Home Chemistry Ionomer-free and recyclable porous-transport electrode for high-performing proton-exchange-membrane water electrolysis

Ionomer-free and recyclable porous-transport electrode for high-performing proton-exchange-membrane water electrolysis

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Ionomer-free and recyclable porous-transport electrode for high-performing proton-exchange-membrane water electrolysis

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Fabrication and construction of ionomer-free PTEs

Fabrication of conventional catalyst-coated membranes (CCMs) or PTEs requires no less than 5 steps: mining, refining, synthesizing, fabricating, and coating (Fig. 1). First, Ir have to be mined from the Earth’s crust, which is on the market solely in a couple of geographical areas together with South Africa, Russia, United States, Canada, and Zimbabwe16. As soon as the Ir ore has been gathered, it should bear sophisticated procedures of refining and processing, comparable to removing of crude ores and separation of PGM focus from different metals and rocks. Then, these concentrates are additional separated to accumulate excessive purity Ir steel appropriate for catalytic functions26. At this stage, high-purity Ir metals are additional processed by sequence of transformations to attain the precursor type, which could be synthesized into nanoparticles for use as water-splitting catalysts, after which, electrolyzer producers combine iridium catalyst, ionomer, and solvents to formulate catalyst inks adopted by ink coating to manufacture PTEs or CCMs. Though an ultrasonic-based coating technique is understood to provide a extra uniform and high-quality Ir catalyst layer, even at ultra-low loadings, it’s a low throughput and economically unfavorable technique on account of using very dilute inks27. In distinction, rod coating28 and roll-to-roll29 strategies can manufacture electrodes at excessive throughput, but they’re restricted to utilization of viscous inks with excessive stable contents for fabricating electrodes at excessive loadings.

Fig. 1: Comparability between standard methods of CCM/PTE fabrication course of and the ionomer-free PTE fabrication.
figure 1

The higher flow-chart describes the standard means of PTE/CCM fabrication, and the decrease flow-chart describes the proposed ionomer-free PTE fabrication. A schematic of the PEM electrolyzer is proven in the fitting backside nook.

Earlier research point out that anode catalyst layer in-plane digital conductivity limits the catalyst utilization particularly at low loading, due to this fact necessitating improved interfacial PTL/catalyst-layer contact30,31,32. Methods comparable to including microporous layer (MPL) or direct interfacial modification have been utilized to enhance PEMWEs efficiency33,34. In comparison with CCMs, PTEs can have benefits in forming a greater contact between catalyst layer and PTL by lowering catalyst layer deformation upon membrane swelling, due to this fact permitting higher catalyst utilization particularly at low loading. Nevertheless, direct ink coating on the PTL to manufacture PTE typically encounters challenges as a result of porous nature of the PTL, resulting in ink intrusion and low PEMWE efficiency. Our proposed PTE fabrication method eliminates two processes from the standard fabrication method: i) catalyst synthesis and ii) catalyst-ink fabrication. After refining of the iridium, it’s made right into a type of a goal, which we use to straight coat a layer of iridium to the PTL by bodily vapor deposition (PVD), forming a nanosized catalyst layer nicely adhered to the PTL, eliminating using ionomer binder to keep up catalyst-layer integrity, which reduces capital value. The direct Ir PVD technique is scalable as it’s facile and is a mature course of for industrial makes use of comparable to in anti-reflective35, textile36, and solar-cell functions37, and since it’s a line-of-sight technique, it solely coats catalyst on the interface as an alternative of coating all the PTL internal and outer surfaces as it could do in different processes (e.g., electroplating).

The floor morphology of the ionomer-free Ir PTE is proven in Fig. 2. Microscopically, a skinny layer of Ir particles is coated on high of the PTL floor (Fig. 2a vs. 2b, PTE vs. PTL). Zoomed-out scanning electron microscope (SEM) picture (Fig. 2c) of PTE signifies that the floor macroscopic options of PTL stays unchanged after PVD coating. The correspondent Vitality-dispersive spectroscopy (EDS) measurement (Fig. second) confirms a uniform Ir coating solely on the floor of the PTL, whereas exhibiting no presence of Ir at open floor pores; due to this fact, indicating negligible Ir penetrating into the PTL throughout coating. A small oxide peak within the Ir part is proven by X-ray photoelectron spectroscopy (XPS) measurement, nonetheless the X-ray diffraction (XRD) measurement solely reveals Ir steel diffraction peaks – indicating that the Ir oxide layer shaped on the PTE is both too skinny to be detected by XRD or amorphous (Supplementary Fig. 1). The cross-sectional picture from FIB-SEM verifies {that a} skinny Ir layer is achieved after coating at typical thickness of 90–100 nm (Supplementary Fig. 2), and it signifies the uniformity of the coated catalyst layer at such a low loading of 0.085 mgIr cm−2 (Fig. 2e), as does the synchrotron X-ray computed tomography (XCT) photographs of the PTE (Fig. 2f, g). The affect of thermal annealing at varied temperatures on ionomer-free PTE can also be investigated, the outcome signifies that further posttreatment is just not wanted, which simplifies the ionomer-free PTE fabrication (Supplementary Dialogue 1, Supplementary Figs. 3 and 4).

