by W₂TiC₂ under TEM and cannot be observed clearly except by mass thickness contrast on very thin areas of sample edge. In Fig. 2c, small dark patches were found at the edge of a MXene flake. Similar dark regions of 1-2 nm size can be observed on very thin regions of the Co/W₂TiC₂-500 sample (Supplementary Fig. 3). Lattice fringes distinguished from the lattice of W₂TiC₂ were marked and measured to be 0.204 nm and 0.177 nm, matching the (111) and (200) planes of face-centered-cubic Co respectively. The grain boundaries were not sufficiently distinct owing to the same reason mentioned above, necessitating the employment of additional characterization techniques to validate the presence of Co particles or patches. The aforementioned small dark regions were not observed in Co/W₂TiC₂-600 and Co/W₂TiC₂-700 samples, which could potentially be attributed to the further reduced size of Co in these catalysts. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) with energy dispersive X-ray spectroscopy (EDS) was employed to characterize the Co/W₂TiC₂ catalysts (Figs. 2e, f). The EDS mappings show the signals of W, Ti and Co uniformly distributed through the sample without obvious phase segregation. The two-dimensional structure of W₂TiC₂ featuring orderly arranged metal layers was clearly presented from the side view STEM image (Fig. 2e). As the contrast in STEM images was directly related to the atomic number of the elements, the outermost layers with brighter contrast were identified as W, while the inner layers with slightly darker contrast were attributed as Ti. The ordered three metal layers agreed well with the simulated W₂TiC₂ structure.

The structure of Co/W₂TiC₂ and the impact of reduction temperature on the metal-support interaction were further investigated by X-ray photoelectron spectroscopy (XPS) and ion scattering spectroscopy (ISS). The ex-situ XPS survey and high-resolution spectra were shown in Supplementary Figs. 4 and 3a-c. The survey spectra confirmed the presence of W, Ti, C, O, and Co. The W 4f spectrum displayed characteristic peaks at around 32.3 eV, corresponding to W in MXene environment (W-C). The slight shift to higher binding energy compared with pure tungsten is attributed to the redistribution of electron density between tungsten and carbon, resulting in the electron density of tungsten to decrease. Conversely to this trend, the binding energy of W-C decreases from 32.3 eV of W₂TiC₂ to 32.2 eV, 32.1 eV, and 32.1 eV for Co/W₂TiC₂-500, 600, and 700 samples, respectively, indicating more electrons drawn from Co towards W, which suggested an enhanced interaction between Co and surface W as the reduction temperature increased. The Co 2p spectrum showed characteristic peaks of metallic Co, Co-O_x and a pair of shake-up satellite of Co-O_x 2p_3/2 and 2p_1/2, indicating the presence of metallic Co and slightly oxidized surface. The Co peaks shifted slightly to higher binding energy from 777.5 eV to 777.8 eV and 778.0 eV (Co/W₂TiC₂-500, 600, and 700, respectively), which is consistent with the trend observed in the W spectrum, agreeing with the more electrons drawn from Co with increased reduction temperature and stronger metal-support interaction. Same positive binding energy shift of Ti with the increasing temperature further validated the regulation of reduction condition on the electronic structure and interaction.

Ion scattering spectroscopy (Fig. 3d) revealed the continuous scattered ions intensity profile change on the Co/W₂TiC₂-700 catalyst with the helium (He⁺) exposure time. At t = 0s, only Co and W peaks were observed with a barely noticeable Ti peak, indicateing the position of Co and W on the top surface of Co/W₂TiC₂. With the increasing exposure time, the He⁺ beam gradually etched the surface and the Ti peak became stronger, while the peak of Co gradually vanished. As ISS provided elemental information from the top monolayer of the surface, this depth profile further confirmed the presence of Co on the surface as well as the Ti sandwiched between W atom layers in the W₂TiC₂ MXene.

Co structure transformation between single sites and nanoparticles on W ₂ TiC ₂. XANES and EXAFS studies were conducted to gain more accurate information about the structures and coordination environments of Co under different reduction temperatures. The Co K-edge XANES energy was the electromagnetic transition from 1s electron to the 4p empty orbital. The XANES energy is determined fromthe first inflection point of the leading edge. For 3d transition metals, a pre-edge peak is often observed, which is due to the 1s to 3d forbidden transition. The oxidation state could be determined by pre-edge peak and edge energy ⁵⁴. Co foil (Co⁰) has no pre-edge peak and XANES energy is 7.7090 keV, while CoO reference (Co²⁺) has a pre-edge peak at 7.7084 keV and XANES energy at 7.7166 keV (Supplementary Fig. 5a). The edge energy of all our Co/W₂TiC₂ samples were very similar and nearly identical to that of Co⁰, indicating the metallic nature of Co (Supplementary Table 2). The similarity in the shapes of the XANES spectra indicated that the local structures on the catalysts are also similar (Fig. 3e).

