Mechanochemical one-pot synthesis of heterostructured pentlandite-carbon composites for the hydrogen evolution reaction

We have utilized carbon sources as milling additives to enable a direct mechanochemical one-pot synthesis of Fe3Co3Ni3S8/carbon (Pn/C) materials using elemental reaction mixtures. The obtained Pn/C materials are thoroughly characterized and their carbon content could be adjusted up to 50 wt%. In addition to carbon black (CB) as an additive, Pn/C materials were produced using graphite, reduced graphene oxide (rGO), and carbon nanotubes (CNTs), which allows the overall physicochemical properties of materials for energy storage applications to be adjusted. By employing the Pn/C materials as electrocatalysts for the HER in a zero-gap proton exchange membrane (PEM) electrolyzer, we were able to reach a current density of 1 A cm−2 at a cell potential as low as 2.12 V using Pn, which was synthesized with 25 wt% CB. Furthermore, electrolysis at an applied current density of 1 A cm−2 for 100 h displays a stable performance, thus providing a sustainable synthesis procedure for potential future energy storage applications. Herein, we show that catalyst supports play an important role in the overall performance.

EASY-GTM gas pressure and temperature measurements during the synthesis of Pn/CB materials and time-resolved PXRD-Analysis of Pn 50%CB

Calculation of maximal gas evolution from carbon black
The assumptions made are: 1. Monolayer adsorption of only N2 at the carbon BET surface a. Monolayer thickness: hmonolayer 0.354 nm. 1 b.Area per molecule: AN2 0.162 nm 2 . 2 2. Ideal gas behavior 3.No volume occupied in the beaker by carbon 4. 23 g of ZrO2 balls in 20 mL beaker with constant volume totaling in 16 cm 3 free volume.5. Two separate state changes: a. Isochoric release of adsorbed species and pore volume b.Isochoric temperature increasec Nitrogen physisorption data for BET surface area and pore volume of the used carbon black are published by Grützmacher et al. 3 .Start (T1) and end temperatures (T2) were taken as measured (Fig. S2) for 1 g of carbon black material.
This calculation provides an estimate pressure increase of 4.95 bar for total pore collapse under mechanochemical conditions.The measured pressure increase of 2.5 bar (Fig. S2) would thus correspond to a collapse of half of the pore structure with desorption of gas at 50 % of available surface area (1.3 mmol).
X-ray fluorescence (XRF) analysis of Pn XCB materials  X-ray photoelectron spectroscopy (XPS) spectra of the Pn XCB materials   Transmission electron microscopy (TEM) images coupled with energy dispersive X-ray emission spectroscopy (EDX) mappings of Pn

Process Flexibility
One opportunity to tune the properties of the heterostructured Pn/C material lies in the possibility to synthesize materials with distinct Pn stoichiometries including mono-, bimetallic and sulfoselenide pentlandite materials.The second opportunity to tune the properties of heterostructured Pn/C material involves changing the carbon material.We have substituted carbon black in the trimetallic reaction mixture by graphite, carbon nanotubes (CNT) or reduced graphene oxide (rGO) for the formation of Pn/C materials with different carbon materials.Here, keeping the milling parameters identical to the one for the CB containing reaction mixture, the synthesis of Pn/graphite, Pn/CNT and Pn/rGO resulted in unfinished products.Instead, samples required an increased milling time to achieve a full Pn-conversion, which can be attributed to the tribilogical properties of the aforementioned carbon materials.Thus, the herein reported synthesis method enables the synthesis of pentlandite materials with a broad stoichiometry and choice of carbon materials.It is expected that other bi-, trimetallic and sulfoselenide pentlandites including other carbon sources can be prepared as well.

Environmental Analysis
To analyze the environmental impact of the presented process two different parameters were employed.While the process mass intensity (PMI, (1)) allows for comparison of the effective utilization of reactants the global warming potential (GWP, ( 2)) provides a metric for energetic contributions.
With mprocess being the total mass added during a process or process step, mProduct the mass of product yielded by the process, Econsumed the consumed energy in kWh and CIPK the carbon emission intensity per kilowatt-hour in gCO2equivalents/kWh. 1,2 The CIPK can be found for most countries and depends on how the energy is generated.For the present calculations a CIPK of 471 gCO2-equivalents/kWh was assumed. 3For both PMI and GWP a low number is the desired outcome.A process with PMI below 15 is generally considered as green, but should rather be considered in comparison to alternative processes. 4The same principle applies for the GWP, where acceptable amounts strongly vary, depending on the perceived value of product.
We identified six common routes to synthesize a range of Pn-and heterostructured Pn-materials that we compared with the developed process.6][7][8][9] Both, the ball mill route as well as the ampoule route have a very low process mass intensity of one, because all reactants are completely converted into product.Mixed metal coprecipitation depend highly on the starting salts.Nitrates result in the highest PMI of this group while sulfates have the lowest.Dicarbamates as precursor result in comparable PMI as ammonium sulfates at about 90.Of all compared processes, the most complex and thus environmentally malign is the direct carbonization technique.Comparing the global warming potential of the reactions caused by energy consumption ball milling is lowest as well (Fig. S14 & Table S2).The low powered electric motor of a ball mill uses only 200 Wh while the next lowest consumption is during the carbamate process that consists of a short length heating step.All other processes depend on high power furnaces with significant higher global warming potential as a result.Again, the direct carbonization has the highest GWP with 8850 gCO2/gProduct.However, this process results in nanostructured carbon wires with homogeneous Pn-distribution across the surface.So overall, the proposed ball milling process has the lowest environmental impact if GWP and PMI are compared while being able to produce pure pentlandite as well as heterostructured samples.