Fig. 2: Structural and morphological options of the ionomer-free PTE.
figure 2

a SEM picture of the Ir coated PTE, exhibiting the presence of Ir nanoparticles. b Uncoated PTL. c Zoomed-out SEM picture of the PTE. d Correspondent EDS picture of c, exhibiting the distribution of Ir throughout PTE floor. e Focus-ion beam scanning electron microscope (FIB-SEM) picture exhibiting the cross part of the ionomer-free Ir PTE. f Synchrotron XCT picture of the PTE. g Coated Ir-layer obtained from f.

PEMWE efficiency and the position of ionomer

We first evaluate the ionomer-free PTE with that of a standard PTE on the similar anode loading situations of ~0.1 mgIr cm−2, fabricated by coating catalyst ink on to PTL floor (outlined as sprayed-PTE) utilizing an ultrasonic sprayer. As proven in Fig. 3a, the sprayed-PTE carried out considerably worse in comparison with the ionomer-free PTE, with a lot increased ohmic and mass-transport overpotentials in comparison with the ionomer-free Ir PTE (Supplementary Fig. 5). Furthermore, the ionomer-free Ir PTE supplies considerably higher electrode kinetics, as demonstrated by decrease kinetic overpotential (Fig. 3b) and decrease Tafel slope (Fig. 3c). Direct ink coating to PTL floor results in catalyst-ink penetration by floor pores of the PTL and depositing inside the PTL—thereby shedding energetic catalysts and leading to inhomogeneous coating in addition to extraordinarily low catalyst utilization (Supplementary Fig. 6). In distinction, the ionomer-free PTE has Ir coating solely on the interfacial floor of the PTL, resulting in negligible catalyst loss and thus maximizing deposited catalyst utilization. The strategy of direct Ir coating not solely curtails the price of PTE manufacturing processes but additionally eliminates security hazards as a result of it doesn’t require catalyst-ink fabrication, which use extremely flammable solvents at industrial scales38.

Fig. 3: Electrochemical efficiency of assorted PTEs.
figure 3

Comparability of a polarization curves and b kinetic overpotentials. c Measured Tafel slopes among the many ionomer-free PTE, ionomer-coated PTE, and conventional ultrasonic spray coated PTE. d Polarization curves of the ionomer-free and ionomer-coated Ir for oxygen-evolution response in a microelectrode setup. The error bars characterize the unfold between two unbiased experiments.