Fitting the EXAFS spectra gave information about the coordination numbers (CNs) and bond distance, as summarized in Supplementary Table 2. The Co K-edge EXAFS spectra for Co foil and CoO reference were shown in Supplementary Fig. 5b and S5c. EXAFS results suggested the Co was highly metallic with small amounts of oxidized Co. No alloy formation with W was found by fitting the Fourier transform of the first shell (Fig. 3f). The NPs sizes were estimated based on their CN ⁵⁵ and the results suggested the major phase was extremely small Co nanoparticles (NPs).

The further fittings of all Co/W₂TiC₂ samples showed Co-O bond distance of 1.97 Å. Unlike the typical Co-O bond distance of 2.13 Å in small CoO clusters reference, the much shorter Co-O bond distance observed in our Co/W₂TiC₂ samples was very similar to previously reported single site Co structure which had Co^+ 2 with 4 Co-N(C) at 1.95 Å ⁵⁶. Fitting of the Co/W₂TiC₂-500 showed a Co-O CN of 2.1, suggesting about 50% of the Co existing as single site Co^+ 2 on the MXene surface and 50% existing as metallic Co NPs. With the fraction of oxidized Co equal to fit CN/CN of the pure species, metallic Co with Co-Co fitted CN of 2.8 should have a true CN of 5.6 and metallic NPs with this CN should have a size of about 1.5 nm. As the reduction temperature increased, the proportion of oxidized Co decreased, resulting in nearly all metallic Co for Co/W₂TiC₂-700. For Co/W₂TiC₂-600 sample, the Co-O CN was about 1, indicating 25% single site Co^+ 2 and the true Co-Co CN for the rest 75% metallic Co being 4.4 with NP sized at about 1.0 nm. Co/W₂TiC₂-700 had a Co-Co CN of 3.8, corresponding to the NPs about 0.9 nm. The summary of percentage of single sites, nanoparticles and nanoparticle sizes were listed in Supplementary Table 3. The higher shell Co-(O)-Co peak of Co-oxide nanoparticles was not detectable due to overlap with the Co-Co scattering path. Subtraction of the Co-Co scattering indicated a very small amount Co-(O)-Co consistent few CoO clusters. Combined with the bond distance, the results suggested the Co on W₂TiC₂ was highly metallic with certain portion existing as single site Co^+ 2 with little surface oxidation. Unlike the rapid oxidation of normal small metallic nanoparticles obtained by other synthesis methods, the tiny Co NPs on W₂TiC₂ were highly resistive to oxidation even when exposed to air. This further confirmed the strong metal-support interaction between Co and W₂TiC₂, i.e., the electrons being drawn from Co to W₂TiC₂ making Co electronically “more positive” and less vulnerable to oxidation. As the reduction temperature increased from 500 ℃ to 700 ℃, the Co structure progressively homogenized, changing from a mixture of single sites and metallic NPs (~ 1.5 nm at 500 ℃) to smaller metallic NPs (~ 1 nm at 600 ℃), and ultimately forming well-dispersed sub-1nm Co NPs (~ 0.9 nm at 700 ℃).

Electrochemical HER evaluation of Co/W ₂ TiC ₂ catalysts. To explore the catalytic activity of the Co/W₂TiC₂ catalysts, the HER performance was evaluated in an H-cell using the three-electrode system. The HER performance of the same Co/W₂TiC₂ catalyst (i.e., Co/W₂TiC₂-700 sample) in acidic, neutral, and alkaline media were first compared. As shown in Supplementary Fig. 6, the current density in 1 M KOH and 0.5 M H₂SO₄ significantly exceeds that in 1 M PBS, suggesting the catalyst was more suitable for catalyzing hydrogen evolution in alkaline and acidic environments. Under acidic condition, the HER performance gave the smallest overpotential of 133.92 mV at 10 mA/cm² with Tafel slope of 72.72 mV/dec. However, a significant degradation in the HER performance was observed after the electrode was immersed in the H₂SO₄ solution for 30 min without applying current, as shown in Supplementary Fig. 7 and S8. TEM images of the catalyst post-reaction in 0.5 M H₂SO₄ revealed the degradation of MXene structure and the aggregation of Co into larger particles, ranging from 5–10 nm in size. This was because Co trimer/W₂TiC₂ and Co tetramer/W₂TiC₂ were more stable than isolated Co/W₂TiC₂ in the presence of H*, as seen in our DFT-calculated formation energy of Co clusters at different H* coverages in Supplementary Fig. 17a. In contrast, the Co/W₂TiC₂-700 sample exhibited no structural damage or performance degradation after the reaction under alkaline conditions, as displayed in Supplementary Fig. 9a. Given the current industrial demand for alkaline water electrolysis catalysts and the observed stability of our Co/W₂TiC₂-700 sample for alkaline HER, a detailed analysis was conducted to determine the electrocatalytic performance of all the Co/W₂TiC₂ catalysts under alkaline conditions.

