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With high scalability and independent control over energy and power, redox flow batteries (RFBs) stand out as an important large-scale energy storage system. However, the widespread application of conventional RFBs is limited by the uncompetitive performance, as well as the high cost and environmental concerns associated with the use of metal-based redox species. In consideration of advantageous features such as potentially low cost, vast molecular diversity, and highly tailorable properties, organic and organometallic molecules emerge as promising alternative electroactive species for building sustainable RFBs. This review presents a systematic molecular engineering scheme for designing these novel redox species. We provide detailed synthetic strategies for modifying the organic and organometallic redox species in terms of solubility, redox potential, and molecular size. Recent advances are then introduced covering the reaction mechanisms, specific functionalization methods, and electrochemical performances of redox species classified by their molecular structures. Finally, we conclude with an analysis of the current challenges and perspectives on future directions in this emerging research field.

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Patients with acute-on-chronic liver failure (ACLF) represent a heterogeneous population. The aim of the study is to identify distinct groups according to the etiologies of precipitating events. A total of 405 ACLF patients were identified from 1,361 patients with cirrhosis with acute decompensation and categorized according to the types of acute insults. Clinical characteristics and prognosis between the hepatic group and extrahepatic group were compared, and the performance of prognostic models was tested in different groups. Two distinct groups (hepatic-ACLF and extrahepatic-ACLF) were identified among the ACLF population. Hepatic-ACLF was precipitated by hepatic insults and had relatively wellcompensated cirrhosis with frequent liver and coagulation failure. In contrast, extrahepatic-ACLF was exclusively precipitated by extrahepatic insults, characterized by more severe underlying cirrhosis and high occurrence of extrahepatic organ failures (kidney, cerebral, circulation, and respiratory systems). Both groups had comparably high short-term mortality (28-day transplant-free mortality: 48.3% vs. 50.7%; P 5 0.22); however, the extrahepatic-ACLF group had significantly higher 90-day and 1-year mortality (90-day: 58.9% vs. 68.3%, P 5 0.035; 1-year: 63.9% vs. 74.6%, P 5 0.019). In hepatic-ACLF group, the integrated Model for End-Stage Liver Disease (iMELD) score had the highest area under the receiver operating characteristic curve (auROC 5 0.787) among various prognostic models in predicting 28-day mortality, whereas CLIF-Consortium scores for ACLF patients (CLIF-C-ACLF) had the highest predictive value in the other group (auROC 5 0.779). Conclusions: ACLF precipitated by hepatic insults is distinct from ACLF precipitated by extrahepatic insults in clinical presentation and prognosis. The iMELD score may be a better predictor for hepatic-ACLF short-term prognosis, whereas CLIF-C-ACLF may be better for extrahepatic-ACLF patients. (HEPATOLOGY 2015;62:232-242)