To analyze the affect of ionomer in PTEs on PEMWE efficiency, we coat an extra layer of perfluorosulfonic-acid (PFSA) ionomer (Nafion) on the Ir PTE floor (outlined as ionomer Ir PTE) on the similar anode loading situations (Supplementary Fig. 6). Surprisingly, the ionomer Ir PTE exhibited increased overpotentials in comparison with the ionomer-free Ir PTE (Fig. 3a), with elevated ohmic loss (Supplementary Fig. 5) particularly at excessive present densities. That is doubtless on account of that increased oxygen-transport resistance by the ionomer part results in oxygen bubble accumulation close to catalyst floor at excessive currents, which in return can affect reactant water provide and leads to native dehydration of the polymer electrolyte. Moreover, the ionomer-coated Ir PTE displays extra sluggish electrode kinetics in comparison with ionomer-free PTE, as indicated by increased kinetic overpotential (Fig. 3b) and Tafel slope (Fig. 3c). The upper kinetic overpotential is suspected to be on account of potential ionomer poisoning of the catalyst floor. Within the case of PEM gasoline cells, the particular adsorption of ionomer sulfonic-acid teams on Pt floor has additionally been instructed to poison catalyst energetic websites, impacting the oxygen-reduction-reaction kinetics no less than from rotating disk electrode (RDE) checks39,40,41. We due to this fact hypothesize that related ionomer adsorption conduct can happen to Ir floor, which may impair OER kinetics. A microelectrode setup (Supplementary Fig. 7) is used to additional reveal the ionomer poisoning impact to Ir42,43. Microelectrode has a really small energetic space (75 µm diameter) and thus has a really low whole present (Supplementary Fig. 8), minimizing the contribution from ohmic losses. Due to this fact, the distinction within the efficiency is dominated by electrode kinetics and mass transport. In comparison with standard RDE measurement, microelectrode primarily depends on the catalyst/polymer-electrolyte interface for cost switch, which is extra consultant to a membrane-electrode meeting (MEA) units. A naked Ir microelectrode (ionomer-free) was first utilized to measure the OER exercise adopted by re-measuring after dipping in Nafion ionomer dispersion to make the ionomer-coated microelectrode. The OER polarization curve obtained utilizing microelectrode signifies an inhibited OER exercise for ionomer-coated Ir in comparison with the ionomer-free Ir (Fig. 3d). Each the MEA and microelectrode outcomes point out that ionomer within the catalyst layer can affect electrode kinetics, probably by a poisoning impact for oxygen evolution response.

Impression of Ir loadings and interfaces on PEMWE efficiency

Iridium loading discount is an important step in the direction of realizing GW-scale electrolyzers. Our proposed ionomer-free Ir PTE is nicely suited to low loading and ultra-low loading situations. The electrochemical efficiency of the ionomer-free Ir PTEs underneath 4 completely different Ir loadings of 0.033, 0.050, 0.085, and 0.187 mgIr cm−2 is given in Fig. 4. The applied-voltage breakdown of ultra-low loaded ionomer-free Ir PTEs is present in Supplementary Fig. 9. Because the Ir loading will increase from 0.033 to 0.085 mgIr cm−2, there’s an enhancement in electrolyzer efficiency; nonetheless, additional improve in loading to 0.187 mgIr cm−2 results in negligible efficiency enchancment, which is probably going on account of a formation of a easy and dense catalyst layer on the PTE (Supplementary Fig. 10), lowering the floor space of the Ir layer which dwindles the variety of energetic websites accessible for the OER. The rise in double-layer capacitance plateaus after 0.085 mgIr cm−2 (Fig. 4b), suggesting that growing Ir loading is just not essentially the most environment friendly manner to enhance the ionomer-free Ir PTE efficiency. As a substitute, we discover different attainable methods by engineering the PTL floor.

Fig. 4: The affect of catalyst loading on the ionomer-free Ir PTE and the improved electrolyzer efficiency with enhanced interface by way of laser ablation.
figure 4

a Polarization curves for ionomer-free Ir PTE at 0.033, 0.050, 0.085, and 0.187 mgIr cm−2. b double-layer capacitance measured at completely different loadings and the laser ablated ionomer-free Ir PTE. c Polarization curve. d Kinetic overpotential measured for the laser ablated ionomer-free Ir PTE at 0.085 mgIr cm−2. The inset picture plots Tafel slopes for the laser ablated and baseline ionomer-free Ir PTE (47.2 mV dec−1 vs 58.3 mV dec−1). e SEM picture of naked PTL floor. f SEM picture of laser ablated PTL floor. The error bars characterize the unfold between two unbiased experiments.