To evaluate the contribution of Co metal and W₂TiC₂ MXene to the catalytic performances, control experiments were conducted using pure W₂TiC₂ MXene and Co/WC (commercial tungsten carbide) reduced at 700 ℃. The metal loading on the catalysts was confirmed by inductively coupled plasma optical emission spectrometry (ICP-OES), the Co contents in Co/W₂TiC₂-500, 600, and 700 were determined to be 4.1%, 4.0%, and 4.3%, respectively, with an approximate average atomic ratio of 4.13%, confirming the total Co amounts in the catalysts are close with no obvious variation across all the three reduction temperatures (Supplementary Table 4). To provide a fair comparison, uniform Co and overall catalyst loading mass were applied on each electrode, as detailed in Supplementary Table 5. All the Co-containing catalysts (Figs. 4a and 4b) exhibited higher HER activities compared to the pristine W₂TiC₂, underscoring the critical role of Co in the HER activity. Furthermore, all three Co/W₂TiC₂ catalysts showed superior activity of below 100 mV overpotential compared to Co/WC, which exhibited a 211.93 mV overpotential at current density of 10 mA/cm² (η @10 mA/cm²). This performance difference could be attributed to the two-dimensional nature of W₂TiC₂, providing more active sites than the bulk WC. Noticeably, Co/W₂TiC₂-700 stands out as the best catalyst, with the smallest overpotential of 62.58 mV at 10 mA/cm², followed by Co/W₂TiC₂-600 (88.78 mV at 10 mA/cm²) and Co/W₂TiC₂-500 (99.34 mV at 10 mA/cm²). The relatively subdued HER performance of Co/W₂TiC₂-500 among the three catalysts can be attributed to the lower fraction and slightly larger metallic NP size among all the three Co/W₂TiC₂ samples, as explained in the following theoretical modeling section. Notably, the advantage of small overpotential became even more pronounced at higher current densities. Co/W₂TiC₂-700 showed overpotential of 191.81 mV @ 100 mA/cm², outperforming commercial 40% Pt/C which had an overpotential 238.02 mV @ 100 mA/cm² by 46.21 mV. At higher current density, Co/W₂TiC₂-700 continued to exhibit excellent low overpotentials as 330.43 mV @ 500 mA/cm² and 407.72 mV @ 1000 mA/cm², highlighting its great potential in industrial-level high current density alkaline hydrogen production.

The Tafel slopes (Fig. 4d) of Co/W₂TiC₂-700, Co/W₂TiC₂-600, Co/W₂TiC₂-500, Co/WC and pristine W₂TiC₂ were 44.34, 50.78, 71.00, 166.30, and 268.04 mV/dec, respectively. The smallest slope of Co/W₂TiC₂-700 showed rapid change of current density with slight increases in applied voltage, indicates fast reaction kinetics. The small Tafel slope of Co/W₂TiC₂-700 also outperformed most of reported non-noble metal catalysts within the realm of alkaline HER, as compared in Supplementary Table 9. The electrochemical impedance spectra (EIS) were collected to study the charge transfer efficiency (Fig. 4e). Nyquist plots showed close solution resistance (R_s) values for all the tests and distinguished charge transfer resistance (R_ct) of the catalysts, suggesting similar electrolyte solution environment. Co/W₂TiC₂-700 and Co/W₂TiC₂-600 exhibited small semi-circles of the curve, indicating faster charge transfer property. The reduced charge transfer resistance of the Co/W₂TiC₂ catalysts in comparison to Co/WC and pristine W₂TiC₂ demonstrated the optimized electron transfer facilitated by the synergistic metal-support interaction between Co and W₂TiC₂, which agreed with the diminished surface oxidation at higher annealing temperature as observed from XPS and EXAFS analyses.

