While the current pause in global activity due to the coronavirus pandemic is a medical and economic tragedy, it is perhaps a breath of fresh air for planet Earth in terms of fossil fuel use and the emission of greenhouse gases which are responsible for long-term climate change. I am sure that I am not alone in hoping that this crisis can serve as an opportunity to redefine how we consume energy so that we can sustain our temporary dip in CO₂ emissions. However, if history is a guide, then I am hesitant to place too much hope in this possibility. Indeed, looking at the 2008–2009 global financial crisis, the significant economic slowdown and concurrent decrease in CO₂ emission rate that occurred during this period was quickly followed by strong growth in emissions from emerging economies and a resumption of emissions growth in developed economies—reverting global CO₂ emissions to before-crisis rates after only one year.(1) Thus, perhaps it is more reasonable to assume that once the current crisis subsides, our challenge to mitigate the effects of anthropomorphic climate change will remain clear and present. A sought-after route toward eliminating our dependence on fossil fuels has long been the development of scalable and economically feasible methods to convert energy from our largest renewable source, the Sun, into chemical fuels.(2) The splitting of water into H₂ and O₂ is the natural target transformation in this regard, and as such, photoelectrochemical (PEC)(3) and heterogeneous photocatalytic (PC)(4) approaches to split water using sunlight (shown schematically in Figure 1) have attracted great attention, as these systems integrate the light harvesting and fuel-producing reactions at a direct semiconductor/liquid junction—potentially enabling solar water splitting at lower cost compared to photovoltaic-electrolyzer technology.(5) Briefly, to optimize photopotential and solar light harvesting with these approaches, two subsystems are typically used: a hydrogen evolving photocatalyst (HEP) and an oxygen evolving photocatalyst (OEP) in the PC approach, and analogously a photocathode and a photoanode in tandem(6) for the PEC approach. Included cocatalysts lower energetic barriers for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). Because the overall process mimics the “Z-scheme” of natural photosynthesis it is often termed “Z-scheme” artificial photosynthesis. Figure 1. Schematic of the prototypical artificial photosynthetic reaction (solar water splitting) with the (a) photoelectrochemical tandem cell and (b) PC Z-Scheme approaches. The OER/HER catalysts are not shown in panel a for clarity. While PEC systems are undeniably useful for research, from a practical perspective (namely the difficulties in managing large-area installations of photoelectrodes in contact with water) attaining economic viability with the PEC approach may be an insurmountable challenge. I am personally not convinced that we will ever see PEC tandem cells producing H₂ at a global scale. However, the PC approach seems more feasible because of its intrinsic simplicity. Nanometer or micrometer sized photocatalyst particles can be simply dispersed in a covered pond, trough, or pipe-based photoreactor and the evolved H₂ is easily collected. A dual-bed system allows separate operation of the HEP and the OEP, which communicate via a redox shuttle. This straightforwardness means the PC approach can be the most feasible global-scale solar-to-hydrogen production method, but only if photocatalyst materials with a solar-to-hydrogen conversion efficiency of 5–10% and a lifetime on the order of years can be identified.(7) However, despite decades of research applying conventional inorganic semiconductors as light harvesters to both PEC and PC approaches (e.g., silicon,(8) III–V compounds,(9) chalcogenides,(10) and metal oxides(11)), systems that combine economic viability, sufficient stability, and high efficiency have yet to be identified. In an effort to explore alternatives, π-conjugated organic semiconductors (OSs) have come into view as possible light-harvesting materials for artificial photosynthesis.(12−16) Certainly, their unique ability to be engineered at the molecular level (affording precise control over optoelectronic properties), and their solubility in common solvents (allowing inexpensive processability) are advantages, but could they really reach the level of performance required for practical implementation? Indeed, this question is by no means clear when looking at the published reports. Considering OS-based photocathodes for H₂ production, a typical system, consisting of a bulk-heterojunction of poly(3-hexylthiophene) and phenyl-(C₆₁ or C₇₁ fullerene)-butyric acid methyl ester (P3HT:PCBM), shows irreversible photocurrent decrease after only minutes of operation due to material corrosion. The use of metal oxide overlayers protecting the OSs can somewhat increase stability,(17) but it is not clear if this strategy is suitable to maintain stability for the year time scale. When applying OSs as photocatalyst particles, a typical strategy has been to use covalent network polymers, cross-linked polymers, or covalent organic frameworks as they form robust porous materials with high surface area for catalysis,(12) and residual Pd from their synthesis can drive the HER.(18) While the tunability of these systems has been demonstrated,(19) the solar-to-hydrogen efficiency has been well below that of the state-of-the-art inorganic particle photocatalysts. Indeed, similar to inorganic photocatalyst particles, organic network polymers and framework photocatalyst materials do not integrate a strategy for the separation of photogenerated charge carriers. To their further detriment, given the dielectric properties of OSs, an electrostatically bound electron–hole pair (an exciton) is created upon light absorption, which normally must be separated into free charge carriers before energy can be harvested. This adds another challenge toward the high-efficiency operation. However, recently a solution to the problem of charge separation in OS-based PC particles was elegantly demonstrated in a contribution by Kosco et al., wherein an intermixed heterojunction between a donor polymer and small-molecule acceptor was integrated into organic nanoparticles prepared via a simple solution-processing mini-emulsion technique.