Because the Ir layer is supported by the PTL to type PTE, one pathway to maximise the variety of energetic websites is to enhance the floor roughness of PTL, the place extra Ir can adhere to. Right here we make the most of a laser ablation method to attain a rougher PTL floor. Particularly, the molten construction of the titanium created by the warmth from the laser ablation closes smaller pores present on the PTL floor, leading to an elevated floor space for electrochemical response to happen (Fig. 4e vs. 4f, naked PTL vs. laser ablated PTL). In comparison with the baseline ionomer-free Ir PTE, the PTE fabricated utilizing laser ablated PTL (outlined as laser ablated ionomer-free Ir PTE) displays improved efficiency (most voltage discount of 56 mV at 4 A cm−2) all through the present densities examined within the polarization curve (Fig. 4c). That is principally on account of enhanced electrochemical floor space of the Ir layer (Fig. 4b), resulting in decrease kinetic overpotential (Fig. 4d) and Tafel slope (47.2 mV dec−1 in comparison with 58.3 mV dec−1, inset of Fig. 4d). To additional reveal the effectiveness of laser ablation to enhance ionomer-free Ir PTE efficiency, we ablated fiber primarily based PTL as proven in Supplementary Fig. 11. The ionomer-free Ir PTE fabricated utilizing laser ablated fiber PTL (Supplementary Fig. 11b) exhibits improved electrolysis efficiency at most voltage discount of 53 mV at 4 A cm−2 (Supplementary Fig. 11c) in comparison with the baseline fiber PTL (Supplementary Fig. 11a), which is basically pushed by improved electrode kinetics (Supplementary Fig. 11d) by improved interfacial space. The efficiency of ionomer-free Ir PTE underneath ultra-low loading offered on this work outperforms state-of-the-art PTEs working at related situations (N117, 80 °C) reported in literature with a 28-fold lower in Ir loading (see Supplementary Desk 1)33,44,45,46,47,48,49,50.

Sturdiness of the ionomer-free Ir PTE

As long-term stability is a vital piece of the electrode design, the soundness of ionomer-free PTE is evaluated by an accelerated stress take a look at (AST). A sq. wave potential biking between 1.45–2.2 V was utilized to the ionomer-free Ir PTE with 5 s maintain at every potential. The AST protocol was chosen primarily based on a complete examine by Alia et al. 51, the place they discovered that it induced extra catalyst-layer degradation in comparison with fixed present maintain. Therefore, this AST protocol is an indicator of the catalyst sturdiness of the ionomer-free Ir PTE electrolyzer. A 5 cm2 ionomer-free Ir PTE was assembled in a PEMWE and underwent a complete of 50k AST cycles with polarization curve and galvanostatic electrochemical impedance spectroscopy (EIS) measurements recorded in between.

The polarization curve acquired from the AST exhibits change of solely 29 mV distinction at 4 A cm−2 after 50k cycles (Fig. 5a) at common price of 0.58 mV per 1000-cycles, indicating glorious sturdiness of the ionomer-free PTE for PEMWE utility when evaluating to the literature reporting degradation price of 4.51 mV per 1000-cycles at 2 A cm−2 in a CCM configuration with Ir loading of 0.1 mgIr cm−2 at related situations51. Despite the fact that a small gradual improve in Tafel slopes is noticed throughout ASTs, its affect on efficiency has been largely offset by lower in excessive frequency resistance (HFR) (Fig. 5b) maybe by continued conditioning, resulting in an total insignificant efficiency penalty throughout AST. Measured EIS additionally demonstrated no signal of noticeable degradation after 50k cycles (Supplementary Fig. 12). The XRD patterns of PTE after AST exhibited negligible distinction in comparison with pristine PTE (Fig. 5c), indicating the majority part of ionomer-free Ir PTE remained intact by the AST. XPS of the Ir after the AST exhibits dominant presence of Ir oxides in comparison with pristine PTE (Fig. 5d), whereas no apparent Ir oxides peak is noticed by XRD, indicating a development of amorphous floor oxides thickness, which explains the gradual improve of Tafel slopes talked about above. X-ray fluorescence (XRF) mapping of Ir illustrates that the PTE nonetheless has uniform distribution of Ir-layer coated on the PTL floor even after 50k AST cycles (Supplementary Fig. 13). The remaining Ir loading of the PTE after AST combines the Ir loading transferred to membrane aspect make as much as a complete of 0.075 mgIr cm−2 measured utilizing XRF, exhibiting a complete lack of simply 0.01 mgIr cm−2 (vs. 0.085 mgIr cm−2) after 50k cycles. A continuing-current maintain sturdiness take a look at was additionally carried out utilizing ionomer-free PTE to make sure secure operation (Supplementary Fig. 14). These outcomes clearly point out that the ionomer-free Ir PTE displays excellent sturdiness for PEMWE functions.

Fig. 5: Sturdiness analysis of ionomer-free Ir PTE in PEMWE utilizing accelerated stress take a look at as much as 50k cycle.
figure 5

a Polarization curves recorded at varied cycles by ASTs. b Tafel slopes and HFR measured by AST. c Comparability of XRD patterns. d XPS measurements of Ir earlier than and after 50k cycles of AST.