To further understand the performance of Co/W₂TiC₂ catalysts, the electrochemical active area (ECSA) was investigated by measuring the double layer capacitance (C_dl). Cyclic voltammograms (CV) were collected at 10 to 100 mV/s scan rates in a non-faradaic region (Supplementary Figs. 12 and 13), and the C_dl values were calculated from the slope of charging current density differences plotted against the scan rates plots. ECSA was determined by dividing the C_dl by the specific capacitance (C_s, assumed as 40 µF/cm²). Supplementary Fig. 12 showed the C_dl of Co/WC and W₂TiC₂ were 10.43 and 20.56 mF/cm². The higher C_dl of W₂TiC₂ was attributable to its inherently larger specific surface area, the characteristic nature of two-dimensional structures. Supplementary Fig. 13 showed C_dl of Co/W₂TiC₂-500, 600 and 700 are 11.76, 38.61 and 55.47 mF/cm², respectively. The decrease in C_dl values on Co/W₂TiC₂ compared with pure W₂TiC₂ was due to the interaction between the MXene flakes and metal precursors (Co here), as also observed in previous study ⁴². The high C_dl value of Co/W₂TiC₂-700 indicated that it had the largest electrochemical active area among all the three catalysts, which could be due to smaller Co nanoparticle size and relatively larger numbers of Co nanoparticle for a constant Co loading in our Co/W₂TiC₂-700 sample (as evident from Supplementary Table 3). This results also hinted the active sites of Co/W₂TiC₂ were either Co NPs or the interfaces between Co NPs and W₂TiC₂ supports, which will be discussed in the next section.

Effect of Co coordination environment on the HER activity and H* binding. Previous literature suggested that the adsorption free energy of H* (${G}_{H*}$) was a good descriptor of the HER activity on transition metal electrodes, finding that Pt(111) lies closest to the observed activity peak ^{57, 58}. We therefore calculated relative adsorption free energies (${\Delta }{G}_{rel}$) for H* on different Co-modified W₂TiC₂ materials, using the corresponding value on Pt(111) as a reference: ${\varDelta G}_{rel}={G}_{H*@\text{C}\text{o}/{W}_{2}Ti{C}_{2}}-{G}_{H*@\text{P}\text{t}\left(111\right)}={G}_{H*}+0.21 \text{e}\text{V}$. Smaller magnitudes of ${\varDelta G}_{rel}$ indicate H* binding closer to the previously-observed HER activity peak on pure Pt. The ${\varDelta G}_{rel}$ values calculated based on the most stable binding site of H* on the respective surfaces suggested that the theoretically expected trend in the HER activity was: isolated Co/W₂TiC₂ (-0.13 eV) > W₂TiC₂ (-0.32 eV) > Co NP/W₂TiC₂ (-0.40 eV). This was contrary to our experimental data (Supplementary Table 7) showing a lower overpotential on Co NP/W₂TiC₂ than on clean W₂TiC₂. Experimentally, the overpotential for W₂TiC₂ shifted by -0.24 V relative to Pt, which is in good agreement with the DFT-calculated ${\varDelta G}_{rel}$ on the clean W₂TiC₂ surface (-0.32 eV). We therefore ascribe the deviation in trend to the strong binding found on the Co NP model: the most stable H-binding site on Co NPs is poisoned by strong binding of H, thereby rendering such sites inactive. We considered ${G}_{H*}$ at a number of possible binding sites in our theoretical model for Co NP/W₂TiC₂ (Fig. 5a). These suggested that the ${{\Delta }\text{G}}_{rel}$ value (0.02 eV) for H* binding at the Co-W interface was the closest to 0 and therefore is the likely active site. Further, this value is in good agreement with our experimental data showing a shift of + 0.05 V in overpotential relative to Pt. Accounting for these interfacial sites as most active, the theoretically expected HER activity (based on ${\varDelta G}_{rel}$, eV) should be: Co NP/W₂TiC₂ (0.02) > isolated Co/W₂TiC₂ (-0.13) > W₂TiC₂ (-0.32), agreeing with our experimental predictions.

We find that the d-band center of the interfacial W atom acts as a strong descriptor of ${G}_{H*}$ across a series of models, as shown in Fig. 5b, suggesting that Co plays an important role in tuning the electronic structure of W toward favorable interactions with H*. Further discussion on the effect of Co-Co coordination environment and H* surface coverage on the H* binding strength is provided in Supplementary Note 2.