(20) The resulting nanoparticle dispersions with included Pt cocatalyst resulted in H₂ evolution at an order-of-magnitude greater rate than control particles (over 60 mmol h^–1 g^–1 with illumination from 350 to 800 nm) and reached external quantum efficiencies over 6% in the region of maximum solar photon flux. While this “bulk-heterojunction nanoparticle” approach represents a promising new path for organic semiconductor photocatalysts, there are certainly important points to consider regarding its development toward practical application. The first is stability. Could these nanoparticles actually be stable for years under operation? Looking at the field of organic photovoltaics, the stability of OSs has been an important point of investigation.(21) Under operation in inert environments (even under harsh heating up to 150 °C), optimized organic semiconductor components show excellent intrinsic photostability(22) and the bulk-heterojunction appears to be very robust.(23) Encapsulated organic photovoltaics have achieved T₈₀ lifetimes of 2 years in large-scale outdoor tests,(24) suggesting that organic semiconductors are inherently stable to a degree sufficient for functioning as photocatalysts for the time scales required. However, because their operation in the presence of water and O₂ remains a concern for the long-term lifetime of organic photovoltaics, is it reasonable to consider that OSs could actually survive the harsh conditions of PC or PEC water splitting? Recently, our lab looked closely at criteria for stable operation of bulk-heterojunction photocathodes for solar H₂ production.(25) We found that replacing the commonly used fullerene-based electron acceptor with a perylene diimide-based polymer significantly increases performance (exhibiting external quantum efficiency under bias close to a maximum 50%) and operational stability. Moreover, limiting the photogenerated electron accumulation at the OS/water interface to values below ∼100 nC cm^–2 was identified as a condition to avoid the irreversible reduction of the organic semiconductors in contact with water. While this is a challenge considering the kinetic bottleneck for the HER, optimizing the composition and morphology of the cocatalyst overlayer was found to mitigate charge accumulation. In the best case, without an encapsulating overlayer, continuous solar-driven H₂ production for over 20 h resulted in only a one-third drop from the initial performance, which was due to the poor adhesion of the HER catalyst to the OSs. While this performance is still far from the established performance of organic photovoltaic devices, it represents a significant step toward increasing the stability of direct OS/liquid junction-based systems. A second aspect to consider regarding the newly demonstrated bulk-heterojunction photocatalyst approach is O₂ evolution. Indeed, it should be emphasized that the overall water-splitting reaction was not demonstrated with the approach of Kosco et al., as ascorbic acid was used as a sacrificial electron donor (as is common in photocatalyst research). To fully realize the promise of organic photocatalysts by completing the Z-Scheme with bulk heterojunction OEPs, much more effort will be needed to develop OSs for stable water oxidation, because their current performance is even poorer than for H₂ production. Indeed, with photoanodes,(26) while single-component thin films of various OS materials have shown suitable energy levels to drive solar water oxidation, external quantum efficiencies have been limited to ∼1% in part because of the poor generation of free charge carriers. Certain OS materials appear stable over 30 min of continuous operation,(27) but the typical stability of an OS-based photoanode is only a few minutes. Thus, a logical next step for the field will be to investigate the stability of bulk-heterojunction OS-based photoanodes with nonfullerene components and quantify the relationship between charge accumulation and stability. However, given the known sensitivity of OS materials to O₂ under operation from efforts in the organic photovoltaic community, and the larger kinetic bottleneck posed by the sluggish OER, a new generation of OS materials may be needed to overcome intrinsic stability issues in this case. Overall, many open questions remain regarding the long-term stability of using OSs for PEC and PC artificial photosynthesis. For example, what are the limits to operational stability if charge accumulation is continuously kept low during operation by using suitable charge extraction layers/catalyst overlayers? How can tuning hydrophilicity or hydrophobicity via molecular design be leveraged to enhance stability? Can encapsulation techniques using protective oxide overlayers be implemented on OS nanoparticles to stabilize PC operation? Can the organic/inorganic interface be engineered to prevent detachment of cocatalysts? I am optimistic that pursuing answers to these questions will not only reveal important insights into the stability of OSs under the harsh conditions of PEC or PC water splitting but may also identify routes to enabling performance at the level required to make solar water splitting with OS nanoparticle dispersions a practical alternative to photovoltaic-electrolyzer technology. There is a significant amount of work left to do in this field and in all fields looking to develop energy technologies for our sustainable future, so I hope we can all get back to the lab quickly and safely. Views expressed in this editorial are those of the author and not necessarily the views of the ACS. This article references 27 other publications.