Cost analysis
Table S3 Resource cost analysis of pentlandite compared to platinum (all data obtained from stock prices on 13 th October 2022).

Resource
Price per ton (€) Amount needed for one ton catalyst (t) Total price (€) Cross sectional SEM image of a catalyst coated membrane using Pn 25%CB as cathodic catalyst Cross sectional SEM image of a catalyst coated membrane using Pn 25%CNT as cathodic catalyst Cross sectional SEM image of a catalyst coated membrane using Pn 25%rGO as cathodic catalyst    Water vapor sorption analysis of Pn 25%CB  Water vapor sorption analysis of Pn 25%graphite Water vapor sorption analysis of Pn 25%CNT  Water vapor sorption analysis of Pn 25%rGO Electrochemical MEA data

Fig. S3 :
Fig. S3: Time-resolved Powder X-ray analysis of the synthesized Pn50%CB displaying the structural changes from NiFeS2 to the pentlandite phase.

Fig. S4 :
Fig. S4: Thermogravimetric measurements of the synthesized PnXCB materials for the determination of the carbon amount in the samples.

Fig. S5 :
Fig. S5: X-ray photoelectron spectroscopy (XPS) survey spectra of the PnXCB materials including carbon black as reference.

Fig. S8 :
Fig. S8: High resolution TEM image of Pn (top left).Dark field scanning tunnelling electron microscopy (DF-STEM) image of Pn (top middle) including energy dispersive X-ray spectroscopy (EDX) element maps displaying a homogeneous distribution of the elements.

Fig. S13 :
Fig. S13: Powder X-ray analysis of the synthesized Pn25%C materials synthesized with different carbon sources.

Fig. S14 :
Fig. S14: Comparison of different Pn synthesis pathways in process mass intensity (PMI) and global warming potential (GWP).Ball milling describes the present method.High temperature abbreviates the most common ampoule syntheses.Coprecipitation was calculated from a process described by Danot et al.Decomposition of dithiocarbamates was developed by Hogarth et al.Kim et al. developed the Pn-decorated carbon sheets.Carbonization represents the decomposition route of Prussian blue analogues.

Fig. S15 : 2 Fig. S16 :
Fig. S15: Cross sectional SEM image of a catalyst coated membrane using Pn25%CB as cathodic catalyst and IrO2 as anodic catalyst.The employed membrane is a Nafion 212.50 µm

Fig. S17 :
Fig. S17: Cross sectional SEM image of a catalyst coated membrane using Pn25%CNT as cathodic catalyst and IrO2 as anodic catalyst.The employed membrane is a Nafion 212.The scale is 20 µm.

Fig. S18 :
Fig. S18: Cross sectional SEM image of a catalyst coated membrane using Pn25%rGO as cathodic catalyst.The employed membrane is a Nafion 212.The scale is 20 µm.

Fig. S26 :
Fig. S26: CV curves of two independently prepared Pn25%CBMEAs in an in-house built PEM electrolyzer recorded with a scan rate of 0.5 mV s -1 at 80 °C.

Fig. S27 :
Fig. S27: Polarization curve of an independently prepared Pn25%CB MEA in an in-house built PEM electrolyzer recorded at 80 °C.

Fig. S28 :Fig. S29 :
Fig. S28: Representative GC-MS-trace of product gas stream obtained with MEAs featuring the Pn-type cathodes.The Signals correspond to H2 and O2, respectively.

Fig. S30 :
Fig. S30: Chronopotentiometry of a benchmark Pt/C|IrO2-MEA at 80 °C for 100 h at an applied current density of 1 A cm -2 .

Table S1 :
X-ray fluorescence (XRF) analysis of PnXCB materials for the determination of the elemental composition.

Table S2
Environmental analysis of different synthesis routes towards pentlandite materials and heterostructured pentlandites.
Table of catalyst layer thicknesses of the prepared cathodes determined via SEM

Table S4 : Calculated thicknesses of the cathodic catalysts layers from SEM.
X-ray photoelectron spectroscopy (XPS) spectra of the Pn 25%CB CCM catalyst layer.