Co/W ₂ TiC ₂ alkaline HER catalyst under industrially relevant conditions. Stability of the catalyst under reaction environment is one of the crucial factors in evaluating the performance of a HER catalyst for industrial-scale applications. To this end, the stability of Co/W₂TiC₂-700 was assessed by stepwise and continuous chronopotentiometry tests, as shown in Fig. 4c and 4f. In a 12-hour test, when the current densities were sequentially increased from − 10 to -50 mA/cm² in the intervals of 4h, the potential remained stable at each setting, and quickly returned to the original values when the current density was reduced from − 50 to -10 mA/cm². Continuous operation at -10 mA/cm² showed no decrease in potential even after 500 h continuous operation. Moreover, chronopotentiometry test was further conducted under high current density of 1000 mA/cm², no obvious decay was found within 100 h. In contrast to the rapid degradation of MXene under acidic condition, SEM image of Co/W₂TiC₂-700 after the stability test in 1M KOH (Supplementary Fig. 9) showed no change in morphology, highlighting the superior stability of Co/W₂TiC₂-700 under alkaline catalytic condition.

From the temperature-induced structure modulation and metal-support interaction as well as meticulous mechanistic investigations, Co/W₂TiC₂-700C demonstrated its great potential and scientific grounding to serve as a high-performance water electrolysis catalyst. Considering the practical industrial environment applications that require high current density and the low overpotential exhibited by our Co/W₂TiC₂-700 at 1000 mA in linear sweep voltammetry, this optimized material was applied in a 2 × 2 cm² flow cell electrolyzer for alkaline water electrolysis tests and hydrogen quantification. Co/W₂TiC₂-700 was employed as the cathode, paired with non-noble metal material Ni foam as the anode on which oxygen evolution happened, and 1M KOH was used as electrolyte for both sides. Using a potentiostat with a ± 2A/±30V booster, a maximum current of 1.6A (400 mA/cm²) was applied. As shown in Fig. 6b, one piece of Co/W₂TiC₂-700 electrode was continuously used in chronopotentiometry tests at current densities of 50, 100, 200, 300, and 400 mA/m² for 3 hours each, exhibiting stable performance throughout the 15 hours of operation. The overall cell voltages of Co/W₂TiC₂-700 working as the cathode from 50 to 400 mA/cm² were all lower than that of Pt/C with comparable loading amount (0.57 mg Pt/cm²) on the identical carbon paper substrate. The lower cell voltage under the same current indicated less voltage was required to drive the electrolysis reaction beyond its thermodynamic potential, resulting from the synergistic effects of intrinsic catalytic activity, conductivity, and surface property. The hydrogen produced was quantified using on-line gas chromatography (GC), as shown in Fig. 6c. The Co/W₂TiC₂-700 catalyst achieved nearly 100% H₂ faradaic efficiency and comparable hydrogen production rates as commercial Pt/C at the conditions of 50 to 400 mA/cm², with the highest hydrogen production rate of 706.75 mL/h attained at 1.6 A. Furthermore, to verify the possibility of further reducing the cell voltage, IrO₂ anode and high operation temperature of 60 ℃ were applied to the same flow cell, and cell voltage was further reduced to 2.85 mV at 400 mA/cm² (Supplementary Fig. 21). Energy efficiency analysis is provided in Supplementary Note 3, the IrO₂||Co/W₂TiC₂-700 demonstrated 46.98%-65.33% iR-free cell energy efficiency, higher than 44.63%-58.31% compared to Pt, substantiated its feasibility for efficient hydrogen production at practical high current applications.

In summary, a new W₂TiC₂ MXene was successfully synthesized by spark plasma sintering followed by the loading of Co through reduction at various temperatures. The configuration and electronic structure of Co on the W₂TiC₂ substrate were modulated by the metal-support interaction during the reduction process, which furthermore fine-tuned its hydrogen evolution performance, yielding high performance for alkaline HER. The re-distribution of single sites and large NPs to uniform small NPs of Co was confirmed by EXAFS. With the optimized Co distribution, Co/W₂TiC₂-700 achieved the best alkaline HER performance of 62.58 mV small overpotential and astonishing stability of 500 h at 10 mA/cm², as well as small Tafel slope of 44.34 mV/dec. Moreover, at higher current densities, the catalyst displayed excellent low overpotential of 191.18 mV and 407.72 mV at 100 mA/cm² and 1000 mA/cm², and stability of 100 h under 1000 mA/cm² in alkaline solution, with close to 100% H₂ faradaic efficiency through 100 to 400 mA/cm², making it suitable for industrial application for long-term large-current hydrogen production. DFT calculations combined with EXFAS studies revealed that the probable active site for HER chemistry on Co/W₂TiC₂ materials was the Co-W interface. The adsorption free energy of H* on these materials was strongly correlated with the d-band center of W, which can be used as a probe to design more HER active W₂TiC₂-based materials in future studies. This work explored the construction of high-performance MXene-support metal catalyst through the metal-support interaction, providing an avenue to further extending the applications of MXene-support metal catalysts to wide range industrially important catalytic reactions.