中文翻译：

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有机半导体是否有能力进行稳固，高效的人工光合作用？

虽然目前由于冠状病毒大流行而导致的全球活动停顿是医学和经济方面的悲剧，但就化石燃料的使用和造成长期气候的温室气体的排放而言，这也许是地球呼吸新鲜空气的原因更改。我敢肯定，我并不孤单地希望这场危机能够为我们提供一个机会，以重新定义我们的能源消耗方式，从而使我们能够暂时减少CO ₂排放量。但是，如果以历史为指导，那么我会为这种可能性寄予太多希望。实际上，纵观2008-2009年的全球金融危机，经济显着放缓，同时CO ₂减少在此期间出现的排放速率之后，新兴经济体的排放量迅速增长，而发达经济体的排放量又恢复了增长，仅在一年后将全球CO ₂排放量恢复为危机前的水平。（1）因此，也许假设当前危机消退，我们减轻人类拟人化气候变化影响的挑战将仍然是明确的和合理的。长期以来，寻求消除对化石燃料的依赖的寻求之路是开发可扩展且经济上可行的方法，以将我们最大的可再生资源太阳转化为化学燃料。（2）将水分解为H ₂和H Ø ₂所包括的助催化剂降低了氢气释放反应（HER）和氧气释放反应（OER）的能量屏障。由于整个过程模仿自然光合作用的“ Z方案”，因此通常称为“ Z方案”人工光合作用。图1.使用（a）光电化学串联电池和（b）PC Z-Scheme方法进行的典型人工光合作用反应（太阳能水分解）的示意图。为了清楚起见，在面板a中未示出OER / HER催化剂。尽管PEC系统无疑对研究有用，但从实际角度（即管理与水接触的大面积光电极安装方面的困难），使用PEC方法实现经济可行性可能是一项不可克服的挑战。我个人不相信我们会见过PEC串联细胞产生H 由于整个过程模仿自然光合作用的“ Z方案”，因此通常称为“ Z方案”人工光合作用。图1.使用（a）光电化学串联电池和（b）PC Z-Scheme方法进行的典型人工光合作用反应（太阳能水分解）的示意图。为了清楚起见，在面板a中未示出OER / HER催化剂。尽管PEC系统无疑对研究有用，但从实际角度（即管理与水接触的大面积光电极安装方面的困难），使用PEC方法实现经济可行性可能是一项不可克服的挑战。我个人不相信我们会见过PEC串联细胞产生H 由于整个过程模仿自然光合作用的“ Z方案”，因此通常称为“ Z方案”人工光合作用。图1.使用（a）光电化学串联电池和（b）PC Z-Scheme方法进行的典型人工光合作用反应（太阳能水分解）的示意图。为了清楚起见，在面板a中未示出OER / HER催化剂。尽管PEC系统无疑对研究有用，但从实际角度（即管理与水接触的大面积光电极安装方面的困难），使用PEC方法获得经济可行性可能是一个不可克服的挑战。我个人不相信我们会见过PEC串联细胞产生H 用（a）光电化学串联电池和（b）PC Z-方案方法进行的典型人工光合作用反应（太阳能水分解）的示意图。为了清楚起见，在面板a中未示出OER / HER催化剂。尽管PEC系统无疑对研究有用，但从实际角度（即管理与水接触的大面积光电极安装方面的困难），使用PEC方法实现经济可行性可能是一项不可克服的挑战。我个人不相信我们会见过PEC串联细胞产生H 用（a）光电化学串联电池和（b）PC Z-方案方法进行的典型人工光合作用反应（太阳能水分解）的示意图。为了清楚起见，在面板a中未示出OER / HER催化剂。