Recyclability of the Ionomer-free Ir PTEs

Recycling is a prerequisite to deployment of GW-scale electrolysis crops however has been pretty underneath investigated within the subject. Whereas recycling PGM supplies utilized in catalysts are of utmost significance, recycling different pricey parts, comparable to membrane or porous transport layers, additionally considerably reduces value for giant scale functions. This part research the feasibility of recycling PTEs and half CCMs used within the PEM electrolyzer with out going by extreme processes comparable to pyrometallurgical or hydrometallurgical extractions. Eliminating the ionomer within the catalyst layer tremendously simplifies the recycling course of in comparison with standard MEA, because it avoids the implications of producing poisonous pollutant and want for ionomer recycling from fluorine moieties52. Moreover, the PTE configuration naturally has advantages in recycling the PEM because it avoids catalyst coating to the membrane aspect throughout fabrication. Put up AST testing, we appeared into three potential recycling situations: i) changing the degraded PTE with a pristine PTE and pairing with the used half CCM (Nafion 117 + cathode), ii) recoating Ir catalysts over degraded Ir catalyst layer of PTE to after AST and pairing with recent half CCM, and iii) recoating the Ir catalysts on the opposite aspect of the PTE after AST (i.e., coating on the aspect initially dealing with stream subject) and pairing with recent half CCM. These three situations have related Ir loadings of 0.085 mgIr cm−2 and examine completely different pathways of recyclability. The primary state of affairs seeks for the feasibility of increasing the lifetime of half CCM by changing the degraded anode PTE with a recent PTE, as degradation of MEA typically comes from anode aspect on account of harsh OER environments. Bettering lifetime of half CCM additionally provides considerably to the associated fee discount as Pt is used for the cathode catalyst in addition to the membrane. The second state of affairs research direct recycling of the ionomer-free PTE after AST, which permits the associated fee discount of not solely the Ir catalysts but additionally Ti PTLs, if electrolyzer efficiency could be regained after merely recoating the degraded PTE. The final state of affairs is an extension of state of affairs 2, which investigates efficiency enchancment when a recent Ir catalyst layer is coated onto the much less degraded interface between PTL and stream subject.

The polarization curves measured for 3 completely different situations are proven in Fig. 6, compared to the pristine PTE and PTE after 50k AST cycles. Direct comparability is proven in Supplementary Fig. 15. All three recycling situations demonstrated restoration of electrolysis efficiency in comparison with the publish AST PTE and exhibited related efficiency to the pristine ionomer-free Ir PTE (Fig. 6a–c). Moreover, the measured values of Tafel slopes in comparison with PTE after AST signifies a restoration of electrode kinetics for all three situations (Fig. 6d). Eventualities 1 and three exhibit higher restoration in comparison with state of affairs 2, which signifies a extra extreme degradation on the PTL/CL interfaces after ASTs. Total, three situations reveal the feasibility of facile course of to recycle varied degraded parts of the ionomer-free PTE MEA, which is essential to create a round hydrogen financial system.

Fig. 6: Evaluating electrolyzer efficiency with recycled catalysts primarily based on completely different recycling situations.
figure 6

a Changing degraded PTE with a pristine PTE. b Recycling degraded PTE by reapplying Ir coating and pairing with a pristine half CCM. c Recycling PTL by reapplying Ir coating on the again aspect and utilizing a pristine half CCM. d Measured Tafel slopes for all examined situations.

In abstract, we current a novel ionomer-free porous-transport electrode (PTE) exhibiting excessive efficiency and sturdiness for proton-exchange-membrane water electrolyzers. This ionomer-free Ir PTE outperforms a conventional PTE by 652 mV at 1.8 A cm−2 at an analogous loading and displays glorious sturdiness of solely 29 mV distinction at 4 A cm−2 after 50k cycles of accelerated stress take a look at. From each the membrane-electrode-assembly and microelectrode measurements, there’s a potential ionomer poisoning impact on Ir impacting oxygen-evolution response. Eliminating the ionomer from the catalyst layer not solely improves efficiency of the electrolyzer by mitigating the impact of ionomer poisoning, but additionally considerably eases processes of fabrication and recycling. The design of the ionomer-free Ir PTE permits facile alternative of electrodes after long-term operation, with efficiency restoration to that of the pristine electrolyzer. This ionomer-free Ir PTE presents a promising paradigm shift within the electrolyzer business that probably permits profitable deployment of GW-scale crops to supply cost-efficient clear hydrogen for decarbonization in varied sectors.

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