Synthesis of W ₂ TiAlC ₂ and W₂TiC₂ MXene. The W₂TiAlC₂ was synthesized by spark plasma sintering (SPS). Commercial powders of tungsten carbide (WC, 45 µm, GoodFellow Alritch, 99.5%), titanium (Ti, 325 mesh, Alfa Aesar, 99%) and aluminum (Al, 17–30 µm, Alfa Aesar, 99%) were mixed with a molar ratio of WC/Ti/Al = 2:1:1.5 in a graphite die coated with boron nitride (BN). The sample in graphite die was then loaded in a Fuji-211lx spark plasma sintering (SPS) and sintered at 1200 ℃ under 30 MPa for 4 h. The sintered bulk W₂TiAlC₂ was pulverized in a synthetic sapphire mortar and screened through a 325-mesh sieve to fine W₂TiAlC₂ powder.

W₂TiC₂ MXene was prepared by etching W₂TiAlC₂ powder with hydrofluoric acid. 1.0 g of the obtained W₂TiC₂ MXene was added into 10 mL of hydrofluoric acid (HF, Sigma Aldrich, 48% in water) and stirred for 72 h at 55°C in a high-density polyethylene centrifugal tube. The sample was then centrifuged and washed by deionized water (DI) until pH reached 5–6. The obtained powder was delaminated with 10 mL tetramethylammonium hydroxide (TMAOH, Acros Organics, 25% in water) at 80 ℃ for 12 h, followed by 6 h sonication. W₂TiC₂ MXene was collected by centrifugation at 9000 rpm and dried in vacuum. The obtained W₂TiC₂ MXene was stored in the glove box filled with N₂ for future use.

Synthesis of Co/W ₂ TiC ₂ catalysts. Co was loaded on WC and W₂TiC₂ MXene by the incipient-wetness impregnation method. Cobalt (Ⅱ) chloride (Sigma Aldrich, 98%) was dissolved with DI water to form a solution of 0.125 g Co/ml. 100 µL of CoCl₂ solution was mixed with 250 mg of W₂TiC₂ MXene powder by consisting stirring in a polyethylene weighing boat. The mixture was dried in vacuum at room temperature, then transferred into a tube furnace and reduced under 5% H₂/N₂ flow at different temperatures (500 ℃, 600 ℃, 700 ℃) for 2 h with a ramping rate of 10 ℃ min^-1 and natural cooling off. The prepared Co/W₂TiC₂-500, Co/W₂TiC₂-600, and Co/W₂TiC₂-700 catalysts were stored in the glove box filled with N₂ for future use.

Characterizations. The X-ray diffraction (XRD) patterns were conducted on a Siemens D500 X-ray diffractometer with Cu Kα radiation source (λ = 1.5406 Å). The simulated XRD was obtained by Material Studio and VESTA and refined using GSAS software. Field-emission scanning electron microscopy (SEM) images were acquired on FEI Quanta 250. Transmission electron microscopy (TEM) imaging was acquired on a 200kV JEOL 2100 TEM. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and energy-dispersive X-ray spectroscopy (EDS) mapping were performed using a Thermo Fisher Scientific Talos F200X S/TEM operated at 200 kV and equipped with a super-X EDS system. X-ray photoelectron spectroscopy (XPS) data was collected on a Kratos Amicus XPS instrument. Ion scattering spectroscopy (ISS) was collected on a Thermo Fisher Scientific Nexsa G2. The inductively coupled plasma-optical emission spectrometry (ICP-OES) tests were conducted on a PerkinElmer Optima 8000 ICP-OES Spectrometer. Co K-edge X-ray absorption spectroscopy (XAS) was measured at 8-ID at the National Synchrotron Light Source II (NSLS-II) at Brookhaven National Laboratory (BNL). Samples were ground into a fine powder and added between quartz wool in Kapton sample tubes with 10 ccm He continuous flow. XAS data were fitted using the WinXAS 3.1 software package. Least-squares fit in R-space of the k²-weighted Fourier transform data from 2.6 to 11.0 Å⁻¹ was used to obtain the EXAFS coordination parameters. The first shell was used to fit the EXAFS spectra. The amplitude reduction factor (S₀²) was determined as 0.70 by fitting a reference spectrum of the Co foil and was used for fitting the other EXAFS spectra.