尽管PEC系统无疑对研究有用，但从实际角度（即管理与水接触的大面积光电极安装方面的困难），使用PEC方法实现经济可行性可能是一项不可克服的挑战。我个人不相信我们会见过PEC串联细胞产生H 从实践的角度（即在管理与水接触的大面积光电极安装方面的困难），使用PEC方法获得经济可行性可能是一个无法克服的挑战。我个人不相信我们会见过PEC串联细胞产生H 从实践的角度（即在管理与水接触的大面积光电极安装方面的困难），使用PEC方法获得经济可行性可能是一个无法克服的挑战。我个人不相信我们会见过PEC串联细胞产生H₂在全球范围内。但是，由于PC方法固有的简单性，因此似乎更可行。纳米或微米级的光催化剂颗粒可以简单地分散在有盖的池塘，槽或管式光反应器中，并且放出的H ₂很容易收集。双床系统允许HEP和OEP分别运行，它们通过氧化还原梭进行通信。这种直接性意味着PC方法可能是最可行的全球规模的太阳能制氢方法，但前提是光催化材料的太阳能制氢效率为5–10％，寿命可长达数年之久。 （7）然而，尽管数十年来的研究将传统的无机半导体作为光收集器应用于PEC和PC方法（例如，硅，（8）III–V化合物，（9）硫族化物，（10）和金属氧化物（ 11）），尚未确定结合经济可行性，足够的稳定性和高效率的系统。为了探索替代方案，π共轭有机半导体（OSs）已被视为可能用于人造光合作用的光收集材料。（12-16）当然，它们在分子水平上具有独特的工程设计能力（能够精确控制光电子性能）以及在普通溶剂中的溶解性（允许廉价的可加工性）是优点，但是它们真的能达到实际应用所需的性能水平吗？确实，在查看已发布的报告时，这个问题并不明确。考虑基于H的基于OS的光电阴极 但是它们真的可以达到实际实施所需的性能水平吗？确实，在查看已发布的报告时，这个问题并不明确。考虑基于H的基于OS的光电阴极 但是它们真的可以达到实际实施所需的性能水平吗？确实，在查看已发布的报告时，这个问题并不明确。考虑基于OS的H的光电阴极₂生产中，一个典型的系统，由聚（3-己基噻吩）和苯基-（C ₆₁或C ₇₁）的本体-异质结组成富勒烯-丁酸甲酯（P3HT：PCBM），由于材料腐蚀，仅在操作几分钟后就显示出不可逆的光电流下降。保护操作系统的金属氧化物覆盖层的使用可以在某种程度上增加稳定性，[17]但尚不清楚该策略是否适合在一年中维持稳定性。当将OS用作光催化剂颗粒时，一种典型的策略是使用共价网络聚合物，交联聚合物或共价有机骨架，因为它们会形成具有高表面积的坚固多孔材料以进行催化（12），并且合成过程中会残留Pd （18）虽然已经证明了这些系统的可调性，但（19）太阳能到氢的效率已经远远低于最新的无机颗粒光催化剂的效率。确实，与无机光催化剂颗粒相似，有机网络聚合物和骨架光催化剂材料没有整合光生载流子分离策略。考虑到OS的介电性能，对它们的进一步损害是，在吸收光后会形成一个静电结合的电子-空穴对（激子），通常必须将其分离成自由电荷载流子，然后才能收集能量。这给高效运行增加了另一个挑战。但是，最近，Kosco等人的一篇论文很好地证明了基于OS的PC颗粒中电荷分离问题的解决方案，其中将供体聚合物和小分子受体之间的混合异质结整合到通过纳米管制备的有机纳米颗粒中简单的溶液处理微乳液技术。₂以比控制粒子大的数量级速率演化（超过60 mmol h ^–1 g ^–1（在350至800 nm的光照下），并且在最大太阳光子通量范围内达到超过6％的外部量子效率。虽然这种“本体-异质结纳米粒子”方法代表了有机半导体光催化剂的有前途的新途径，但在其向实际应用的发展方面肯定要考虑一些重要的观点。首先是稳定性。这些纳米粒子实际上可以在运行多年后保持稳定吗？纵观有机光伏领域，OS的稳定性一直是研究的重点。（21）在惰性环境下操作（即使在高达150°C的苛刻加热条件下），优化的有机半导体组件也具有出色的固有光稳定性（22） ），并且体异质结似乎非常坚固。（23）封装的有机光伏已经实现在大规模室外测试中，T _80的寿命为2年，[24]表明有机半导体固有地稳定到一定程度，足以在所需的时间范围内用作光催化剂。但是，由于它们在水和O ₂的存在下的操作仍然是有机光伏电池长期使用寿命的考虑因素，是否合理地考虑到OS可以在PC或PEC分解水的苛刻条件下生存？最近，我们的实验室仔细研究了用于太阳能H ₂的大体积异质结光电阴极的稳定运行标准（25）我们发现，用基于di二酰亚胺的聚合物代替常用的基于富勒烯的电子受体显着提高了性能（在接近最大50％的偏压下展示了外部量子效率）和操作稳定性。此外，确定将OS /水界面处的光生电子积累限制在约100 nC cm ^-2以下是避免与水接触的有机半导体不可逆还原的条件。尽管考虑到HER的动力学瓶颈是一个挑战，但发现优化助催化剂覆盖层的组成和形态可减轻电荷积累。在最好的情况下，在没有封装覆盖层的情况下，连续的太阳能驱动的H ₂超过20小时的生产仅使初始性能下降了三分之一，这是由于HER催化剂对OS的粘附力差所致。尽管此性能仍远未达到有机光伏器件的既定性能，但它代表了提高直接基于OS /液结的直接系统稳定性的重要一步。关于新证明的本体-异质结光催化剂方法要考虑的第二个方面是O ₂演化。实际上，应该强调的是，由于抗坏血酸被用作牺牲电子给体（在光催化剂研究中很常见），因此没有用Kosco等人的方法证明整体的水分解反应。要通过完成带有本体异质结OEP的Z方案来完全实现有机光催化剂的前景，将需要花费更多的精力来开发用于稳定水氧化的OS，因为它们的当前性能甚至比H ₂还要差。生产。的确，使用光阳极，（26）虽然各种OS材料的单组分薄膜显示出合适的能级来驱动太阳能氧化，但外部量子效率被限制在〜1％，部分原因是由于自由电荷载流子的生成较差。某些OS材料在连续运行30分钟后似乎很稳定（27），但是基于OS的光电阳极的典型稳定性只有几分钟。因此，该领域的逻辑下一步将是研究具有非富勒烯组分的基于本体异质结OS的光阳极的稳定性，并量化电荷积累与稳定性之间的关系。但是，已知OS材料对O _2的敏感性在有机光伏界的努力下运行，以及缓慢的OER造成更大的动力学瓶颈，在这种情况下可能需要新一代OS材料来克服固有的稳定性问题。总体而言，关于将操作系统用于PEC和PC人工光合作用的长期稳定性，仍然存在许多悬而未决的问题。例如，如果使用合适的电荷提取层/催化剂覆盖层，在运行过程中将电荷积累持续保持在较低水平，对运行稳定性有何限制？如何通过分子设计调节亲水性或疏水性以增强稳定性？可以在OS纳米颗粒上实施使用保护性氧化物覆盖层的封装技术来稳定PC操作吗？是否可以设计有机/无机界面来防止助催化剂的分离？我很乐观，寻求这些问题的答案不仅可以揭示在PEC或PC水分解的苛刻条件下对OS稳定性的重要见解，而且还可以确定实现OS纳米颗粒使太阳能水分解所需水平的性能的途径。分散液的替代品 光伏电解技术。在这个领域以及在为可持续发展的未来开发能源技术的所有领域中，还有大量工作要做，因此，我希望我们所有人都能安全快捷地回到实验室。本社论中表达的观点只是作者的观点，不一定是ACS的观点。本文引用了其他27个出版物。