Electrochemical Measurements. The working electrodes were prepared by spray coating. 200 mg of catalysts were dispersed in 20 mL of isopropanol by sonication for 5 min, 20 wt% Nafion solution (LQ-1105 1100EW, 5% wt., Ion Power) was then added into the catalyst ink and sonicated for another 5 min to obtain a homogeneous ink. The prepared catalyst inks were sprayed on a 2 × 2 cm² area on a hydrophilic carbon paper (Freudenberg H23, Fuelcell Store) with 0.1 mg/cm² Vulcan carbon nanoparticle layer to increase the contact between catalysts and the carbon paper, the Vulcan layer was verified as inert by the polarization curve test (Supplementary Fig. 10). The electrodes were heating at 80 ℃ during the spray to vaporize the isopropanol and were left on the heat plated at 80 ℃ for 10 min after the spray for complete drying. For comparative experiments, commercial 40% Pt/C (40% Platinum on Vulcan XC-72R, 2–3 nm, Fuelcell Store) was sprayed using the same method. The electrochemical tests are conducted in 1 M potassium hydroxide (KOH), 0.5 M sulfuric acid (H₂SO₄), and 1 M phosphate buffered saline (PBS) for evaluating HER performance in alkaline, acidic, and neutral environments, all the electrolytes are purged by N2 for 30 min.

The H-cell tests were carried out on an electrochemical workstation (PMC 200, AMETEK scientific instruments) using a three-electrode H-cell system, with Ag/AgCl as the reference electrode in acidic and neutral media and Hg/HgO as the reference electrode in alkaline media. A 0.5 × 0.5 cm² Pt foil was used as the counter electrode. Nafion membrane (Nafion-115, Fuelcell Store) pre-treated according to the type of ions transferred (K⁺ or H⁺) was used between two counterparts of the H-cell to prevent Pt contamination from the anode side. The flow cell tests were performed with a 2 × 2 cm² two-electrode flow cell on a Biologic SP-300 potentiostat with a ± 2 A/±30 V booster. A 2 × 2 cm² Ni foam (1.6 mm thick, MTI corporation) was used as the counter electrode. Pre-treated Nafion membrane was used between two counterparts of the flow cell. The H₂ quantification was carried out by an SRI GC (Multiple gas analyzer #5) with TCD and FID detector. All the test potentials were converted to reversible hydrogen electrodes (RHE) and were automatically 85% IR corrected by the potentiostats.

Computational Methods. All DFT calculations were performed using non-collinear spin-polarized plane-wave DFT ⁵⁹ as implemented in the Vienna Ab initio Simulation Package (VASP) ^{60, 61}. The electron-ion interactions were described by projector augmented wave (PAW) potentials ^{62, 63}, and the generalized gradient approximation (GGA-PW91) was used to describe the exchange-correlation functionals ⁶⁴. The electron wavefunction was expanded using plane waves with an energy cutoff of 500 eV, with electronic convergence to 10^− 4 eV. The W₂TiC₂ surface was modeled with a single MXene “sandwich” structure in a (3 × 3) periodic slab, as shown in the inset image of Fig. 5b. Approximately 18 Å of vacuum separation was provided between the periodic images along the “z” dimension. All atoms were fully relaxed. A 6 × 6 × 1 Monkhorst-Pack k-point grid was used for these slab calculations ⁶⁵. The W₂TiC₂ surface lattice parameters were optimized, resulting in a W-W in-plane distance of 2.98 Å used to construct all models.⁶⁵. The ionic forces on each atom were converged to 0.01 eV/Å during geometric optimizations. The DFT + U approach by Dudarev was used to correct the on-site Coulomb interactions between d orbitals of Co, W, and Ti ⁶⁶, with parameters for each of these atoms set to U – J = 4 eV ^{67, 68}.

To match the concentration of Co in the experimental samples of Co/W₂TiC₂, four Co atoms were added to a (3 × 3) W₂TiC₂ surface in four different configurations to replicate different Co-Co coordination environments ranging from (i) isolated Co, (ii) Co dimer, (iii) Co trimer, and (iv) Co tetramer. The relative position(s) of Co atom(s) on the other side of the MXene model were varied and geometry optimizations were performed for all possible configurations to find the most stable Co arrangements for each of the four systems. The most stable geometries for each of these four different Co arrangements are shown in Supplementary Fig. 16. The formation energy of a Co cluster (${E}_{Co}$) on the W₂TiC₂ surface at different H* coverage was referenced to the energy of Co bulk, and the energy of the most stable configuration of H* adsorbed on W₂TiC₂:

$${E}_{Co}={E}_{4Co\text{*}+nH\text{*}+{W}_{2}Ti{C}_{2}}-{E}_{nH\text{*}+{W}_{2}Ti{C}_{2}}-{E}_{Co,bulk}$$

Here, ${E}_{4Co\text{*}+nH\text{*}+{W}_{2}Ti{C}_{2}}$is the total energy of W₂TiC₂ with four Co atoms and “n” atoms of H*, ${E}_{nH*+{W}_{2}Ti{C}_{2}}$ is the total energy of the most stable configuration of “n” H* on W₂TiC₂, and ${E}_{Co,bulk}$ is the total energy of a bulk Co atom.

To find the most stable binding geometries of H*, geometry optimizations were performed for an isolated H* (i.e., 0.11 ML) at all unique surface sites. Assuming the same most stable binding site as an isolated H*, geometry optimizations were then performed for different possible configurations of H* at surface coverages varying from 0.11 to 0.44 ML, to find the most stable configuration of H* at a given surface coverage. The cumulative adsorption free energy of “n” species of H* (${G}_{nH*}$) was referenced to the gas phase free energies of H₂ and the energy of clean slabs:

$${G}_{nH\text{*}}=\left({E}_{nH\text{*}}+{ZPE}_{nH\text{*}}-T{S}_{nH\text{*}}\right)-{E}_{surf}-\frac{n}{2}({E}_{{H}_{2},g}+{ZPE}_{{H}_{2},g}-T{S}_{{H}_{2},g})$$

Here, ${E}_{i}$ are total energies, ${ZPE}_{i}$ are calculated zero-point energies, and ${S}_{i}$ are calculated entropies. Systems denoted by nH* contain “n” atoms of H* adsorbed on the surface; the subscript “g” denotes respective gas-phase quantities. ${E}_{surf}$ is the total energy of the clean slab. ${G}_{H*}$, as used in this manuscript, refers to the n = 1 case. The harmonic oscillator approximation was used to calculate $ZPE$ and $S$. The temperature (T) was set to 298 K, in accordance with experimental conditions for electrocatalytic performance evaluation. We note that the value of the gas-phase reference energy of H₂ does not impact trends related to ${G}_{H*}$ on a given material referenced to Pt(111) in Fig. 5.

A (5 × 5) unit cell of W₂TiC₂ with periodic boundaries and ~ 18 Å of vacuum separation between the W₂TiC₂ slabs was used for the Co nanoparticle (NP) calculations. A 4-layered Co NP with 22 Co atoms was constructed on a (5 × 5) unit cell of W₂TiC₂, as shown in Supplementary Fig. 19, to resemble Co NPs in our experimental Co/W₂TiC₂-700 sample (Supplementary Table 3). A 2 × 2 × 1 Monkhorst-Pack k-point grid was used for all the Co NP/W₂TiC₂ calculations ⁶⁵. The remaining parameters for the DFT setup were the same as used for the slab calculations.

Data availability

All data are available from the authors upon reasonable request.

Acknowledgements

Y. W. acknowledged the support from the Herbert L. Stiles Professorship from Iowa State University. This research used resources at the 8-ID beamline of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704. R.C. and W.H. were supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division, Separation Science program for the adsorption studies. Ames National Laboratory is operated for the U.S. Department of Energy by Iowa State University under Contract No. DEAC02-07CH11358. W. L. acknowledged funding supports of USDA-NIFA (2021-67021-34650) and NSF (CBET-1947435). The computations reported in this paper are partially supported by HPC@ISU equipment at Iowa State University, some of which has been purchased through funding provided by NSF under MRI grant numbers 1726447 and 2018594.

Author contributions

Y. W., W. L., L. T. R, and J. T. M. supervised the project. X. L., F. Y., and D. M. P. planned and developed the project. F. Y. and X. L. developed and prepared the catalysts and performed the catalytic experiments. D. M. P. performed all the DFT calculations. M. A. assisted with testing the flow cell device. W. H. oversaw and R. C. helped with the ICP tests of the catalysts. S. J. and C. J. B. conducted the XAS characterization and analyzed the results. T. M. characterized the catalysts by TEM. D. J. helped characterize and analyze the catalysts by XPS. X. L., D. M. P., and F. Y. wrote the paper with contributions from S. J., C. J. B., and C. W. S.

Competing interests

The authors declare no competing interests.

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Cobalt on Tungsten Titanium Carbide MXene: A Electrocatalyst for Industrial-Scale High Current Density Alkaline Hydrogen Evolution Reaction

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